Comparison of the mechanisms of latency shift in pattern reversal visual evoked potential induced by blurring and contrast reduction.

Reduction of visual acuity or of the contrast of the stimulus induces a prolongation of the pattern reversal visual evoked potential (PR-VEP) latencies, perhaps because these conditions cause deterioration of the visual capacity to recognize objects and may preferentially activate the slower central retina channel. The PR-VEP was obtained with a video stimulator and 3 kinds of stimuli: total video field, video with a central scotoma and a restricted central stimulus. The subjects were tested under conditions of normal (20/20) and reduced visual acuity (20/200) with 14' and 56' checks and 60% contrast, and under conditions of normal visual acuity (20/20) with 14' checks and with stimulus contrast of 60% and 25%. Blurring increased latencies and decreased amplitudes only with the 14' checks stimulus but no with 56' checks, and the amplitudes obtained with the central stimulus became greater than those obtained with a central scotoma. Reducing contrast increased only latency, and there was not difference between amplitudes obtained with a central stimulus or a central scotoma. We conclude that blurring small checks induces a preferential stimulation of receptors in the central retina, but the same effect was not observed when stimulus contrast was reduced.