Learning to silence saccadic suppression.

Perceptual stability is facilitated by a decrease in visual sensitivity during rapid eye movements, called saccadic suppression. While a large body of evidence demonstrates that saccadic programming is plastic, little is known about whether the perceptual consequences of saccades can be modified. Here, we demonstrate that saccadic suppression is attenuated during learning on a standard visual detection-in-noise task, to the point that it is effectively silenced. Across a period of 7 days, 44 participants were trained to detect brief, low-contrast stimuli embedded within dynamic noise, while eye position was tracked. Although instructed to fixate, participants regularly made small fixational saccades. Data were accumulated over a large number of trials, allowing us to assess changes in performance as a function of the temporal proximity of stimuli and saccades. This analysis revealed that improvements in sensitivity over the training period were accompanied by a systematic change in the impact of saccades on performance-robust saccadic suppression on day 1 declined gradually over subsequent days until its magnitude became indistinguishable from zero. This silencing of suppression was not explained by learning-related changes in saccade characteristics and generalized to an untrained retinal location and stimulus orientation. Suppression was restored when learned stimulus timing was perturbed, consistent with the operation of a mechanism that temporarily reduces or eliminates saccadic suppression, but only when it is behaviorally advantageous to do so. Our results indicate that learning can circumvent saccadic suppression to improve performance, without compromising its functional benefits in other viewing contexts.

The spatial properties of adaptation-induced distance compression.

Exposure to a dynamic texture reduces the perceived separation between objects, altering the mapping between physical relations in the environment and their neural representations. Here we investigated the spatial tuning and spatial frame of reference of this aftereffect to understand the stage(s) of processing where adaptation-induced changes occur. In Experiment 1, we measured apparent separation at different positions relative to the adapted area, revealing a strong but tightly tuned compression effect. We next tested the spatial frame of reference of the effect, either by introducing a gaze shift between adaptation and test phase (Experiment 2) or by decoupling the spatial selectivity of adaptation in retinotopic and world-centered coordinates (Experiment 3). Results across the two experiments indicated that both retinotopic and world-centered adaptation effects can occur independently. Spatial attention to the location of the adaptor alone could not account for the world-centered transfer we observed, and retinotopic adaptation did not transfer to world-centered coordinates after a saccade (Experiment 4). Finally, we found that aftereffects in different reference frames have a similar, narrow spatial tuning profile (Experiment 5). Together, our results suggest that the neural representation of local separation resides early in the visual cortex, but it can also be modulated by activity in higher visual areas.

Impact of microsaccades on visual shape processing.

Sensitivity to subtle changes in the shape of visual objects has been attributed to the existence of global pooling mechanisms that integrate local form information across space. Although global pooling is typically demonstrated under steady fixation, other work suggests prolonged fixation can lead to a collapse of global structure. Here, we ask whether small ballistic eye movements that naturally occur during periods of fixation affect the global processing of radial frequency (RF) patterns-closed contours created by sinusoidally modulating the radius of a circle. Observers were asked to discriminate the shapes of circular patterns and RF-modulated patterns while fixational eye movements were recorded binocularly at 500 Hz. Microsaccades were detected using a velocity-based algorithm, allowing trials to be sorted according to the relative timing of stimulus and microsaccade onset. Results revealed clear perisaccadic changes in shape discrimination thresholds. Performance was impaired when microsaccades occurred close to stimulus onset, but facilitated when they occurred shortly afterward. In contrast, global integration of shape was unaffected by the timing of microsaccades. These findings suggest that microsaccades alter the discrimination sensitivity to briefly presented shapes but do not disrupt the spatial pooling of local form signals.NEW & NOTEWORTHY Microsaccades cause rapid displacement of visual images during fixation and dramatically alter the perception of basic image features. However, their effect on more complex aspects of visual processing is not well understood. Here, we demonstrate a dissociation in the impact of microsaccades on shape perception. Although overall shape discrimination performance is modulated around the time of microsaccades, the pooling efficiency of global mechanisms that combine local form information across space remains unaffected.

Does physical exercise and congruent visual stimulation enhance perceptual learning?

PURPOSE: There is currently great interest in methods that can modulate brain plasticity, both in terms of understanding the basic mechanisms, and in the remedial application to situations of sensory loss. Recent work has focussed on how different manipulations might be combined to produce new settings that reveal synergistic actions. Here we ask whether a prominent example of adult visual plasticity, called perceptual learning, is modified by other environmental factors, such as visual stimulation and physical exercise. METHODS: We quantified the magnitude, rate and transfer of perceptual learning using a peripheral Vernier alignment task, in two groups of subjects matched for a range of baseline factors (e.g. age, starting Vernier threshold, baseline fitness). We trained subjects for 5 days on a Vernier alignment task. In one group, we introduced an exercise protocol with congruent visual stimulation. The control group received the same visual stimulation, but did not exercise prior to measurement of Vernier thresholds. RESULTS: Although the task generated large amounts of learning (~40%) and some transfer to untrained conditions in both groups, there were no specific benefits associated with either the addition of an exercise schedule or congruent visual stimulation. CONCLUSION: In adults, short periods of physical exercise and visual stimulation do not enhance perceptual learning.

Generalization of prior information for rapid Bayesian time estimation.

To enable effective interaction with the environment, the brain combines noisy sensory information with expectations based on prior experience. There is ample evidence showing that humans can learn statistical regularities in sensory input and exploit this knowledge to improve perceptual decisions and actions. However, fundamental questions remain regarding how priors are learned and how they generalize to different sensory and behavioral contexts. In principle, maintaining a large set of highly specific priors may be inefficient and restrict the speed at which expectations can be formed and updated in response to changes in the environment. However, priors formed by generalizing across varying contexts may not be accurate. Here, we exploit rapidly induced contextual biases in duration reproduction to reveal how these competing demands are resolved during the early stages of prior acquisition. We show that observers initially form a single prior by generalizing across duration distributions coupled with distinct sensory signals. In contrast, they form multiple priors if distributions are coupled with distinct motor outputs. Together, our findings suggest that rapid prior acquisition is facilitated by generalization across experiences of different sensory inputs but organized according to how that sensory information is acted on.

Temporal modulation improves dynamic peripheral acuity.

Macular degeneration and related visual disorders greatly limit foveal function, resulting in reliance on the peripheral retina for tasks requiring fine spatial vision. Here we investigate stimulus manipulations intended to maximize peripheral acuity for dynamic targets. Acuity was measured using a single interval orientation discrimination task at 10° eccentricity. Two types of image motion were investigated along with two different forms of temporal manipulation. Smooth object motion was generated by translating targets along an isoeccentric path at a constant speed (0-20°/s). Ocular motion was simulated by jittering target location using previously recorded fixational eye movement data, amplified by a variable gain factor (0-8). In one stimulus manipulation, the sequence was temporally subsampled by displaying the target on an evenly spaced subset of video frames. In the other, the contrast polarity of the stimulus was reversed at a variable rate. We found that threshold under object motion was improved at all speeds by reversing contrast polarity, while temporal subsampling improved resolution at high speeds but impaired performance at low speeds. With simulated ocular motion, thresholds were consistently improved by contrast polarity reversal, but impaired by temporal subsampling. We find that contrast polarity reversal and temporal subsampling produce differential effects on peripheral acuity. Applying contrast polarity reversal may offer a relatively simple image manipulation that could enhance visual performance in individuals with central vision loss.

Selective modulation of visual sensitivity during fixation.

During periods of steady fixation, we make small-amplitude ocular movements, termed microsaccades, at a rate of 1-2 every second. Early studies provided evidence that visual sensitivity is reduced during microsaccades-akin to the well-established suppression associated with larger saccades. However, the results of more recent work suggest that microsaccades may alter retinal input in a manner that enhances visual sensitivity to some stimuli. Here we parametrically varied the spatial frequency of a stimulus during a detection task and tracked contrast sensitivity as a function of time relative to microsaccades. Our data reveal two distinct modulations of sensitivity: suppression during the eye movement itself and facilitation after the eye has stopped moving. The magnitude of suppression and facilitation of visual sensitivity is related to the spatial content of the stimulus: suppression is greatest for low spatial frequencies, while sensitivity is enhanced most for stimuli of 1-2 cycles/°, spatial frequencies at which we are already most sensitive in the absence of eye movements. We present a model in which the tuning of suppression and facilitation is explained by delayed lateral inhibition between spatial frequency channels. Our data show that eye movements actively modulate visual sensitivity even during fixation: the detectability of images at different spatial scales can be increased or decreased depending on when the image occurs relative to a microsaccade. NEW & NOTEWORTHY Given the frequency with which we make microsaccades during periods of fixation, it is vital that we understand how they affect visual processing. We demonstrate two selective modulations of contrast sensitivity that are time-locked to the occurrence of a microsaccade: suppression of low spatial frequencies during each eye movement and enhancement of higher spatial frequencies after the eye has stopped moving. These complementary changes may arise naturally because of sluggish gain control between spatial channels.

Position matching between the visual fields in strabismus.

The misalignment of visual input in strabismus disrupts positional judgments. We measured positional accuracy in the extrafoveal visual field (1°-7° eccentricity) of a large group of strabismic subjects and a normal control group to identify positional distortions associated with the direction of strabismus. Subjects performed a free localization task in which targets were matched in opposite hemifields whilst fixating on a central cross. The constant horizontal error of each response was taken as a measure of accuracy, in addition to radial and angular error. In monocular conditions, all stimuli were viewed by one eye; thus, the error reflected spatial bias. In dichoptic conditions, the targets were seen by separate eyes; thus, the error reflected the perceived stimulus shift produced by ocular misalignment in addition to spatial bias. In both viewing conditions, both groups showed reliable over- and underestimations of visual field position, here termed a compression of response coordinates. The normal group showed compression in the left periphery, regardless of eye of stimulation. The strabismic group showed a visual field-specific compression that was clearly associated with direction of strabismus. The variation in perceived shift of strabismic subjects was largely accounted for by the biases present in monocular viewing, suggesting that binocular correspondence was uniform in the tested region. The asymmetric strabismic compression could not be reproduced in normal subjects through prism viewing, and its presence across viewing conditions suggests a hemifield-specific change in spatial coding induced by long-standing ocular misalignment.

Motion-based super-resolution in the peripheral visual field.

Improvements in foveal acuity for moving targets have been interpreted as evidence for the ability of the visual system to combine information over space and time, in order to reconstruct the image at a higher resolution (super-resolution). Here, we directly test whether this occurs in the peripheral visual field and discuss its potential for improving functional capacity in ocular disease. The effect of motion on visual acuity was first compared under conditions in which performance was limited either by natural undersampling in the retinal periphery or by the presence of overlaid masks with opaque elements to simulate retinal loss. To equate the information content of moving and static sequences, we next manipulated the dynamic properties of the masks. Finally, we determined the dependence of motion-related improvements on the object of motion (target or mask) and its trajectory (smooth or jittered). Motion improved visual acuity for masked but not unmasked peripheral targets. Equating the information content of moving and static conditions removed some but not all of this benefit. Residual motion-related improvements were largest in conditions in which the target moved along a consistent and predictable path. Our results show that motion can improve peripheral acuity in situations in which performance is limited by abnormal undersampling. These findings are consistent with the operation of a super-resolution system and could have important implications for any pathology that alters the regular sampling properties of the retinal mosaic.

Do perceptual biases emerge early or late in visual processing? Decision-biases in motion perception.

Visual perception is strongly influenced by contextual information. A good example is reference repulsion, where subjective reports about the direction of motion of a stimulus are significantly biased by the presence of an explicit reference. These perceptual biases could arise early, during sensory encoding, or alternatively, they may reflect decision-related processes occurring relatively late in the task sequence. To separate these two competing possibilities, we asked (human) subjects to perform a fine motion-discrimination task and then estimate the direction of motion in the presence or absence of an oriented reference line. When subjects performed the discrimination task with the reference, but subsequently estimated motion direction in its absence, direction estimates were unbiased. However, when subjects viewed the same stimuli but performed the estimation task only, with the orientation of the reference line jittered on every trial, the directions estimated by subjects were biased and yoked to the orientation of the shifted reference line. These results show that judgements made relative to a reference are subject to late, decision-related biases A model in which information about motion is integrated with that of an explicit reference cue, resulting in a late, decision-related re-weighting of the sensory representation, can account for these results.

Object size determines the spatial spread of visual time.

A key question for temporal processing research is how the nervous system extracts event duration, despite a notable lack of neural structures dedicated to duration encoding. This is in stark contrast with the orderly arrangement of neurons tasked with spatial processing. In this study, we examine the linkage between the spatial and temporal domains. We use sensory adaptation techniques to generate after-effects where perceived duration is either compressed or expanded in the opposite direction to the adapting stimulus' duration. Our results indicate that these after-effects are broadly tuned, extending over an area approximately five times the size of the stimulus. This region is directly related to the size of the adapting stimulus-the larger the adapting stimulus the greater the spatial spread of the after-effect. We construct a simple model to test predictions based on overlapping adapted versus non-adapted neuronal populations and show that our effects cannot be explained by any single, fixed-scale neural filtering. Rather, our effects are best explained by a self-scaled mechanism underpinned by duration selective neurons that also pool spatial information across earlier stages of visual processing.

Is the frequency of adult strabismus surgery increasing?

PURPOSE: In recent years there has been an increase in evidence for the functional and psychosocial benefits of correcting strabismus/heterotropia in adults. This study aimed to establish whether there has been an associated change in the frequency of strabismus surgery performed on adults in England since 2000. METHODS: Data on strabismus surgery performed in England between 2000 and 2014 were obtained from Hospital Episode Statistics, Health and Social Care Information Centre, England. The frequency of strabismus surgery was analysed for different age groups. Data were considered in the context of total population data for England, obtained from the Office for National Statistics. RESULTS: There was little change in the total number of strabismus operations performed in 2000-2014 (1% reduction). In the same period the number of operations performed on children aged 0-15 years decreased by 17%. In contrast, there was a 24% increase in the number of strabismus operations performed on patients aged 15 years or older. CONCLUSIONS: Although strabismus surgery is still most commonly performed on children, the data show there has been a significant increase in the number of strabismus operations performed on adults. We speculate that this increase is connected to the growing weight of evidence detailing the functional and psychosocial consequences of strabismus and the benefits of correction. These results have potential implications for the delivery of future care.

Fixational eye movements predict visual sensitivity.

During steady fixation, observers make small fixational saccades at a rate of around 1-2 per second. Presentation of a visual stimulus triggers a biphasic modulation in fixational saccade rate-an initial inhibition followed by a period of elevated rate and a subsequent return to baseline. Here we show that, during passive viewing, this rate signature is highly sensitive to small changes in stimulus contrast. By training a linear support vector machine to classify trials in which a stimulus is either present or absent, we directly compared the contrast sensitivity of fixational eye movements with individuals' psychophysical judgements. Classification accuracy closely matched psychophysical performance, and predicted individuals' threshold estimates with less bias and overall error than those obtained using specific features of the signature. Performance of the classifier was robust to changes in the training set (novel subjects and/or contrasts) and good prediction accuracy was obtained with a practicable number of trials. Our results indicate a tight coupling between the sensitivity of visual perceptual judgements and fixational eye control mechanisms. This raises the possibility that fixational saccades could provide a novel and objective means of estimating visual contrast sensitivity without the need for observers to make any explicit judgement.

The effect of normal aging and age-related macular degeneration on perceptual learning.

We investigated whether perceptual learning could be used to improve peripheral word identification speed. The relationship between the magnitude of learning and age was established in normal participants to determine whether perceptual learning effects are age invariant. We then investigated whether training could lead to improvements in patients with age-related macular degeneration (AMD). Twenty-eight participants with normal vision and five participants with AMD trained on a word identification task. They were required to identify three-letter words, presented 10° from fixation. To standardize crowding across each of the letters that made up the word, words were flanked laterally by randomly chosen letters. Word identification performance was measured psychophysically using a staircase procedure. Significant improvements in peripheral word identification speed were demonstrated following training (71% ± 18%). Initial task performance was correlated with age, with older participants having poorer performance. However, older adults learned more rapidly such that, following training, they reached the same level of performance as their younger counterparts. As a function of number of trials completed, patients with AMD learned at an equivalent rate as age-matched participants with normal vision. Improvements in word identification speed were maintained at least 6 months after training. We have demonstrated that temporal aspects of word recognition can be improved in peripheral vision with training across a range of ages and these learned improvements are relatively enduring. However, training targeted at other bottlenecks to peripheral reading ability, such as visual crowding, may need to be incorporated to optimize this approach.

Age-related changes in auditory and visual interactions in temporal rate perception.

We investigated how aging affects the integration of temporal rate for auditory flutter (amplitude modulation) presented with visual flicker. Since older adults were poorer at detecting auditory amplitude modulation, modulation depth was individually adjusted so that temporal rate was equally discriminable for 10 Hz flutter and flicker, thereby balancing the reliability of rate information available to each sensory modality. With age-related sensory differences normalized in this way, rate asynchrony skewed both auditory and visual rate judgments to the same extent in younger and older adults. Therefore, reliability-based weighting of temporal rate is preserved in older adults. Concurrent presentation of synchronous 10 Hz flicker and flutter improved temporal rate discrimination consistent with statistically optimal integration in younger but not older adults. In a control experiment, younger adults were presented with the same physical auditory stimulus as older adults. This time, rate asynchrony skewed perceived rate with greater auditory weighting rather than balanced integration. Taken together, our results indicate that integration of discrepant auditory and visual rates is not altered due to the healthy aging process once sensory deficits are accounted for, but that aging does abolish the minor improvement in discrimination performance seen in younger observers when concordant rates are integrated.

Estimation of cortical magnification from positional error in normally sighted and amblyopic subjects.

We describe a method for deriving the linear cortical magnification factor from positional error across the visual field. We compared magnification obtained from this method between normally sighted individuals and amblyopic individuals, who receive atypical visual input during development. The cortical magnification factor was derived for each subject from positional error at 32 locations in the visual field, using an established model of conformal mapping between retinal and cortical coordinates. Magnification of the normally sighted group matched estimates from previous physiological and neuroimaging studies in humans, confirming the validity of the approach. The estimate of magnification for the amblyopic group was significantly lower than the normal group: by 4.4 mm deg(-1) at 1° eccentricity, assuming a constant scaling factor for both groups. These estimates, if correct, suggest a role for early visual experience in establishing retinotopic mapping in cortex. We discuss the implications of altered cortical magnification for cortical size, and consider other neural changes that may account for the amblyopic results.

Decoding working memory of stimulus contrast in early visual cortex.

Most studies of the early stages of visual analysis (V1-V3) have focused on the properties of neurons that support processing of elemental features of a visual stimulus or scene, such as local contrast, orientation, or direction of motion. Recent evidence from electrophysiology and neuroimaging studies, however, suggests that early visual cortex may also play a role in retaining stimulus representations in memory for short periods. For example, fMRI responses obtained during the delay period between two presentations of an oriented visual stimulus can be used to decode the remembered stimulus orientation with multivariate pattern analysis. Here, we investigated whether orientation is a special case or if this phenomenon generalizes to working memory traces of other visual features. We found that multivariate classification of fMRI signals from human visual cortex could be used to decode the contrast of a perceived stimulus even when the mean response changes were accounted for, suggesting some consistent spatial signal for contrast in these areas. Strikingly, we found that fMRI responses also supported decoding of contrast when the stimulus had to be remembered. Furthermore, classification generalized from perceived to remembered stimuli and vice versa, implying that the corresponding pattern of responses in early visual cortex were highly consistent. In additional analyses, we show that stimulus decoding here is driven by biases depending on stimulus eccentricity. This places important constraints on the interpretation for decoding stimulus properties for which cortical processing is known to vary with eccentricity, such as contrast, color, spatial frequency, and temporal frequency.

Characterizing the role of disparity information in alleviating visual crowding.

The ability to identify a target is reduced by the presence of nearby objects, a phenomenon known as visual crowding. The extent to which crowding impairs our perception is generally governed by the degree of similarity between a target stimulus and its surrounding flankers. Here we investigated the influence of disparity differences between target and flankers on crowding. Orientation discrimination thresholds for a parafoveal target were first measured when the target and flankers were presented at the same depth to establish a flanker separation that induced a significant elevation in threshold for each individual. Flankers were subsequently fixed at this spatial separation while the disparity of the flankers relative to the target was altered. For all participants, thresholds showed a systematic decrease as flanker-target disparity increased. The resulting tuning function was asymmetric: Crowding was lower when the target was perceived to be in front of the flankers rather than behind. A series of control experiments confirmed that these effects were driven by disparity, as opposed to other factors such as flanker-target separation in three-dimensional (3-D) space or monocular positional offsets used to create disparity. When flankers were distributed over a range of crossed and uncrossed disparities, such that the mean was in the plane of the target, there was an equivalent or greater release of crowding compared to when all flankers were presented at the maximum disparity of that range. Overall, our results suggest that depth cues can reduce the effects of visual crowding, and that this reduction is unlikely to be caused by grouping of flankers or positional shifts in the monocular image.

Collective plasticity of binocular interactions in the adult visual system.

Binocular visual plasticity can be initiated via either bottom-up or top-down mechanisms, but it is unknown if these two forms of adult plasticity can be independently combined. In seven participants with normal binocular vision, sensory eye dominance was assessed using a binocular rivalry task, before and after a period of monocular deprivation and with and without selective attention directed towards one eye. On each trial, participants reported the dominant monocular target and the inter-ocular contrast difference between the stimuli was systematically altered to obtain estimates of ocular dominance. We found that both monocular light- and pattern-deprivation shifted dominance in favour of the deprived eye. However, this shift was completely counteracted if the non-deprived eye's stimulus was selectively attended. These results reveal that shifts in ocular dominance, driven by bottom-up and top-down selection, appear to act independently to regulate the relative contrast gain between the two eyes.