Light responses of neurons in several brain areas are affected by eye position (Andersen et al, 1990 Journal of Neuroscience 10 1176 – 1196). To find out if eye position may influence contrast sensitivity, measurements were made of the spatial contrast modulation transfer function at two different eye positions: with the gaze directed straight ahead and horizontally deflected by 30 deg, respectively. Eleven gratings (1.05 – 36.8 cycles deg−1) were presented so that their retinal position was identical at both eye positions. The data obtained show a significant effect of eye position on contrast sensitivity in 4 subjects out of 8. We found a mean decrease in contrast sensitivity by 10.8% in the ‘deflected’ condition. The eye-position effect was pronounced at low (14.8%) and high (29.6%) frequencies, being insignificant at middle frequencies. The maximal sensitivity decrease observed was 62.1% at 25 cycles deg−1 in one subject. In 4 of the subjects, no significant eye-position effect was found.
Thus a horizontal deflection of the gaze direction can narrow the visual transfer function in some observers and consequently impair perception of near-threshold objects dominated by high or low spatial frequencies. Further studies will show what may account for this phenomenon: the interaction of retinal and eye position signals and/or optical changes due to compression of the eye ball.
