METHOD

Subjects

Two groups were studied with 10 subjects in each group, i.e., congenitally blind and normal vision. The congenitally blind subjects (group average age ± SD, 22.4 ± 4.9 years) had a diagnosed peripheral visual deficit from birth and no other abnormality. Blindness was confirmed by the absence of visual evoked responses. The normal vision group was matched for age (exact matching) and sex. They all had normal visual evoked responses elicited by light flashes. The congenitally blind group received no formal rehabilitation (i.e., standard education using Braille or auditory cues) till the age of sixteen years, when they started to receive vocational training (e.g., weaving).

Design of the Study

Subjects were assessed in a single sitting with two consecutive assessments (RI, R2) of the AEP-MLR, to estimate the reproducibility of the recordings. This was followed by a recording of Visual Evoked Potentials.

Recording of Evoked Potentials

Auditory middle latency evoked potentials were recorded in the 100ms. 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. The rejection level was expressed as a percentage of the full scale range of the analog-to-digital convertor. This level was set at 85%. The number of sweeps was displayed on the monitor. Click stimuli of 40 msec. duration and alternating polarity at the rate of 5 Hz, were delivered binaurally through acoustically shielded earphones (Elga DR-531, Japan). The intensity was kept at 85 dB for all assessments. The threshold of hearing was noted.

Visual evoked potentials were recorded in the 200 msec. time period from Oz, referenced to the right earlobe, with the ground electrode on the forehead. The preamplifier band width was set at 1 to 100Hz and 100 responses were averaged for each period. The method for artifact rejection was the same as for AEP-MLR, as described above. Light flashes were given binocularly, using a LED visual stimulator (Nihon Kohden SLS 3500).

AEP-MLRs Components

Peak amplitudes of short latency wave V, and middle latency Na, Pa and Nb waves were measured from the baseline existing at the beginning of the sweep. Peak latency was measured from the time of click delivery. Also the peak amplitudes of AEP-MLR cortical components (i.e., Pa and Nb) recorded from the conventional site (i.e., vertex) were compared with recordings made from the occiput.

The auditory evoked response components were described as follows: wave V was the maximum positive peak between 5 and 8 ms, the Na wave was the maximum negative peak between 10 and 18ms, the maximum positive peak between Na and 35 ms was described as the Pa wave, and the maximum negative peak between 38 and 52ms was described as the Nb wave. These descriptions are similar to other descriptions of AEP components (Erwin and Buchwald, 1986; McPherson, Tures and Starr, 1989; Naveen et al., 1997).

Data Analysis

Comparison of peak latencies between congenitally blind subjects and those with normal vision. The data were analysed using two factor analyses of variance (ANOVA), with Factor A = groups, i.e., congenitally blind versus normal vision and Factor B = repeat recordings (R1, R2). The Tukey multiple comparison test was used to detect significant differences between group mean values.

Similar analyses (as mentioned above) were done for the peak amplitudes. The peak amplitudes of the cortically generated Pa and Nb components recorded from Cz and Oz, were compared using two factor analyses of variance. Factor A = groups (congenitally blind, normal sighted) and Factor (B) = sites of recording (Cz, Oz). For these analyses the data of RI and R2 were pooled.