Eye blinks as a visual processing stage

Humans blink their eyes frequently during normal viewing, more often than it seems necessary for keeping the cornea well lubricated. Since the closure of the eyelid disrupts the image on the retina, eye blinks are commonly assumed to be detrimental to visual processing. However, blinks also provide luminance transients rich in spatial information to neural pathways highly sensitive to temporal changes. Here, we report that the luminance modulations from blinks enhance visual sensitivity. By coupling high-resolution eye tracking in human observers with modeling of blink transients and spectral analysis of visual input signals, we show that blinking increases the power of retinal stimulation and that this effect significantly enhances visibility despite the time lost in exposure to the external scene. We further show that, as predicted from the spectral content of input signals, this enhancement is selective for stimuli at low spatial frequencies and occurs irrespective of whether the luminance transients are actively generated or passively experienced. These findings indicate that, like eye movements, blinking acts as a computational component of a visual processing strategy that uses motor behavior to reformat spatial information into the temporal domain.

Poster Session: Oculomotor influences on retinal input signals in myopia

Studies of emmetropization have traditionally focused on the spatial characteristics of visual input signals. Yet the input to the retina is not a two-dimensional pattern but a temporally-varying luminance flow. The temporal structure of this flow is predominately determined by eye movements, as the human eyes move incessantly. Even when fixating on a single point, a persistent motion known as ocular drift reformats the luminance flow in a way that counterbalances the spectra of natural scenes. It is established that emmetropes are highly sensitive to these luminance modulations. However, their visual consequences in myopia and hyperopia are unknown. Here, we first review how the temporal-frequency distribution of retinal input signals varies with the amount of ocular drift. We then use a detailed optical/geometrical model of the eye to study how the eye movements jointly shape retinal input as a function of refraction. We show that, within the temporal range of sensitivity of the retina, the spatial frequency distribution of the input signals conveys signed information about defocus. Specifically, for a given degree of defocus, myopic retinas experience more power from low spatial frequency stimuli than hyperopic retinas. These redistribution of input power may have a consequence during eye growth supporting the proposal that eye movements should be taken into consideration in the process of emmetropization.

Poster Session: Spatial-frequency-selective enhancement of visual sensitivity from saccade dynamics

Eye movements transform a spatial scene into luminance modulations on the retina. Recent work has shown that this transformation is highly structured: within human temporal sensitivity, saccades deliver power that increases in proportion to spatial frequency (SF) up to a critical frequency and remains constant beyond that. Importantly, the critical SF increases with decreasing amplitude. Therefore, at sufficiently low SFs, larger saccades effectively deliver stronger input signals to the retina. Here we tested whether this input reformatting has the predicted perceptual consequences, by examining how large and small saccades (6o & 1o) affect contrast sensitivity. We measured relative sensitivity at two SFs: a reference (0.5 cpd), equal to the critical SF for the small saccade, and a probe at either a lower or higher SF (0.1/2.5 cpd). We predicted that large saccades enhance visibility only when the probe has a lower SF than the reference. Subjects (N=7) made instructed saccades while presented with a plaid of overlapping orthogonal gratings at the two SFs and reported which grating was more visible. Results closely follow theoretical predictions: psychometric functions following small and large saccades only differed with the lower SF probe, in which case the larger saccade significantly enhanced visibility. In sum, saccades enable selectivity not only in the spatial domain, but also in the spatial-frequency domain.

Poster Session: Fixational Eye Movements in Myopia

During fixation, an incessant drift of the eye keeps the image impinging on the retina always in motion. Previous work indicated that luminance modulations from ocular drift serve important visual functions in emmetropes (Intoy & Rucci, 2020; Clark et al 2022). However, it remains unknown how ocular drift varies under myopia, a visual impairment commonly caused by eye elongation. We measured eye movements in 19 individuals with varying degrees of myopia (-0.25D to -6.5D) using a digital Dual-Purkinje Image eye-tracker, a recently developed system with sub-arcminute resolution. Subjects observed stimuli monocularly with vision corrected via a Badal optometer. They engaged in two high-acuity tasks: (a) resolution of a 20/20 line of an eye chart (5 evenly spaced tumbling E optotypes); and (b) a more natural task where subjects were presented with images of distant faces (1⁰) and asked to report the image's gaze direction. We show ocular drift characteristics differ in myopes relative to emmetropes. Drift was faster and less curved in myopic observers. On the retina, these changes result in luminance modulations that amplify low spatial frequencies at the expense of high spatial frequencies, so that high-frequency signals are effectively weaker in myopes These results are consistent with the proposal that fine spatial vision strongly relies on oculomotor-induced luminance modulations and emphasize the importance of considering fine eye movements in myopia.

High-resolution eye-tracking via digital imaging of Purkinje reflections

Reliably measuring eye movements and determining where the observer looks are fundamental needs in vision science. A classical approach to achieve high-resolution oculomotor measurements is the so-called dual Purkinje image (DPI) method, a technique that relies on the relative motion of the reflections generated by two distinct surfaces in the eye, the cornea and the back of the lens. This technique has been traditionally implemented in fragile and difficult to operate analog devices, which have remained exclusive use of specialized oculomotor laboratories. Here we describe progress on the development of a digital DPI, a system that builds on recent advances in digital imaging to enable fast, highly precise eye-tracking without the complications of previous analog devices. This system integrates an optical setup with no moving components with a digital imaging module and dedicated software on a fast processing unit. Data from both artificial and human eyes demonstrate subarcminute resolution at 1 kHz. Furthermore, when coupled with previously developed gaze-contingent calibration methods, this system enables localization of the line of sight within a few arcminutes.

Cognitive influences on fixational eye movements

We perceive the world based on visual information acquired via oculomotor control,¹ an activity intertwined with ongoing cognitive processes.^2,3,4 Cognitive influences have been primarily studied in the context of macroscopic movements, like saccades and smooth pursuits. However, our eyes are never still, even during periods of fixation. One of the fixational eye movements, ocular drifts, shifts the stimulus over hundreds of receptors on the retina, a motion that has been argued to enhance the processing of spatial detail by translating spatial into temporal information.⁵ Despite their apparent randomness, ocular drifts are under neural control.^6,7,8 However little is known about the control of drift beyond the brainstem circuitry of the vestibulo-ocular reflex.^9,10 Here, we investigated the cognitive control of ocular drifts with a letter discrimination task. The experiment was designed to reveal open-loop effects, i.e., cognitive oculomotor control driven by specific prior knowledge of the task, independent of incoming sensory information. Open-loop influences were isolated by randomly presenting pure noise fields (no letters) while subjects engaged in discriminating specific letter pairs. Our results show open-loop control of drift direction in human observers.

Accuracy and precision of small saccades

Despite recent advances on the mechanisms and purposes of fine oculomotor behavior, a rigorous assessment of the precision and accuracy of the smallest saccades is still lacking. Yet knowledge of how effectively these movements shift gaze is necessary for understanding their functions and is helpful in further elucidating their motor underpinnings. Using a combination of high-resolution eye-tracking and gaze-contingent control, here we examined the accuracy and precision of saccades aimed toward targets ranging from [Formula: see text] to [Formula: see text] eccentricity. We show that even small saccades of just 14-[Formula: see text] are very effective in centering the stimulus on the retina. Furthermore, we show that for a target at any given eccentricity, the probability of eliciting a saccade depends on its efficacy in reducing the foveal offset. The pattern of results reported here is consistent with current knowledge on the motor mechanisms of microsaccade production.

Spatiotemporal Content of Saccade Transients

Humans use rapid gaze shifts, known as saccades, to explore visual scenes. These movements yield abrupt luminance changes on the retina, which elicit robust neural discharges at fixation onsets. Yet little is known about the spatial content of saccade transients. Here, we show that saccades redistribute spatial information within the temporal range of retinal sensitivity following two distinct regimes: saccade modulations counterbalance (whiten) the spectral density of natural scenes at low spatial frequencies and follow the external power distribution at higher frequencies. This redistribution is a consequence of saccade dynamics, particularly the speed/amplitude/duration relation known as the main sequence. It resembles the redistribution resulting from inter-saccadic eye drifts, revealing a continuum in the modulations given by different eye movements, with oculomotor transitions primarily acting by regulating the bandwidth of whitening. Our findings suggest important computational roles for saccade transients in the establishment of spatial representations and lead to testable predictions about their consequences for visual functions and encoding mechanisms.

Finely tuned eye movements enhance visual acuity

High visual acuity is essential for many tasks, from recognizing distant friends to driving a car. While much is known about how the eye’s optics and anatomy contribute to spatial resolution, possible influences from eye movements are rarely considered. Yet humans incessantly move their eyes, and it has long been suggested that oculomotor activity enhances fine pattern vision. Here we examine the role of eye movements in the most common assessment of visual acuity, the Snellen eye chart. By precisely localizing gaze and actively controlling retinal stimulation, we show that fixational behavior improves acuity by more than 0.15 logMAR, at least 2 lines of the Snellen chart. This improvement is achieved by adapting both microsaccades and ocular drifts to precisely position the image on the retina and adjust its motion. These findings show that humans finely tune their fixational eye movements so that they greatly contribute to normal visual acuity.

Humans are normally not aware that their eyes are always in motion, even when attempting to maintain steady gaze on a point. Here the authors show that these small eye movements are finely controlled and contribute more than two lines in a standard eye-chart test of visual acuity.

Visual attractiveness is leaky: the asymmetrical relationship between face and hair

Predicting personality is crucial when communicating with people. It has been revealed that the perceived attractiveness or beauty of the face is a cue. As shown in the well-known "what is beautiful is good" stereotype, perceived attractiveness is often associated with desirable personality. Although such research on attractiveness used mainly the face isolated from other body parts, the face is not always seen in isolation in the real world. Rather, it is surrounded by one's hairstyle, and is perceived as a part of total presence. In human vision, perceptual organization/integration occurs mostly in a bottom up, task-irrelevant fashion. This raises an intriguing possibility that task-irrelevant stimulus that is perceptually integrated with a target may influence our affective evaluation. In such a case, there should be a mutual influence between attractiveness perception of the face and surrounding hair, since they are assumed to share strong and unique perceptual organization. In the current study, we examined the influence of a task-irrelevant stimulus on our attractiveness evaluation, using face and hair as stimuli. The results revealed asymmetrical influences in the evaluation of one while ignoring the other. When hair was task-irrelevant, it still affected attractiveness of the face, but only if the hair itself had never been evaluated by the same evaluator. On the other hand, the face affected the hair regardless of whether the face itself was evaluated before. This has intriguing implications on the asymmetry between face and hair, and perceptual integration between them in general. Together with data from a post hoc questionnaire, it is suggested that both implicit non-selective and explicit selective processes contribute to attractiveness evaluation. The findings provide an understanding of attractiveness perception in real-life situations, as well as a new paradigm to reveal unknown implicit aspects of information integration for emotional judgment.