Neuropsychological Views

Neuropsychological Views

Neuropsychologists have been trying to understand what the underlying brain structures involved with language

are, where they are located, and how they operate.

Interest in localizing language function in the brain dates back at least to the 1800s, when a French physician with interests in anthropology and ethnography, Pierre Paul Broca, read a paper in 1861 at the meeting of the Société d’Anthropologie in Paris. The paper reported on a patient, nicknamed “Tan” because he had lost the ability to speak any words save for tan. Shortly after the patient died, his brain was examined and found to have a lesion in the left frontal lobe. The very next day, Broca reported this exciting (for science, not for the patient or his family, probably) finding (Posner & Raichle, 1994). The area of the brain, henceforth known as Broca’s area.

Subsequently, several other patients were reported who had similar difficulties in speaking and who were found to have lesions in the same brain region.

About 13 years later, a German neurologist Carl Wernicke identified another brain area that, if damaged by a small lesion (often the result of a stroke), left patients with extreme difficulty comprehending (but not producing) spoken language. (Not surprisingly, this area has come to be called Wernicke’s area).

Both these language disorders were termed aphasia, although the first was called expressive aphasia (or Broca’s aphasia) and the second receptive aphasia (or Wernicke’s aphasia). Broca’s aphasia appeared to leave language reception and processing undisturbed; Wernicke’s, to spare fluent production of words and sentences (although the language was often gibberish). More recent evidence provides qualifications to these statements, suggesting, for example, that patients with Broca’s aphasia do have some difficulties in understanding spoken language. Thus our understanding of different kinds of aphasia is becoming more elaborated. Other kinds of aphasia have also been reported and correlated with brain damage in specific brain regions, often ones adjacent to Broca’s or Wernicke’s areas (Banich, 1997).

Researchers studying aphasia also noticed an interesting generalization about aphasic patients: Usually the area of damage to the brain was in the left and not the right hemisphere. This led to the idea that the two cerebral hemispheres of the brain play different roles and have different functions. The term for this specialization of function between the two hemispheres is lateralization.

Briefly, it appears that in most people the left cerebral hemisphere is associated with the ability to produce and comprehend  language and the right hemisphere, with the ability to process complex spatial relationships (Springer & Deutsch, 1998). Evidence for this lateralization began with the clinical observation (beginning with Broca) of aphasic patients. Other  evidence comes from a test used with people about to undergo brain surgery for epilepsy, called the Wada test. This involves injecting a barbiturate drug, sodium amobarbital, into one of two carotid arteries: either the one going to the left hemisphere or the one going to the right hemisphere. The injection anesthetizes one of the hemispheres. The patient is kept conscious during this procedure and, just before the injection, is asked to hold up his or her two arms and to start counting. When the drug reaches the intended hemisphere, the patient drops the arm that is on the opposite side of the body from the side anesthetized.

Technologies such as CAT and PET scans have also been used to study language functioning in both aphasic and nonaphasic people. Kempler et al. (1990) studied three patients with an aphasia known as slowly progressive aphasia,

noting either normal or mild atrophy of the left language regions (shown by CAT scans) and hypometabolism (that is, less use of glucose) by the left hemispheres of the three patients.

Caplan (1994) concluded that the localization of specific language processing in particular brain regions is not straightforward. One possible idea entertained by Caplan is that language processes do not necessarily have a specific

location in the brain. Instead, they may be distributed across a region of the brain in a neural network configuration.