Characteristics of anisometropic suppression: Simple reaction time measurements

Abstract

The characteristics of artificially induced anisometropic suppression were investigated in observers with normal and abnormal binocular vision (anisometropic amblyopia) by using a simple reaction time paradigm. Reaction time was measured as a function of stimulus intensity for various stimulus durations. For all conditions, the reaction time increased as stimulus intensity decreased toward threshold. Wefound that traditional techniques for modeling this trend were inadequate, so we developed a simple visuogram method for comparing these functions. Using this technique, reaction time versus intensity functions are shown to be shape-invariant for all conditions examined. This means that, although reaction times are longer during induced anisometropic suppression or in anisometropic amblyopia, they are the same if contrast is normalized to equate threshold. The shapeinvariant nature of these functions is also consistent with the notion that a single mechanism mediates detection under these conditions. Temporal summation was investigated at both threshold (method of limits) and suprathreshold (criterion reaction time) levels. Again, because of shape invariance, the suprathreshold results mirror the threshold results. The critical duration (the duration at the intersection of the complete summation and zero summation regions) is not affected by any of the conditions. However, the critical intensity (the intensity for the zero summation region) is higher for the amblyopic eyes, as compared with the normal or nonamblyopic eyes. Induced anisometropic suppression always increases the critical intensity, with a smaller increase occurring for the amblyopic eyes. This suggests that amblyopic eyes do not have a need for strong suppression.

Impaired Temporal, Not Just Spatial, Resolution in Amblyopia

“…36 Reaction times for stimulus appearance and latencies of evoked potentials were somewhat slower through amblyopic eyes (anisometropic amblyopia) than through their normal partner eyes. 37,38 In line with these results, latencies of evoked potentials for high spatial frequencies increase in human amblyopia, 39 and stimulation latencies of single cells increase in the feline primary visual cortex. 40 However, as mentioned earlier, the differences between amblyopic and normal eyes of patients usually disappear when the contrast of the stimuli is equated to accommodate the lower contrast sensitivity of the amblyopic eye, indicating that there is no specific loss in the time domain but just an expected loss due to decreased contrast sensitivity.…”

Saccadic latency in amblyopia

“…To explore whether binocularity affected their saccadic latency, we divided the anisometropic amblyopes into binocular and nonbinocular groups. The mean for the binocular anisometropic amblyopes (green circle) shows a small, but significant, increase in the interocular latency difference of about 15 ms compared to the normal observers—a value that is small enough that it may arise from their interocular difference in contrast sensitivity (see Pianta & Kalloniatis, 1998). However, with the loss of binocular vision, the latency difference for the anisometropic amblyopes (green diamond) doubles to roughly 30 ms, and is now roughly equal to the interocular latency difference of one of the strabismic groups.…”

“…Reaction time decreases as a power function of stimulus intensity (Piéron, 1914, 1952), so if the effective stimulus were weaker in the amblyopic eye than in the fellow eye, one would predict a slower response (Cuiffreda et al, 1991; Pianta & Kalloniatis, 1998). In the studies described above, the targets were small luminous features presented on a dark background, i.e., high contrast targets.…”

“…The reaction time function in the amblyopic eye is systematically slower than the function of the fellow eye; it is displaced leftward, along the contrast axis, by an amount roughly equal to the ratio of the contrast thresholds for the two eyes (Palmer, Huk, & Shadlen, 2005; Pianta & Kalloniatis, 1998). In the bottom graph of Figure 1, we replot the data in multiples of contrast threshold.…”

Binocular retinal image differences influence eye-position signals for perceived visual direction

“…Subjects with strabismus exhibit clinical suppression when one eye is deviated, to avoid the confusion that would occur when dissimilar images fall on corresponding retinal points and diplopia when similar images fall on non-corresponding retinal points (Cooper, Feldman & Pasner, 2000; Hess, 1991; Holopigian, 1989; Schor, 1977; Serrano-Pedraza, Clarke & Reed, 2011; Sireteanu, 1982; Smith et al, 1994; Steinbach, 1981; Travers, 1940). In observers without strabismus, suppression is fostered by dissimilarity between the images in the two eyes, as occurs for example when the image in one eye is blurred as a result of anisometropia (e.g., Heath, Hines & Schwartz, 1986; Humphriss, 1982; Liu & Schor, 1994; Pianta & Kalloniatis, 1998; Schor, Landsman & Erickson, 1987; Shors, Wright & Greene, 1992; Simpson, 1991). The deviation of one eye has been reported not to influence perceived EVD in individuals with constant strabismus, who typically behave as if they are unaware of both the retinal and eye-position information from the deviated eye (Gauthier, Berard, Deransard, Semmlow & Vercher, 1985; Mann, Hein & Diamond, 1979).…”