Temporal integration in diseased eyes

Abstract

We examined critical duration for visual acuity in eyes with central serous retinopathy (CSR), macular edema (ME) and glaucoma. Critical duration for visual acuity is the minimum period of time to perceive the acuity chart of his best. Visual acuity was measured at several limited exposure durations and the results were then compared with that of normal eyes. The acuity target was a single Landolt ring. The size, direction, and exposure duration of the target were computer controlled. The mean critical duration for visual acuity of the CSR and ME groups was 1.78 s and 2.69 s. These were significantly longer than that of the normal group (0.62 s). The critical duration of the glaucoma group was 0.42 s, which was not significantly prolonged. Critical duration for the increment or static threshold was measured for the purpose of comparison, and no significant differences were found between the diseased eyes (the CSR and ME groups) and the normal eyes, although the thresholds were significantly higher in the diseased eyes. Next, fixation movements which occurred during visual acuity testing were observed in order to investigate their role in acuity testing. In both normal and ME eyes the frequency and amplitude of microsaccades were smaller while the visual acity chart was shown. This suggests that microsaccades play no positive role in the reading of acuity charts, and that they bear little relation to the phenomenon of critical duration for visual acuity. From these results we hypothesized that in the CSR and ME groups the X-type ganglion cells send incomplete information to the central neural system. Thus, a longer time is required to obtain complete information before responding. Furthermore, one glaucoma case suggested that in diseased eyes in which mainly the Y-cell systems are damaged, temporal specificity in visual acuity for shorter exposure duration may reveal different response patterns.