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 t he visual capacity to recognize objects and may preferentially activat e the slower central retina channel. The PR-VEP was obtained with a vi deo 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 (2 0/200) with 14' and 56' checks and 60% contrast, and under conditions of normal visual acuity (20/20) with 14/ checks and with stimulus cont rast of 60% and 25%. Blurring increased latencies and decreased amplit udes only with the 14' checks stimulus but no with 56' checks, and the amplitudes obtained with the central stimulus became greater than tho se obtained with a central scotoma. Reducing contrast increased only l atency, and there was not difference between amplitudes obtained with a central stimulus or a central scotoma. We conclude that blurring sma ll checks induces a preferential stimulation of receptors in the centr al retina, but the same effect was not observed when stimulus contrast was reduced. (C) 1997 Elsevier Science ireland Ltd.