Transsaccadic processing: stability, integration, and the potential role of remapping

The perceptual timescape: Perceptual history on the sub-second scale

There is a high-capacity store of brief time span (∼1000 ms) which information enters from perceptual processing, often called iconic memory or sensory memory. It is proposed that a main function of this store is to hold recent perceptual information in a temporally segregated representation, named the perceptual timescape. The perceptual timescape is a continually active representation of change and continuity over time that endows the perceived present with a perceived history. This is accomplished primarily by two kinds of time marking information: time distance information, which marks all items of information in the perceptual timescape according to how far in the past they occurred, and ordinal temporal information, which organises items of information in terms of their temporal order. Added to that is information about connectivity of perceptual objects over time. These kinds of information connect individual items over a brief span of time so as to represent change, persistence, and continuity over time. It is argued that there is a one-way street of information flow from perceptual processing either to the perceived present or directly into the perceptual timescape, and thence to working memory. Consistent with that, the information structure of the perceptual timescape supports postdictive reinterpretations of recent perceptual information. Temporal integration on a time scale of hundreds of milliseconds takes place in perceptual processing and does not draw on information in the perceptual timescape, which is concerned with temporal segregation, not integration.

A presaccadic perceptual impairment at the postsaccadic location of the blindspot

Saccadic eye movements are preceded by profound changes in visual perception. These changes have been linked to the phenomenon of 'forward remapping', in which cells begin to respond to stimuli that appear in their post-saccadic receptive field before the eye has moved. Few studies have examined the perceptual consequences of remapping of areas of impaired sensory acuity, such as the blindspot. Understanding the perceptual consequences of remapping of scotomas may produce important insights into why some neurovisual deficits, such as hemianopia are so intractable for rehabilitation. The current study took advantage of a naturally occurring scotoma in healthy participants (the blindspot) to examine pre-saccadic perception at the upcoming location of the blindspot. Participants viewed stimuli monocularly and were required to make stimulus-driven vertical eye-movements. At a variable latency between the onset of saccade target (ST) and saccade execution a discrimination target (DT) was presented at one of 4 possible locations; within the blindspot, contralateral to the blindspot, in post-saccadic location of the blindspot and contralateral to the post-saccadic location of the blindspot. There was a significant perceptual impairment at the post-saccadic location of the blindspot relative to the contralateral post-saccadic location of the blindspot and the post-saccadic location of the blindspot in a no-saccade control condition. These data are consistent with the idea that the visual system includes a representation of the blindspot which is remapped prior to saccade onset.

Transsaccadic visual perception of foveal compared to peripheral environmental changes

The maintenance of stable visual perception across eye movements is hypothesized to be aided by extraretinal information (e.g., corollary discharge [CD]). Previous studies have focused on the benefits of this information for perception at the fovea. However, there is little information on the extent that CD benefits peripheral visual perception. Here we systematically examined the extent that CD supports the ability to perceive transsaccadic changes at the fovea compared to peripheral changes. Human subjects made saccades to targets positioned at different amplitudes (4° or 8°) and directions (rightward or upward). On each trial there was a reference point located either at (fovea) or 4° away (periphery) from the target. During the saccade the target and reference disappeared and, after a blank period, the reference reappeared at a shifted location. Subjects reported the perceived shift direction, and we determined the perceptual threshold for detection and estimate of the reference location. We also simulated the detection and location if subjects solely relied on the visual error of the shifted reference experienced after the saccade. The comparison of the reference location under these two conditions showed that overall the perceptual estimate was approximately 53% more accurate and 30% less variable than estimates based solely on visual information at the fovea. These values for peripheral shifts were consistently lower than that at the fovea: 34% more accurate and 9% less variable. Overall, the results suggest that CD information does support stable visual perception in the periphery, but is consistently less beneficial compared to the fovea.

A review of interactions between peripheral and foveal vision

Visual processing varies dramatically across the visual field. These differences start in the retina and continue all the way to the visual cortex. Despite these differences in processing, the perceptual experience of humans is remarkably stable and continuous across the visual field. Research in the last decade has shown that processing in peripheral and foveal vision is not independent, but is more directly connected than previously thought. We address three core questions on how peripheral and foveal vision interact, and review recent findings on potentially related phenomena that could provide answers to these questions. First, how is the processing of peripheral and foveal signals related during fixation? Peripheral signals seem to be processed in foveal retinotopic areas to facilitate peripheral object recognition, and foveal information seems to be extrapolated toward the periphery to generate a homogeneous representation of the environment. Second, how are peripheral and foveal signals re-calibrated? Transsaccadic changes in object features lead to a reduction in the discrepancy between peripheral and foveal appearance. Third, how is peripheral and foveal information stitched together across saccades? Peripheral and foveal signals are integrated across saccadic eye movements to average percepts and to reduce uncertainty. Together, these findings illustrate that peripheral and foveal processing are closely connected, mastering the compromise between a large peripheral visual field and high resolution at the fovea.

Intra-saccadic motion streaks as cues to linking object locations across saccades

When visual objects shift rapidly across the retina, they produce motion blur. Intra-saccadic visual signals, caused incessantly by our own saccades, are thought to be eliminated at early stages of visual processing. Here we investigate whether they are still available to the visual system and could—in principle—be used as cues for localizing objects as they change locations on the retina. Using a high-speed projection system, we developed a trans-saccadic identification task in which brief but continuous intra-saccadic object motion was key to successful performance. Observers made a saccade to a target stimulus that moved rapidly either up or down, strictly during the eye movement. Just as the target reached its final position, an identical distractor stimulus appeared on the opposite side, resulting in a display of two identical stimuli upon saccade landing. Observers had to identify the original target using the only available clue: the target's intra-saccadic movement. In an additional replay condition, we presented the observers’ own intra-saccadic retinal stimulus trajectories during fixation. Compared to the replay condition, task performance was impaired during saccades but recovered fully when a post-saccadic blank was introduced. Reverse regression analyses and confirmatory experiments showed that performance increased markedly when targets had long movement durations, low spatial frequencies, and orientations parallel to their retinal trajectory—features that promote intra-saccadic motion streaks. Although the potential functional role of intra-saccadic visual signals is still unclear, our results suggest that they could provide cues to tracking objects that rapidly change locations across saccades.

An adaptive algorithm for fast and reliable online saccade detection

To investigate visual perception around the time of eye movements, vision scientists manipulate stimuli contingent upon the onset of a saccade. For these experimental paradigms, timing is especially crucial, because saccade offset imposes a deadline on the display change. Although efficient online saccade detection can greatly improve timing, most algorithms rely on spatial-boundary techniques or absolute-velocity thresholds, which both suffer from weaknesses: late detections and false alarms, respectively. We propose an adaptive, velocity-based algorithm for online saccade detection that surpasses both standard techniques in speed and accuracy and allows the user to freely define the detection criteria. Inspired by the Engbert-Kliegl algorithm for microsaccade detection, our algorithm computes two-dimensional velocity thresholds from variance in the preceding fixation samples, while compensating for noisy or missing data samples. An optional direction criterion limits detection to the instructed saccade direction, further increasing robustness. We validated the algorithm by simulating its performance on a large saccade dataset and found that high detection accuracy (false-alarm rates of < 1%) could be achieved with detection latencies of only 3 ms. High accuracy was maintained even under simulated high-noise conditions. To demonstrate that purely intrasaccadic presentations are technically feasible, we devised an experimental test in which a Gabor patch drifted at saccadic peak velocities. Whereas this stimulus was invisible when presented during fixation, observers reliably detected it during saccades. Photodiode measurements verified that-including all system delays-the stimuli were physically displayed on average 20 ms after saccade onset. Thus, the proposed algorithm provides a valuable tool for gaze-contingent paradigms.

Dynamics of gaze control during prey capture in freely moving mice

Many studies of visual processing are conducted in constrained conditions such as head- and gaze-fixation, and therefore less is known about how animals actively acquire visual information in natural contexts. To determine how mice target their gaze during natural behavior, we measured head and bilateral eye movements in mice performing prey capture, an ethological behavior that engages vision. We found that the majority of eye movements are compensatory for head movements, thereby serving to stabilize the visual scene. During movement, however, periods of stabilization are interspersed with non-compensatory saccades that abruptly shift gaze position. Notably, these saccades do not preferentially target the prey location. Rather, orienting movements are driven by the head, with the eyes following in coordination to sequentially stabilize and recenter the gaze. These findings relate eye movements in the mouse to other species, and provide a foundation for studying active vision during ethological behaviors in the mouse.

Extra-foveal Processing of Object Semantics Guides Early Overt Attention During Visual Search

Eye-tracking studies using arrays of objects have demonstrated that some high-level processing of object semantics can occur in extra-foveal vision, but its role on the allocation of early overt attention is still unclear. This eye-tracking visual search study contributes novel findings by examining the role of object-to-object semantic relatedness and visual saliency on search responses and eye-movement behaviour across arrays of increasing size (3, 5, 7). Our data show that a critical object was looked at earlier and for longer when it was semantically unrelated than related to the other objects in the display, both when it was the search target (target-present trials) and when it was a target's semantically related competitor (target-absent trials). Semantic relatedness effects manifested already during the very first fixation after array onset, were consistently found for increasing set sizes, and were independent of low-level visual saliency, which did not play any role. We conclude that object semantics can be extracted early in extra-foveal vision and capture overt attention from the very first fixation. These findings pose a challenge to models of visual attention which assume that overt attention is guided by the visual appearance of stimuli, rather than by their semantics.

Feature-based guidance of attention during post-saccadic selection

Current models of trans-saccadic perception propose that, after a saccade, the saccade target object must be localized among objects near the landing position. However, the nature of the attentional mechanisms supporting this process is currently under debate. In the present study, we tested whether surface properties of the saccade target object automatically bias post-saccadic selection using a variant of the visual search task. Participants executed a saccade to a shape-singleton target in a circular array. During this primary saccade, the array sometimes rotated so that the eyes landed between the target and an adjacent distractor, requiring gaze correction. In addition, each object in the array had an incidental color value. On Switch trials, the target and adjacent distractor switched colors. The accuracy and latency of gaze correction to the target (measures that provide a direct index of target localization) were compared with a control condition in which no color switch occurred (No-switch trials). Gaze correction to the target was substantially impaired in the Switch condition. This result was obtained even when participants had substantial incentive to avoid encoding the color of the saccade target. In addition, similar effects were observed when the roles of the two feature dimensions (color and shape) were reversed. The results indicate that saccade target features are automatically encoded before a saccade, are retained in visual working memory across the saccade, and instantiate a feature-based selection operation when the eyes land, biasing attention toward objects that match target features.

Corollary Discharge Contributions to Perceptual Continuity Across Saccades

Our vision depends upon shifting our high-resolution fovea to objects of interest in the visual field. Each saccade displaces the image on the retina, which should produce a chaotic scene with jerks occurring several times per second. It does not. This review examines how an internal signal in the primate brain (a corollary discharge) contributes to visual continuity across saccades. The article begins with a review of evidence for a corollary discharge in the monkey and evidence from inactivation experiments that it contributes to perception. The next section examines a specific neuronal mechanism for visual continuity, based on corollary discharge that is referred to as visual remapping. Both the basic characteristics of this anticipatory remapping and the factors that control it are enumerated. The last section considers hypotheses relating remapping to the perceived visual continuity across saccades, including remapping's contribution to perceived visual stability across saccades.

Remapping locations and features across saccades: a dual-spotlight theory of attentional updating

How do we maintain visual stability across eye movements? Much work has focused on how visual information is rapidly updated to maintain spatiotopic representations. However, predictive spatial remapping is only part of the story. Here I review key findings, recent debates, and open questions regarding remapping and its implications for visual attention and perception. This review focuses on two key questions: when does remapping occur, and what is the impact on feature perception? Findings are reviewed within the framework of a two-stage, or dual- spotlight, remapping process, where spatial attention must be both updated to the new location (fast, predictive stage) and withdrawn from the previous retinotopic location (slow, post-saccadic stage), with a particular focus on the link between spatial and feature information across eye movements.

Implicit processing during change blindness revealed with mouse-contingent and gaze-contingent displays

People often miss salient events that occur right in front of them. This phenomenon, known as change blindness, reveals the limits of visual awareness. Here, we investigate the role of implicit processing in change blindness using an approach that allows partial dissociation of covert and overt attention. Traditional gaze-contingent paradigms adapt the display in real time according to current gaze position. We compare such a paradigm with a newly designed mouse-contingent paradigm where the visual display changes according to the real-time location of a user-controlled mouse cursor, effectively allowing comparison of change detection with mainly overt attention (gaze-contingent display; Experiment 2) and untethered overt and covert attention (mouse-contingent display; Experiment 1). We investigate implicit indices of target detection during change blindness in eye movement and behavioral data, and test whether affective devaluation of unnoticed targets may contribute to change blindness. The results show that unnoticed targets are processed implicitly, but that the processing is shallower than if the target is consciously detected. Additionally, the partial untethering of covert attention with the mouse-contingent display changes the pattern of search and leads to faster detection of the changing target. Finally, although it remains possible that the deployment of covert attention is linked to implicit processing, the results fall short of establishing a direct connection.

Time course of spatiotopic updating across saccades

Humans move their eyes several times per second, yet we perceive the outside world as continuous despite the sudden disruptions created by each eye movement. To date, the mechanism that the brain employs to achieve visual continuity across eye movements remains unclear. While it has been proposed that the oculomotor system quickly updates and informs the visual system about the upcoming eye movement, behavioral studies investigating the time course of this updating suggest the involvement of a slow mechanism, estimated to take more than 500 ms to operate effectively. This is a surprisingly slow estimate, because both the visual system and the oculomotor system process information faster. If spatiotopic updating is indeed this slow, it cannot contribute to perceptual continuity, because it is outside the temporal regime of typical oculomotor behavior. Here, we argue that the behavioral paradigms that have been used previously are suboptimal to measure the speed of spatiotopic updating. In this study, we used a fast gaze-contingent paradigm, using high phi as a continuous stimulus across eye movements. We observed fast spatiotopic updating within 150 ms after stimulus onset. The results suggest the involvement of a fast updating mechanism that predictively influences visual perception after an eye movement. The temporal characteristics of this mechanism are compatible with the rate at which saccadic eye movements are typically observed in natural viewing.

Scene Buildup From Latent Memory Representations Across Eye Movements

An unresolved problem in eye movement research is how a representation is constructed on-line from several consecutive fixations of a scene. Such a scene representation is generally understood to be sparse; yet, for meeting behavioral goals a certain level of detail is needed. We propose that this is achieved through the buildup of latent representations acquired at fixation. Latent representations are retained in an activity-silent manner, require minimal energy expenditure for their maintenance, and thus allow a larger storage capacity than traditional, activation based, visual working memory. The latent representations accumulate and interact in working memory to form to the scene representation. The result is rich in detail while sparse in the sense that it is restricted to the task-relevant aspects of the scene sampled through fixations. Relevant information can quickly and flexibly be retrieved by dynamical attentional prioritization. Latent representations are observable as transient functional connectivity patterns, which emerge due to short-term changes in synaptic weights. We discuss how observing latent representations could benefit from recent methodological developments in EEG-eye movement co-registration.

Functions of Memory Across Saccadic Eye Movements

Several times per second, humans make rapid eye movements called saccades which redirect their gaze to sample new regions of external space. Saccades present unique challenges to both perceptual and motor systems. During the movement, the visual input is smeared across the retina and severely degraded. Once completed, the projection of the world onto the retina has undergone a large-scale spatial transformation. The vector of this transformation, and the new orientation of the eye in the external world, is uncertain. Memory for the pre-saccadic visual input is thought to play a central role in compensating for the disruption caused by saccades. Here, we review evidence that memory contributes to (1) detecting and identifying changes in the world that occur during a saccade, (2) bridging the gap in input so that visual processing does not have to start anew, and (3) correcting saccade errors and recalibrating the oculomotor system to ensure accuracy of future saccades. We argue that visual working memory (VWM) is the most likely candidate system to underlie these behaviours and assess the consequences of VWM's strict resource limitations for transsaccadic processing. We conclude that a full understanding of these processes will require progress on broader unsolved problems in psychology and neuroscience, in particular how the brain solves the object correspondence problem, to what extent prior beliefs influence visual perception, and how disparate signals arriving with different delays are integrated.

The Relationship Between Spatial Attention and Eye Movements

The nature of the relationship between spatial attention and eye movements has been the subject of intense debate for more than 40 years. Two ideas have dominated this debate. First is the idea that spatial attention shares common neural mechanisms with eye movement programming, characterizing attention as an eye movement that has been prepared but not executed. Second, based on the observation that attention shifts to saccade targets, several theories have proposed that saccade programming necessarily recruits attentional resources. In this chapter, we review the evidence for each of these ideas and discuss some of the limitations and challenges in confirming their predictions. Although they are clearly dependent under some circumstances, dissociations between spatial attention and eye movements, and clear differences in their basic functions, point to the existence of two interconnected, but separate, systems.

Motion Masking by Stationary Objects: A Study of Simulated Saccades

Saccades are crucial to visual information intake by re-orienting the fovea to regions of interest in the visual scene. However, they cause drastic disruptions of the retinal input by shifting the retinal image at very high speeds. The resulting motion and smear are barely noticed, a phenomenon known as saccadic omission. Here, we studied the perception of motion during simulated saccades while observers fixated, moving naturalistic visual scenes across the retina with saccadic speed profiles using a very high temporal frequency display. We found that the mere presence of static pre- and post-saccadic images significantly reduces the perceived amplitude of motion but does not eliminate it entirely. This masking of motion perception could make the intra-saccadic stimulus much less salient and thus easier to ignore.

Decoding Trans-Saccadic Memory

We examine whether peripheral information at a planned saccade target affects immediate postsaccadic processing at the fovea on saccade landing. Current neuroimaging research suggests that presaccadic stimulation has a late effect on postsaccadic processing, in contrast to the early effect seen in behavioral studies. Human participants (both male and female) were instructed to saccade toward a face or a house that, on different trials, remained the same, changed, or disappeared during the saccade. We used a multivariate pattern analysis of electroencephalography data to decode face versus house processing directly after the saccade. The classifier was trained on separate trials without a saccade, where a house or face was presented at the fovea. When the saccade target remained the same across the saccade, we could reliably decode the target 123 ms after saccade offset. In contrast, when the target was changed during the saccade, the new target was decoded at a later time-point, 151 ms after saccade offset. The "same" condition advantage suggests that congruent presaccadic information facilitates processing of the postsaccadic stimulus compared with incongruent information. Finally, the saccade target could be decoded above chance even when it had been removed during the saccade, albeit with a slower time course (162 ms) and poorer signal strength. These findings indicate that information about the (peripheral) presaccadic stimulus is transferred across the saccade so that it becomes quickly available and influences processing at its expected new retinal position (the fovea).SIGNIFICANCE STATEMENT Here we provide neural evidence for early information transfer across saccades. Specifically, we examined the effect of presaccadic sensory information on the initial neuronal processing of a postsaccadic stimuli. Using electroencephalography and multivariate pattern analysis, we found the following: (1) that the identity of the presaccadic stimulus modulated the postsaccadic latency of stimulus relevant information; and (2) that a saccadic neural marker for a saccade target stimulus could be detected even when the stimulus had been removed during saccade. These results demonstrate that information about the peripheral presaccadic stimulus was transferred across the saccade and influenced processing at a new retinal position (the fovea) directly after the saccade landed.

Numerosity estimation benefits from transsaccadic information integration

Humans achieve a stable and homogeneous representation of their visual environment, although visual processing varies across the visual field. Here we investigated the circumstances under which peripheral and foveal information is integrated for numerosity estimation across saccades. We asked our participants to judge the number of black and white dots on a screen. Information was presented either in the periphery before a saccade, in the fovea after a saccade, or in both areas consecutively to measure transsaccadic integration. In contrast to previous findings, we found an underestimation of numerosity for foveal presentation and an overestimation for peripheral presentation. We used a maximum-likelihood model to predict accuracy and reliability in the transsaccadic condition based on peripheral and foveal values. We found near-optimal integration of peripheral and foveal information, consistently with previous findings about orientation integration. In three consecutive experiments, we disrupted object continuity between the peripheral and foveal presentations to probe the limits of transsaccadic integration. Even for global changes on our numerosity stimuli, no influence of object discontinuity was observed. Overall, our results suggest that transsaccadic integration is a robust mechanism that also works for complex visual features such as numerosity and is operative despite internal or external mismatches between foveal and peripheral information. Transsaccadic integration facilitates an accurate and reliable perception of our environment.

Spatiotopic updating facilitates perception immediately after saccades

As the neural representation of visual information is initially coded in retinotopic coordinates, eye movements (saccades) pose a major problem for visual stability. If no visual information were maintained across saccades, retinotopic representations would have to be rebuilt after each saccade. It is currently strongly debated what kind of information (if any at all) is accumulated across saccades, and when this information becomes available after a saccade. Here, we use a motion illusion to examine the accumulation of visual information across saccades. In this illusion, an annulus with a random texture slowly rotates, and is then replaced with a second texture (motion transient). With increasing rotation durations, observers consistently perceive the transient as large rotational jumps in the direction opposite to rotation direction (backward jumps). We first show that accumulated motion information is updated spatiotopically across saccades. Then, we show that this accumulated information is readily available after a saccade, immediately biasing postsaccadic perception. The current findings suggest that presaccadic information is used to facilitate postsaccadic perception and are in support of a forward model of transsaccadic perception, aiming at anticipating the consequences of eye movements and operating within the narrow perisaccadic time window.

A State Space Model for Spatial Updating of Remembered Visual Targets during Eye Movements

In the oculomotor system, spatial updating is the ability to aim a saccade toward a remembered visual target position despite intervening eye movements. Although this has been the subject of extensive experimental investigation, there is still no unifying theoretical framework to explain the neural mechanism for this phenomenon, and how it influences visual signals in the brain. Here, we propose a unified state-space model (SSM) to account for the dynamics of spatial updating during two types of eye movement; saccades and smooth pursuit. Our proposed model is a non-linear SSM and implemented through a recurrent radial-basis-function neural network in a dual Extended Kalman filter (EKF) structure. The model parameters and internal states (remembered target position) are estimated sequentially using the EKF method. The proposed model replicates two fundamental experimental observations: continuous gaze-centered updating of visual memory-related activity during smooth pursuit, and predictive remapping of visual memory activity before and during saccades. Moreover, our model makes the new prediction that, when uncertainty of input signals is incorporated in the model, neural population activity and receptive fields expand just before and during saccades. These results suggest that visual remapping and motor updating are part of a common visuomotor mechanism, and that subjective perceptual constancy arises in part from training the visual system on motor tasks.

Saccadic Corollary Discharge Underlies Stable Visual Perception

Saccadic eye movements direct the high-resolution foveae of our retinas toward objects of interest. With each saccade, the image jumps on the retina, causing a discontinuity in visual input. Our visual perception, however, remains stable. Philosophers and scientists over centuries have proposed that visual stability depends upon an internal neuronal signal that is a copy of the neuronal signal driving the eye movement, now referred to as a corollary discharge (CD) or efference copy. In the old world monkey, such a CD circuit for saccades has been identified extending from superior colliculus through MD thalamus to frontal cortex, but there is little evidence that this circuit actually contributes to visual perception. We tested the influence of this CD circuit on visual perception by first training macaque monkeys to report their perceived eye direction, and then reversibly inactivating the CD as it passes through the thalamus. We found that the monkey's perception changed; during CD inactivation, there was a difference between where the monkey perceived its eyes to be directed and where they were actually directed. Perception and saccade were decoupled. We established that the perceived eye direction at the end of the saccade was not derived from proprioceptive input from eye muscles, and was not altered by contextual visual information. We conclude that the CD provides internal information contributing to the brain's creation of perceived visual stability. More specifically, the CD might provide the internal saccade vector used to unite separate retinal images into a stable visual scene.

SIGNIFICANCE STATEMENT

Visual stability is one of the most remarkable aspects of human vision. The eyes move rapidly several times per second, displacing the retinal image each time. The brain compensates for this disruption, keeping our visual perception stable. A major hypothesis explaining this stability invokes a signal within the brain, a corollary discharge, that informs visual regions of the brain when and where the eyes are about to move. Such a corollary discharge circuit for eye movements has been identified in macaque monkey. We now show that selectively inactivating this brain circuit alters the monkey's visual perception. We conclude that this corollary discharge provides a critical signal that can be used to unite jumping retinal images into a consistent visual scene.

A New Font, Specifically Designed for Peripheral Vision, Improves Peripheral Letter and Word Recognition, but Not Eye-Mediated Reading Performance

Reading speed is dramatically reduced when readers cannot use their central vision. This is because low visual acuity and crowding negatively impact letter recognition in the periphery. In this study, we designed a new font (referred to as the Eido font) in order to reduce inter-letter similarity and consequently to increase peripheral letter recognition performance. We tested this font by running five experiments that compared the Eido font with the standard Courier font. Letter spacing and x-height were identical for the two monospaced fonts. Six normally-sighted subjects used exclusively their peripheral vision to run two aloud reading tasks (with eye movements), a letter recognition task (without eye movements), a word recognition task (without eye movements) and a lexical decision task. Results show that reading speed was not significantly different between the Eido and the Courier font when subjects had to read single sentences with a round simulated gaze-contingent central scotoma (10° diameter). In contrast, Eido significantly decreased perceptual errors in peripheral crowded letter recognition (-30% errors on average for letters briefly presented at 6° eccentricity) and in peripheral word recognition (-32% errors on average for words briefly presented at 6° eccentricity).

The relationship between visual attention and visual working memory encoding: A dissociation between covert and overt orienting

There is substantial debate over whether visual working memory (VWM) and visual attention constitute a single system for the selection of task-relevant perceptual information or whether they are distinct systems that can be dissociated when their representational demands diverge. In the present study, we focused on the relationship between visual attention and the encoding of objects into VWM. Participants performed a color change-detection task. During the retention interval, a secondary object, irrelevant to the memory task, was presented. Participants were instructed either to execute an overt shift of gaze to this object (Experiments 1-3) or to attend it covertly (Experiments 4 and 5). Our goal was to determine whether these overt and covert shifts of attention disrupted the information held in VWM. We hypothesized that saccades, which typically introduce a memorial demand to bridge perceptual disruption, would lead to automatic encoding of the secondary object. However, purely covert shifts of attention, which introduce no such demand, would not result in automatic memory encoding. The results supported these predictions. Saccades to the secondary object produced substantial interference with VWM performance, but covert shifts of attention to this object produced no interference with VWM performance. These results challenge prevailing theories that consider attention and VWM to reflect a common mechanism. In addition, they indicate that the relationship between attention and VWM is dependent on the memorial demands of the orienting behavior. (PsycINFO Database Record

A "blanking effect" for surface features: Transsaccadic spatial-frequency discrimination is improved by postsaccadic blanking

Although saccadic eye movements occur frequently—about three or four times a second—humans are astonishingly blind to transsaccadic changes. Locational displacements of the saccade target of up to 2 deg of visual angle, and even large changes of a visual scene, can go unnoticed. For a long time, this insensitivity was ascribed to deficits in transsaccadic memory: Only a coarse, (spatially) imprecise representation would be retained across a saccade. This assumption was contradicted by Deubel's and Schneider's (Behavioral and Brain Sciences 17:259-260, 1994) striking finding that locational discrimination performance across a saccade is greatly improved by inserting a short postsaccadic blank. Surprisingly, the question of whether blanking effects occur also for other forms of transsaccadic changes (i.e., surface-feature changes) has been widely ignored. We tested this question by means of a transsaccadic change in spatial frequency. Postsaccadic blanking facilitated spatial-frequency discrimination, but to a smaller amount than the usual blanking effects obtained with locational displacements. This finding bears important implications for models of visual stability and transsaccadic memory.

Remapping, Spatial Stability, and Temporal Continuity: From the Pre-Saccadic to Postsaccadic Representation of Visual Space in LIP

As our eyes move, we have a strong percept that the world is stable in space and time; however, the signals in cortex coming from the retina change with each eye movement. It is not known how this changing input produces the visual percept we experience, although the predictive remapping of receptive fields has been described as a likely candidate. To explain how remapping accounts for perceptual stability, we examined responses of neurons in the lateral intraparietal area while animals performed a visual foraging task. When a stimulus was brought into the response field of a neuron that exhibited remapping, the onset of the postsaccadic representation occurred shortly after the saccade ends. Whenever a stimulus was taken out of the response field, the presaccadic representation abruptly ended shortly after the eyes stopped moving. In the 38% (20/52) of neurons that exhibited remapping, there was no more than 30 ms between the end of the presaccadic representation and the start of the postsaccadic representation and, in some neurons, and the population as a whole, it was continuous. We conclude by describing how this seamless shift from a presaccadic to postsaccadic representation could contribute to spatial stability and temporal continuity.

Attentional trade-offs maintain the tracking of moving objects across saccades

In many situations like playing sports or driving a car, we keep track of moving objects, despite the frequent eye movements that drastically interrupt their retinal motion trajectory. Here we report evidence that transsaccadic tracking relies on trade-offs of attentional resources from a tracked object's motion path to its remapped location. While participants covertly tracked a moving object, we presented pulses of coherent motion at different locations to probe the allocation of spatial attention along the object's entire motion path. Changes in the sensitivity for these pulses showed that during fixation attention shifted smoothly in anticipation of the tracked object's displacement. However, just before a saccade, attentional resources were withdrawn from the object's current motion path and reflexively drawn to the retinal location the object would have after saccade. This finding demonstrates the predictive choice the visual system makes to maintain the tracking of moving objects across saccades.