•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