The utility of peripheral stereopsis.

This perspective article makes the case for evaluating and training peripheral stereopsis, particularly when the central visual field is compromised in one or both eyes. Examples of clinical conditions that preferentially affect the central visual field include macular degeneration, which affects the central macular region in one or both eyes, and amblyopia where the central field is often affected in one eye, but the peripheral field is largely intact. While binocular acuity may be preserved when the monocular central field of one eye is affected, fine stereopsis is compromised because it requires intact vision in corresponding locations in the two eyes. Even in these clinical conditions, recent studies that map stereoacuity at locations across the visual field demonstrate that the periphery supports coarse stereopsis, and that training efforts to use residual stereopsis may have greater benefit if they take this finding into account.

Eye movements in visual impairment.

This Special Issue describes the impact of visual impairment on visuomotor function. It includes contributions that examine gaze control in conditions associated with abnormal visual development such as amblyopia, dyslexia and neurofibromatosis as well as disorders associated with field loss later in life, such as macular degeneration and stroke. Specifically, the papers address both gaze holding (fixation), and gaze-following behavior (single saccades, sequences of saccades and smooth-pursuit) that characterize active vision in daily life and evaluate the influence of both pathological and simulated field loss. Several papers address the challenges to reading and visual search; describing how the patterns of eye movements in these real-world tasks adapt to visual impairment and highlighting how they could serve as diagnostic markers of visuomotor function.

Mapping residual stereopsis in macular degeneration.

Individuals with macular degeneration typically lose vision in the central region of one or both eyes. A binocular scotoma occurs when vision loss occurs in overlapping locations in both eyes, but stereopsis is impacted even in the non-overlapping region wherever the visual field in either eye is affected. We used a novel stereoperimetry protocol to measure local stereopsis across the visual field (up to 25° eccentricity) to determine how locations with functional stereopsis relate to the scotomata in the two eyes. Participants included those with monocular or binocular scotomata and age-matched controls with healthy vision. Targets (with or without depth information) were presented on a random dot background. Depth targets had true binocular disparity of 20' (crossed), whereas non-depth targets were defined by monocular cues such as contrast and dot density. Participants reported target location and whether it was in depth or flat. Local depth sensitivity (d') estimates were then combined to generate a stereopsis map. This stereopsis map was compared to the union of the monocular microperimetry estimates that mapped out the functional extent of the scotoma in each eye. The "union" prediction aligned with residual stereopsis, showing impaired stereopsis within this region and residual stereopsis outside this region. Importantly, the stereoblind region was typically more extensive than the binocular scotoma defined by the intersection (overlap) of the scotomata. This explains why individuals may have intact binocular visual fields but be severely compromised in tasks of daily living that benefit from stereopsis, such as eye-hand coordination and navigation.

Saccades during visual search in macular degeneration.

Macular degeneration (MD) compromises both high-acuity vision and eye movements when the foveal regions of both eyes are affected. Individuals with MD adapt to central field loss by adopting a preferred retinal locus (PRL) for fixation. Here, we investigate how individuals with bilateral MD use eye movements to search for targets in a visual scene under realistic binocular viewing conditions. Five individuals with binocular scotomata, 3 individuals with monocular scotomata and 6 age-matched controls participated in our study. We first extensively mapped the binocular scotoma with an eyetracker, while fixation was carefully monitored (Vullings & Verghese, 2020). Participants then completed a visual search task where 0, 1, or 2 Gaussian blobs were distributed randomly across a natural scene. Participants were given 10 s to actively search the display and report the number of blobs. An analysis of saccade characteristics showed that individuals with binocular scotomata made more saccades in the direction of their scotoma than controls for the same directions. Saccades in the direction of the scotoma were typically of small amplitude, and did not fully uncover the region previously hidden by the scotoma. Rather than make more saccades to explore this hidden region, participants frequently made saccades back toward newly uncovered regions. Backward saccades likely serve a similar purpose to regressive saccades exhibited during reading in MD, by inspecting previously covered regions near the direction of gaze. Our analysis suggests that the higher prevalence of backward saccades in individuals with binocular scotomata might be related to the PRL being adjacent to the scotoma.

Saccadic contributions to smooth pursuit in macular degeneration.

Saccades during smooth pursuit can help bring the fovea on target, particularly in cases of low pursuit gain. Individuals with macular degeneration often suffer damage to the central retina including the fovea, which impacts oculomotor function such as fixation, saccadic and smooth pursuit eye movements. We hypothesized that these oculomotor changes in macular degeneration (MD) would make saccades less appropriately directed (even if more numerous). To investigate saccades during pursuit in MD, we conducted a quantitative analysis of smooth pursuit eye movement data from a prior study, Vision Research 141 (2017) 181-190. Here we examined saccade frequency, magnitude, and direction across viewing conditions for MD and control participants during pursuit of a target moving in a modified step-ramp paradigm. Individuals with MD had more variability in saccade directions that included directions orthogonal to the target trajectory. PRL eccentricity significantly correlated with increases in saccades in non-target directions during smooth pursuit. These results suggest that a large number of saccades during pursuit in MD participants are unlikely to be catch-up saccades that serve to keep the eye on the target.

Excitatory Contribution to Binocular Interactions in Human Visual Cortex Is Reduced in Strabismic Amblyopia.

Binocular summation in strabismic amblyopia is typically reported as being absent or greatly reduced in behavioral studies and is thought to be because of a preferential loss of excitatory interactions between the eyes. Here, we studied how excitatory and suppressive interactions contribute to binocular contrast interactions along the visual cortical hierarchy of humans with strabismic and anisometropic amblyopia in both sexes, using source-imaged steady-state visual evoked potentials (SSVEP) over a wide range of relative contrast between the two eyes. Dichoptic parallel grating stimuli modulated at unique temporal frequencies in each eye allowed us to quantify spectral response components associated with monocular inputs (self-terms) and the response components because of interaction of the inputs of the two eyes [intermodulation (IM) terms]. Although anisometropic amblyopes revealed a similar pattern of responses to normal-vision observers, strabismic amblyopes exhibited substantially reduced IM responses across cortical regions of interest (V1, V3a, hV4, hMT+ and lateral occipital cortex), indicating reduced interocular interactions in visual cortex. A contrast gain control model that simultaneously fits self- and IM-term responses within each cortical area revealed different patterns of binocular interactions between individuals with normal and disrupted binocularity. Our model fits show that in strabismic amblyopia, the excitatory contribution to binocular interactions is significantly reduced in both V1 and extra-striate cortex, whereas suppressive contributions remain intact. Our results provide robust electrophysiological evidence supporting the view that disruption of binocular interactions in strabismus or amblyopia is because of preferential loss of excitatory interactions between the eyes.SIGNIFICANCE STATEMENT We studied how excitatory and suppressive interactions contribute to binocular contrast interactions along the visual cortical hierarchy of humans with normal and amblyopic vision, using source-imaged SSVEP and frequency-domain analysis of dichoptic stimuli over a wide range of relative contrast between the two eyes. A dichoptic contrast gain control model was used to characterize these interactions in amblyopia and provided a quantitative comparison to normal vision. Our model fits revealed different patterns of binocular interactions between normal and amblyopic vision. Strabismic amblyopia significantly reduced excitatory contributions to binocular interactions, whereas suppressive contributions remained intact. Our results provide robust evidence supporting the view that the preferential loss of excitatory interactions disrupts binocular interactions in strabismic amblyopia.

Eye Movements in Macular Degeneration.

In healthy vision, the fovea provides high acuity and serves as the locus for fixation achieved through saccadic eye movements. Bilateral loss of the foveal regions in both eyes causes individuals to adopt an eccentric locus for fixation. This review deals with the eye movement consequences of the loss of the foveal oculomotor reference and the ability of individuals to use an eccentric fixation locus as the new oculomotor reference. Eye movements are an integral part of everyday activities, such as reading, searching for an item of interest, eye-hand coordination, navigation, or tracking an approaching car. We consider how these tasks are impacted by the need to use an eccentric locus for fixation and as a reference for eye movements, specifically saccadic and smooth pursuit eye movements.

Mapping the binocular scotoma in macular degeneration.

When the scotoma is binocular in macular degeneration (MD), it often obscures objects of interest, causing individuals to miss information. To map the binocular scotoma as precisely as current methods that map the monocular scotoma, we propose an iterative eye-tracker method. Study participants included nine individuals with MD and four age-matched controls. We measured the extent of the monocular scotomata using a scanning laser ophthalmoscope/optical coherence tomography (SLO/OCT). Then, we precisely mapped monocular and binocular scotomata with an eye tracker, while fixation was monitored. Participants responded whenever they detected briefly flashed dots, which were first presented on a coarse grid, and then at manually selected points to refine the shape and edges of the scotoma. Monocular scotomata measured in the SLO and eye tracker are highly similar, validating the eye-tracking method for scotoma mapping. Moreover, all participants used clustered fixation loci corresponding to their dominant preferred fixation locus. Critically, for individuals with binocular scotomata, the binocular map from the eye tracker was consistent with the overlap of the monocular scotoma profiles from the SLO. Thus, eye-tracker-based perimetry offers a reliable and sensitive tool for measuring both monocular and binocular scotomata, unlike the SLO/OCT that is limited to monocular viewing.

Predicting Stereopsis in Macular Degeneration.

Each of our eyes sees a slightly different view of the physical world. Disparity is the small difference in position of features in the retinal images; stereopsis is the percept of depth from disparity. A distance between corresponding features in the retinal images of the two eyes smaller than the "upper disparity limit" yields a percept of depth; distances greater than this limit cause the two unfused monocular features to appear flattened into the fixation plane. This behavioral disparity limit is consistent with neurophysiological estimates of the largest disparity scale in primate, allowing us to relate physiological limits on plausible binocular interactions to separation between retinal locations. Here we test the hypothesis that this upper disparity limit predicts the presence of coarse stereopsis in humans with macular degeneration (MD), which affects the central retina but typically spares the periphery. The pattern of vision loss can be highly asymmetric, such that an intact location in one eye has a corresponding point in the other eye that lies within affected retina. Nevertheless, some individuals with MD have coarse stereopsis that is useful for eye-hand coordination. Our results show that individuals with MD (n = 25, male and female) have coarse stereopsis when the distance between intact retinal locations is less than the behavioral and physiological upper disparity limit at the corresponding eccentricity. Furthermore, for those without stereopsis, we can predict whether they can achieve stereopsis by using alternate retinal loci at further eccentricities whose separation is below the upper disparity limit.SIGNIFICANCE STATEMENT We show that the largest separation between features in the two eyes that yields a percept of depth in humans is related to the largest disparity scale in macaque medial temporal area and to the estimated size of the receptive fields in human depth-sensitive cortical regions. This upper disparity limit also predicts whether individuals with retinal damage due to macular degeneration will have stereopsis. Individuals have stereopsis when the separation between intact retinal locations in the two eyes is smaller than the upper disparity limit measured behaviorally. Our results indicate the importance of the behavioral upper disparity limit as a predictor for stereopsis in populations with retinal damage.

Contrast Normalization Accounts for Binocular Interactions in Human Striate and Extra-striate Visual Cortex.

During binocular viewing, visual inputs from the two eyes interact at the level of visual cortex. Here we studied binocular interactions in human visual cortex, including both sexes, using source-imaged steady-state visual evoked potentials over a wide range of relative contrast between two eyes. The ROIs included areas V1, V3a, hV4, hMT+, and lateral occipital cortex. Dichoptic parallel grating stimuli in each eye modulated at distinct temporal frequencies allowed us to quantify spectral components associated with the individual stimuli from monocular inputs (self-terms) and responses due to interaction between the inputs from the two eyes (intermodulation [IM] terms). Data with self-terms revealed an interocular suppression effect, in which the responses to the stimulus in one eye were reduced when a stimulus was presented simultaneously to the other eye. The suppression magnitude varied depending on visual area, and the relative contrast between the two eyes. Suppression was strongest in V1 and V3a (50% reduction) and was least in lateral occipital cortex (20% reduction). Data with IM terms revealed another form of binocular interaction, compared with self-terms. IM response was strongest at V1 and was least in hV4. Fits of a family of divisive gain control models to both self- and IM-term responses within each cortical area indicated that both forms of binocular interaction shared a common gain control nonlinearity. However, our model fits revealed different patterns of binocular interaction along the cortical hierarchy, particularly in terms of excitatory and suppressive contributions.SIGNIFICANCE STATEMENT Using source-imaged steady-state visual evoked potentials and frequency-domain analysis of dichoptic stimuli, we measured two forms of binocular interactions: one is associated with the individual stimuli that represent interocular suppression from each eye, and the other is a direct measure of interocular interaction between inputs from the two eyes. We demonstrated that both forms of binocular interactions share a common gain control mechanism in striate and extra-striate cortex. Furthermore, our model fits revealed different patterns of binocular interaction along the visual cortical hierarchy, particularly in terms of excitatory and suppressive contributions.

Motion perception in central field loss.

Motion information is essential in daily life because it provides cues to depth, timing, object identification, and self-motion, as well as input to the oculomotor system. As the peripheral visual field is exquisitely sensitive to motion, we investigated the periphery of individuals with central visual field loss (CFL) to determine whether speed and direction discrimination are intact in this population. We compared CFL participants' (N = 8), older (N = 6), and young controls' (N = 6) ability to discriminate motion speed and direction in a two-spatial-alternative forced-choice design. Participants viewed moving dots on the left and right of a fixation marker and judged which side had the faster speed or more clockwise direction. For the young control group, we repeated the experiment with the stimulus limited to thin strips of fixed width at eccentricities of 5°, 10°, and 15°. There was no significant difference in mean speed or direction discrimination thresholds of CFL participants and older controls for either velocity. Young controls had significantly lower thresholds than the CFL group for both tasks. We did not find an effect of visual acuity, viewing eccentricity, or scotoma location on individuals' ability to discriminate speed or direction. Our results indicate that for high-visibility stimuli moving at 5°-10°/s, speed and direction discrimination are intact in the periphery of individuals with CFL.

The upper disparity limit increases gradually with eccentricity.

Stereopsis is important for tasks of daily living such as eye-hand coordination. It is best in central vision but is also mediated by the periphery. Previously we have shown that individuals with central-field loss who have residual stereopsis in the periphery perform better at an eye-hand-coordination task when they perform the task binocularly rather than monocularly. Here we seek to determine what sets the limit of stereopsis, defined as the largest disparity that supports the sustained appearance of depth, in the near periphery in healthy individuals. While stereoacuity thresholds increase sharply with eccentricity, Panum's area increases much more slowly. We used a rigorous method to determine the uppermost limit of disparity. At long durations, the two half-images that define a large disparity appear as two isolated targets in the same flat plane; small incremental changes in disparity produce changes in the separation between the half-images, and disparity magnitude can be judged on the basis of separation, like a monocular width judgment. The disparity limit is the point at which the threshold for judging dichoptic separation between the half-images is equal to the monocular width-discrimination threshold. The disparity limit at 10° was a factor of 2-4 times larger than the fovea, regardless of the meridian tested. The increase in the disparity limit with eccentricity was shallow, similar to that of Panum's area. Within this disparity limit, disparity increment thresholds were comparable for foveal and peripheral targets, illustrating the significance and utility of peripheral stereopsis, especially in the absence of foveal stereopsis.

The spatial and temporal properties of attentional selectivity for saccades and reaches.

The preparation and execution of saccades and goal-directed movements elicits an accompanying shift in attention at the locus of the impending movement. However, some key aspects of the spatiotemporal profile of this attentional shift between eye and hand movements are not resolved. While there is evidence that attention is improved at the target location when making a reach, it is not clear how attention shifts over space and time around the movement target as a saccade and a reach are made to that target. Determining this spread of attention is an important aspect in understanding how attentional resources are used in relation to movement planning and guidance in real world tasks. We compared performance on a perceptual discrimination paradigm during a saccade-alone task, reach-alone task, and a saccade-plus-reach task to map the temporal profile of the premotor attentional shift at the goal of the movement and at three surrounding locations. We measured performance relative to a valid baseline level to determine whether motor planning induces additional attentional facilitation compared to mere covert attention. Sensitivity increased relative to movement onset at the target and at the surrounding locations, for both the saccade-alone and saccade-plus-reach conditions. The results suggest that the temporal profile of the attentional shift is similar for the two tasks involving saccades (saccade-alone and saccade-plus-reach tasks), but is very different when the influence of the saccade is removed. In this case, performance in the saccade-plus-reach task reflects the lower sensitivity observed when a reach-alone task is being conducted. In addition, the spatial profile of this spread of attention is not symmetrical around the target. This suggests that when a saccade and reach are being planned together, the saccade drives the attentional shift, and the reach-alone carries little attentional weight.

Attention deficits in Amblyopia.

Amblyopia is a neuro-developmental abnormality associated with deficits in a broad range of both low-level and high-level visual tasks. This is particularly true in strabismic amblyopia where fixation is unstable and there is an increased frequency of microsaccades. In light of the close association between eye movements and attention, we propose a novel hypothesis: that the cost of unstable fixation in amblyopia is a deficit in selective attention. The increased latency for saccades and manual response time with amblyopic-eye viewing is consistent with attention being distracted by unwanted fixational eye movements. We review other attention deficits in amblyopia and discuss whether they are explained by fixation instability, or whether they involve a form of neglect or suppression of the visual input from the amblyopic eye.

Cortical sources of Vernier acuity in the human visual system: An EEG-source imaging study.

Vernier acuity determines the relative position of visual features with a precision better than the sampling resolution of cone receptors in the retina. Because Vernier displacement is thought to be mediated by orientation-tuned mechanisms, Vernier acuity is presumed to be processed in striate visual cortex (V1). However, there is considerable evidence suggesting that Vernier acuity is dependent not only on structures in V1 but also on processing in extrastriate cortical regions. Here we used functional magnetic resonance imaging-informed electroencephalogram source imaging to localize the cortical sources of Vernier acuity in observers with normal vision. We measured suprathreshold and near-threshold responses to Vernier onset/offset stimuli at different stages of the visual cortical hierarchy, including V1, hV4, lateral occipital cortex (LOC), and middle temporal cortex (hMT+). These responses were compared with responses to grating on/off stimuli, as well as to stimuli that control for lateral motion in the Vernier task. Our results show that all visual cortical regions of interest (ROIs) responded to both suprathreshold Vernier and grating stimuli. However, thresholds for Vernier displacement (Vernier acuity) were lowest in V1 and LOC compared with hV4 and hMT+, whereas all visual ROIs had identical thresholds for spatial frequency (grating acuity) and for relative motion. The cortical selectivity of sensitivity to Vernier displacement provides strong evidence that LOC, in addition to V1, is involved in Vernier acuity processing. The robust activation of LOC might be related to the sensitivity to the relative position of features, which is common to Vernier displacement and to some kinds of texture segmentation.

Attention to Multiple Objects Facilitates Their Integration in Prefrontal and Parietal Cortex.

Selective attention is known to interact with perceptual organization. In visual scenes, individual objects that are distinct and discriminable may occur on their own, or in groups such as a stack of books. The main objective of this study is to probe the neural interaction that occurs between individual objects when attention is directed toward one or more objects. Here we record steady-state visual evoked potentials via electrocorticography to directly assess the responses to individual stimuli and to their interaction. When human participants attend to two adjacent stimuli, prefrontal and parietal cortex shows a selective enhancement of only the neural interaction between stimuli, but not the responses to individual stimuli. When only one stimulus is attended, the neural response to that stimulus is selectively enhanced in prefrontal and parietal cortex. In contrast, early visual areas generally manifest responses to individual stimuli and to their interaction regardless of attentional task, although a subset of the responses is modulated similarly to prefrontal and parietal cortex. Thus, the neural representation of the visual scene as one progresses up the cortical hierarchy becomes more highly task-specific and represents either individual stimuli or their interaction, depending on the behavioral goal. Attention to multiple objects facilitates an integration of objects akin to perceptual grouping.SIGNIFICANCE STATEMENT Individual objects in a visual scene are seen as distinct entities or as parts of a whole. Here we examine how attention to multiple objects affects their neural representation. Previous studies measured single-cell or fMRI responses and obtained only aggregate measures that combined the activity to individual stimuli as well as their potential interaction. Here, we directly measure electrocorticographic steady-state responses corresponding to individual objects and to their interaction using a frequency-tagging technique. Attention to two stimuli increases the interaction component that is a hallmark for perceptual integration of stimuli. Furthermore, this stimulus-specific interaction is represented in prefrontal and parietal cortex in a task-dependent manner.

Texture segmentation influences the spatial profile of presaccadic attention.

Attention is important for selecting targets for action. Several studies have shown that attentional selection precedes eye movements to a target, and results in an enhanced sensitivity at the saccade goal. Typically these studies have used isolated targets on blank backgrounds, which are rare in real-world situations. Here, we examine the spatial profile of sensitivity around a saccade target on a textured background and how the influence of the surrounding context develops over time. We used two textured backgrounds: a uniform texture, and a concentric arrangement of an inner and an outer texture with orthogonal orientations. For comparison, we also measured sensitivity around the target on a blank background. The spatial profile of sensitivity was measured with a brief, dim, probe flashed around the saccade target. When the target was on a blank or a uniformly textured background, spatial sensitivity peaked near the target location around 350 ms after cue onset and declined with distance from the target. However, when the background was made up of an inner and outer texture, sensitivity to the inner texture was uniformly high, peaking at about 350 ms after cue onset, suggesting that the entire inner texture was selected along with the target. The enhancement of sensitivity on the inner texture was much smaller when observers attended the target covertly and performed the probe-detection task. Thus, our results suggest that the surface representation around the target is taken into account when an observer actively plans to interact with the target.

Monocular and binocular smooth pursuit in central field loss.

Macular degeneration results in heterogeneous central field loss (CFL) and often has asymmetrical effects in the two eyes. As such, it is not clear to what degree the movements of the two eyes are coordinated. To address this issue, we examined smooth pursuit quantitatively in CFL participants during binocular viewing and compared it to the monocular viewing case. We also examined coordination of the two eyes during smooth pursuit and how this coordination was affected by interocular ratios of acuity and contrast, as well as CFL-specific interocular differences, such as scotoma sizes and degree of binocular overlap. We hypothesized that the coordination of eye movements would depend on the binocularity of the two eyes. To test our hypotheses, we used a modified step-ramp paradigm, and measured pursuit in both eyes while viewing was binocular, or monocular with the dominant or non-dominant eye. Data for CFL participants and age-matched controls were examined at the group, within-group, and individual levels. We found that CFL participants had a broader range of smooth pursuit gains and a significantly lower correlation between the two eyes, as compared to controls. Across both CFL and control groups, smooth pursuit gain and correlation between the eyes are best predicted by the ratio of contrast sensitivity between the eyes. For the subgroup of participants with measurable stereopsis, both smooth pursuit gain and correlation are best predicted by stereoacuity. Therefore, our results suggest that coordination between the eyes during smooth pursuit depends on binocular cooperation between the eyes.

Training eye movements for visual search in individuals with macular degeneration.

We report a method to train individuals with central field loss due to macular degeneration to improve the efficiency of visual search. Our method requires participants to make a same/different judgment on two simple silhouettes. One silhouette is presented in an area that falls within the binocular scotoma while they are fixating the center of the screen with their preferred retinal locus (PRL); the other silhouette is presented diametrically opposite within the intact visual field. Over the course of 480 trials (approximately 6 hr), we gradually reduced the amount of time that participants have to make a saccade and judge the similarity of stimuli. This requires that they direct their PRL first toward the stimulus that is initially hidden behind the scotoma. Results from nine participants show that all participants could complete the task faster with training without sacrificing accuracy on the same/different judgment task. Although a majority of participants were able to direct their PRL toward the initially hidden stimulus, the ability to do so varied between participants. Specifically, six of nine participants made faster saccades with training. A smaller set (four of nine) made accurate saccades inside or close to the target area and retained this strategy 2 to 3 months after training. Subjective reports suggest that training increased awareness of the scotoma location for some individuals. However, training did not transfer to a different visual search task. Nevertheless, our study suggests that increasing scotoma awareness and training participants to look toward their scotoma may help them acquire missing information.

Accuracy of eye position for saccades and smooth pursuit.

In this study, we address the question of whether a target is foveated during smooth pursuit. Specifically, we examine whether smooth pursuit eye movements land near the center-of-mass of the target, as is the case for saccades. To that end, we instructed eight untrained, healthy participants to follow moving targets, presented monocularly in a scanning laser ophthalmoscope. Stimuli moved either in a modified step-ramp (smooth pursuit), or made a single step (saccade), stepping 6° from the center. Targets were ring-shaped and either 0.6° or 1.7° in diameter. In an additional set of experiments, two participants collected more extensive data on smooth pursuit and saccades for a larger range of target sizes (0.6°, 1.7°, or 4.3°). During pursuit, eyes were rarely placed at target center, even when participants' fixational stability was taken into account. Furthermore, there was a clear tendency for distance from target center to increase with target size. This outcome was in contrast to saccades, where there was no effect of target size across participants. The difference in foveal placement between the two types of eye movements is consistent with their different purposes: closer inspection of the target for saccades versus maintenance of the target in the visual field for smooth pursuit.