- Section of the corpus callosum has come to be used increasingly as a treatment for medically intractable epilepsy (23, 101, 102, 105, 117). When the callosotomy is complete, it is regularly followed by a wide variety of neurological and neuropsychological deficits (13, 22, 142, 143). The deficits that are most apparent immediately after operation make up the acute disconnection syndrome (133). Those deficits that persist constitute the stabilized syndrome of hemisphere disconnection (13, 15, 16, 103, 104, 122). Partial section of the corpus callosum, as the usual first stage for treating epilepsy with callosotomy or to facilitate the operative approach to deep midline lesions, may produce a fraction of the full syndrome, depending on the part of the corpus callosum that has been cut as well as the amount and nature of any associated extracallosal damage (107).
- In general, most of the disconnection deficits, both acute and chronic, are not present as long as the splenium is spared (4, 35, 60, 99). However, certain partial sections do entail a few inevitable deficits and some common complications, the recognition of which is important for the surgeon using the transcallosal approaches (38, 59, 78, 103, 106). In this chapter, J present first the syndromes (acute and chronic) consequent to a complete cerebral commissurotomy (including the corpus callosum and the underlying dorsal hippocampal commissure, the ventral hippocampal commissure between the fornices, the massa intermedia, and the anterior commissure). This is followed by comments on those fractions of the full-blown syndrome that are apt to appear with different partial sections. One should keep in mind that callosal lesions--surgical and naturally occurring--are often accompanied by damage to or pressure on neighboring structures, resulting in several distinct types of signs: (a) signs of hemisphere disconnection, (b) neighborhood signs, and (c) nonlocalizing signs such as meningismus or signs of increased intracranial pressure. In this chapter, attention is given mainly to signs of the first type. A recent history of studies of the corpus callosum explains how the disconnection signs have come to be understood and emphasized (13). Other extensive reviews are available (8, 18, 30, 31, 43, 48, 55, 60, 91, 92, 128).
Historically Salient Signs
- Before callosal syndromes are considered in detail, it is useful to have an overall view of those signs of hemispheric disconnection that have been emphasized historically and whose reality has been confirmed by more recent investigations.
- After hemisphere disconnection in the human, unilateral tactile anemia, left hemialexia, and unilateral apraxia are typical. That is, a right-hander with complete cerebral commissurotomy cannot name aloud objects correctly manipulated (hence, recognized) with the left hand, cannot read aloud written material presented solely to the left half-field of vision, and cannot execute with the left hand actions verbally named or described by the examiner, although these actions are readily imitated when demonstrated. The apraxia usually recedes within a few months, whereas the hemialexia and unilateral anemia can persist for years.
- The relationship of apraxia to the corpus callosum was pointed out by Liepmann and Mass (81). They considered the corpus callosum instrumental in left-hand responses to verbal command. The verbal instruction was comprehended only by the left hemisphere, and the left hand followed instructions delivered by a route involving callosal interhemispheric transfer from left to right and then by what we now call "contralateral control" (i.e., by right hemisphere control of the left hand). Necessarily then, callosal interruption would result in an inability to follow verbal commands with the left hand, although there would be no loss of comprehension (as expected from a left hemisphere lesion) and there would be no weakness or incoordination of the left hand (as expected from a right hemisphere lesion). The notion that spatial or pictorial instructions understood by the right hemisphere may require callosally mediated interhemispheric communication for correct right-hand execution is now recognized. This right-to-left aspect of callosal function was not part of Liepmann's original callosal concept, although in retrospect it seems a natural corollary. Liepmann himself considered the left hemisphere to be the organizer of complex (particularly learned) motor behavior. Whether and in what way the left hemisphere is dominant for skilled movements generally (and not just those linguistically related) are currently matters of active controversy (64, 66, 72, 75, 141).
- Acute Disconnection Syndrome (As Observed in Right-handers Operated on From the Right Side)
- During the first few days after complete cerebral commissurotomy, patients commonly respond reasonably well with their right limbs to simple commands. However, they are easily confused by three or even two-part commands, each part of which is obviously understood. Patients often lie quietly and may seem mildly "akinetic," although cooperating when stimulated. There is sometimes an "imperviousness" resembling that often seen with naturally occurring genu lesions (2, 13). Patients are often mute even when willing to write short (usually one word) answers. This mutism is of special interest and is discussed in more detail in the section, "Postcallosotomy Mutism," in this chapter.
- The left-sided apraxia to verbal command is usually severe and can be mistaken for hemiplegia. Similarly, a disregard for the left half-field of vision can be mistaken for a hemianopsia. Left side weakness in the first week or so due to retraction edema in the right hemisphere sometimes confounds the picture. The neurological status is difficult to evaluate not only because the seemingly flaccid and unresponsive left extremities may exhibit coordinated movements, but also because there may be competitive movements between the left and right hands as the patient improves. Moreover, some patients have focal motor seizures, manifested by clonic contractions on alternating sides of the body and without loss of consciousness, occasionally followed by transient unresponsiveness of whichever limbs were involved. Patients commonly have bilateral Babinski signs and bilaterally absent superficial abdominal reflexes.
- Well-coordinated but repetitive reaching, groping, and grasping with the left hand sometimes resemble a grasp reflex; grasp reflexes may actually be present bilaterally for a day or two. When forced grasping cannot be elicited (by inserting two fingers into the patient's palm from the ulnar side, with some distal stroking), it is nevertheless possible in most cases to demonstrate a proximal traction response (PTR) (i.e., the patient is unable to relax the hand grip when the examiner pulls so as to exert traction on the elbow and shoulder flexors). The left arm hypotonia, the left arm PTR, the responses bilaterally to plantar stimulation, and the mutism were regularly observed in our patients (Figs. 5.1 and 5.2).
- Figure 5.1 indicates the intensity and duration of mutism and other features of the acute disconnection syndrome in our first patient, W.J. (24-26). His commissural section was done without entering the third ventricle; two exposures, before and behind the Rolandic veins, made it possible to confirm under direct vision that his callosal section was complete. The patient's condition worsened at the end of the first week, associated with an alkalotic hyponatremia present on days S to 7 (the patient had had diabetes insipidus since his previous head injury, and vasopressin was used throughout his hospital stay). When the hyponatremia was corrected, the patient rapidly improved with respect to alertness and left side coordination, with subsidence of the hypotonia and PTR. Nevertheless, some degree of mutism persisted for another month. This patient's mutism and that of several others has been described in detail elsewhere (12). As discussed below, the duration (,f mutism seems to be related to the extent of extracallosal damage; in this case, there was distinct atrophy of the right frontal lobe evident at the time of operation and a left parietotemporal electroencephalogram (EEG) focus and bitemporal EEG foci on sphenoid recordings (110).
- FIGURE 5.1. Features of the acute disconnection syndrome as observed postoperatively in W.J., a man aged 48 years at the time of operation on February 2, 1962. "N" means "none" (i.e., normal). A solid block in black indicates a marked deficit, whereas interrupted black bars indicate that the deficit was present but mild, the observations having been made by the author. Where the solid block or interrupted bars are gray, the observation was made by someone else, usually a resident.
- FIGURE 5.2. Features of the acute disconnection syndrome as observed postoperatively in N.G., a woman aged 30 years at the time of operation on September 5, 1963. The symbols are the same as in Figure 5.1.
- Figure 5.2 shows the daily postoperative observations of key features of the acute disconnection syndrome in N.G. (14, 20, 26). Her anterior commissure and massa intermedia were sectioned under direct vision after opening into the third ventricle between the fornices. Recent magnetic resonance imaging studies have confirmed that her callosotomy was complete and that there was a paucity of extracommissural damage. That her extracommissural damage was slight is probably important with respect to the relatively brief duration of her mutism. Her postoperative course overall was quite smooth (small amounts of vasopressin were used on days 3 to 6). Even so, there was a marked left apraxia to verbal command for many weeks. There was also, as in every 1 of our 12 right-handers tested after complete commissurotomy, a persistent anemia in the left hand.
- The left hand anemia has been such a regularly appearing, easily tested, and persistently present phenomenon that a separate section, "Unilateral (Left) Tactile Anemia," is devoted to its further description in this chapter.
- N.G.'s postoperative course was summarized at the end of a month by the author as follows.
- Two hours after operation the patient did not react to pain, and 1 hour later there was bilateral flexion to sternal rub. One hour later, there was a good grip on the right to request, the right toe sign was positive, and the entire left side was flaccid. In succeeding hours, the left side remained flaccid to passive motion and to verbal request, but the patient used both hands together to pull the covers over her chest. Hyperreflexia and positive toe sign on the right side subsided after several days; a tonic grasp and PTR on the right rapidly decreased but could be intermittently elicited as late as 1 week postoperatively. On the first postoperative day, there was occasional mumbling including "yes" and "no." On the second day, there was intelligible speech; on the third day, the patient was well oriented for place and person, although not for time and did not recognize her husband. Disorientation for time and date and intermittent inability to recognize the doctors persisted for nearly 2 weeks. Complete apraxia, tonic grasp, and PTR remained pronounced on the left side for 2 weeks. By the end of the third week, the left grasping had largely subsided; no verbal instructions were followed with the left hand but poses and gestures were copied excellently. The patient graduated from wheelchair to walker after 2 weeks but continued to have difficulty controlling the left leg. By the fourth week, the patient was walking very well although still apraxic with the left hand.
- A positive Babinski sign has been present not only contralateral to the retracted hemisphere but also, for at least a day or two, on the ipsilateral side. An ipsilateral (and contralateral) toe sign were also seen in our one patient operated on from the left--C.C. (10, 26). The best explanation for this ipsilateral Babinski sign is probably that the diaschistic shock effect on the unexposed hemisphere after an extensive callosal section is sufficient to depress for a few days the corticofugal inhibition of the primitive extensor response (11, 129). In any event, the Babinski signs rapidly subside bilaterally and are not present over the long term (27). An ipsilateral Babinski sign is usually not observed when the callosal section is partial.
- It is not possible to include here all of the daily progress notes on this patient, who had a rather smooth and almost ideal postoperative course after a complete section. However, the following provides some idea of the patient's status at the end of 2.5 weeks.
PROGRESS NOTE OF SEPTEMBER 23, 1963
- The patient has progressed rapidly in the past week and is now up walking. She eats by herself without difficulty and discusses things at some length, although after a lengthy conversation she tends to become a little bit confused and says, "You people must have taken out my brains."
- When the patient walks, she tends to have a slight shuffle but there is no asymmetry. However, if one asks her to walk on her heels, she does this readily with the right foot but does not do it with the left foot. If one asks her to walk standing up on her tiptoes, she again does this only with the right foot; she could not rise up on either the heel or the toe of the left foot. The hand is progressing. Whereas some days ago there was a PTR on the left, the patient now has no difficulty letting go with the left hand when asked to do so, even if considerable stretch is applied to the flexor joints. However, apraxia of the left hand continues in that the patient has considerable difficulty carrying out any command with the left hand and often, while laughing, reaches over with the right hand to make the left hand behave in a more appropriate or precise fashion. When the patient was asked to put her left hand on her head, she could not do so. However, when the patient was asked to imitate the examiner when he put his left hand on his head, she did so without any difficulty. She was then instructed, "Now put your left hand on your head," and she did this right away. She was then instructed, "Put your left hand on your left ear," and the left hand went on top of the head. This could easily be duplicated with any combination of movements, that is, the patient would be shown how to do something and do it excellently with the left hand and then she would be asked to do something else and the hand would do what she had previously just done ... Without warning, J put my hand in front of my forehead so that a finger was placed in each supra-orbital notch, with the palm of my hand up over my forehead and the little finger sticking out to the side. No sooner had J done this without saying anything, when the patient's left arm went up in complete imitation, to the exact positioning of the fingers. After her arm was lowered, the patient was then told, just lift your arm up in the air." The left arm went up with palm on the forehead, in an imitation of the previous peculiar position.
- Reflex testing showed no asymmetry; the knee jerks were quite hyperactive bilaterally, but the reflexes at the ankle, biceps, and triceps were normal. Plantar stimulation produced some withdrawal on each side with no suggestion of dorsiflexion. Testing with the pinwheel disclosed that on the left the patient felt the pinwheel to be somewhat less sharp than the corresponding parts of the right side; this hypalgesia is similar to what she had before surgery and is quite distinct from the "shock-like" sensation that W.J. has on the left side of his body. Vibratory sense is apparently quite normal on the right side, but the patient is not cooperative enough to give reliable results over the left side.
- As the severity of the acute disconnection syndrome subsides in a week or so, a phenomenon variously called "intermanual conflict" or "the alien hand" appears. Almost all complete commissurotomy patients manifest some degree of intermanual conflict during the early postoperative period. For example, a few weeks after operation, R.Y. (26) was seen buttoning his shirt with one hand while the other hand was following along just behind undoing : the buttons.
- Intermanual conflict was observed after commissurotomy by Wilson et al. (133) and by Akelaitis (1), who called it "diagnostic dyspraxia." The phenomenon has been described in many case reports of callosal infarcts or tumors (40). Even our youngest patient (L.B.) with no appreciable apraxia to verbal command in the long term manifested this "alienation" 3 weeks after operation. While doing the block design test unimanually with his right hand, his left hand came up from beneath the table and was reaching for the blocks when he slapped it with his right hand and said, "That will keep it quiet for a while." Such behavior progressively subsides after callosotomy, probably because of other integrative mechanisms supplementing or replacing commissural function. Examples of prominent intermanual conflict have been reported by Rayport et al. (100), Ferguson (49), and Reeves and Risse (104).
- In rare instances, the intermanual conflict may reappear even years later. An example is the following from a follow-up examination performed on February 26, 1973 (A.M. underwent operation on July 7, 1964, at age 31).
- The most interesting finding of the entire examination is the frequent occurrence of well-coordinated movements of the left arm that are at cross-purposes with whatever else is going on. These sometimes seem to occur spontaneously but on other occasions are clearly in conflict with the behavior of the right arm. For example, when attempting a lendrassic reinforcement, the patient reached with his right hand to hold his left, but the left hand actually pushed his right hand away. While testing finger-to-nose test (with the patient sitting), his left hand suddenly started slapping his chest like Tarzan.
- The patient follows verbal instructions readily with the right hand but poorly if at all with the left hand, although he can raise and lower the left arm with considerable reliability the more distal the joints involved in the requested act, the more obvious the defect. After a variety of maneuvers (the left hand often following a visual demonstration fairly well), it was possible to test the grip of the left hand while keeping the thumb extended, so it was apparent that there is a negative Wartenberg thumb sign on the left and on the right.
- In most patients, the intermanual conflict progressively subsides in a few months or even weeks. N.G. had only mild intermanual conflicts in the hospital, and none were noted after her return home. Her status 5 months postoperatively was as follows.
PROGRESS NOTE OF FEBRUARY 2, 1964
- N.G. is doing excellently. There is still some lack of initial live and an occasional defect in memory, but otherwise her husband and she were both delighted with her progress when J visited them in their home today. The surgical wound has healed well and has a good cosmetic appearance except for a small depression in the most anterior burr hole. She has no headaches.
- The patient is eating well and has gained weight although she has been doing calisthenics each morning to the TV. She sleeps well except for an occasional nocturia (she needs no help with personal care); she recently had a urethral dilatation and takes Mandelamine 4 times a day along with her Dilantin (100 mg) and Mebaral (50 mg).
- She has had no seizures (in particular no convulsive movements), unless one wishes to suspect as seizures an occasional inattention and some numbness of the left side. This absence of seizures is despite the fact that the patient has had two menstrual periods since surgery, the most recent a few weeks ago. The numbness of the left side of the body is apparently continuous at a low level, but once or twice a day it is quite bothersome for 5 to 10 minutes at a time. It involves more the leg (thus resembling her aura before surgery) and also the arm, but not the face. Despite the "numbness" the patient has no disability with the left hand, which she uses quite naturally for dressing, cooking, etc. Although she occasionally has trouble fastening her bra behind her back, she says that she dresses with ease and demonstrated for me two-handed tying of shoelaces, this being done in a manner indistinguishable from normal. The patient also shuffled a pack of cards and dealt them deftly. She dealt one-handed with either hand (slightly better with the left), and at my request she unbuttoned and rebuttoned the top of her blouse with her right hand while simultaneously dealing cards with the left hand.
- The patient wrote well with the right hand but could make no legible characters with the left hand on several trials. However, copying of designs was somewhat better with the left. Her deficit with the right hand appeared to be a deficit of conceptualization, whereas the deficit in the left hand appeared to be one of muscular coordination. It is of interest that neither the patient nor her husband could recall any incident of intermanual conflict since her arrival home.
- On blindfold testing today, the patient followed my verbal commands with the left quite well, her unilateral apraxia seemingly overcome. However, the anemia was still present in the left hand to full extent. J repeatedly demonstrated this with various objects while her husband sat watching in open-mouthed silence (Fig. 5.2). The patient's personality seems to me essentially unchanged from before surgery; she laughs easily and appropriately. Her husband says she talks less ("She used to talk my ear off") but socializes well with her neighbors. She is perhaps a little more interested in sexual activity than before the operation. Her memory occasionally fails in that she may sometimes accompany her husband somewhere and on the next day ask him what they did. The memory loss occasionally takes the form of absent-mindedness; while helping her husband prepare lunches for the family, "She sometimes puts two or three slices of lunch meat in a sandwich instead of one."
- During my visit, the patient was quite interested and alert and not only responded appropriately to questions, but spoke spontaneously on a number of subjects and did not appear to have any memory difficulties at this time. Each night, her husband makes a list of housework for her to do the next day. He reported that, "She does everything well; but if J forget to make the list, she will just sit around the next day and do nothing." He makes out a menu and she cooks the meals. "I used to have trouble getting things to come out at the same time," she said, "but the meals are better now." (Her husband nodded agreement.)
- N.G. remained seizure-free for 8 years and then had status epilepticus a few months after her family physician discontinued her anticonvulsant medication (14). Her seizures were readily controlled when her medication was reinstituted, and her EEG abnormalities cleared during the next 18 months. N.G. has been examined often, continues to be seizure-free, and continues to participate in laboratory studies at Caltech and UCLA. Her left-handed anemia was still readily demonstrable 30 years postoperatively. This persistence of unilateral tactile anemia has also been true of all of our other patients (19).
Chronic, Stabilized Syndrome of Hemisphere Disconnection Overall Effects
- Within a few months after operation, the symptoms of acute hemisphere disconnection become compensated to a remarkable degree. In personality and social situations the patient appears much as before. However, when input is lateralized (i.e., presented to only one hemisphere), each hemisphere can be shown to operate independently to a large extent. Each hemisphere seems to have its own learning processes and its own separate memories. Split-brain patients soon accept the idea that they have capacities of which they are not conscious, such as left-hand retrieval of objects not nameable. They may quickly rationalize such acts, sometimes in a transparently erroneous way (53). Even many years after operation, patients will occasionally be quite surprised when some well-coordinated or obviously well-informed act has just been carried out by the left hand. This is particularly common under conditions of continuously lateralized input (137, 139).
Visual Effects: Double Hemianopia
- Visual material can be presented selectively to a single hemisphere by having the patient fix his or her gaze on a projection screen onto which pictures of objects or symbols are back-projected to the right, left, or both visual half-fields using exposure times of 0.1 seconds or less. Patients can read and describe material of various kinds in the right half-field essentially as before operation. When stimuli are presented to the left half-field, however, patients usually report that they see "nothing" or at most "a flash of light." The disconnection (if it includes the splenium) can sometimes be demonstrated with simple confrontation testing. The patient is allowed to have both eyes open but does not speak and is allowed to use only one hand (sitting on the other hand, for example). Using the free hand, the subject indicates the onset of a stimulus, such as the wiggling of the examiner's fingers. With such testing there may seem to be a homonymous hemianopia in the half-field contralateral to the indicating hand. When the patient is tested with the other hand, there seems to be a homonymous hemianopia in the other half-field.
- This situation must be distinguished from the much more commonly occurring extinction or hemi inattention deficit from a hemispheric lesion, in which the patient tends to indicate only one stimulus when the stimuli are in fact bilateral (132). An observable difference is that the double hemianopia is a symmetrical phenomenon (the deficit occurs on each side), whereas extinction or hemi inattention is typically one-sided and more common for the left side.
- In most patients, there eventually appears a condition in which no field defect can be demonstrated by a casual confrontation technique. This apparently depends mainly on the ability of each hemisphere to direct the head and eyes. For example, if the patient is instructed to point with the right hand and if the examiner then wiggles the fingers in the patient's left visual half-field, the patient's head and eyes quickly turn to the left and then the right hand points to the correct target.
- If turning of the head is prevented, a leftward glance will suffice for the patient's need for a cue, or the right hand may point to the left visual half field as soon as it is apparent that there is no suitable stimulus in the right visual half-field. This "cheating" by the left hemisphere can often be detected by providing no stimulus at all. Furthermore, if stimuli are presented simultaneously in both visual half fields, only the stimulus in the right half-field is described by the patient, that is, by the left hemisphere. There is usually no verbal response to the stimulus in the left visual half-field until the left hemisphere realizes that the patient's left hand is also in action, pointing to the left half-field stimulus.
- When a patient has been tested repeatedly so that the occurrence of bilateral stimuli can be anticipated, both stimuli may be identified by a single hand; if using the right hand, the patient will point first to the right and then to the left half-field. In patients who have been frequently tested, the appearance of a stimulus in the left visual half-field is occasionally recognized despite the examiner's attempts to circumvent the various cross-cueing strategies. However, even in our patient who does the best (L.B.), performance on confrontation testing of visual fields is still distinguishable from normal.
Auditory Suppression
- After cerebral commissurotomy, the patient readily identifies single words (and other sounds) if they are presented to one ear at a time. If different words are presented to the two ears simultaneously (dichotic listening), only the words presented to the right ear will be reliably reported (46, 90, 115).
- This large right ear advantage is usually considered the result of two concurrent circumstances: (a) the ipsilateral pathway (from the left ear to the left hemisphere) is suppressed by the presence of the simultaneous but differing inputs, as it is in intact individuals during dichotic listening (74, 112, 125), and (b) the contralateral pathway from the left ear to the right hemisphere conveys information that ordinarily reaches the left (speaking) hemisphere by the callosal pathway that has now been severed. Although left ear words are rarely reported, their perception by the right hemisphere is occasionally evidenced by appropriate actions of the left hand (57).
- Contralateral ear suppression commonly appears after hemispherectomy or the creation of another large hemispheric lesion. Because there is usually suppression of the right ear by left hemisphere lesions, suppression of the left ear by a left hemisphere lesion has been called "paradoxical ipsilateral extinction." Further observations have led to the conclusion that whether the lesion is in the left or the right hemisphere, if it is close to the midline, then the suppression of left ear stimuli is probably attributable to interruption of interhemispheric pathways (37, 87, 116).
Unilateral Apraxia
- The degree of left hand dyspraxia is subject to large individual differences. Immediately after operation, all our patients showed some left-sided apraxia to verbal commands such as, "Wiggle your left toes," or "Make a fist with your left hand." Left limb dyspraxia can be attributed to the simultaneous presence of two deficits: poor comprehension by the right hemisphere (which has good control of the left hand) and poor ipsilateral control by the left hemisphere (which understands the commands). Subsidence of the dyspraxia, therefore, can result from two compensatory mechanisms: increased right hemisphere comprehension of words or increased left hemisphere control of the left hand. The extent of ipsilateral motor control can be tested by flashing to right or left visual half-field sketches of thumb and fingers in different postures for the subject to mimic with one or the other hand. Responses are poor with the hand on the side opposite the visual input, simple postures such as a closed fist or an open hand being attainable after further recovery. As recovery proceeds, good ipsilateral control is first attained for responses performed by the more proximal musculature. After several months, most patients can form a variety of hand and finger postures with either hand to verbal instructions such as, "Make a circle with your thumb and little finger," and the like. Even many years later, however, some degree of apraxia can be demonstrated (141).
Somesthetic Effects
- The lack of interhemispheric transfer after hemisphere disconnection can be demonstrated with respect to somesthesis (including touch, pressure, and proprioception) in a variety of ways.
CROSS-RETRIEVAL OF SMALL TEST OBJECTS
Unseen objects in the right hand are handled, named, and described in normal fashion. In contrast, attempts to name or describe the same objects held out of sight in the left hand consistently fail. Despite the patient's inability to name an unseen object in his or her left hand, identification of the object by the right hemisphere is evident from appropriate manipulation of the item, showing how it is used, or by retrieval of the same object with the left hand from among a collection of other objects screened from sight. What distinguishes the split brain patient from normal is that excellent same hand retrieval (with either hand) is not accompanied by the ability to retrieve with one hand objects felt with the other.
CROSS-REPLICATION OF HAND POSTURES
Specific postures impressed on one (unseen) hand by the examiner cannot be mimicked in the opposite hand. Also, if a hand posture in outline form is flashed by tachistoscope to one visual half-field, it can be copied easily by the hand on that side but usually not by the other hand.
- A convenient way to test for lack of interhemispheric transfer of proprioceptive information is as follows. The patient extends both hands beneath an opaque screen (or vision is otherwise excluded), and the examiner impresses a particular posture on one hand. For example, one can put the tip of the thumb against the tip of the little finger and have the other three fingers fully extended and separated. The split brain patient cannot mimic with the other hand a posture being held by the first hand. This procedure should be repeated with various postures and in both directions.
CROSS-LOCALIZATION OF FINGERTIPS
After complete cerebral commissurotomy, there is a partial loss of the ability to name exact points stimulated on the left side of the body. This defect is least apparent (if at all) on the face and most apparent on the distal parts (especially the fingertips). This deficit is not dependent on language; it can be shown in a nonverbal (picture identification) fashion, in which case the deficit is present in both directions (right to-left and vice versa).
- An easy way to demonstrate the defect is to have the subject's hands extended, palms up (with vision excluded). One touches the tip of one of the four fingers with the point of a pencil, asking the patient to then touch the same point with the tip of the thumb of the same hand. Repeating this maneuver many times produces a numerical score, approximately 100% in normal subjects for either hand. In the absence of a parietal lesion, identification of any of the four fingertips by putting the thumb tip on the particular finger can be done at a nearly 100% level by split-brain patients. One then changes the task so that the fingertip is to be indicated, not by touching it with the thumb of the same hand, but by touching the corresponding fingertip of the other hand with the thumb of that (other) hand. Sometimes the procedure should be demonstrated with the patient's hand in full vision until the patient understands what is required. This cross-localization cannot be done by split-brain patients at much better than chance level (25%), whereas most normal adults do better than 90%.
- An inability to cross-localize or cross-match has been found in young children (51), possibly because their commissures are not yet fully functioning (135).
Verbal Comprehension by the Right Hemisphere
- Auditory comprehension of words by the disconnected right hemisphere is suggested by the subject's ability to retrieve with the left hand various objects if they are named aloud by the examiner. Visual comprehension of printed words by the right hemisphere is often present; after a printed word is flashed to the left visual half-field, the subject is often able to retrieve with the left hand the designated item from among an array of hidden objects. Control by the left hemisphere is excluded in these tests because incorrect verbal descriptions given immediately after a correct response by the left hand show that only the right hemisphere knew the answer.
- Although the disconnected right hemisphere's receptive vocabulary can increase considerably over the years, this single-word comprehension is rarely accompanied by speech. The most extreme cases (to date) of right hemisphere language ability in right-handed (and left-hemisphere-speaking) split-brain subjects are two patients with right hemisphere speech, both with the anterior commissure uncut (5, 86, 113). Right hemisphere language in the split brain subject has other limitations, with syntactic ability being rudimentary at best. After studying a few cases in great depth for more than 10 years, Zaidel concluded:
- Whereas phonetic and syntactic analysis seem to specialize heavily in the left hemisphere, there is a rich lexical structure in the right hemisphere. The structure of the right hemisphere lexicon appears to be unique in that it has access to a severely limited short term verbal memory, and it has neither phonetic encoding nor grapheme-to-phoneme correspondence rules ... [this] represents the limited linguistic competence that can be acquired by a nonlinguistic, more general purpose (or other purpose) cognitive apparatus (138).
Right Hemisphere Dominance
- After commissurotomy, each hemisphere can be tested separately to demonstrate in a positive way those things that each hemisphere can do better than the other, rather than inferring what a hemisphere does from loss of function when it is injured. There are several representative reviews on this subject (18, 79, 95, 121, 139). The rapidly growing literature on hemispheric specialization was thoroughly summarized by Bradshaw and Nettleton (28) and by Trevarthen (127) and was more recently discussed by Gordon (58).
Unilateral (Left) Agraphia
- Right-handers can write legibly, albeit not fluently, with the left hand. This ability is commonly lost with callosal lesions, especially those that cause unilateral apraxia. An inability to write to dictation is common with left hemisphere lesions, almost always affecting both hands. The left hand may be dysgraphic if affected by a right hemispheric lesion, such as a frontal lesion causing forced grasping. That the left dysgraphia after callosal sectioning is not simply attributable to an incoordination or paresis can be established if one can demonstrate some other ability in the left hand requiring as much control as would be required for writing. The left hand may spontaneously doodle or it may copy various designs or diagrams. It is not so much the presence of a deficit, but rather the contrast between certain deficits and certain retained abilities that is most informative.
- Simple or even complex geometric figures can often be copied by a left hand that cannot write or even copy writing previously made with the patient's right hand (10, 21, 76, 141).
- Unilateral (Left) Tactile Anemia The most useful single sign of hemisphere disconnection is unilateral tactile anemia. This is an inability to name or to describe an object when it is felt by one hand, whereas it is readily named (or well described if the name is unknown) when it is placed into the other hand or when it is presented either to vision or to audition. This unilateral tactile anemia was present in every one of our complete commissurotomy patients, in the left hands of 12 right-handers, and in the right hand of our one left-handed patient. Others have repeatedly confirmed this finding, and it has also been observed when the patient has had a callosotomy sparing the anterior commissure (53, 85).
- Not only is unilateral anemia quite regular in its appearance, it is also quite persistent (whenever appropriately tested) for as long as 20 years (19) (and longer, as shown by my more recent testing). In addition to its regularity and its persistence, the demonstration of this sign requires a minimum of equipment and time, and the interpretation of results is usually quite clear. Of the many maneuvers developed in the laboratory to test split-brain patients, this is the principal one to be adopted as part of a routine neurological examination. Although there are many signs of brain bisection (;1\ described), one of the most convincing ways to demonstrate hemisphere disconnection is to ask the patient to feel with one hand and then name various small, common objects such as a button, coin, safety pin, paper clip, pencil stub, rubber band, or key. Vision must be excluded. A blindfold is notoriously unreliable it is better to hold the patient's eyelids closed, put a pillowcase over the patient's head, or use an opaque screen.
- The patient with a hemisphere disconnection is generally unable to name or describe an object in the left hand although he or she readily names objects in the right hand. Sometimes the patient will give a J vague description of the object although unable to name it, but there is a contrast with the ability to name the object readily when it is placed into the right hand.
- To establish hemisphere disconnection, other causes of unilateral anemia--particularly astereognosis (or a gross sensory deficit)--must be excluded. The most certain proof that the object has been identified is for the subject to retrieve it correctly from a collection of similar objects. Such a collection is most conveniently placed on a paper plate about 12 to 15 cm in diameter, around which the subject can shuffle the objects with one hand while exploring for the test object. Even without evidence of correct retrieval, astereognosis can be reasonably excluded by observing the rapid, facile, and appropriate manipulation of an object despite an inability to name or verbally describe it.
- In testing for unilateral anemia, the examiner must be aware of strategies for circumventing the defect. For example, the patient may manipulate it to produce a characteristic noise or may identify a pipe or some other object by a characteristic smell and thus circumvent the inability of the left hemisphere to identify, by palpation alone, an object placed in the left hand.
Syndrome After Corpus Callosotomy
- When the corpus callosum is sectioned in its entirety at one time, the acute disconnection syndrome appears in almost all respects as it is seen after a complete cerebral commissurotomy including the anterior commissure and massa intermedia. This state after callosotomy includes transient mutism; hence, this symptom when it follows a complete section is not reasonably ascribed to molestation of third ventricular structures (Fig. 5.3). Moreover, mutism does not necessarily follow a frontal commissurotomy that does include molestation of the third ventricular structures but spares the splenium. These two observations taken together suggest that postcommissurotomy mutism requires explanation on some other basis than in terms of third ventricle retraction.
- Staying out of the third ventricle does avoid some problems, including transient diabetes insipidus. In addition, there may be less likelihood of aseptic meningitis or other complications leading to hydrocephalus, a point that was often advanced by Harbaugh et al. and Wilson et al. (67, 133).
- Experiments with monkeys have shown that if the anterior commissure is left intact, it can compensate for loss of the splenium with respect to interhemispheric transfer of certain kinds of visual information (65). However, the anterior commissure cannot compensate completely for splenial loss in the human because hemialexia usually is present after splenial section. Indeed, most of the stabilized syndrome seen after a complete cerebral commissurotomy is also seen (i.e., has not been compensated for) after a callosotomy sparing the anterior commissure (53, 85). This is perhaps not surprising because the anterior commissure is only 1/100 the size of the corpus callosum. On the other hand, we can appreciate how significant it might be when we consider the wealth of information that is conveyed over one optic nerve, the diameter of which is about the same as that of the anterior commissure. This question is complicated by the fact that the size of the anterior commissure is quite variable; a diameter difference of three or four times has been reported (41, 136). The discrepancy between monkeys (transfer of learning by the anterior commissure) and humans (inability of the anterior commissure to compensate for callosotomy) may reflect differences between recently acquired memories as opposed to long-standing ones. Current evidence suggests that memory deficits occasionally appear after callosotomy, even when the splenium is spared (33, 88, 104, 140).
- FIGURE 5.3. Degree of mutism (marked, mild, or none) after operation on 15 patients with medically intractable epilepsy. Thirteen patients underwent complete commissurotomy, and two patients (D.M. and N.F.) underwent frontal commissurotomy (52). The symbols are the same as in Figure 5.1.
Syndrome After Frontal Commissurotomy
- By "frontal commissurotomy" we refer to section of the anterior two thirds of the corpus callosum together with anterior commissurotomy (60); section of the anterior commissure was done under direct vision and the third ventricle was entered between the two fornices so the section also included the ventral hippocampal commissure (Fig. 5.4). The same operation was used on a few occasions by Wilson et al. (133).
- When the splenium is spared, as it is in the case of frontal commissurotomy, very little of either the acute or the chronic disconnection syndrome is seen. In particular, mutism did not occur in our four operations for intractable seizures with bitemporal foci. Because these patients had retraction within the third ventricle to allow section of the anterior commissure under direct vision, the mutism in cases with more extensive section (i.e., complete commissurotomy) was not likely of third ventricular origin. Moreover, all of these four had retraction of the medial aspect of the right frontal lobe, comparable with that in the complete cases; therefore, retraction on the supplementary motor cortex does not seem to be a sufficient explanation for the mutism after callosotomy, an issue discussed in more detail in the section, "Postcallosotomy Mutism," in this chapter.
- In the long term, Preilowski (98) could show some deficits of motor coordination in two of our patients with frontal commissurotomy. Nevertheless, exhaustive testing for the usual disconnection deficits (as described in the previous section, "Chronic, Stabilized Syndrome of Hemisphere Disconnection," in this chapter) has shown that with retention of the splenium, such deficits should not be expected (19, 54, 60, 63). Memory deficits have been observed after complete commissurotomy (88, 140) and also in lesser degree after frontal commissurotomy sparing the splenium. This may be in part attributable to deficits in combining linguistic representations with visual or spatial images. Further research using a wide variety of memory tests before and after operation, particularly with prolonged follow-up, is needed.
Syndrome After Callosotomy Sparing the Splenium
- Section of the corpus callosum that spares the splenium (and the anterior commissure) but is otherwise complete can be readily accomplished via an exposure anterior to the Rolandic bridging veins (Fig. 5.5). This procedure, which avoids entry into the third ventricle, has come to be the most popular version of commissural section for epilepsy. After this operation, one does not observe most of the acute disconnection syndrome. This smoother postoperative course is not surprising because the acute disconnection syndrome does not usually occur after frontal commissurotomy, which is essentially the same procedure plus section of the anterior commissure and massa intermedia.
- Absence of significant mutism with callosotomy sparing the splenium has been reported by Rayport et al. (100) and in personal communications to myself from M. Rayport, M. Apuzzo, J. Vries, J. Wade, and G. Yasargil (1987). On the other hand, an abstract by Ross et al. (108) reported transient (1- to l0-day) mutism with this procedure. This difference could depend on the amount of retraction, particularly if there has been retraction of the left hemisphere below the fair and the usual right hemisphere retraction, or it might reflect differences in the criteria of the observers. Perhaps most important are differences in the patients; even a lesser callosal section may be followed by mutism when it is associated with a concurrent thalamic lesion.
- FIGURE 5.4. Frontal commissurotomy. Sketched here are the structures sectioned during a frontal commissurotomy. This includes most of the corpus callosum (sparing the splenium), the ventral hippocampal commissure (between the fornices), and the anterior commissure.
- FIGURE 5.5. Anterior callosotomy. This sketch shows how anterior callosotomy differs from frontal commissurotomy (illustrated in Fig. 5.4). There are two principal differences: the anterior commissure is spared, and third ventricle is not entered by separating the fornices.
Midcallosal Section
- The most common variant of callosal section is an incision through the trunk sparing not only the splenium but also most of the genu as well. Such an incision affords ready access to both foramina of Monro with practically no obligatory physiological cost as currently assessable (3, 4, 60, 82, 96, 114, 134). With some possible exceptions (occasional brief mutism, inconstant auditory effects, tactile transfer deficits), any impairments after callosal trunk section are typically of the neighborhood type. For example, the language deficits allegedly secondary to callosal trunk section have been reported in only one case, one in which the callosum was approached in a right-hander by retraction of the left hemisphere (42). Mutism in Special Cases
- Mutism can be expected, at least for a time, if partial callosal section is done in a right-hander to remove or to biopsy a right thalamic lesion; this conclusion is based on three personal cases and two cases of others on which J was consulted. For several weeks mutism followed callosal trunk incision for the removal of a colloid cyst in a case in which right thalamic injury had occurred during a previous emergency ventriculostomy (P. LaPrade, personal communication, 1983). Significant mutism did not appear after callosal trunk section in five other personal cases including a small left thalamic arteriovenous malformation (AVM) (not resected) and two small AVMs of the septum (with resection in both cases). As mentioned in the section, "Syndrome After Frontal Commissurotomy," no mutism appeared after frontal commissurotomy in four of our epilepsy patients, and no mutism was seen in two personal tumor patients (both still being followed after 15 and 18 years) with quite extensive midcallosal section for a third ventricle craniopharyngioma and a third ventricle glioma (96).
Inconstant Auditory Effects
- As noted in the "Auditory Suppression" section of this chapter, a naturally occurring lesion near the callosal trunk can result in suppression of one ear when tested dichotically. Hence, one might expect similar changes after section of the callosal trunk. In any case, the auditory effects are unlikely to be clinically important, although one can anticipate certain patients (for example, a symphony conductor) in whom slight auditory alterations might be quite important.
Effects on Tactile Interaction?
- The somesthetic nontransfer described under "Somesthetic Effects" has not been detected in most cases of midcallosal section. However, if the task is made more difficult (so that normal subjects commonly make mistakes), some deficits compared with normal subjects have been elicited (7, 73). Further : studies along this line will no doubt be forthcoming; they are clearly needed, and the growing popularity of this approach will afford frequent opportunities for both preoperative and postoperative testing.
Posterior Callosal (Splenial) Section
- Whereas more anterior section of the corpus callosum entails minimal long-term physiological cost, section of the splenium (Fig. 5.6) commonly causes a left hemialexia (36, 71, 84, 126, 131). This inability to read in the left visual half-field may be accompanied by so-called color anemia. This is an inability to give the names of colors presented to the patient's view, although the colors can be matched and the patient can give (speak or write) the color names of objects (e.g., answer "yellow" when asked the color of a banana) (56). When the hemialexia and associated deficits are most severe, they can be mistaken for a left homonymous hemianopsia. Although use of a tachistoscope is more precise, it is not necessary because the deficit has often been demonstrated \without such equipment. Hemialexia can be considered to be of two types: an inability to match written words with objects and an inability to read aloud written words or letters. Both of these are said to be mimicked by the condition of visual hemineglect (123).
- Splenial section stops interhemispheric transfer riot only of color and alphabetical data, but of most :visual information. For example, patients with splenial section cannot mimic with the ipsilateral hand the picture of a hand posture projected to only one hemisphere (130). However, some tasks may not be affected by splenial section. Simple reaction time, such as pressing a button to a lateralized flash of light, is typically 2 to 3 msec longer with the ipsilateral hand than with the hand contralateral to the hemisphere stimulated. Since the time of Poffenberger (1912), this 2- to 3-msec difference has been attributed to interhemisphere transmission time (IHTT) through the corpus callosum. After callosotomy, the IHTT is approximately 20 times as long (40 to 60 msec), probably because the transmission occurs via multisynaptic, subcallosal pathways (34). Recent results that require confirmation suggest that partial callosal section, whether anterior or splenial, results in only slight increases in IHTT (9). If this is correct, it may mean that fast crossed visuomotor responses to simple stimuli are not dependent on specific callosal channels, as are some other types of information transfer (70).
- The forgoing hemideficits may not prove totally disabling for most persons, although they can be so in individuals whose occupations involve seeing large groups of symbols more or less simultaneously, such as a symphonic score. When the splenial disconnection can be quite crippling for almost any literate person is when the callosal lesion is combined with a left occipital lesion or indeed any lesion causing a right hemianopsia. Such individuals typically have alexia without agraphia (i.e., they can write but are unable to read, even what they have just written correctly to dictation). This remarkable dissociation of reading from writing has been known for nearly a century (6, 39). One explanation is as follows. When the patient has a right homonymous hemianopsia resulting from a left occipital lobe lesion, nothing can be seen, much less read, in the right half-field. Hence, visual information can reach the left hemisphere language zone only from the left half-field via the right occipital cortex and transfer via the splenium. If another (or confluent) splenial lesion has disconnected the right occipital cortex from the left hemisphere, the left hemisphere retains competence to write to dictation but no longer has access to information arriving in the right occipital lobe from the left visual half-field.
- As its proponents have recognized, there are some difficulties with this explanation of alexia without agraphia. These patients can often name objects or pictures of objects visualized in the left half-field, showing that information can reach the language zone from the left half-field. Moreover, alexia without agraphia can occur without an accompanying loss of the right visual half-field (62, 69). Alexia without agraphia can occur in cases with the splenium largely intact. Reading seems to be a multistage process that can be disturbed in a variety of ways (62, 68, 77).
- Figure 5.6 Posterior callosotomy. In this operation, in addition to the splenium, part of the dorsal hippocampal commissure has been divided as it is usually adherent to the undersurface of the corpus callosum.
- One can readily see that if splenial section were accompanied by left occipital damage, the patient might be unable to read altogether and not just be hemialexic. Hence, retraction of the left occipital pole is to be avoided in approaching the splenium, although there may be situations (such as an AVM in the medial aspect of the left occipital lobe) in which it seems necessary. In one such personal case, the patient was "cured" of her AVM and was still able to read, albeit with difficulty. However, she lost entirely her ability, which was said to have been phenomenal, to sight read music for the piano. Approaching the splenium from the left may be preferable in the rare case in which a left-hander has been shown to be a right-hemisphere speaker, preferably by use of the Wada carotid amobarbital technique.
Completion of Callosotomy
- In the 1990s, partial callosotomy has become a widely used treatment for medically intractable epilepsy. In the words of Spencer et al. (117):
- Over the past decade, corpus callosum section has become a widely accepted, relatively safe, clearly needed, broadly practiced, and continually evolving addition to the medical treatment of certain patients who are not candidates for resective procedures.
- At present, it is usual to do a section of the anterior two thirds of the corpus callosum first, carrying little risk of obligatory loss as described above in "Syndrome After Callosotomy Sparing the Splenium" and "Midcallosal Section." If this procedure provides insufficient amelioration of the seizure disorder, a second procedure that completes the callosotomy may be in order (50, 104, 118). When the callosotomy has been completed by a second operation, there routinely has appeared (103, 104) the collection of interhemispheric transfer deficits described above as the "Chronic, Stabilized Syndrome of Hemisphere Disconnection." It has been reported that completion of callosotomy need not result in a disconnection syndrome (83). One explanation offered for these unusual results was that the second operations were performed more than 1 year after the first, rather than after several months, as commonly done by others. This interesting result needs further substantiation in the form of details of the testing and magnetic resonance imaging evidence of completion of the section (17). As pointed out elsewhere (13), insufficient testing and/or incompleteness of section were responsible for a false impression of callosal nonfiction for more than half a century.
Postcallosotomy Mutism
- After complete section of the cerebral commissures, in almost every case there was a postoperative period of mutism of varying duration during which speech was absent or extremely sparse, although comprehension and writing were retained. The patient did riot talk even when quite cooperative and having some ability to write. This emissive deficit lasted, in milder form at least, in 11 right-handed patients for nearly 3 months, 3 days, 20 years, 4 weeks, 4 months 2 days, 4 weeks, 2 weeks, 1 week, 4 weeks, and 4 weeks, respectively (Fig. 5.3). In one lefthander (PD) mutism lasted 8 months. In all cases, there was little if any paraphasia; when the ability to talk returned, there was no nominal amnesia. In some cases there was a definite lack of bodily spontaneity and motor initiative for a time, but this was only partially correlated in duration with the loss of speech. As the mutism subsided, there was a stage of partial recovery that usually included hoarseness or whispering--but without paraphasia, anemia, or novel semantic or syntactic errors--except for one right-hander (C.C.) operated on from the left.
- The notable exception in our series (Fig. 5.3) was M.K. Her convulsive disorder, beginning on the left side, was associated with a right ventricular dilatalion and a left footdrop present since childhood. (:compensation as a result of this right cerebral atrophic change may have contributed to the absence (,f postoperative mutism. On the other hand, this was our only case in which a very large bone plate (nearly a hemicraniotomy) was left out and replaced later to minimize pressure effects during postoperalive swelling.
- Clearing of the mutism occurred in all cases but one. This patient (A.M.) continues, even years later, to speak very little. When he does speak, his speech i\ poorly phonated and badly articulated. He had severe brain damage before the operation, and we believe that he was--to an unusual degree--dependent on a compensatory function of his corpus callosum for articulatory control. In this case, the prolonged speechlessness was almost certainly not attributable to callosal section alone. This case further illustrates the previously emphasized point that postcommissurotomy deficits depend in large part on the nature and amount of preexisting, extracallosal damage (12, 60, 122).
- Postcommissurotomy mutism was originally considered a simple neighborhood sign, a partial akinetic mutism from retraction affecting the anterior end of the third ventricle (32, 109) during section of the anterior commissure. However, a number of patients have now been seen with similar retraction but a spared splenium who did not become mute, including N.F. and D.M. (Fig. 5.3).
- In contrast to the cases of complete commissurotomy, in these (and two other patients, D.B. and B.K.), all of the same structures (including the massa intermedia and the anterior commissure) were severed except the splenium and there was no immediate postoperative mutism (60). Such cases afford evidence not only against a third ventricle origin for the mutism, but also against a right supplementary cortex origin. On the other hand, Ross et al. (108) reported mutism after anterior callosal section but not after posterior callosal section. How to reconcile this apparent conflict is one task of this section and for the future.
- After complete callosotomy in two stages, Rayport et al. (100) observed a significant decrease in spontaneous speech unaccompanied by paraphasia or comprehension deficit or inability to sing in three of eight cases. The authors suggested that in these most affected cases, mixed hand dominance may have been an important consideration. Of particular importance was the absence of any language or speech problem after the first stage (rostrum, genu, and most of the trunk). The mutism appeared only after the second stage (splenium and remainder of the trunk). This result does not totally eliminate retraction on supplementary motor cortex as a partial contributory cause, but edema probably did not play a role because the second stage followed the first by 2 months, 36 months, and 19 days, respectively. Rayport et al. (100) have suggested that mutism could result from an interhemispheric conflict (see also Bogen [121). However, they emphasize more the aspect of mixed dominance, which might indicate a greater role than usual of the corpus callosum in the production of speech, as was probably the situation in A.M. (Fig. 5.3) and the patients reported by Sussman et al. (124) and Sass et al. (111).
- The role of the corpus callosum in speech may be much greater in certain patients. With respect to the risk of severe postoperative mutism, we can probably be reassured by a favorable response to carotid amobarbital testing. This test can give ambiguous results, but J believe that it can also be used to exclude a critical dependence on the commissures. That is, if the patient continues to speak intelligibly when the hemisphere minor for speech is narcotized, then disconnection of this minor hemisphere will probably not deprive the patient of an essential resource with respect to speech production.
- Although severe, long-term mutism may occur only when callosal section is done in certain patients who are peculiarly dependent on these pathways for speech, mutism lasting several weeks in callosotomy patients without either widespread damage or anomalous lateralization from other causes remains to be explained.
- A diaschisis secondary to deafferentation of speech areas accounts for the deficits in terms of left hemisphere dysfunction even when the left hemisphere is unmolested. This interpretation is consistent with the appearance of a right Babinski sign (a sign of left hemisphere malfunction) and may help to explain why our patients (and those of Rayport) whose spleniums were spared had no mutism. This means that the diaschisis of the left hemisphere (from a complete section) must affect the speech "centers" or "circuits" more than it affects writing "centers" or "circuits." This implies, in turn, that the writing function is more robust or resistant in some sense than that for speech despite having developed later in both phylogenesis and ontogenesis.
- A more speculative possibility is that speech usually requires interhemispheric integration for control of the larynx and other midline structures in the following sense. One can suppose that left hemisphere speech ordinarily includes a corollary discharge (119) to the other hemisphere. When the commissures are completely severed, a downstream interhemispheric conflict occurs at the level of the motor nuclei for the larynx, which results in dysphonia. This explanation is consistent with most of the evidence. In particular, it fits the case of one patient (M.K.) who had no mutism at all (this was the patient with right hemisphere atrophy from childhood). It also fits the report of Sussman et al. (124) that their patient uttered several complex sentences shortly after operation although he was otherwise speechless for 16 months.
- Those concerned with the normal physiology of speech will be more interested in the temporary (several weeks to months) mutism after splenial section (either as part of a total callosotomy or as a second stage) than the long-lasting mutism (many months to years) of those whose anomalous laterality is associated with long-standing cortical lesions. For the surgeon using a transcallosal approach to the third ventricle, neither of these is apt to be as important as transient mutism (several days to weeks) after section of callosal segments (such as the trunk) well anterior to the splenium.
- The mutism reported by Ross et al. (108) lasted 1 to 10 days; hence, it is more relevant to the problems of third ventricle access than the severe, prolonged type of mutism. J have already alluded to the notion that the lack of speech could be considered a mutism of the type often associated with akinesia of either cingulate or subfrontal origin. This suggests that we think of the postcommissurotomy state as a sort of "forme fruste" of akinetic mutism, in which the mutism is much more evident than is the akinesia. The occurrence of mutism in callosotomy cases without entry into the third ventricle could be explained as the result of subfrontal retraction during section of the rostrum. Arguing against this is the paucity of postoperative mutism in four of our patients (D.M., N.F., D.B., and B.K.) whose spleniums were spared but who underwent section of both the rostrum and the anterior commissure under direct vision.
- The cause for transient mutism may be different from one case to the next, and in any given case there may be more than one operative factor (94).
- (a) The commissural fibers may have been acting in a compensatory fashion because of previous brain injury.
- (b) Speech may require some absolute number of relevant brain connections. In a marginal case, the callosotomy may be sufficient to bring the number of connections below the minimum required.
- (c) The callosotomy may produce a diaschisis in the speaking hemisphere, from which it recovers only slowly.
- (d) The callosum may carry a corollary discharge for speech output whose sudden loss results in downstream interhemispheric conflict.
- (e) Temporary circulatory alterations (particularly of the internal cerebral veins) may transiently derange the more sensitive functions of the basal ganglia.
- (f) Damage to one or the other fornix may be contributory.
- (g) Trauma to the anterior third ventricle during division of the anterior commissure or during division of the rostrum from in front may be relevant.
- (h) Supplementary motor cortex dysfunction may be contributory.
- (i) Decussating fibers (e.g., from medial frontal cortex to opposite basal ganglia) may be relevant so that their interruption plus medial frontal cortex edema combines to produce mutism when associated with a thalamic or striatal lesion.
Concluding Remarks
- The multifactorial causation of postcallosotomy mutism affords a measure of our ignorance, representing as it does the difficulties in predicting the likelihood of this usually transient but nonetheless distressing malfunction. On the other hand, the very existence of this riddle affords an opportunity for someone to unravel it. Likewise, the lack of replicate signs or symptoms from section of the anterior two thirds of the corpus callosum represents a territory still unknown and awaiting exploration.
- That the transcallosal approach to the third ventricle is largely without obligatory physiological cost is both a boon to the operator and a challenge to the scientist. Neurosurgeons have historically been both therapeutic and investigative, and we can expect that continued exploitation of the transcallosal approach will benefit our patients both at the time of treatment and by augmenting our understanding of what Bremer (29) once called, "the highest and most elaborate activities of the brain."
Acknowledgments
- The author thanks S. Zeind and staff, including P. Logan and J. Thangarajah of the Huntington Memorial Hospital, for library assistance and S. Johnstone for word processing.