METHOD

Subjects

Two groups of 9 subjects each, congenitally blind and with normal vision, were tested. The congenitally blind group (M age 14.1 yr., SD 1.4) had a diagnosed peripheral visual deficit from birth and no other abnormality. This was confirmed by an absence of visual evoked responses. The normal vision group (M age 14.0 yr., SD = 1.1) had normal visual evoked responses elicited by light flashes.

Design of Study, and Testing Procedure

Subjects were assessed in a single sitting with two consecutive assess ments or recordings (R1, R2).

Auditory middle latency evoked potentials were recorded in the 100 msec. poststimulus time period, from the vertex referenced to the right earlobe, with the ground electrode on the forehead. The preamplifier band width (Nihon Kohden, Neuropack 8, Japan) was set at 10 to 1500 Hz. Altogether 1500 responses were averaged for each assessment. Click stimuli of 40 msec., duration and alternating polarity were delivered through acoustically shielded earphones (Elga DR-531, Japan). The intensity was kept at 80 dB for all assessments. The threshold of bearing was noted. Visual evoked potentials were recorded in the 200-msec. time period, using parameters as described elsewhere (Naveen, et al, 1997).

Middle Latency Auditory Evoked Potentials, P1 Component

The peak latency and amplitude of the P1 component were measured from the baseline existing at the beginning of the sweep. The P1 wave is a positive component following the Nb wave (Cacace, Satya-Murti. & Wolpaw, 1990) and occurring between 30 and 100 msec. (Picton, Champagne, & Kellett, 1992). The components of the P1 wave for congenitally blind subjects and those with normal vision were compared using separate two factor analyses of variance, for the peak latency and the peak amplitude. For this, the first factor was the different groups, i.e., congenitally blind versus normal vision, and the second factor was repeated recordings (R1, R2).