Consequences of eye movements for spatial selectivity

Gamma synchrony is ubiquitous in visual cortex, but whether it contributes to perceptual grouping remains contentious based on observations that gamma frequency is not consistent across stimulus features and that gamma synchrony depends on distances between image elements. These stimulus dependencies have been argued to render synchrony among neural assemblies encoding components of the same object difficult. Alternatively, these dependencies may shape synchrony in meaningful ways. Using the theory of weakly coupled oscillators (TWCO), we demonstrate that stimulus dependence is crucial for gamma's role in perception. Synchronization among coupled oscillators depends on frequency dissimilarity and coupling strength, which in early visual cortex relate to local feature dissimilarity and physical distance, respectively. We manipulated these factors in a texture segregation experiment wherein human observers identified the orientation of a figure defined by reduced contrast heterogeneity compared to the background. Human performance followed TWCO predictions both qualitatively and quantitatively, as formalized in a computational model. Moreover, we found that when enriched with a Hebbian learning rule, our model also predicted human learning effects. Increases in gamma synchrony due to perceptual learning predicted improvements in behavioral performance across sessions. This suggests that the stimulus-dependence of gamma synchrony is adaptable to the statistics of visual experiences, providing a viable neural grouping mechanism that can improve with visual experience. Together our results highlight the functional role of gamma synchrony in visual scene segmentation and provide a mechanistic explanation for its stimulus-dependent variability.

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Oculomotor Contributions to Foveal Crowding

Clark,

Huynh,

Poletti

2024

J. Neurosci.

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Crowding, the phenomenon of impaired visual discrimination due to nearby objects, has been extensively studied and linked to cortical mechanisms. Traditionally, crowding has been studied extrafoveally; its underlying mechanisms in the central fovea, where acuity is highest, remain debated. While low-level oculomotor factors are not thought to play a role in crowding, this study shows that they are key factors in defining foveal crowding. Here we investigate the influence of fixational behavior on foveal crowding and provide a comprehensive assessment of the magnitude and extent of this phenomenon (N=13 human participants, 4 males). Leveraging on a unique blend of tools for high-precision eyetracking and retinal stabilization, we show that removing the retinal motion introduced by oculomotor behavior with retinal stabilization, diminishes the negative effects of crowding. Ultimately, these results indicate that ocular drift contributes to foveal crowding resulting in the same pooling region being stimulated both by the target and nearby objects over the course of time, not just in space. The temporal aspect of this phenomenon is peculiar to crowding at this scale and indicates that the mechanisms contributing to foveal and extrafoveal crowding differ.Significance Statement:Foveated stimuli are often crowded. The effects of crowding have been extensively studied in the visual periphery and are thought to have a cortical origin. Nonetheless, foveal crowding mechanisms remain elusive. Here we show that acuity drops by two lines on a Snellen Chart when flankers surround a stimulus presented at the very center of gaze. Further, at this scale, crowding cannot be regarded as a purely cortical phenomenon. Because foveal neurons' receptive fields are the smallest, eye jitter during fixation introduces spatial uncertainty by sweeping target and surrounding distractors over the same cortical pooling region even during short fixation periods, exacerbating crowding effects.

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Consequences of eye movements for spatial selectivity

Cited by 3 publications

References 62 publications

Building egocentric models of local space from retinal input

Building egocentric models of local space from retinal input

Gamma Synchrony Mediates Figure-Ground Perception

Oculomotor Contributions to Foveal Crowding

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