DISCUSSION

The present results show that the Nb wave peak latency was significantly lower in congenitally blind (CB) subjects compared to age matched subjects with normal vision (NV). The peak amplitude of the Pa wave was significantly lower for the CB group compared to the NV group when the recording was made from Oz. There was no obvious difference in the threshold of hearing of CB and NV groups.

The lower peak latency of the Nb wave in the CB group is in keeping with previous reports of lower 1atency of the N1 wave in CB subjects (Niemeyer & Starlinger, 1981; Woods et al., 1985). However these reports also described enhanced amplitude in the blind. In the present study there was no significant difference in peak amplitudes of the two groups when recordings were made from the vertex (Cz). Another study (Alho et al., 1993), had described a larger processing negativity (PN) to attended tones at occipital scalp sites in CB subjects compared to subjects with normal vision. These results suggested that in blind subjects in addition to the participation of conventional neural areas (auditory cortex, frontal cortex) the parietal and even occipital cortices may contribute to the PN.

In the present results no such effects were observed. In fact the peak amplitude of the Pa wave recorded from the occipital area of the congenitally blind subjects was significantly less than that of the normal sighted subjects, recorded from the same site. The Pa wave is understood to be related to simultaneous activation of both supratemporal auditory cortices (Deiber, Ibanez, Fischer, Perrin & Mauguiere, 1988). Intracerebral recording in man has shown that the generator of the Nb wave is localized with relatively focal topology, in the dorsoposteromedial part of Heschl's gyrus, or primary auditory cortex (Liegeois-Chauvel, Musolino, Badier, Marquis & Chauvel, 1994). In the present study lower Nb wave peak 1atency in the blind implies that facilitation of processing of auditory information occurs at the level of the primary auditory cortex. The absence of change in the wave V and Na wave may mean that brainstem and diencephalic areas, which are known generators of these waves (Deiber et al., 1988), appear unchanged.

The results of the present study also suggest that the occipital areas do not participate in the information processing at primary cortical areas. This result is in contrast to the previous study (Alho et al., 1993) which demonstrated participation of more posterior areas in generation of potentials during auditory selective attention.

The lower peak amplitude of the Pa wave recorded from Oz in the blind compared to the normal sighted is contrary to the above cited study (Alho et al., 1993). None of the blind subjects had central (cortical) blindness, making it unlikely that underlying neuropathology could explain the lower amplitude recorded at Oz in the CB group. Hence it appears as though the occipital area does not play a role ill auditory information processing at primary cortical areas, in the congenitally blind. Compensatory changes at this level occur as shortened latency of the corresponding components.