Vernier Offset Discrimination Research Articles

Delayed foveal noise affects performance in a vernier offset discrimination task

Delayed foveal noise affects performance in a vernier offset discrimination task

Unlocking crowding by ensemble statistics

In crowding,1,2,3,4,5,6,7 objects that can be easily recognized in isolation appear jumbled when surrounded by other elements.8 Traditionally, crowding is explained by local pooling mechanisms,3,6,9,10,11,12,13,14,15 but many findings have shown that the global configuration of the entire stimulus display, rather than local aspects, determines crowding.8,16,17,18,19,20,21,22,23,24,25,26,27,28 However, understanding global configurations is challenging because even slight changes can lead from crowding to uncrowding and vice versa.23,25,28,29 Unfortunately, the number of configurations to explore is virtually infinite. Here, we show that one does not need to know the specific configuration of flankers to determine crowding strength but only their ensemble statistics, which allow for the rapid computation of groups within the stimulus display.30,31,32,33,34,35,36,37 To investigate the role of ensemble statistics in (un)crowding, we used a classic vernier offset discrimination task in which the vernier was flanked by multiple squares. We manipulated the orientation statistics of the squares based on the following rationale: a central square with an orientation different from the mean orientation of the other squares stands out from the rest and groups with the vernier, causing strong crowding. If, on the other hand, all squares group together, the vernier is the only element that stands out, and crowding is weak. These effects should depend exclusively on the perceived ensemble statistics, i.e., on the mean orientation of the squares and not on their individual orientations. In two experiments, we confirmed these predictions.

Identifying Long- and Short-Term Processes in Perceptual Learning

Practice makes perfect in almost all perceptual tasks, but how perceptual improvements accumulate remains unknown. Here, we developed a multicomponent theoretical framework to model contributions of both long- and short-term processes in perceptual learning. Applications of the framework to the block-by-block learning curves of 49 adult participants in seven perceptual tasks identified ubiquitous long-term general learning and within-session relearning in most tasks. More importantly, we also found between-session forgetting in the vernier-offset discrimination, face-view discrimination, and auditory-frequency discrimination tasks; between-session off-line gain in the visual shape search task; and within-session adaptation and both between-session forgetting and off-line gain in the contrast detection task. The main results of the vernier-offset discrimination and visual shape search tasks were replicated in a new experiment. The multicomponent model provides a theoretical framework to identify component processes in perceptual learning and a potential tool to optimize learning in normal and clinical populations.

What crowding can tell us about object representations.

In crowding, perception of a target usually deteriorates when flanking elements are presented next to the target. Surprisingly, adding further flankers can lead to a release from crowding. In previous work we showed that, for example, vernier offset discrimination at 9° of eccentricity deteriorated when a vernier was embedded in a square. Adding further squares improved performance. The more squares presented, the better the performance, extending across 20° of the visual field. Here, we show that very similar results hold true for shapes other than squares, including unfamiliar, irregular shapes. Hence, uncrowding is not restricted to simple and familiar shapes. Our results provoke the question of whether any type of shape is represented at any location in the visual field. Moreover, small changes in the orientation of the flanking shapes led to strong increases in crowding strength. Hence, highly specific shape-specific interactions across large parts of the visual field determine vernier acuity.

Intact crowding and temporal masking in dyslexia.

Phonological deficits in dyslexia are well documented. However, there is an ongoing discussion about whether visual deficits limit the reading skills of people with dyslexia. Here, we investigated visual crowding and backward masking. We presented a Vernier (i.e., two vertical bars slightly offset to the left or right) and asked observers to indicate the offset direction. Vernier stimuli are visually similar to letters and are strongly affected by crowding, even in the fovea. To increase task difficulty, Verniers are often followed by a mask (i.e., backward masking). We measured Vernier offset discrimination thresholds for the basic Vernier task, under crowding, and under backward masking, in students with dyslexia (n = 19) and age and intelligence matched students (n = 27). We found no group differences in any of these conditions. Controls with fast visual processing (good backward masking performance), were faster readers. By contrast, no such correlation was found among the students with dyslexia, suggesting that backward masking does not limit their reading efficiency. These findings indicate that neither elevated crowding nor elevated backward masking pose a bottleneck to reading skills of people with dyslexia.

Targets but not flankers are suppressed in crowding as revealed by EEG frequency tagging

Perception of a visual target can strongly deteriorate in the presence of flanking elements (crowding). For example, adding lines next to a vernier makes vernier offset discrimination difficult. Crowding is often considered a bottleneck of low-level vision, determined by the unavoidable limitations of the early visual system. In accordance with this proposal, neural processing of the flankers should be impaired in crowding as much as that of the target. To test this prediction, we used steady-state visually evoked potentials (ssVEPs) to separate target responses from flanker responses. We presented a vernier target either alone or flanked by lines, which had the same color as the vernier or a different color. Crowding by same-color flankers was stronger than by different-color flankers. Mirroring the behavioral results, ssVEP amplitudes corresponding to the target were higher for different-color flankers than for same-color flankers. Flanker related ssVEPs, however, did not depend on crowding strength. It seems that target, but not flanker processing, is susceptible to crowding. In line with previous results, we suggest that crowding is not caused by low-level interferences but is linked to target-flanker grouping instead.

Perceptual grouping determines haptic contextual modulation

Since the early phenomenological demonstrations of Gestalt principles, one of the major challenges of Gestalt psychology has been to quantify these principles. Here, we show that contextual modulation, i.e. the influence of context on target perception, can be used as a tool to quantify perceptual grouping in the haptic domain, similar to the visual domain. We investigated the influence of target–flanker grouping on performance in haptic vernier offset discrimination. We hypothesized that when, despite the apparent differences between vision and haptics, similar grouping principles are operational, a similar pattern of flanker interference would be observed in the haptic as in the visual domain. Participants discriminated the offset of a haptic vernier. The vernier was flanked by different flanker configurations: no flankers, single flanking lines, 10 flanking lines, rectangles and single perpendicular lines, varying the degree to which the vernier grouped with the flankers. Additionally, we used two different flanker widths (same width as and narrower than the target), again to vary target–flanker grouping. Our results show a clear effect of flankers: performance was much better when the vernier was presented alone compared to when it was presented with flankers. In the majority of flanker configurations, grouping between the target and the flankers determined the strength of interference, similar to the visual domain. However, in the same width rectangular flanker condition we found aberrant results. We discuss the results of our study in light of similarities and differences between vision and haptics and the interaction between different grouping principles. We conclude that in haptics, similar organization principles apply as in visual perception and argue that grouping and Gestalt are key organization principles not only of vision, but of the perceptual system in general.

Crowding, grouping, timing

In crowding, target perception is deteriorated by flanking elements. For example, when a vernier is flanked by two lines of the same length, vernier offset discrimination strongly deteriorates. Interestingly, changing the color of the flankers can reduce crowding (uncrowding). Similarly, when the flanking lines are part of a cube, i.e., a good Gestalt, crowding is weak. Conversely, however, scrambling the lines of the cubes, i.e., a “bad” Gestalt, leads to strong crowding. We proposed that crowding is strong when target and flankers group (two-lines, scrambled cubes). Crowding is weak when the target ungroups from the flankers (two flanking lines with different color, cubes). Here, we show, first, that when target and flankers group (strong crowding), crowding is unaffected by stimulus duration. Two same-length flankers presented for 20 ms lead to similar performance as when presented for 150 ms. Second, in uncrowding with flankers of different color (and other basic feature differences), performance is again unaffected by stimulus duration. Third, in uncrowding with cubes, duration matters. For short durations (20 ms), crowding is strong and only for longer stimulus durations (from 120 ms on) does crowding decrease. We suggest that, for short durations, the brain cannot process the good Gestalt of the cubes. The representation of the cube’s lines is “unstructured”, as in the scrambled cubes, and hence crowding is strong. Interestingly, a short preview (20 ms duration) of only the cubes strongly reduces crowding, even when the preview is presented one second before the “crowded” stimulus. Our results suggest that uncrowding emerges in a slow, recurrent manner, with iconic memory playing an important role.

Comparing Response Times and Error Rates in a Simultaneous Masking Paradigm

In simultaneous masking, performance on a foveally presented target is impaired by one or more flanking elements. Previous studies have demonstrated strong effects of the grouping of the target and the flankers on the strength of masking (e.g., Malania, Herzog & Westheimer, 2007). These studies have predominantly examined performance by measuring offset discrimination thresholds as a measure of performance, and it is therefore unclear whether other measures of performance provide similar outcomes. A recent study, which examined the role of grouping on error rates and response times in a speeded vernier offset discrimination task, similar to that used by Malania et al. (2007), suggested a possible dissociation between the two measures, with error rates mimicking threshold performance, but response times showing differential results (Panis & Hermens, 2014). We here report the outcomes of three experiments examining this possible dissociation, and demonstrate an overall similar pattern of results for error rates and response times across a broad range of mask layouts. Moreover, the pattern of results in our experiments strongly correlates with threshold performance reported earlier (Malania et al., 2007). Our results suggest that outcomes in a simultaneous masking paradigm do not critically depend on the outcome measure used, and therefore provide evidence for a common underlying mechanism.

Electrophysiological signatures of crowding are similar in foveal and peripheral vision

Flankers can strongly deteriorate performance on a visual target (crowding). For example, vernier offset discrimination strongly deteriorates when neighbouring flankers are presented. Interestingly, performance for longer and shorter flankers is better than performance for flankers of the same length as the vernier. Based on these findings, we proposed that crowding is strongest when the vernier and the flankers group (same length flankers) and weaker when the vernier ungroups from the flankers (shorter or longer flankers). These effects were observed both in foveal and peripheral vision. Here, using high-density EEG, we show that electrophysiological signatures of crowding are also similar in foveal and peripheral vision. In both foveal and peripheral (3.9°) vision, the N1 wave correlated well with performance levels and, hence, with crowding. Amplitudes were highest for the long flankers, intermediate for the short flankers and lowest for the equal length flankers. This effect was observed neither at earlier stages of processing, nor in control conditions matched for stimulus energy. Effects are more pronounced in the fovea than in the periphery. These similarities are evidence for a common mechanism of crowding in both foveal and peripheral vision.

The N1 wave amplitude reflects perceptual grouping and correlates with crowding

In crowding, flanking elements strongly deteriorate performance. For example, vernier offset discrimination is strongly affected by neighboring flankers. Performance is worst when the flankers have the same length as the vernier. Surprisingly, performance improves for longer and shorter flankers [Malania et al., 2007, Journal of Vision, 7(2):1, 1-7]. We proposed that crowding is strongest when the vernier and the flankers group (same length flankers) and weaker when they ungroup (shorter or longer flankers). Here, we used high density EEG to investigate the time course of crowding. First we replicated previous findings. Performance was best in the long flankers condition, intermediate in the short flankers condition, and worst in the medium flankers condition. The P1 wave amplitude correlated with flanker length being highest in the long flankers condition, intermediate in the medium flankers condition, and lowest in the short flankers condition. The N1 wave amplitude correlated well with performance being highest in the long flankers condition, intermediate in the short flankers condition, and lowest in the medium flankers condition. Our study shows that the N1 wave is a good predictor for perceptual grouping and hence crowding. These processes seem to occur after the P1 wave, i.e. after basic feature extraction.

How Recurrent Dynamics Explain Crowding

In crowding, flankers impair perception of a target. For example, Vernier offset discrimination deteriorates when the Vernier is flanked by parallel lines. Pooling models explain crowding by averaging of neural activity corresponding to the Vernier and the flankers, thus, reducing signal to noise ratio. However, recently, it was shown that flankers, longer than the Vernier, lead to less crowding than equal length flankers. Adding additional long flankers reduced crowding almost fully- in stark contrast to pooling models which predict just the opposite result. These and other findings clearly show that crowding cannot be explained by local spatial interactions, but global computations are needed. Here, we show that a Wilson-Cowan type model can explain both classical, local and recent, global aspects of crowding. The Wilson-Cowan type model employs end-stopped receptive fields with isotropic excitatory connections and anisotropic lateral inhibitory connections that are reciprocal between adjacent neurons. The key feature of the models is spread of neural activity across similar elements which are eliminated during recurrent inhibition. For example, crowding strength decreases the more long flankers are presented because these similar, long flankers inhibit each other during time consuming processing and, thus, reduce inhibition on the dissimilar Vernier. For equal length flankers, the Vernier is “treated” similarly to a flanker and inhibited. For this reason, and in accordance with psychophysical data, crowding does not vary with the number of equal length flankers.

How Recurrent Dynamics Explain Crowding

In crowding, flankers impair perception of a target. For example, Vernier offset discrimination deteriorates when the Vernier is flanked by parallel lines. Pooling models explain crowding by averaging of neural activity corresponding to the Vernier and the flankers, thus, reducing signal to noise ratio. However, recently, it was shown that flankers, longer than the Vernier, lead to less crowding than equal length flankers. Adding additional long flankers reduced crowding almost fully- in stark contrast to pooling models which predict just the opposite result. These and other findings clearly show that crowding cannot be explained by local spatial interactions, but global computations are needed. Here, we show that a Wilson-Cowan type model can explain both classical, local and recent, global aspects of crowding. The Wilson-Cowan type model employs end-stopped receptive fields with isotropic excitatory connections and anisotropic lateral inhibitory connections that are reciprocal between adjacent neurons. The key feature of the models is a spread of neural activity across similar elements which are eliminated during recurrent inhibition. For example, crowding strength decreases the more long flankers are presented because these similar, long flankers inhibit each other during time consuming processing and, thus, reduce inhibition on the dissimilar Vernier. For equal length flankers, the Vernier is "treated" similarly to a flanker and is inhibited. For this reason, and in accordance with psychophysical data, crowding does not vary with the number of equal length flankers. Meeting abstract presented at VSS 2012

How alcohol intake affects visual temporal processing

Alcohol affects vision. However, the influence of alcohol on visual processing is largely unknown. Here, we investigated the effects of alcohol on visual spatiotemporal processing. We employed a visual paradigm, the shine through backward masking paradigm, in which a vernier is either presented alone or followed by a variety of mask. We investigated performance for women at blood alcohol levels of 0mg/kg, 400mg/kg and 600mg/kg and for men at 0mg/kg, 400mg/kg and 800mg/kg. When the vernier was presented alone, vernier offset discrimination was not affected by alcohol. When the vernier was followed by a mask, stimulus onset asynchronies (SOAs) between target and mask were significantly longer after alcohol intake. However, as a second experiment showed, spatial and temporal processing per se were not impaired by alcohol. In addition, spatial processing was not affected by moderate alcohol consumption. Hence, moderate consumption of alcohol does not affect visual processing per se. We propose that the longer SOAs after alcohol intake are related to changes in mechanisms of target stabilization rather than changes in spatial and temporal sensitivity as has been previously suggested.

Crowding suppresses the N1 wave

Flankers can strongly deteriorate performance on a visual target (crowding). For example, vernier offset discrimination is strongly affected by neighboring flankers. Interestingly, performance for longer and shorter flankers is better than performance for equal length flankers. We previously found that crowding is strongest when the vernier and the flankers group (same length flankers) and weaker when they ungroup (shorter or longer flankers). Here, we used high density EEG to investigate the mechanisms underlying crowding. The P1 wave amplitude correlated with the length of flankers (stimulus energy). The N1 wave amplitude correlated with performance, with the highest amplitudes occurring for long flankers, intermediate ones for short flankers, and low amplitudes for medium flankers. The N1 amplitude did not depend on the task difficulty per se but on target-flanker grouping, because increasing vernier offset size did not change the N1 amplitude. We show that the N1 reduction corresponds to suppression in crowding.

When bigger is better

One of the main factors of crowding is the spacing between target and flankers. The closer the flankers are to the target, the stronger is crowding. Recently, it was proposed that crowding strength is determined by the distance between target and flanker centroids (Levi & Carney, 2009). Here, we determined vernier offset discrimination in the periphery with different flanker configurations. When the vernier was flanked by two vertical lines, thresholds increased. Thresholds decreased compared to the two-lines condition when each of the lines was complemented to form a rectangle. This is in line with the centroid hypothesis because the rectangles' centroids are further away than the centroids of the single flankers. However, when crossing the upper and lower horizontal lines of the rectangles, performance deteriorated even though centroids are the same in this and the rectangle condition. These results can neither be explained by the spacing between the target and the flankers nor by centroid distance. Also simple pooling models fail to account for these results. We propose instead that grouping is a key factor in crowding: crowding decreases when target and flankers ungroup, crowding increases when target and flankers group.

Quantifying target conspicuity in contextual modulation by visual search

Contextual elements can strongly modulate visual performance. For example, performance deteriorates when a vernier is flanked by neighboring lines. On a neural level, such contextual modulation is often explained by local spatial interactions such as lateral inhibition or pooling. However, these mechanisms cannot account for a number of recent results which showed that global rather than local factors play a key role in contextual modulation. On a level of perceptual organization, we proposed that contextual modulation increases when the target groups with the flankers and decreases when the target stands out from the flankers. To quantify this "standing out" in foveal vision, here, we performed both a visual search and a vernier offset discrimination task on the same stimulus configurations. Stimulus configurations yielding short reaction times in visual search yielded good vernier discrimination performance. Stimulus configurations yielding long reaction times yielded weaker discrimination. Hence, vernier offset discrimination is superior for targets that are efficiently searched and vice versa.

The time course of perceptual grouping: a high density ERP study

Performance on a target can be strongly modified by context. For example, vernier offset discrimination is strongly deteriorated by neighboring flankers. Performance is worst when the flankers have the same length as the vernier. Surprisingly, performance improves for longer and shorter flankers [Malania et al, 2007 Journal of Vision 7(2):1, 1–7]. It was proposed that interference is strongest when the vernier and the flankers group, and weaker when they ungroup. Here, we used high density EEG to investigate the time course of this contextual modulation. A vernier was flanked on both sides by ten lines which were either shorter, longer, or of the same length as the vernier. Performance was worst for equal length flankers, and best for longer flankers. The P1 amplitude monotonically increased with flanker size, reflecting the stimulus layout. The N1 amplitude was highly correlated with performance and, hence, with the strength of grouping: longer flankers elicited the highest amplitude of the N1 wave, shorter flankers medium amplitude, and the equal length flankers elicited the lowest one. Hence, perceptual grouping occurs before the N1 onset, ie before 150–160 ms.

Age-related differences in temporal processing

Visual backward masking paradigm, as determined with the shine-through effect, is a very sensitive tool to detect age related differences in visual temporal processing. We presented a vernier offset to the left or right followed by a grating mask which impedes vernier offset discrimination. Because of the use of such a vernier acuity task, visual deficits are unlikely caused by optical deficiencies. In the first experiment, we found strongly deteriorated masking performance of elderly from 60 years on and teenagers basically from 7 to 13 years compared to controls aged between 18 and 32 years. In a second experiment, we varied the spatial and temporal layout of the mask, for example, by inserting a small gap in the grating mask. With these manipulations, processing deficits in elderly turned out to be very different from the teenagers' ones. Teenagers were sensitive to the gaps (and other manipulations) whereas elderly were not--possibly because elderly low-pass filter visual information, such as the gaps, related to a loss of retinal and other neurons.

Hemispheric asymmetries for simple visual judgments in the split brain

While it is commonly noted that the right cerebral hemisphere is specialized for visuospatial processing, the scope and nature of this specialization remain somewhat ill defined. Our previous research with callosotomy (‘split-brain’) patients has suggested that the asymmetry may be limited to conditions that have an explicit spatial component. To investigate this we compared the performance of the divided hemispheres of two callosotomy patients on four simple visual-matching tasks. These tasks were orientation discrimination, vernier offset discrimination, size discrimination, and luminance discrimination. In each task, two stimuli were presented briefly to one visual hemifield and the patient was asked to discriminate whether they were the same or different. The first three tasks (orientation, vernier, and size) were all spatial in nature and were performed better by the right hemisphere. The luminance discrimination task, which is non-spatial, was performed equivalently by the two hemispheres. These results support the view that the fundamental difference in visual function between the hemispheres is in the ability to perform spatial discriminations.