Stimulus duration, neural adaptation, and sweep visual evoked potential acuity estimates

PURPOSE. Results in several studies have suggested that the visual evoked p otential (VEP) amplitude can vary with stimulus duration. The purpose of th is study was to determine whether acuity estimates obtained by extrapolatio n of the sweep VEP are altered by this adaptation effect. METHODS. Sweep VEP data were obtained from 16 healthy observers under binoc ular viewing conditions. Data were acquired with a commercially available V EP unit using standard electrode recording techniques. Three sweeps (high s patial frequencies , medium spatial frequencies, and low spatial frequencie s) were run. The subjects' visual acuity at the monitor distance was 6/6 fo r the high spatial frequency sweep. For the medium and low spatial frequenc y sweeps, the subjects were dioptrically blurred to 6/15 (medium spatial fr equencies) or 6/30 (low spatial frequencies) at the monitor distance. Each sweep consisted of six spatial frequencies (contrast 80%; temporal frequenc y (TF) = 7.5 Hz; screen luminance = 100 candela [cd]/m(2)). For each spatia l frequency, the stimulus duration was 8 seconds, partitioned into 1-second bins. A minimum of eight sweeps were obtained per subject. An acuity estim ate nas obtained for each second's data by fitting a line to the high spati al frequencies (excluding noise) and extrapolating this line to the x-axis. With this technique, estimates could not be obtained for 29 of 384 possibl e acuities. RESULTS. The sweep VEP acuities for the 16 subjects did not change signific antly over the 8 seconds of data collection for the high, medium, or low sp atial frequency sweep (repeated measures analysis of variance [ANOVA]: high , P = 0.25; medium, P = 0.50; low, P = 0.23). In any given subject, there w as a 1- to 2-octave range in acuity estimates over the 8 seconds of stimulu s presentation (high, 1.23 +/- 0.417 octaves; medium, 1.41 +/- 0.593 octave s; low, 1.52 +/- 0.475 octaves; mean +/- SD). CONCLUSIONS. These results suggest that there is not a significant change i n sweep VEP acuity estimates over an 8-second stimulus presentation. Thus, neural adaptation does not significantly affect the clinical use of the swe ep VEP.