Finely tuned eye movements enhance visual acuity

Sub-cone visual resolution by active, adaptive sampling in the human foveola

The foveated architecture of the human retina and the eye's mobility enables prime spatial vision, yet the interplay between photoreceptor cell topography and the constant motion of the eye during fixation remains unexplored. With in vivo foveal cone-resolved imaging and simultaneous microscopic photo stimulation, we examined visual acuity in both eyes of 16 participants while precisely recording the stimulus path on the retina. We find that resolution thresholds were correlated with the individual retina's sampling capacity, and exceeded what static sampling limits would predict by 18%, on average. The length and direction of fixational drift motion, previously thought to be primarily random, played a key role in achieving this sub-cone diameter resolution. The oculomotor system finely adjusts drift behavior towards retinal areas with higher cone densities within only a few hundred milliseconds to enhance retinal sampling.

Seeing on the fly: Physiological and behavioral evidence show that space-to-space representation and processing enable fast and efficient performance by the visual system

When we view the world, our eyes saccade quickly between points of interest. Even when fixating a target our eyes are not completely at rest but execute small fixational eye movements (FEMs). That vision is not blurred despite this ever-present jitter has seemingly motivated an increasingly popular theory denying the reliance of the visual system on pure spatial processing in favor of a space-to-time mechanism generated by the eye drifting across the image. Accordingly, FEMs are not detrimental but rather essential to good visibility. However, the space-to-time theory is incompatible with physiological data showing that all information is conveyed by the short neural volleys generated when the eyes land on a target, and with our faithful perception of briefly displayed objects, during which time FEMs have no effect. Another difficulty in rejecting the idea of image representation by the locations and nature of responding cells in favor of a timecode, is that somewhere, somehow, this code must be decoded into a parallel spatial one when reaching perception. Thus, in addition to the implausibility of generating meaningful responses during retinal drift, the space-to-time hypothesis calls for replacing efficient point-to-point parallel transmission with a cumbersome, delayed, space-to-time-to-space process. A novel physiological framework is presented here wherein the ability of the visual system to quickly process information is mediated by the short, powerful neural volleys generated by the landing saccades. These volleys are necessary and sufficient for normal perception without FEMs contribution. This mechanism enables our excellent perception of brief stimuli and explains that visibility is not blurred by FEMs because they do not generate useful information.

Investigating causal effects of pupil size on visual discrimination and visually evoked potentials in an optotype discrimination task

Pupil size primarily changes to regulate the amount of light entering the retina, optimizing the balance between visual acuity and sensitivity for effective visual processing. However, research directly examining the relationship between pupil size and visual processing has been limited. While a few studies have recorded pupil size and EEG signals to investigate the role of pupil size in visual processing, these studies have predominantly focused on the domain of visual sensitivity. Causal effects of pupil size on visual acuity, therefore, remain poorly understood. By manipulating peripheral background luminance levels and target stimulus contrast while simultaneously recording pupillometry and EEG signals, we examined how absolute pupil size affects visual discrimination and visually evoked potentials (VEP) in a task using optotype mimicking the Snellen eye chart, the most common assessment of visual acuity. Our findings indicate that both higher background luminance levels and higher target contrast were associated with improved target discrimination and faster correct reaction times. Moreover, while higher contrast visual stimuli evoked larger VEPs, the effects of pupil size on VEPs were not significant. Additionally, we did not observe inter-individual correlations between absolute pupil size and discrimination performance or VEP amplitude. Together, our results demonstrate that absolute pupil size, regulated by global luminance level, played a functional role in enhancing visual discrimination performance in an optotype discrimination task. The differential VEP effects of pupil size compared to those of stimulus contrast further suggested distinct neural mechanisms involved in facilitating visual acuity under small pupils.

Benefits of postural sway to succeed in goal-directed visual tasks

When individuals stand, they sway and so have to maintain their balance. It is generally expected that task performance is worse when standing and swaying than when sitting and therefore not swaying. In contrast, we hypothesized that greater sway is associated with better task performance in the absence of external perturbations of posture. Twenty-four healthy, young adults performed two goal-directed, modified Stroop tasks (incongruent and reversed incongruent) in four body position conditions (standing against a vertical surface, and standing freely with a wide, standard or narrow stance). Centre of pressure (COP) sway, head sway, eye movements, visual attention, and task performance were recorded. Partial correlation analyses showed significant positive associations between task performance and some COP and head sway variables, after controlling for the level of visual attention. Analyses of variance with three factors (body position, task difficulty, target distance) also showed significant interaction effects between body position (and therefore postural sway) and the number of accurate target findings. The presence of these interactions showed that narrow stance was both the best body position for performing the incongruent task and the worst body position for performing the reversed incongruent task. Overall, COP sway and head sway can increase task performance. Hence, healthy, young adults in quiet stance appear to use sway to explore their environment more effectively. However, it should be borne in mind that our hypothesis was formulated solely with regard to healthy, young adults standing in quiet stance.

Correcting visual acuity beyond 20/20 improves contour element detection and integration: A cautionary tale for studies of special populations

Contrary to popular lore, optimal visual acuity is typically better than 20/20. Could correcting acuity beyond 20/20 offer any benefit? An affirmative answer could present new confounds in studies of aging, development, psychiatric illness, neurodegenerative disorders, or any other population where refractive error might be more likely. An affirmative answer would also offer a novel explanation of inter-observer variability in visual performance. To address the question, we had individuals perform two well-studied visual tasks, once with 20/20 vision and once with optical correction, so that observers could see one line better on an eye chart. In the contour integration task, observers sought to identify the screen quadrant location of a sparsely defined (integrated) shape embedded in varying quantities of randomly oriented "noise" elements. In the collinear facilitation task, observers sought to detect a low-contrast element flanked by collinear or orthogonal high-contrast elements. In each case, displays were scaled in size to modulate element visibility and spatial frequency (4-12 cycles/deg). We found that improving acuity beyond 20/20 improved contour integration for the high spatial frequency displays. Although improving visual acuity did not affect collinear facilitation, it did improve detection of the central low-contrast target, especially at high spatial frequencies. These results, which were large in magnitude, suggest that optically correcting beyond 20/20 improves the detection and integration of contour elements, especially those that are smaller and of higher spatial frequency. Refractive blur within the normal range may confound special population studies, explain inter-observer differences, and meaningfully impact performance in low-visibility environments.

Fixational eye movements enhance the precision of visual information transmitted by the primate retina

Fixational eye movements alter the number and timing of spikes transmitted from the retina to the brain, but whether these changes enhance or degrade the retinal signal is unclear. To quantify this, we developed a Bayesian method for reconstructing natural images from the recorded spikes of hundreds of retinal ganglion cells (RGCs) in the macaque retina (male), combining a likelihood model for RGC light responses with the natural image prior implicitly embedded in an artificial neural network optimized for denoising. The method matched or surpassed the performance of previous reconstruction algorithms, and provides an interpretable framework for characterizing the retinal signal. Reconstructions were improved with artificial stimulus jitter that emulated fixational eye movements, even when the eye movement trajectory was assumed to be unknown and had to be inferred from retinal spikes. Reconstructions were degraded by small artificial perturbations of spike times, revealing more precise temporal encoding than suggested by previous studies. Finally, reconstructions were substantially degraded when derived from a model that ignored cell-to-cell interactions, indicating the importance of stimulus-evoked correlations. Thus, fixational eye movements enhance the precision of the retinal representation.

Flicker adaptation improves acuity for briefly presented stimuli by reducing crowding

Adaptation to flickering/dynamic noise improves visual acuity for briefly presented stimuli (Arnold et al., 2016). Here, we investigate whether such adaptation operates directly on our ability to see detail or by changing fixational eye movements and pupil size or by reducing visual crowding. Following earlier work, visual acuity was measured in observers who were either unadapted or who had adapted to a 60-Hz flickering noise pattern. Participants reported the orientation of a white tumbling-T target (four-alternative forced choice [4AFC], ⊤⊣⊥⊢). The target was presented for 110 ms either in isolation or flanked by randomly oriented T's (e.g., ⊣⊤⊢) followed by an isolated (+) or flanked (+++) mask, respectively. We measured fixation stability (using an infrared eye tracker) while observers performed the task (with and without adaptation). Visual acuity improved modestly (around 8.4%) for flanked optotypes following adaptation to flicker (mean, -0.038 ± 0.063 logMAR; p = 0.015; BF10 = 3.66) but did not when measured with isolated letters (mean, -0.008 ± 0.055 logMAR; p = 0.5; BF10 = 0.29). The magnitude of acuity improvement was associated with individuals' (unadapted) susceptibility to crowding (the ratio of crowded to uncrowded acuity; r = -0.58, p = 0.008, BF10 = 7.70) but to neither fixation stability nor pupil size. Confirming previous reports, flicker improved acuity for briefly presented stimuli, but we show that this was only the case for crowded letters. These improvements likely arise from attenuation of sensitivity to a transient low spatial frequency (SF) image structure (Arnold et al., 2016; Tagoh et al., 2022), which may, for example, reduce masking of high SFs by low SFs. We also suggest that this attenuation could reduce backward masking and so reduce foveal crowding.

Consequences of eye movements for spatial selectivity

What determines spatial tuning in the visual system? Standard views rely on the assumption that spatial information is directly inherited from the relative position of photoreceptors and shaped by neuronal connectivity.1,2 However, human eyes are always in motion during fixation,3,4,5,6 so retinal neurons receive temporal modulations that depend on the interaction of the spatial structure of the stimulus with eye movements. It has long been hypothesized that these modulations might contribute to spatial encoding,7,8,9,10,11,12 a proposal supported by several recent observations.13,14,15,16 A fundamental, yet untested, consequence of this encoding strategy is that spatial tuning is not hard-wired in the visual system but critically depends on how the fixational motion of the eye shapes the temporal structure of the signals impinging onto the retina. Here we used high-resolution techniques for eye-tracking17 and gaze-contingent display control18 to quantitatively test this distinctive prediction. We examined how contrast sensitivity, a hallmark of spatial vision, is influenced by fixational motion, both during normal active fixation and when the spatiotemporal stimulus on the retina is altered to mimic changes in fixational control. We showed that visual sensitivity closely follows the strength of the luminance modulations delivered within a narrow temporal bandwidth, so changes in fixational motion have opposite visual effects at low and high spatial frequencies. By identifying a key role for oculomotor activity in spatial selectivity, these findings have important implications for the perceptual consequences of abnormal eye movements, the sources of perceptual variability, and the function of oculomotor control.

Foveolar Drusen Decrease Fixation Stability in Pre-Symptomatic AMD

Purpose

This study aims at linking subtle changes of fixational eye movements (FEM) in controls and in patients with foveal drusen using adaptive optics retinal imaging in order to find anatomo-functional markers for pre-symptomatic age-related macular degeneration (AMD).

Methods

We recruited 7 young controls, 4 older controls, and 16 patients with presymptomatic AMD with foveal drusen from the Silversight Cohort. A high-speed research-grade adaptive optics flood illumination ophthalmoscope (AO-FIO) was used for monocular retinal tracking of fixational eye movements. The system allows for sub-arcminute resolution, and high-speed and distortion-free imaging of the foveal area. Foveal drusen position and size were documented using gaze-dependent imaging on a clinical-grade AO-FIO.

Results

FEM were measured with high precision (RMS-S2S = 0.0015 degrees on human eyes) and small foveal drusen (median diameter = 60 µm) were detected with high contrast imaging. Microsaccade amplitude, drift diffusion coefficient, and ISOline area (ISOA) were significantly larger for patients with foveal drusen compared with controls. Among the drusen participants, microsaccade amplitude was correlated to drusen eccentricity from the center of the fovea.

Conclusions

A novel high-speed high-precision retinal tracking technique allowed for the characterization of FEM at the microscopic level. Foveal drusen altered fixation stability, resulting in compensatory FEM changes. Particularly, drusen at the foveolar level seemed to have a stronger impact on microsaccade amplitudes and ISOA. The unexpected anatomo-functional link between small foveal drusen and fixation stability opens up a new perspective of detecting oculomotor signatures of eye diseases at the presymptomatic stage.

High refresh rate display for natural monocular viewing in AOSLO psychophysics experiments

By combining an external display operating at 360 frames per second with an Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) for human foveal imaging, we demonstrate color stimulus delivery at high spatial and temporal resolution in AOSLO psychophysics experiments. A custom pupil relay enables viewing of the stimulus through a 3-mm effective pupil diameter and provides refractive error correction from -8 to +4 diopters. Performance of the assembled and aligned pupil relay was validated by measuring the wavefront error across the field of view and correction range, and the as-built Strehl ratio was 0.64 or better. High-acuity stimuli were rendered on the external display and imaged through the pupil relay to demonstrate that spatial frequencies up to 54 cycles per degree, corresponding to 20/11 visual acuity, are resolved. The completed external display was then used to render fixation markers across the field of view of the monitor, and a continuous retinal montage spanning 9.4 by 5.4 degrees of visual angle was acquired with the AOSLO. We conducted eye-tracking experiments during free-viewing and high-acuity tasks with polychromatic images presented on the external display. Sub-arcminute eye position uncertainty was achieved, enabling precise localization of the line of sight on the monitor while simultaneously imaging the fine structure of the human central fovea. This high refresh rate display overcomes the temporal, spectral, and field of view limitations of AOSLO-based stimulus presentation, enabling natural monocular viewing of stimuli in psychophysics experiments conducted with AOSLO.

Theoretical impact of chromatic aberration correction on visual acuity

It has been known for more than 220 years that the image quality of the human eye is significantly degraded by chromatic aberrations. Recently, it was shown experimentally that correcting chromatic aberrations results in a 0.2- to 0.8-line improvement in visual acuity. Here we ask, is this expected? We developed tools that enable simulations of the optical impact of physiologically relevant amounts of chromatic aberration in real human eyes and combined these with tools that compute the visual acuity of an ideal observer. This allows us to characterize the theoretical impact of chromatic aberration correction on visual acuity. Results indicate a substantive improvement of 0.4- to 2-lines in ideal observer visual acuity with chromatic aberration correction. Ideal observer thresholds benefit significantly more from correction of longitudinal than correction of transverse chromatic aberration. Finally, improvements in ideal observer visual acuity are greater for subjects with less monochromatic aberration, such that subjects with better baseline optical quality benefit most from correction of chromatic aberrations.

Oculomotor routines for perceptual judgments

In everyday life we frequently make simple visual judgments about object properties, for example, how big or wide is a certain object? Our goal is to test whether there are also task-specific oculomotor routines that support perceptual judgments, similar to the well-established exploratory routines for haptic perception. In a first study, observers saw different scenes with two objects presented in a photorealistic virtual reality environment. Observers were asked to judge which of two objects was taller or wider while gaze was tracked. All tasks were performed with the same set of virtual objects in the same scenes, so that we can compare spatial characteristics of exploratory gaze behavior to quantify oculomotor routines for each task. Width judgments showed fixations around the center of the objects with larger horizontal spread. In contrast, for height judgments, gaze was shifted toward the top of the objects with larger vertical spread. These results suggest specific strategies in gaze behavior that presumably are used for perceptual judgments. To test the causal link between oculomotor behavior and perception, in a second study, observers could freely gaze at the object or we introduced a gaze-contingent setup forcing observers to fixate specific positions on the object. Discrimination performance was similar between free-gaze and the gaze-contingent conditions for width and height judgments. These results suggest that although gaze is adapted for different tasks, performance seems to be based on a perceptual strategy, independent of potential cues that can be provided by the oculomotor system.

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.

Visual Feature Tuning Properties of Short-Latency Stimulus-Driven Ocular Position Drift Responses during Gaze Fixation

Ocular position drifts during gaze fixation are significantly less well understood than microsaccades. We recently identified a short-latency ocular position drift response, of ∼1 min arc amplitude, that is triggered within <100 ms by visual onsets. This systematic eye movement response is feature-tuned and seems to be coordinated with a simultaneous resetting of the saccadic system by visual stimuli. However, much remains to be learned about the drift response, especially for designing better-informed neurophysiological experiments unraveling its mechanistic substrates. Here we systematically tested multiple new feature tuning properties of drift responses. Using highly precise eye tracking in three male rhesus macaque monkeys, we found that drift responses still occur for tiny foveal visual stimuli. Moreover, the responses exhibit size tuning, scaling their amplitude (both up and down) as a function of stimulus size, and they also possess a monotonically increasing contrast sensitivity curve. Importantly, short-latency drift responses still occur for small peripheral visual targets, which additionally introduce spatially directed modulations in drift trajectories toward the appearing peripheral stimuli. Drift responses also remain predominantly upward even for stimuli exclusively located in the lower visual field and even when starting gaze position is upward. When we checked the timing of drift responses, we found it was better synchronized to stimulus-induced saccadic inhibition than to stimulus onset. These results, along with a suppression of drift response amplitudes by peristimulus saccades, suggest that drift responses reflect the rapid impacts of short-latency and feature-tuned visual neural activity on final oculomotor control circuitry in the brain.

Mammals Achieve Common Neural Coverage of Visual Scenes Using Distinct Sampling Behaviors

Most vertebrates use head and eye movements to quickly change gaze orientation and sample different portions of the environment with periods of stable fixation. Visual information must be integrated across fixations to construct a complete perspective of the visual environment. In concert with this sampling strategy, neurons adapt to unchanging input to conserve energy and ensure that only novel information from each fixation is processed. We demonstrate how adaptation recovery times and saccade properties interact and thus shape spatiotemporal tradeoffs observed in the motor and visual systems of mice, cats, marmosets, macaques, and humans. These tradeoffs predict that in order to achieve similar visual coverage over time, animals with smaller receptive field sizes require faster saccade rates. Indeed, we find comparable sampling of the visual environment by neuronal populations across mammals when integrating measurements of saccadic behavior with receptive field sizes and V1 neuronal density. We propose that these mammals share a common statistically driven strategy of maintaining coverage of their visual environment over time calibrated to their respective visual system characteristics.

Ultra-fine resolution of pre-saccadic attention in the fovea

Microsaccades, the tiny gaze relocations that occurr during fixation, have been linked to covert attention deployed degrees away from the center of gaze. However, the link between attention and microsaccades is deeper in that it also unfolds at the foveal scale. Here, we have examined the spatial grain of pre-microsaccadic attention across the 1° foveola. Through the use of high-precision eye-tracking and gaze-contingent display system that achieves arcminute precision in gaze localization, we have shown that the spotlight of attention at this scale can reach a strikingly high resolution, in the order of 0.17°. Further, when a microsaccade occurs, vision is modulated in a peculiar way across the foveola; whereas fine spatial vision is enhanced at the microsaccade goal location, it drops at the very center of gaze, where acuity is normally highest. These results reveal the finesse of the visuomotor system and of the interplay between eye movements and attention.

Alterations to foveal crowding with microsaccade preparation

Visual stimuli presented around the time of a saccade have been shown to be perceived differently by the visual system, including a reduction in the harmful impact of flankers (crowding). However, whether the effects observed are due strictly to crowding remains controversial, and the effects have only been measured with large saccades in peripheral vision. Here we investigate how crowded stimuli placed 20 arc minutes from the center of gaze are affected by an upcoming microsaccade. The stimulus consisted of a rotated T of size 6.25 arcminutes that was either unflanked, surrounded by four flankers (Experiment 1), or surrounded by two flankers that were positioned either radially or tangentially (Experiments 2 and 3). In 80 % of trials, subjects made voluntary microsaccades to the target when cued, and in the remaining 20 % of the trials subjects continued to maintain fixation. In Experiments 1 and 2, subjects were required to saccade to the same location as the target, while in Experiment 3 subjects saccaded to a different location ∼ 20 arc min to the upper left of the target. Thus, we provide evidence for two separable pre-saccadic benefits for crowded parafoveal targets: one isotropizes the crowding zone for stimuli presented 200 to 125 ms before microsaccadic onset, and another provides a benefit exclusively for microsaccade targets surrounded by tangential flankers in the presence of imminent microsaccades. Two possible mechanisms are attentional enhancement and predictive remapping of receptive fields, respectively.

Eye movements in Parkinson's disease: from neurophysiological mechanisms to diagnostic tools

Movement disorders such as Parkinson's disease (PD) impact oculomotor function - the ability to move the eyes accurately and purposefully to serve a multitude of sensory, cognitive, and secondary motor tasks. Decades of neurophysiological research in monkeys and behavioral studies in humans have characterized the neural basis of healthy oculomotor control. This review links eye movement abnormalities in persons living with PD to the underlying neurophysiological mechanisms and pathways. Building on this foundation, we highlight recent progress in using eye movements to gauge symptom severity, assess treatment effects, and serve as potential precision biomarkers. We conclude that whereas eye movements provide insights into PD mechanisms, based on current evidence they appear to lack sufficient sensitivity and specificity to serve as a standalone diagnostic tool. Their full potential may be realized when combined with other disease indicators in big datasets.

An eye for detail: Eye movements and attention at the foveal scale

Human vision relies on a tiny region of the retina, the 1-deg foveola, to achieve high spatial resolution. Foveal vision is of paramount importance in daily activities, yet its study is challenging, as eye movements incessantly displace stimuli across this region. Here I will review work that, building on recent advances in eye-tracking and gaze-contingent display, examines how attention and eye movements operate at the foveal level. This research highlights how exploration of fine spatial detail unfolds following visuomotor strategies reminiscent of those occurring at larger scales. It shows that, together with highly precise control of attention, this motor activity is linked to non-homogenous processing within the foveola and selectively modulates sensitivity both in space and time. Overall, the picture emerges of a highly dynamic foveal perception in which fine spatial vision, rather than simply being the result of placing a stimulus at the center of gaze, is the result of a finely tuned and orchestrated synergy of motor, cognitive, and attentional processes.

Inferential eye movement control while following dynamic gaze

Attending to other people's gaze is evolutionary important to make inferences about intentions and actions. Gaze influences covert attention and triggers eye movements. However, we know little about how the brain controls the fine-grain dynamics of eye movements during gaze following. Observers followed people's gaze shifts in videos during search and we related the observer eye movement dynamics to the time course of gazer head movements extracted by a deep neural network. We show that the observers' brains use information in the visual periphery to execute predictive saccades that anticipate the information in the gazer's head direction by 190-350ms. The brain simultaneously monitors moment-to-moment changes in the gazer's head velocity to dynamically alter eye movements and re-fixate the gazer (reverse saccades) when the head accelerates before the initiation of the first forward gaze-following saccade. Using saccade-contingent manipulations of the videos, we experimentally show that the reverse saccades are planned concurrently with the first forward gaze-following saccade and have a functional role in reducing subsequent errors fixating on the gaze goal. Together, our findings characterize the inferential and functional nature of social attention's fine-grain eye movement dynamics.

Human microsaccade cueing modulation in visual- and memory-delay saccade tasks after theta burst transcranial magnetic stimulation over the frontal eye field

Microsaccades that occur during periods of fixation are modulated by various cognitive processes and have an impact on visual processing. A network of brain areas is involved in microsaccade generation, including the superior colliculus and frontal eye field (FEF) which are involved in modulating microsaccade rate and direction after the appearance of a visual cue (referred to as microsaccade cueing modulation). Although the neural mechanisms underlying microsaccade cueing modulations have been demonstrated in monkeys, limited research has investigated a causal role of these areas in humans. By applying continuous theta-burst transcranial magnetic stimulation (cTBS) over the right FEF and vertex, we investigated the role of human FEF in modulating microsaccade responses after the appearance of a visual target in a visual- and memory-delay saccade task. After target appearance, microsaccade rate was initially suppressed but then increased in both cTBS conditions. More importantly, in the visual-delay task, microsaccades after target appearance were directed to the ipsilateral side more often with FEF, compared to vertex stimulation. Moreover, microsaccades were directed towards the target location, then to the opposite location of the target in both tasks, with larger effects in the visual-, compared to, memory-delay task. This microsaccade direction modulation was delayed after FEF stimulation in the memory-delay task. Overall, some microsaccade cueing modulations were moderately disrupted after FEF cTBS, suggesting a causal role for involvement of the human FEF in microsaccade generation after presentation of salient stimuli.

Mammals achieve common neural coverage of visual scenes using distinct sampling behaviors

Most vertebrates use head and eye movements to quickly change gaze orientation and sample different portions of the environment with periods of stable fixation. Visual information must be integrated across several fixations to construct a more complete perspective of the visual environment. In concert with this sampling strategy, neurons adapt to unchanging input to conserve energy and ensure that only novel information from each fixation is processed. We demonstrate how adaptation recovery times and saccade properties interact, and thus shape spatiotemporal tradeoffs observed in the motor and visual systems of different species. These tradeoffs predict that in order to achieve similar visual coverage over time, animals with smaller receptive field sizes require faster saccade rates. Indeed, we find comparable sampling of the visual environment by neuronal populations across mammals when integrating measurements of saccadic behavior with receptive field sizes and V1 neuronal density. We propose that these mammals share a common statistically driven strategy of maintaining coverage of their visual environment over time calibrated to their respective visual system characteristics.

Detailed characterization of neural selectivity in free viewing primates

Fixation constraints in visual tasks are ubiquitous in visual and cognitive neuroscience. Despite its widespread use, fixation requires trained subjects, is limited by the accuracy of fixational eye movements, and ignores the role of eye movements in shaping visual input. To overcome these limitations, we developed a suite of hardware and software tools to study vision during natural behavior in untrained subjects. We measured visual receptive fields and tuning properties from multiple cortical areas of marmoset monkeys who freely viewed full-field noise stimuli. The resulting receptive fields and tuning curves from primary visual cortex (V1) and area MT match reported selectivity from the literature which was measured using conventional approaches. We then combined free viewing with high-resolution eye tracking to make the first detailed 2D spatiotemporal measurements of foveal receptive fields in V1. These findings demonstrate the power of free viewing to characterize neural responses in untrained animals while simultaneously studying the dynamics of natural behavior.

High-resolution structural and functional retinal imaging in the awake behaving mouse

The laboratory mouse has provided tremendous insight to the underpinnings of mammalian central nervous system physiology. In recent years, it has become possible to image single neurons, glia and vascular cells in vivo by using head-fixed preparations combined with cranial windows to study local networks of activity in the living brain. Such approaches have also succeeded without the use of general anesthesia providing insights to the natural behaviors of the central nervous system. However, the same has not yet been developed for the eye, which is constantly in motion. Here we characterize a novel head-fixed preparation that enables high-resolution adaptive optics retinal imaging at the single-cell level in awake-behaving mice. We reveal three new functional attributes of the normal eye that are overlooked by anesthesia: 1) High-frequency, low-amplitude eye motion of the mouse that is only present in the awake state 2) Single-cell blood flow in the mouse retina is reduced under anesthesia and 3) Mouse retinae thicken in response to ketamine/xylazine anesthesia. Here we show key benefits of the awake-behaving preparation that enables study of retinal physiology without anesthesia to study the normal retinal physiology in the mouse.

Active fixation as an efficient coding strategy for neuromorphic vision

Contrary to a photographer, who puts a great effort in keeping the lens still, eyes insistently move even during fixation. This benefits signal decorrelation, which underlies an efficient encoding of visual information. Yet, camera motion is not sufficient alone; it must be coupled with a sensor specifically selective to temporal changes. Indeed, motion induced on standard imagers only results in burring effects. Neuromorphic sensors represent a valuable solution. Here we characterize the response of an event-based camera equipped with fixational eye movements (FEMs) on both synthetic and natural images. Our analyses prove that the system starts an early stage of redundancy suppression, as a precursor of subsequent whitening processes on the amplitude spectrum. This does not come at the price of corrupting structural information contained in local spatial phase across oriented axes. Isotropy of FEMs ensures proper representations of image features without introducing biases towards specific contrast orientations.

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.

Active vision: How you look reflects what you are looking for

While we fixate an object, our eyes are never stationary but constantly drifting, with miniature movements traditionally thought to be random and involuntary. A new study shows that the orientation of such drift in humans is actually not random but is influenced by the task demands to improve performance.

Eye movements during optic flow perception

Optic flow is an important visual cue for human perception and locomotion and naturally triggers eye movements. Here we investigate whether the perception of optic flow direction is limited or enhanced by eye movements. In Exp. 1, 23 human observers localized the focus of expansion (FOE) of an optic flow pattern; in Exp. 2, 18 observers had to detect brief visual changes at the FOE. Both tasks were completed during free viewing and fixation conditions while eye movements were recorded. Task difficulty was varied by manipulating the coherence of radial motion from the FOE (4 %-90 %). During free viewing, observers tracked the optic flow pattern with a combination of saccades and smooth eye movements. During fixation, observers nevertheless made small-scale eye movements. Despite differences in spatial scale, eye movements during free viewing and fixation were similarly directed toward the FOE (saccades) and away from the FOE (smooth tracking). Whereas FOE localization sensitivity was not affected by eye movement instructions (Exp. 1), observers' sensitivity to detect brief changes at the FOE was 27 % higher (p <.001) during free-viewing compared to fixation (Exp. 2). This performance benefit was linked to reduced saccade endpoint errors, indicating the direct beneficial impact of foveating eye movements on performance in a fine-grain perceptual task, but not during coarse perceptual localization.

AttentionMNIST: a mouse-click attention tracking dataset for handwritten numeral and alphabet recognition

Multiple attention-based models that recognize objects via a sequence of glimpses have reported results on handwritten numeral recognition. However, no attention-tracking data for handwritten numeral or alphabet recognition is available. Availability of such data would allow attention-based models to be evaluated in comparison to human performance. We collect mouse-click attention tracking data from 382 participants trying to recognize handwritten numerals and alphabets (upper and lowercase) from images via sequential sampling. Images from benchmark datasets are presented as stimuli. The collected dataset, called AttentionMNIST, consists of a sequence of sample (mouse click) locations, predicted class label(s) at each sampling, and the duration of each sampling. On average, our participants observe only 12.8% of an image for recognition. We propose a baseline model to predict the location and the class(es) a participant will select at the next sampling. When exposed to the same stimuli and experimental conditions as our participants, a highly-cited attention-based reinforcement model falls short of human efficiency.

Inferring visual space from ultra-fine extra-retinal knowledge of gaze position

It has long been debated how humans resolve fine details and perceive a stable visual world despite the incessant fixational motion of their eyes. Current theories assume these processes to rely solely on the visual input to the retina, without contributions from motor and/or proprioceptive sources. Here we show that contrary to this widespread assumption, the visual system has access to high-resolution extra-retinal knowledge of fixational eye motion and uses it to deduce spatial relations. Building on recent advances in gaze-contingent display control, we created a spatial discrimination task in which the stimulus configuration was entirely determined by oculomotor activity. Our results show that humans correctly infer geometrical relations in the absence of spatial information on the retina and accurately combine high-resolution extraretinal monitoring of gaze displacement with retinal signals. These findings reveal a sensory-motor strategy for encoding space, in which fine oculomotor knowledge is used to interpret the fixational input to the retina.

A Narrative Literature Review About the Role of Microsaccades in Sports

In many daily and sport situations, people have to simultaneously perceive and process multiple objects and scenes in a short amount of time. A wrong decision may lead to a disadvantage for a team or for a single athlete, and during daily life (i.e., driving, surgery), it could have more dangerous consequences. Considering the results of different studies, the ability to distribute visual attention depends on different levels of expertise and environment-related constraints. This article is a narrative review of the current scientific evidence in the field of eye movements in sports, focusing on the role of microsaccades in sporting task situations. Over the past 10 years, microsaccades have become one of the most increasing areas of research in visual and oculomotor studies and even in the area of sport science. Here, we review the latest findings and discuss the relationships between microsaccades and attention, perception, and action in sports.

Eye drift during fixation predicts visual acuity

Visual acuity is commonly assumed to be determined by the eye optics and spatial sampling in the retina. Unlike a camera, however, the eyes are never stationary during the acquisition of visual information; a jittery motion known as ocular drift incessantly displaces stimuli over many photoreceptors. Previous studies have shown that acuity is impaired in the absence of retinal image motion caused by eye drift. However, the relation between individual drift characteristics and acuity remains unknown. Here, we show that a) healthy emmetropes exhibit a large variability in their amount of drift and that b) these differences profoundly affect the structure of spatiotemporal signals to the retina. We further show that c) the spectral distribution of the resulting luminance modulations strongly correlates with individual visual acuity and that d) natural intertrial fluctuations in the amount of drift modulate acuity. As a consequence, in healthy emmetropes, acuity can be predicted from the motor behavior elicited by a simple fixation task, without directly measuring it. These results shed new light on how oculomotor behavior contributes to fine spatial vision.

Fixational eye movements enable robust edge detection

Human vision relies on mechanisms that respond to luminance edges in space and time. Most edge models use orientation-selective mechanisms on multiple spatial scales and operate on static inputs assuming that edge processing occurs within a single fixational instance. Recent studies, however, demonstrate functionally relevant temporal modulations of the sensory input due to fixational eye movements. Here we propose a spatiotemporal model of human edge detection that combines elements of spatial and active vision. The model augments a spatial vision model by temporal filtering and shifts the input images over time, mimicking an active sampling scheme via fixational eye movements. The first model test was White's illusion, a lightness effect that has been shown to depend on edges. The model reproduced the spatial-frequency-specific interference with the edges by superimposing narrowband noise (1–5 cpd), similar to the psychophysical interference observed in White's effect. Second, we compare the model's edge detection performance in natural images in the presence and absence of Gaussian white noise with human-labeled contours for the same (noise-free) images. Notably, the model detects edges robustly against noise in both test cases without relying on orientation-selective processes. Eliminating model components, we demonstrate the relevance of multiscale spatiotemporal filtering and scale-specific normalization for edge detection. The proposed model facilitates efficient edge detection in (artificial) vision systems and challenges the notion that orientation-selective mechanisms are required for edge detection.

Fixational drift is driven by diffusive dynamics in central neural circuitry

During fixation and between saccades, our eyes undergo diffusive random motion called fixational drift. The role of fixational drift in visual coding and inference has been debated in the past few decades, but the mechanisms that underlie this motion remained unknown. In particular, it has been unclear whether fixational drift arises from peripheral sources, or from central sources within the brain. Here we show that fixational drift is correlated with neural activity, and identify its origin in central neural circuitry within the oculomotor system, upstream to the ocular motoneurons (OMNs). We analyzed a large data set of OMN recordings in the rhesus monkey, alongside precise measurements of eye position, and found that most of the variance of fixational eye drifts must arise upstream of the OMNs. The diffusive statistics of the motion points to the oculomotor integrator, a memory circuit responsible for holding the eyes still between saccades, as a likely source of the motion. Theoretical modeling, constrained by the parameters of the primate oculomotor system, supports this hypothesis by accounting for the amplitude as well as the statistics of the motion. Thus, we propose that fixational ocular drift provides a direct observation of diffusive dynamics in a neural circuit responsible for storage of continuous parameter memory in persistent neural activity. The identification of a mechanistic origin for fixational drift is likely to advance the understanding of its role in visual processing and inference.

All eyes on attention

Eye movements are neither necessary nor sufficient to account for the neural effects associated with covert attention.

Differentiating global luminance, arousal and cognitive signals on pupil size and microsaccades

Pupil size reflects a proxy for neural activity associated with global luminance, arousal and cognitive processing. Microsaccades are also modulated by arousal and cognitive processing. Are these effects of arousal and cognitive signals on pupil size and microsaccades coordinated? If so, via what neural mechanisms? We hypothesized that if pupil size and microsaccades are coordinately modulated by these processes, pupil size immediately before microsaccade onset, as an index for ongoing cognitive and arousal processing, should correlate with microsaccade responses during tasks alternating these signals. Here, we examined the relationship between pupil size and microsaccade responses in tasks that included variations in global luminance, arousal and inhibitory control. Higher microsaccade peak velocities correlated with larger pre-microsaccade pupil response related to arousal and inhibitory control signals. In contrast, pupil responses evoked by global luminance signals did not correlate with microsaccade responses. Given the central role of the superior colliculus in microsaccade generation, these results suggest the critical involvement of the superior colliculus to coordinate pupil and microsaccade responses for arousal and inhibitory control modulations, but not for the pupil luminance modulation.

Fixational eye movements in passive versus active sustained fixation tasks

Human fixational eye movements are so small and precise that high-speed, accurate tools are needed to fully reveal their properties and functional roles. Where the fixated image lands on the retina and how it moves for different levels of visually demanding tasks is the subject of the current study. An Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) was used to image, track and present a variety of fixation targets (Maltese cross, disk, concentric circles, Vernier and tumbling-E letter) to healthy subjects. During these different passive (static) or active (discriminating) tasks under natural eye motion, the landing position of the target on the retina was tracked in space and time over the retinal image directly with high spatial (<1 arcmin) and temporal (960 Hz) resolution. We computed both the eye motion and the exact trajectory of the fixated target's motion over the retina. We confirmed that compared to passive tasks, active tasks elicited a partial inhibition of microsaccades, leading to longer drift periods compensated by larger corrective saccades. Consequently, the overall fixation stability during active tasks was on average 57% larger than during passive tasks. The preferred retinal locus of fixation was the same for each task and did not coincide with the location of the peak cone density.

Mobile Computational Photography: A Tour

The first mobile camera phone was sold only 20 years ago, when taking pictures with one's phone was an oddity, and sharing pictures online was unheard of. Today, the smartphone is more camera than phone. How did this happen? This transformation was enabled by advances in computational photography-the science and engineering of making great images from small-form-factor, mobile cameras. Modern algorithmic and computing advances, including machine learning, have changed the rules of photography, bringing to it new modes of capture, postprocessing, storage, and sharing. In this review, we give a brief history of mobile computational photography and describe some of the key technological components, including burst photography, noise reduction, and super-resolution. At each step, we can draw naive parallels to the human visual system.

Fast and nonuniform dynamics of perisaccadic vision in the central fovea

Humans shift their gaze more frequently than their heart beats. These rapid eye movements (saccades) enable high visual acuity by redirecting the tiny high-resolution region of the retina (the foveola). But in doing so, they abruptly sweep the image across receptors, raising questions on how the visual system achieves stable percepts. It is well established that visual sensitivity is transiently attenuated during saccades. However, little is known about the time course of foveal vision despite its disproportionate importance, as technical challenges have so far prevented study of how saccades affect the foveola. Here we show that saccades modulate this region in a nonuniform manner, providing stronger and faster changes at its very center, a locus with higher sensitivity.

Humans use rapid eye movements (saccades) to inspect stimuli with the foveola, the region of the retina where receptors are most densely packed. It is well established that visual sensitivity is generally attenuated during these movements, a phenomenon known as saccadic suppression. This effect is commonly studied with large, often peripheral, stimuli presented during instructed saccades. However, little is known about how saccades modulate the foveola and how the resulting dynamics unfold during natural visual exploration. Here we measured the foveal dynamics of saccadic suppression in a naturalistic high-acuity task, a task designed after primates’ social grooming, which—like most explorations of fine patterns—primarily elicits minute saccades (microsaccades). Leveraging on recent advances in gaze-contingent display control, we were able to systematically map the perisaccadic time course of sensitivity across the foveola. We show that contrast sensitivity is not uniform across this region and that both the extent and dynamics of saccadic suppression vary within the foveola. Suppression is stronger and faster in the most central portion, where sensitivity is generally higher and selectively rebounds at the onset of a new fixation. These results shed light on the modulations experienced by foveal vision during the saccade-fixation cycle and explain some of the benefits of microsaccades.

Pupil dynamics: A potential proxy of neural preparation for goal-directed eye movement

The pupils reflexively constrict or dilate to regulate the influx of light on the retinae. Pupillary light reflex (PLR) is susceptible to many non-visual cognitive processes including covert orientation of attention and planning rapid saccadic eye movement. The frontal eye field (FEF) and superior colliculus (SC), which also send projections to the PLR pathway, are two important areas in primate's brain for planning saccade and orientation of attention. The saccadic reaction time (SRT) and the rate of increase in activity of movement neurons in these areas are inversely correlated. This study addressed how pupil dynamics, activity in the FEF and SC and SRT are related in a saccadic decision-making task. The rate of visually evoked pupil constriction was found inversely related to SRT. This was further verified by simulating a homeomorphic biomechanical model of pupillary muscle plants, wherein we projected signals similar to build-up activity in the FEF and SC to the parasympathetic (constriction) and sympathetic (dilation) division of the PLR pathway, respectively. A striking similarity between simulated and observed dynamics of pupil constriction suggests that PLR is a potential proxy of saccade planning by movement neurons in the FEF and SC. Indistinguishable pupil dynamics when planned saccades were elicited versus when they were cancelled eliminated the possibility that the obligatory pre-saccadic shift of attention alone influenced the rate of pupil constriction. Our study envisages a mechanism of how the oculomotor system influences the autonomic activity in an attempt to timely minimize saccadic visual transients by regulating the influx of light.

A change in perspective: The interaction of saccadic and pursuit eye movements in oculomotor control and perception

Due to the close relationship between oculomotor behavior and visual processing, eye movements have been studied in many different areas of research over the last few decades. While these studies have brought interesting insights, specialization within each research area comes at the potential cost of a narrow and isolated view of the oculomotor system. In this review, we want to expand this perspective by looking at the interactions between the two most important types of voluntary eye movements: saccades and pursuit. Recent evidence indicates multiple interactions and shared signals at the behavioral and neurophysiological level for oculomotor control and for visual perception during pursuit and saccades. Oculomotor control seems to be based on shared position- and velocity-related information, which leads to multiple behavioral interactions and synergies. The distinction between position- and velocity-related information seems to be also present at the neurophysiological level. In addition, visual perception seems to be based on shared efferent signals about upcoming eye positions and velocities, which are to some degree independent of the actual oculomotor response. This review suggests an interactive perspective on the oculomotor system, based mainly on different types of sensory input, and less so on separate subsystems for saccadic or pursuit eye movements.

Micropursuit and the control of attention and eye movements in dynamic environments

It is more challenging to plan eye movements during perceptual tasks performed in dynamic displays than in static displays. Decisions about the timing of saccades become more critical, and decisions must also involve smooth eye movements, as well as saccades. The present study examined eye movements when judging which of two moving discs would arrive first, or collide, at a common meeting point. Perceptual discrimination after training was precise (Weber fractions < 6%). Strategies reflected a combined contribution of saccades and smooth eye movements. The preferred strategy was to look near the meeting point when strategies were freely chosen. When strategies were assigned, looking near the meeting point produced better performance than switching between the discs. Smooth eye movements were engaged in two ways: (a) low-velocity smooth eye movements correlated with the motion of each disc (micropursuit) were found while the line of sight remained between the discs; and (b) spontaneous smooth pursuit of the pair of discs occurred after the perceptual report, when the discs moved as a pair along a common path. The results show clear preferences and advantages for those eye movement strategies during dynamic perceptual tasks that require minimal management or effort. In addition, smooth eye movements, whose involvement during perceptual tasks within dynamic displays may have previously escaped notice, provide useful indictors of the strategies used to select information and distribute attention during the performance of dynamic perceptual tasks.

Evaluation of a Wearable AR Platform for Guiding Complex Craniotomies in Neurosurgery

Today, neuronavigation is widely used in daily clinical routine to perform safe and efficient surgery. Augmented reality (AR) interfaces can provide anatomical models and preoperative planning contextually blended with the real surgical scenario, overcoming the limitations of traditional neuronavigators. This study aims to demonstrate the reliability of a new-concept AR headset in navigating complex craniotomies. Moreover, we aim to prove the efficacy of a patient-specific template-based methodology for fast, non-invasive, and fully automatic planning-to-patient registration. The AR platform navigation performance was assessed with an in-vitro study whose goal was twofold: to measure the real-to-virtual 3D target visualization error (TVE), and assess the navigation accuracy through a user study involving 10 subjects in tracing a complex craniotomy. The feasibility of the template-based registration was preliminarily tested on a volunteer. The TVE mean and standard deviation were 1.3 and 0.6 mm. The results of the user study, over 30 traced craniotomies, showed that 97% of the trajectory length was traced within an error margin of 1.5 mm, and 92% within a margin of 1 mm. The in-vivo test confirmed the feasibility and reliability of the patient-specific template for registration. The proposed AR headset allows ergonomic and intuitive fruition of preoperative planning, and it can represent a valid option to support neurosurgical tasks.

Transient perceptual enhancements resulting from selective shifts of exogenous attention in the central fovea

Exogenous attention, a powerful adaptive tool that quickly and involuntarily orients processing resources to salient stimuli, has traditionally been studied in the lower-resolution parafoveal and peripheral visual field.1-4 It is not known whether and how it operates across the 1° central fovea where visual resolution peaks.5,6 Here we investigated the dynamics of exogenous attention in the foveola. To circumvent the challenges posed by fixational eye movements at this scale, we used high-precision eye-tracking and gaze-contingent display control for retinal stabilization.7 High-acuity stimuli were briefly presented foveally at varying delays following an exogenous cue. Attended and unattended locations were just a few arcminutes away from the preferred locus of fixation. Our results show that for short temporal delays, observers' ability to discriminate fine detail is enhanced at the cued location. This enhancement is highly localized and does not extend to the nearby locations only 16' away. On a longer timescale, instead, we report an inverse effect: paradoxically, acuity is sharper at the unattended locations, resembling the phenomenon of inhibition of return at much larger eccentricities.8-10 Although exogenous attention represents a mechanism for low-cost monitoring of the environment in the extrafoveal space, these findings show that, in the foveola, it transiently modulates vision of detail with a high degree of resolution. Together with inhibition of return, it may aid visual exploration of complex foveal stimuli.11.

Head-Mounted Augmented Reality Platform for Markerless Orthopaedic Navigation

Visual augmented reality (AR) has the potential to improve the accuracy, efficiency and reproducibility of computer-assisted orthopaedic surgery (CAOS). AR Head-mounted displays (HMDs) further allow non-eye-shift target observation and egocentric view. Recently, a markerless tracking and registration (MTR) algorithm was proposed to avoid the artificial markers that are conventionally pinned into the target anatomy for tracking, as their use prolongs surgical workflow, introduces human-induced errors, and necessitates additional surgical invasion in patients. However, such an MTR-based method has neither been explored for surgical applications nor integrated into current AR HMDs, making the ergonomic HMD-based markerless AR CAOS navigation hard to achieve. To these aims, we present a versatile, device-agnostic and accurate HMD-based AR platform. Our software platform, supporting both video see-through (VST) and optical see-through (OST) modes, integrates two proposed fast calibration procedures using a specially designed calibration tool. According to the camera-based evaluation, our AR platform achieves a display error of 6.31 2.55 arcmin for VST and 7.72 3.73 arcmin for OST. A proof-of-concept markerless surgical navigation system to assist in femoral bone drilling was then developed based on the platform and Microsoft HoloLens 1. According to the user study, both VST and OST markerless navigation systems are reliable, with the OST system providing the best usability. The measured navigation error is 4.90 1.04 mm, 5.96 2.22 for VST system and 4.36 0.80 mm, 5.65 1.42 for OST system.

Foveal vision

William Tuten and Wolf Harmening introduce the anatomical and functional signatures of foveated vision in humans.

Resolution acuity across the visual field for mesopic and scotopic illumination

We investigated the classical question of why visual acuity decreases with decreasing retinal illuminance by holding retinal eccentricity fixed while illumination varied. Our results indicate that acuity is largely independent of illuminance at any given retinal location, which suggests that under classical free-viewing conditions acuity improves as illumination increases from rod threshold to rod saturation because the retinal location of the stimulus is permitted to migrate from a peripheral location of maximum sensitivity but poor acuity to the foveal location of maximum acuity but poor sensitivity. Comparison with anatomical sampling density of retinal neurons suggests that mesopic acuity at all eccentricities and scotopic acuity for eccentricities beyond about 20° is limited by the spacing of midget ganglion cells. In central retina, however, scotopic acuity is further limited by spatial filtering due to spatial summation within the large, overlapping receptive fields of the A-II class of amacrine cells interposed in the rod pathway between rod bipolars and midget ganglion cells. Our results offer a mechanistic interpretation of the clinical metrics for low-luminance visual dysfunction used to monitor progression of retinal disease.

Gaze dynamics of feature-based distractor inhibition under prior-knowledge and expectations

Prior information about distractor facilitates selective attention to task-relevant items and helps the optimization of oculomotor planning. In the present study, we capitalized on gaze-position decoding to examine the dynamics of attentional deployment in a feature-based attentional task that involved two groups of dots (target/distractor dots) moving toward different directions. In Experiment 1, participants were provided with target cues indicating the moving direction of target dots. The results showed that participants were biased toward the cued direction and tracked the target dots throughout the task period. In Experiment 2 and Experiment 3, participants were provided with cues that informed the moving direction of distractor dots. When the distractor cue varied on a trial-by-trial basis (Experiment 2), participants continuously monitored the distractor's direction. However, when the to-be-ignored distractor direction remained constant (Experiment 3), participants would strategically bias their attention to the distractor's direction before the cue onset to reduce the cost of redeployment of attention between trials and reactively suppress further attraction evoked by distractors during the stimulus-on stage. This functional dissociation reflected the distinct influence that expectation produced on ocular control. Taken together, these results suggest that monitoring the distractor's feature is a prerequisite for feature-based attentional inhibition, and this process is facilitated by the predictability of the distractor's feature.

The Role of Blinks, Microsaccades and their Retinal Consequences in Bistable Motion Perception

Eye-related movements such as blinks and microsaccades are modulated during bistable perceptual tasks. However, if they play an active role during internal perceptual switches is not known. We conducted two experiments involving an ambiguous plaid stimulus, wherein participants were asked to continuously report their percept, which could consist of either unidirectional coherent or bidirectional component movement. Our main results show that blinks and microsaccades did not facilitate perceptual switches. On the contrary, a reduction in eye movements preceded the perceptual switch. Blanks, on the other hand, thought to mimic the retinal consequences of a blink, consistently led to a switch. Through the timing of the blank-introduced perceptual change, we were able to estimate the delay between the internal switch and the response. This delay further allowed us to evaluate that the reduction in blink probability co-occurred with the internal perceptual switch. Additionally, our results indicate that distinct internal processes underlie the switch to coherent vs. component percept. Blanks exclusively facilitated a switch to the coherent percept, and only the switch to coherent percept was followed by an increase in blink rate. In a second study, we largely replicated the findings and included a microsaccade analysis. Microsaccades only showed a weak relation with perceptual switches, but their direction was correlated with the perceived motion direction. Nevertheless, our data suggests an interaction between microsaccades and blinks by showing that microsaccades were differently modulated around blinks compared with blanks. This study shows that a reduction in eye movements precedes internal perceptual switches indicating that the rate of blinks can set the stage for a reinterpretation of sensory input. While a perceptual switch based on changed sensory input usually leads to an increase in blink rate, such an increase was only present after the perceptual switch to coherent motion but absent after the switch to component percept. This provides evidence of different underlying mechanism or internal consequence of the two perceptual switches and suggests that blinks can uncover differences in internal percept-related processes that are not evident from the percept itself.