We have used positron emission tomography (PET) to measure regional cerebral blood flow (rCBF) in sighted and congenitally blind subjects performing auditory localization tasks. During scanning, the spectral and binaural cues of localized sound were reproduced by a sound system and delivered via headphones. During tasks that required auditory localization both the sighted and blind subjects strongly activated posterior parietal areas. In addition, the blind subjects activated association areas in the right occipital cortex, the foci of which were similar to areas previously identified in visual location and motion detection experiments in sighted subjects. The blind subjects, therefore, demonstrated visual to auditory cross-modal plasticity with auditory localization activating occipital association areas originally intended for dorsal-stream visual processing. To determine the functional connectivity of pre-selected brain regions in primary and non-primary auditory and posterior parietal cortex in the two cohorts, we performed an inter-regional correlation analysis on the rCBF data set. During auditory localization in the blind subjects, rCBF activity in the right posterior parietal cortex was positively correlated with that in the right occipital region, whereas in sighted subjects correlations were generally negative. There were no significant positive occipital correlations in either cohort when reference regions in temporal or left parietal cortex were chosen. This indicates that in congenitally blind subjects the right occipital cortex participates in a functional network for auditory localization and that occipital activity is more likely to arise from connections with posterior parietal cortex.
Although the importance of the posterior parietal and prefrontal regions in spatial localization of visual stimuli is well established, their role in auditory space perception is less clear. Using positron emission tomography (PET) during auditory and visual spatial localization in the same subjects, modality-specific areas were identified in the superior parietal lobule, middle temporal and lateral prefrontal cortices. These findings suggest that, similar to the visual system, the hierarchical organization of the auditory system extends beyond the temporal lobe to include areas in the posterior parietal and prefrontal regions specialized in auditory spatial processing. Our results may explain the dissociation of visual and auditory spatial localization deficits following lesions involving these regions.
The brain regions activated by simple repetitive and sequential finger movements of different length were localized by measuring regional cerebral blood flow (rCBF) with PET. The experimental design consisted of finger movements cued by auditory pacing at 0.5 Hz. In all conditions of different sequence length the contralateral primary sensorimotor and premotor cortex, supplementary motor area and ipsilateral cerebellar cortex were activated. These areas showed a large increase in activation from rest to simple repetitive movement, and a further increase with the shortest sequence, suggesting an executive role in running sequences. The ipsilateral premotor area (Brodmann area 6), bilateral posterior parietal areas (Brodmann area 7) and precuneus showed an increase in rCBF related only to the length of the sequences, without any change from rest to simple repetitive movement. These areas are more selectively related to sequence performance. This finding is consistent with the hypothesis that these areas function in the storage of motor sequences in spatial working memory. Our results suggest that sequential finger movements recruit discrete sets of brain areas with different functions.
Cross‐modal plasticity in blind subjects contributes to sensory compensation when vision is lost early in life, but it is not known if it does so when visual loss occurs at an older age. We used H215O positron emission tomography to identify cerebral regions activated in association with Braille reading, and repetitive transcranial magnetic stimulation to induce focal transient disruption of function during Braille reading, in 8 subjects who became blind after age 14 years (late‐onset blind), after a lengthy period of normal vision. Results were compared with those previously reported obtained from congenitally and early‐onset blind subjects. As shown by H215O positron emission tomographic scanning, the occipital cortex was strongly activated in the congenitally blind and early‐onset blind groups but not in the late‐onset blind group. Occipital repetitive transcranial magnetic stimulation disrupted the Braille reading task in congenitally blind and early‐onset blind subjects but not in late‐onset blind subjects. These results indicate that the susceptible period for this form of functionally relevant cross‐modal plasticity does not extend beyond 14 years. Ann Neurol 1999;45:451–460
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