Infant VEP acuity measurements: analysis of individual differences and measurement error

The reliability of visual evoked potential (VEP) measurements of acuity was determined by estimating acuity for sinewave luminance gratings which were counterphase modulated at either 12 or 20 reversals/sec (rps). Gratings were swept in spatial frequency beyond the acuity limit and acuity was estimated from an extrapolation based on the last peak in the VEP amplitude versus spatial frequency function. Twenty-five infants ranging in age from 17 to 25 weeks were studied. Individual 10 sec sweeps resulted in records with a criterion response in 65-75% of trials. The reliability of acuities obtained from individual 10 sec sweeps was +/- 0.54 octaves at 95% confidence across 12 and 20 rps recording conditions (RMS error of +/- 0.27 octaves). The best acuity attained by each infant on either a single sweep, or on their vector average, was reliable to +/- 0.38 octaves at 95% confidence (RMS error of +/- 0.19 octaves) compared to a range of individual acuities of about 2 octaves. Much of the variability of sweep VEP acuity in cross-sectional samples of infants is therefore attributable to reliable individual differences rather than to measurement error. In testing individual infants our analysis indicates that choice of temporal frequency accounts for only 14% of the variation in acuity estimates within subjects.