METHOD

Subjects

There were two groups of ten subjects each. One group consisted of blind subjects (group average age ± SD, 14.3 ± 1.4 years), with a single female subject. All subjects had a diagnosed peripheral deficit with an inability to differentiate between light and dark, from birth. This was confirmed by an absence of visual ERPs to light flashes. Apart from visual impairment these subjects had no other physical or mental deficit. The other group consisted of subjects with normal vision without correction, confirmed by normal ERPs elicited by light flashes, who were selected to match the blind subjects for age (± 1 year). The group average age ± SD of this group was 14.1 ± 1.1 years. For both groups the informed consent of the subjects and their guardians was taken in accordance with the ethical guidelines of the Indian Council of Medical Research (New Delhi, India).

Design of the Study

All subjects were assessed in a single sitting. Recordings were made in the following sequence: two consecutive recordings (R1, R2) of AEP-MLR to assess the immediate reproducibility of the waveforms, followed by a recording of VEP.

Recording of Evoked Potentials

(1) Auditory middle 1atency evoked potentials (AEP-MLR) were recorded in the 100 ms postimulus time period, from Cz, referenced to the right ear lobe with the ground electrode on the forehead. The preamplifier band width was set at 10 to 1500 Hz. 1500 responses were averaged for each period. Sweeps containing artifactually large signals were rejected through software specification. The rejection level was expressed as a percentage of the full scale range of the analog-to-digital converter. This level was set at 85%. The number of sweeps was displayed on the monitor. Click stimuli of 40ms duration and alternating polarity were delivered at 5 Hz binaurally, through acoustically shielded earphones (Elga DR-531, Japan). The intensity was kept at 80 dB for all assessments. The threshold of hearing was also noted in all subjects.

Visual evoked potentials were recorded in the 200 ms poststimulus 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 1 00 Hz and 1 00 responses were averaged for each period. Artifact rejection was similar to the method described above for recording auditory evoked potentials. Light flashes were given using a LED visual stimulator (Nihon Kohden SLS 3500). Stimuli were given to each eye separately and also binocularly.

AEP-MLRs Components

Peak amplitudes of short latency wave V, and middle latency Na, Pa and Nb waves were measured from the baseline existing it the beginning of the sweep. Peak latency was measured from the time of click delivery.

The middle latency auditory evoked response components (AEP-MLRs 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 14 and 18 ms, the maximum positive peak between the Na wave and 35 ms was described as the Pa wave, and the maximum negative peak between 38 and 48 ms was described as the Nb wave. These descriptions are similar to those in other studies on AEP-MLRs (Erwin & Buchwald, 1986; McPherson, Tures & Starr, 1989).

Data Analysis

1. Comparison of peak latencies between congenitally blind subjects and those with normal vision. The data were analyzed using a two factor analysis 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.

2. Comparison of peak amplitudes of potentials in congenitally blind subjects and those with normal vision using a two factor analysis of variance (ANOVA), with factor A = groups i.e., congenitally blind versus normal vision, and factor B=repeat recordings (RI, R2). Separate ANOVAs were performed for peak amplitudes of potentials recorded from the two sites, viz Cz and Oz. This analysis was restricted to the Pa and Nb waves, which are known to have cortical generators. The Tukey multiple comparison test was used to detect significant differences between group mean values.