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H. R. Nagendra, Shirley Telles and K. V. Naveen
Vivekananda Kendra Yoga Research Foundation, Bangalore, India.

R. S. Srinivas and K. S. Nirmala
Department of Neurology, M.S. Ramaiah Medical
Teaching Hospital, Bangalore, India.

(Received 29 September 1996)

Middle latency auditory evoked potentials were recorded in two groups of ten subjects each, viz, congenitally blind (CB) and age-matched subjects with normal vision (NV). The age range for both groups was 13 to 16 years. The CB group subject had peripheral deficits, with absence of visual evoked responses. The peak latency, of the Nb wave (the maximum negativity between 38 and 48 ms) was significantly lower in the CB group compared to NV group (P < .05, one-tailed, two factor ANOVA, Tukey test). In addition to these recordings from the vertex, recordings were also made from occipital areas, to test whether the visual cortex contributes to information processing at primary auditory cortical levels in the blind, as was reported in earlier studies on the generation of potentials during auditory selective attention. No such effect was observed. Hence, it appears that in blind subjects changes in generators of auditory middle latency evoked potential, are mainly related to latency, rather than to scalp distribution of these components.

Keywords: Middle latency auditory evoked potentials; congenitally blind; normal sighted, primary cortical areas.

Blindness at an early stage of development has been found to modify nondeprived sensory modalities as a compensatory measure. A study of the long-latency auditory event related potentials (ERPs) in early blind humans showed that the negative deflection 100 ms from stimulus onset (NI) and subsequent deflections (P2 and P3) show shortened latencies and enhanced amplitudes (Niemeyer & Starlinger, 1981; Woods, Clayworth & Bach-y-Rita, 1985). In a subsequent study auditory ERPs were recorded in early blind subjects and sighted Controls when they attended to stimuli delivered to a designated ear under dichotic conditions (Alho, Kujala, Paavilainen, Summala & Naatanen, 1993). The scalp distribution of the processing negativity, i.e., the endogenous negativity elicited by attended stimuli, as well as the mismatch negativity elicited by occasional higher frequency tones, was recorded more posteriorly in the blind compared to the normal sighted. These results suggest that posterior brain areas normally involved in vision participate in auditory selective attention in the blind.

The present study was designed to compare: (1) the peak latencies and amplitudes of middle latency auditory evoked potential (AEP-MLR) components in blind and age-matched normal sighted subjects, also (2) the peak amplitudes of AEP-MLR components recorded from the conventional site (i.e., vertex) with recordings made from posterior brain areas (i.e., occiput). This comparison was made in both blind and normal-sighted subjects.

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