Both saccadic and manual responses in the amblyopic eye of strabismics are irreducibly delayed

Abstract

Abnormal early visual development can result in a constellation of neural and visual deficits collectively known as amblyopia. Among the many deficits, a common finding is that both saccadic and manual reaction times to targets presented to the amblyopic eye are substantially delayed when compared to the fellow eye or to normal eyes. Given the well-known deficits in contrast sensitivity in the amblyopic eye, a natural question is whether the prolonged reaction times are simply a consequence of reduced stimulus visibility. To address this question, in Experiment 1 we measure saccadic reaction times (RT) to perifoveal stimuli as a function of effective stimulus contrast (i.e., contrast scaled by the amblyopic eye's contrast threshold). We find that when sensory differences between the eyes are minimized, the asymptotic RTs of our anisometropic amblyopes were similar in the two eyes. However, our results suggest that some strabismic amblyopes have an irreducible delay at the asymptote. That is, even when the sensory differences of the stimulus were accounted for, these observers still had large interocular differences (on average, 77 ms) in saccadic reaction time. In Experiment 2, to assess the role of fixation on saccadic reaction time we compared reaction time with and without a foveal target (the “gap effect”). Our results suggest that, while removing the fixation target does indeed speed up reaction time in the amblyopic eye, the gap effect is similar in the two eyes. Therefore, the gap effect does not eliminate the irreducible delay in the amblyopic eye. Finally, in Experiment 3 we compared the interocular differences in saccadic and manual reaction times in the same observers. This allowed us to determine the relationship between the latencies in the two modalities. We found a strong correlation between the differences in saccadic and manual reaction times; however, the manual RT difference is about half that of saccadic RT, suggesting that there may be two separable effects on saccadic reaction time: (a) a central problem with directing actions to a target, related to disengagement of attention at the fovea, which results in delays in both saccadic and manual reaction times, and (b) a further delay in saccadic reaction times because of the motor refractory period from a previous saccade or microsaccade, made in an attempt to stabilize the amblyopic eye of strabismics.

Optical treatment of strabismic amblyopia

“…At the moment there are no data available linking changes in attentional processing with treatment outcomes in strabismic amblyopia. Several recent lines of research point to temporal processing delays in the strabismic amblyopic eye relative to the fellow eye . Is it possible that increased retinal contrast from optical correction reduces the latency of neuronal responses in the strabismic eye, bringing the two eyes into better temporal correspondence?…”

“…This seems unlikely; the temporal delays in strabismic amblyopic eyes persist when stimulus contrast is varied 21 and when resolution and contrast deficits associated with the amblyopia are accounted for. 22 Regardless of the mechanism behind optical treatment of strabismic amblyopia, the fact that considerable gains in acuity and stereopsis 23,24 can be achieved places optometrists at the frontline of treatment. A useful field guide, informed by sound empirical evidence, has been complied by Chen and Cotter.…”

Gap effect and express saccades generation in amblyopia

“…Although three of our strabismic amblyopes (No. 3,4, and 8) manifested small anisometropia, we decided not to create a third group of participants with mixed type of amblyopia and to classify subjects with strabismus as only strabismic amblyopes, irrespectively of their refractive status (Gambacorta et al, 2018). The clinical characteristics of strabismic as well as anisometropic individuals is presented in Table 1 and 2.…”

“…Thus, it seems that the stimulus attributes (e.g., luminance, contrast) should not account for this discrepancy. Interestingly, Gambacorta et al (2018) showed that for some amblyopes (especially strabismic), saccadic latencies were increased, even when the signal strength in the two eyes was equated. Thus, future research on a larger group of amblyopic subjects is needed to further explore the source of difference in saccadic latency between the strabismic and anisometropic amblyopes.…”

“…In the discussion section of our recent work (Perdziak, Witkowska, Gryncewicz, & Ober, 2018), we have suggested that it will be interesting to explore the impact of amblyopia on saccades control during the gap condition. Recently Gambacorta, Ding, McKee, and Levi (2018), using a Gabor target, have reported in a very small group of unilateral amblyopes (two anisometropic amblyopes, three strabismic amblyopes, and two control subjects) that the gap effect is similar in both amblyopic and dominant eyes; however, there is still a lack of sufficient data to describe the express saccades production and the existence of gap effect in amblyopic subjects in a more exhaustive manner. We hypothesize that the gap effect will be present in both amblyopic and normal eyes since amblyopia affects rather higher cortical processing associated with saccade initiation than processes related to saccade programming or visual fixation disengagement.…”