Lesions of the Temporal Association Cortex: Deficits of Recognition

Clinical evidence from patients with lesions of theassociation cortex in thetemporal lobe indicate that one of the major functions of this part of the brain is recognition and identification of stimuli that are attended to, particularly complex stimuli. Thus, damage to either temporal lobe can result in difficulty recognizing, identifying, and naming different categories of objects. These disorders, collectively called agnosias (from the Greek for “not knowing”), are quite different from the neglect syndromes. As noted, patients with rightparietal lobe damage often deny awareness ofsensory information in the leftvisual field (and are less attentive to the left sides of objects generally), despite the fact that the sensory systems are intact (for instance, an individual withcontralateral neglect syndrome typically withdraws his left arm in response to a pinprick, even though he may not admit the arm's existence). Patients withagnosia, on the other hand, acknowledge the presence of a stimulus, but are unable to report what it is. These latter disorders have both alexical aspect (a mismatching of verbal or cognitive symbols with sensory stimuli; see Chapter 27) and amnemonic aspect (a failure to recall stimuli when confronted with them again; see Chapter 31).

One of the most thoroughly studied agnosias following damage to the temporalassociation cortex in humans is the inability to recognize and identify faces. This disorder, calledprosopagnosia (prosopo-, from the Greek for “face” or “person”), was recognized by neurologists in the late nineteenth century and remains an area of intense investigation. After damage to the temporallobes, typically the righttemporal lobe, patients are often unable to identify familiar individuals by their facial characteristics, and in some cases cannot recognize a face at all. Nonetheless, such individuals are perfectly aware that some sort of visual stimulus is present and can describe particular aspects or elements of it without difficulty.

An example is the case of L.H., a patient described by the neuropsychologistN. L. Etcoff and colleagues in 1991. (The use of initials to identify neurological patients in published reports is standard practice.) This 40-year-old minister and social worker had sustained a severe head injury as the result of an automobile accident when he was 18. After recovery, L.H. could not recognize familiar faces, report that they were familiar, or answer questions about faces from memory. He was nonetheless able to lead a fairly normal and productive life. He could still identify other common objects, could discriminate subtle shape differences, and could recognize the sex, age, and even the “likability” of faces. Moreover, he could identify particular people by nonfacial cues such as voice, body shape, and gait. The only other category of visual stimuli he had trouble recognizing was animals and their expressions, though these impairments were not as severe as for human faces. Noninvasive brain imaging showed that L.H.'sprosopagnosia was the result of damage to the righttemporal lobe.

More recently, imaging studies in normal subjects have confirmed that the inferiortemporal lobe mediates face recognition and that nearby regions are responsible for categorically different recognition functions (Figure 26.8). In general, lesions of the right temporal lobe lead toagnosia for faces and objects, whereas lesions of the corresponding regions of the left temporal lobe tend to result in difficulties with language-related material (recall that theprimary auditory cortex is on the superior aspect of the temporal lobe; as described in the following chapter, the cortex adjacent to the auditory cortex in the left temporal lobe is specifically concerned with language). The lesions that typically cause recognition deficits are in the inferior temporal lobe in or near the fusiform gyrus (those that cause language-related problems in the left temporal lobe tend to be on the lateral surface of the lobe). Consistant with these conclusions, electrical stimulation in subjects whose temporallobes are being mapped for neurosurgery (typically removal of an epileptic focus) may have a transientprosopagnosia as a consequence of this abnormalactivation of the relevant regions of the right temporal lobe.

Figure 26.8

Functional brain imaging of temporal lobes during face recognition. (A) Face stimulus presented to a normal subject at time indicated by arrow. Graph shows activity change in the relevant area of the right temporal lobe. (B) Location offMRI activity(more...)

Prosopagnosia and related agnosias involving objects are specific instances of a broad range of functional deficits that have as their hallmark the inability to recognize a complexsensory stimulus as familiar, and to identify and name that stimulus as a meaningful entity in the environment. Depending on the laterality, location, and size of the lesion in temporalcortex, agnosias can be as specific as for human faces, or as general as an inability to name most familiar objects.

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