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In a patient with a transected corpus callosum, tests were conducted to assess their abilities. The patient was asked to fixate on a point on a rear projection screen while pictures were projected on either side of the fixation point. The patient could palpate objects corresponding to the projected pictures, but they were unable to see these objects. In one particular test, the patient's left hand responded to a picture of a key in the left field of view, but when asked, the patient verbally described seeing a picture of a ring (Redrawn from Sperry RW. In: Schmitt FO, Worden FG [eds]. The Neurosciences: Third Study Program. Cambridge, MA: MIT Press; 1974). This suggests that the right hemisphere of the brain has specialized functions for spatial tasks and is better at solving three-dimensional puzzles than the left hemisphere. Additionally, functions such as facial expression, body language, and speech intonation are also more associated with the right hemisphere.

Patients with a transected corpus callosum lack normal coordination between the two hemispheres. For example, when dressing, one hand may try to button a shirt while the other hand tries to unbutton it. Observation of these patients has shown that the two hemispheres can operate independently when they are not interconnected. However, one hemisphere can express itself through language, while the other hemisphere communicates nonverbally.

One striking example of interhemispheric differences is the phenomenon of "cortical neglect." This occurs as a result of a lesion in the parietal cortex of the non-dominant hemisphere, usually the right hemisphere. In such cases, the patient ignores objects and individuals in the left visual field, draws incomplete objects on the left, denies the existence of their left arm and leg, and fails to dress the left side of their body. The patient may also deny having any difficulties, a condition known as anosognosia. Although the patient may respond to touch and pinprick on the left side of the body, they cannot identify objects placed in the left hand. This lesion is located adjacent to the first somatosensory (SI) cortex and the visual association cortex, indicating that this region plays a special role in the perception of body image and immediate extrapersonal space. Similar lesions on the dominant side only result in the loss of some higher-order somesthesias, such as agraphesthesia (inability to identify characters drawn on the palm) and astereognosis (inability to identify an object solely by touch).

Learning and memory are major functions of the higher levels of the nervous system. Learning is the neural mechanism through which an organism's behavior changes based on experience, while memory is the mechanism for storing what is learned.

The neural circuitry involved in memory and learning in mammals is complex and challenging to study. Various approaches are used, including animal studies (particularly in simpler nervous systems of invertebrates), analysis of functional consequences of lesions, and anatomical/physiological studies at the cellular and pathway level. For instance, in the marine mollusk Aplysia, researchers have been able to isolate a connection between a single sensory neuron and a motor neuron that exhibits aspects of habituation (learning to not respond to repetitive insignificant stimuli), sensitization (increased responsiveness to harmless stimuli following the presentation of a strong or noxious stimulus), and even associative conditioning (learning to respond to a previously insignificant event after it has been paired with a meaningful stimulus).

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•Fig. 10.8 Illustration of Tests in a Patient With a Transected Corpus Callosum. A, Thepatientfixesonapointonarearprojectionscreen,andpicturesareprojectedtoeithersideofthefixationpoint.Thehandcanpalpateobjectsthatcorrespondtotheprojectedpictures,buttheseobjectscannotbeseen.B, Responsebythelefthandtoapictureofakeyintheleftfieldofview.However,theverbalresponseisthatthepatientseesapictureofaring.(RedrawnfromSperryRW.In:SchmittFO,WordenFG[eds].The Neurosciences: Third Study Program. Cambridge,MA:MITPress;1974.) three-dimensional puzzles better with the right than with the left hemisphere, which suggests that the right hemisphere has specialized functions for spatial tasks. Other functions that seem to be more associated with the right than the left hemisphere are facial expression, body language, and speech intonation (
Fig. 10.9 ). Patients with a transected corpus callosum lack normal interhemispheric coordination. When they are dressing, for example, one hand may button a shirt while the other tries to unbutton it. Observation of these patients indicates that the two hemispheres can operate quite independently when they are no longer interconnected. However, one hemisphere can express itself with language, whereas the other communicates only nonverbally.
Oneofthemorestrikingexamplesofinterhemisphericdifferencesisthephenomenonof“cortical neglect,” whichisaconsequenceofalesionintheparietalcortexofthenondominant(usuallyright)hemisphere.Insuchcases,thepatientignoresobjectsandindividualsintheleftvisualfield,drawsobjectsthatareincompleteontheleft,deniestheexistenceofhisorherleftarmandleg,andfailstodresstheleftsideofhisorherbody.Thepatientalsodenieshavinganysuchdifficulties(anosognosia). Althoughthepatientmayrespondtotouchandpinprickontheleftsideofthebody,heorshecannotidentifyobjectsplacedinthelefthand.Thelesionisadjacenttothefirstsomatosensory(SI)cortex,aswellasthevisualassociationcortex,anditsuggeststhatthisregionplaysaspecialroleintheperceptionofbodyimageandimmediateextrapersonalspace.Similarlesionsonthedominantsideresultonlyinlossofsomehigherordersomesthesias,suchasagraphesthesia (inabilitytoidentifycharactersdrawnonthepalm)andastereognosis (inabilitytoidentifyanobjectonlybytouch).
Major functions of the higher levels of the nervous system are learning and memory. Learning is a neural mechanism by which the organism’s behavior changes as a result of experience. Memory is the storage mechanism for what is learned.
The neural circuitry involved in memory and learning in mammals is complex and difficult to study. Alternative approaches are animal studies (especially in the simpler nervous systems of invertebrates), analysis of the functional consequences of lesions, and anatomical/physiological studies at the cellular and pathway level. For example, in the marine mollusk Aplysia, it has been possible to isolate a connection between a single sensory neuron and a motor neuron, which shows aspects of habituation (learning not to respond to repetitions of an insignificant stimulus), sensitization (increased responsiveness to innocuous stimuli that follow the presentation of a strong or noxious stimulus), and even associative conditioning (learning to respond to a previously insignificant event after it has been