Speech and language in the developing child are so prepotent and resilient that their acquisition is precluded by brain damage only if it affects the perisylvian region bilaterally. Recently, we reported on a child ("Alex") who failed to develop speech and language, in this case because of epilepsy associated with Sturge-Weber disease, a congenital anomaly affecting the blood supply to his left hemisphere. However, at the age of nearly nine and a half, having received a left hemispherectomy for relief of epilepsy ten months earlier, and having had all anticonvulsants withdrawn three months earlier, Alex suddenly began to acquire speech and language for the first time, presumably because his right hemisphere had now been released from functional interference. His early progress resembled that of a normal 1 to 3-year-old child, first producing words, then word combinations, and finally, grammatical phrases, although he accomplished this in about nine months instead of the usual eighteen. Now 16 years old, having improved progressively in both language and other cognitive functions since his speech onset, he performs these functions at a level equivalent to that of an 8 to 10-year-old. Moreover, comparison of his receptive and expressive language ability, memory, and intelligence with those of other left or right hemispherectomized children with early onset of disease, but with early development of speech and language, indicate that, despite his late start, Alex has suffered no permanent disadvantage relative to the others. His case is remarkable in showing just how long the complex neural system underlying linguistic function, and one limited moreover to an isolated right hemisphere, can remain viable despite disuse from birth onward. The results in Alex, as in other children with left hemispherectomy, reemphasize the fact that although speech and language are uniquely human, the uniqueness does not reside in some nonreplicable capacity of the human left hemisphere. This project thus seeks to explore the functional organization of the auditory cortex of rhesus monkeys using physiological, anatomical, and behavioral techniques. Our first such study confirmed that neurons in the rostral area (R) of the supratemporal plane respond preferentially to low-frequency tones, whereas neurons in the caudo-medial area (CM) of the supratemporal plane prefer high frequencies. Subsequent lesion and anatomical tracing experiments have provided convergent evidence that area CM, unlike area R, depends on tonotopic input from primary auditory cortex (AI), which lies between the CM and R. Area R, on the other hand, like A1, appears to receive tonotopic input directly from the ventral division of the medial geniculate body. In the lateral belt areas of auditory cortex, which receive input from both R and AI, most neurons do not respond well to pure tones. However, we have found that units in this region can be driven briskly by bandpass-filtered noise (BPN) bursts. Using this new type of stimulus, cochleotopic maps were revealed along the antero-posterior axis in three lateral areas, which we have termed AL, ML, and CL. The neurons here are tuned to a best center frequency of the BPN bursts, a dimension that is represented along the rostro-caudal axis. They are also tuned to a best bandwidth of these bursts, which is represented orthogonally to best center frequency, i.e. along the medio-lateral axis. Finally, these neurons also respond well to frequency-modulated (FM) sounds and show selectivity for the rate and direction of FM stimuli. Noise bursts and FM sounds are essential components of many species-specific communication calls, and some neurons in the lateral belt areas respond selectively to such calls. Post-mapping injections of tracers into the three tonotopic areas of the lateral belt (AL, ML, and CL) have revealed distinctive patterns of thalamic projections from the medial geniculate, supraorbital, medial pulvinar, and medial dorsal nuclei. The three tonotopic areas project, in turn, to numerous other cortical regions, including the anterior superior temporal gyrus, the parietal lobe, the anterior cingulate gyrus, and five different regions within the prefrontal cortex. Results of the anatomical tracing experiments will help guide our future electrophysiological and lesion studies in monkeys trained on behavioral tasks. The lesion studies have been initiated and have yielded a surprising preliminary finding, namely, combined perirhinal and entorhinal ablation of a type that produces severe impairment in both visual and tactile recognition appears to have little or no effect on auditory recognition. Experiments are currently underway to determine whether this unexpected difference in behavioral effects is due to differences in training techniques (other than the sensory modality being tested) or to a genuine difference in the neural substrate for auditory as compared with visual and tactile memory.
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