Motion Information via the Nonfixating Eye Can Drive Optokinetic Nystagmus in Strabismus

Purpose: Strabismic patients can perceptually suppress information from one eye to avoid double vision. However, evidence from prior studies shows that some parts of the visual field of the deviated eye are not suppressed. Our goal here was to investigate whether motion information available only to the deviated eye can be utilized by the oculomotor system to drive eye movements. Methods: Binocular eye movements were acquired in two exotropic monkeys in a dichoptic viewing task in which the fixating eye viewed a stationary spot and the deviated eye viewed a 10° × 10° stationary patch that contained a drifting grating stimulus moving at 10°/s to the right or left for 20 seconds. Spatial location and contrast of the grating were systematically varied in subsequent trials. For each trial, mean slow-phase velocity of the optokinetic nystagmus (OKN) elicited by grating motion was calculated. Results: We found that OKN responses can be elicited by a motion stimulus presented to the foveal region of the deviated eye. Optokinetic nystagmus magnitude varied depending on which eye was viewing the drifting grating and correlated well with fixation preference and fixation stability (indicators of amblyopia). The magnitude of OKN increased for increased relative contrast of the motion stimulus compared to the fixation spot. Conclusions: Our results show that motion information available only to the deviated eye can drive optokinetic eye movements. We conclude that the brain has access to visual information from portions of the deviated eye (including the fovea) in strabismus that it can use to drive eye movements

A Statistical Approach to Visual Masking and Spatial Attention

A stimulus (mask) reduces the visibility of another stimulus (target) when they are presented in close spatio-temporal vicinity of each other, a phenomenon called visual masking. Visual masking has been extensively studied to understand the dynamics of information processing in the visual system. Visual spatial attention is also known to modulate information processing and transfer within the visual system. Since both processes control the transfer of information from sensory memory to visual short-term memory (VSTM), a natural question is whether these processes interact or operate independently. Here, we modeled visual masking by using a statistical framework, and used this theoretical framework along with psychophysical experiments to determine whether and how masking and attention interact. In a psychophysical experiment, observers were asked to report the orientation of a target bar under three different masking paradigms. The distribution of response errors was modeled by using statistical mixture-models. Our results show that in all three types of masking, the reduction of a target’s signal-to-noise ratio (SNR) was the primary process whereby masking occurred. We interpret these findings as the mask reducing the target’s SNR (i) by suppressing or interrupting the signal of the target in para-/meta- contrast, (ii) by increasing noise in pattern masking by noise, and (iii) a combination of the two in pattern masking by structure. Recent evidence suggests that the studies that reported interactions between masking and attention suffered from ceiling and/or floor effects. We investigated interactions between metacontrast masking and attention by using an experimental design in which saturation effects were avoided. In these experiments, attention was controlled either by set-size or by spatial pre-cues. We examined attention-masking interactions based on two types of dependent-variables: (i) the mean absolute response errors and (ii) the distribution of signed response errors. Our results show that both the voluntary (endogenous) and reflexive (exogenous) mechanisms of attention affect observers’ performance without interacting with masking. Statistical modeling of response errors suggests that attention and metacontrast masking exert their effects mainly through independent modulations of the guessing component of the mixture model. Taken together, our results suggest that visual masking and attention operate independently.Electrical and Computer Engineering, Department o

Effects of Exogenous and Endogenous Attention on Metacontrast Masking

To efficiently use its finite resources, the visual system selects for further processing only a subset of the rich sensory information. Visual masking and spatial attention control the information transfer from visual sensory-memory to visual short-term memory. There is still a debate whether these two processes operate independently or interact, with empirical evidence supporting both arguments. However, recent studies pointed out that earlier studies showing significant interactions between common-onset masking and attention suffered from ceiling and/or floor effects. Our review of previous studies reporting metacontrast-attention interactions revealed similar artifacts. Therefore, we investigated metacontrast-attention interactions by using an experimental paradigm, in which ceiling/floor effects were avoided. We also examined whether metacontrast masking is differently influenced by endogenous and exogenous attention. We analyzed mean absolute-magnitude of response-errors and their statistical distribution. When targets are masked, our results support the hypothesis that manipulations of the levels of metacontrast and of endogenous/exogenous attention have largely independent effects. Moreover, statistical modeling of the distribution of response-errors suggests weak interactions modulating the probability of “guessing” behavior for some observers in both types of attention. Nevertheless, our data suggest that any joint effect of attention and metacontrast can be adequately explained by their independent and additive contributions