The term “disconnection syndrome” is applied to the effects of lesions of association pathways, either those which lie within a single cerebral hemisphere or those which join the two halves of the brain (Geschwind,1965). Often the disconnection syndromes are talked about more generally as Collosal Syndromes. Of course, the corpus callosum is by far the largest of the nerve fiber bundles that directly connect one cerebral hemisphere with the other. These are generally called cerebral commissures and include the anterior and hippocampal commissures. These syndromes occur when the corpus callosum is partially or fully surgically divided. Syndromes of hemisphere disconnection can also occur when there is a partial disconnection such as in naturally occurring diseases/condition such as stroke (thrombosis etc.) Callosal lesions are often accompanied by damage to neighboring structures. Therefore “neighborhood signs” may overshadow signs of callosal disconnection.
- Just after complete cerebral commissurotomy patients respond well and can write letters to simple commands. Easily confused by three or even two-part verbal commands. Seen as mildly akinetic. A seeming imperviousness is observed. Often patients may write short and usually one-word answers. Considerable variation from one patient to another in terms of neurologic outcome and deficits.
- After a few months symptoms of hemisphere disconnection are compensated for to a large degree and seen in improved personality functioning, interactions and social situations and on neurologic exam.
a.) Visual material: visual material presented selectively to a single hemisphere by fixing gaze on some picture or object. Can describe or read various kinds of material in the right half-field at the level substantially the same before surgery. When presented to left half-field usually reports they see nothing or a flash of light.
b.) Inability to describe verbally left half-field stimuli includes hemialexia, where subject can’t read individual words flashed to the left half-field. Some recovery occurs over a period of years where word is recognized by right hemisphere as a symbol for something, albeit diffuse semantic information. Semantic information transferred to the speaking left hemisphere which approximates the stimulus word.
c.) Auditory Suppression: Right-handers identify single words if presented to one ear at a time. If different words presented during dichotic listening tasks, right ear information will be reliably reported. Left ear words poorly reported although occasionally appropriate actions are made by the left hand indicating some perception. Repeated testing indicates that your asymmetry may decrease particularly if patients attention is directed to left ear or if information load is reduced.
d.) Motor function: left-sided apraxia to verbal commands noted. Left limb apraxia seen because of poor comprehension by right hemisphere and poor ipsilateral control by left hemisphere. Compensatory mechanisms may emerge including increased right-hemisphere comprehension of words and increased left-hemisphere control of left hand.
e.) Somesthetic Effects (including touch, pressure, and proprioception). Unseen objects in right hand are handled, named and described in normal fashion. In contrast, naming of objects held out of sight in left hand are consistently failed. Despite inability to name the object in left hand, the left hand is able to manipulate the item and show how it’s used ;or they retrieve the object with the left hand from among the collection of objects screened from the patient sight (implicating that the object is also being identified by the right hemisphere.)
f.) Cross-replication of hand postures: One patient told to make postures with one hand, can’t mimic same posture with the opposite hand . When a specific posture is flashed in one visual half-field, can be copied by hand on the same side but not by the other hand.
g.) Inter-manual point localization: After a complete commissurotomy, there is a partial loss of ability to name exact points stimulated on left-side of body. Defect is least apparent in the face and most apparent and distal parts (e.g. fingertips). To test for this, while hand is shielded, the patient is asked to identify which finger tip was touched by examiner and to touch that finger with their own thumb (on the same side touched by examiner). Then when asked to identify the same fingertip on the opposite hand, (and once again to touch it with the thumb on that hand) they are unable to do it. Cross localization can be done by split-brain patients at a level not much better than chance.
h.) Right-hemisphere Verbal Comprehension: Auditory comprehension of words by disconnected right hemisphere is seen in the patients ability to retrieve, with the left hand, various objects if they are named aloud. Visual comprehension of words is accurate especially with short, and high-frequency words. So if a word name of an object (e.g. the word key) is flashed to the left visual half-field, patient can retrieve the item with the left hand from a group of objects hidden from view. Typically incorrect verbal descriptions are given after the word is presented, implicating that only the right hemisphere knew the answer. Right hemisphere language capabilities in right-handers are more evident when left hemisphere has been removed. If removed during infancy these capabilities may seem nearly normal as child develops language capabilities. Left hemisphere disease later in childhood that affects language abilities typically shows less intact language function since right hemisphere does not compensate as well. After split brain surgery and the left hemisphere is present and relatively intact, most linguistic abilities of disconnected right hemisphere are largely absent or suppressed. The disconnected right hemisphere’s receptive vocabulary can grow considerably and reach levels comparable to that of the vocabulary of the 10-to 16 year-old. Right hemisphere language has limitations: such as poor syntactic ability which is rudimentary at best. The rich lexical structure of language in right hemisphere diminishes significantly.
Other Disorders Following Hemisphere Disconnection
- Unilateral verbal anosmia:unable to named odors presentd to the right nostril even when readily presented to the left. Not a defect of smell in right nostril.
- Double hemianopia
- Hemialexia and Alexia without agraphia: When printed letters or short words are presented to left half-field patient unable to read information. Can readily read it when presented in the right half-field. Left hemialexia when combined with right homonymous hemianopia results in Alexia without Agraphia. For example, stroke patients can write but are unable to read even when they have just written correctly to dictation. Occurs in about 75% of righthanders with left posterior cerebral artery infarcts. Typically the patient has a right homonymous hemianopia resulting from a left occipital lobe lesion. In this case nothing can be seen, much less read in the right half-field. Visual information can reach the left hemisphere language only from the left half-field via the right occipital cortex and spelenium. Since there is a lesion typically in this region, it has disconnected the right occipital cortex from the left hemisphere. The left hemisphere still retains the ability to right to dictation even though it does no longer have access to information arriving in the right occipital lobe from the left visual half-field.
- Auditory Suppression
- Unilateral (left) ideomotor apraxia: This was the first described callosal symptom in the literature. In response to a verbal command the right-handed individual can’t carry out with the left hand some behavior which the right hand can do. Important to distinguish upon examination that the problem doesn’t have to do with a loss of dexterity or mild weakness. Also distinguished from ideational apraxia
- Unilateral (right) constructional apraxia: The inability to organize several parts into a configuration despite normal ability to handle or draw the individual parts. This type of apraxia can be quite prominent in the right hand of right-handers with callosal lesions. Have the patient copy with one hand, and then with the other hand, various geometric figures. Hemisphere disconnection in a right-hander is strongly suggested if patient can copy designs better with left hand.This may only be the case when there is no other right hemisphere involvement (e.g., left hand is paretic or ataxic)…in this instance they probably do no better with one hand vs. the other.
- Spatial acalculia: Since hemisphere disconnection can cause right-handed disability for spatial forms patient may have difficulty using pencil and paper to solve arithmetic problems. This deficit typically progressively received with recovery. Sometimes they can do mental calculations even though they cant to written ones.
Summary of Disconnection Syndromes based on the work of NORMAN GESCHWIND..
(if you want to read this these are the syndromes he covers)
- Word blindness without agraphia: Regarded as the first example of a callosal disconnection syndromes for which clear anatomical evidence was established.
- Pure Word Deafness: A lesion, located subcortically in the left temporal lesion typically destroys the left auditory radiation as well as callosal fibers from the opposite auditory region. The lesion has the effect of preventing the speech area (that part of the auditory association cortex that is Wernicke’s area) from receiving auditory stimulation. Although the right primary auditory cortex could receive auditory stimuli it can’t convey them to the speech area because the callosal connections from the right side were destroyed in the left temporal lobe.
- Lesions of Wernicke’s Area: although verbal comprehension is impaired, speech is also impaired. The loss of Wernicke’s area can be regarded as the destruction of a memory store as it was in fact regarded classically. Presumably it functions importantly as the “storehouse” of auditory associations. Geschwind describes that “a name” passes through Wernicke’s area then via the angular gyrus which arouses associations and other parts of the brain. Wernicke’s area attains its essential importance and comprehension as it is also involved in the arousal of associations. (maybe we should discuss this and talk about whether this is phenomena is discussed in this way anymore).
- Tactile aphasia: A disturbance characterized by an inability to name objects tactilely in the preservation of the ability to name on the basis of visual or auditory stimulation and spontaneous speech.