Variation of the multifocal visual evoked potential with eccentricity in isometropic amblyopes

To investigate the variation of visual function in different eccentricities of the visual field in isometropic amblyopes.

The stimulus matrix containing 61 hexagons was generated on a monitor. The diameter of the entire stimulating field was approximately 13.6 deg of arc; the frame rate of the monitor was 67 Hz. Every hexagon of the display contained a number of black and white small hexagonal patches which reversed during stimulation. These hexagons were simultaneously but independently modulated in time by the controlling computer program. The flashed elements were selected differently on each frame according to a computer-generated binary pseudo-random time series (m-sequence); the response contributions from each of the individual stimulus elements could be extracted from the cross corre-ation function. Subjects were asked to maintain fixation at the center of the stimulus pattern while each of the hexagons of the display reversed. The VERIS system extracted the local responses by cross-correlating the input (binary m-sequence) and output (evoked response) signals. The latencies and amplitudes of the responses from the central 8.6 deg of arc in the visual field were analyzed.

Mean latency is about 22 ms longer at the fovea, and 10 ms longer in the first ring for the isometropic amblyopes than for the normal (control) subjects. The differences are highly significant at the fovea and in the first ring. In the second ring and the third ring, the difference have reduced to 3 to 4 ms. The latencies at the second ring and the third ring are not significantly different. In isometropic amblyopes, the P1-N2 amplitude is about half of that in the normal subjects for the fovea and is similar to that for the normal subjects in the third ring. The P1-N2 amplitudes in the isometropic amblyopes are significant lower than the normal values from the center to the second ring.

In isometropic amblyopia, the prolongation of latencies and the attenuation of the amplitudes of topographic VEPs are marked in the central region of the visual field and diminished in the periphery. The topographic VEP can objectively reflect the variation of visual response with eccentricity in the visual field.