What crowds in crowding?

Peer reviewedPublisher PD

Vision in clutter:neural correlates of visual crowding

Perception of a visual target can strongly deteriorate in the presence of flanking elements (crowding). Crowding is the main limiting factor in reading and a major contributor to poor vision in amblyopia. In crowding, elements appear jumbled and thus are hard to recognize. Classic pooling models propose that crowding occurs via pooling of information from low-level to high-level visual areas. Because of pooling, receptive fields become larger and, at final stages, encompass both the target's and flankers' locations. As a result, the brain is unable to perceive the target separately from the flankers. Pooling models predict that adding flankers or increasing the size of the flankers should always increase crowding strength. In a series of previous studies from our laboratory and other groups, this view was shown to be over-simplistic and incomplete. In many situations, crowding can be reduced and good performance recovered by adding flanking elements or by increasing their size. This can occur when flankers ungroup from the target and group with each other. Grouping between elements is thus a key component in crowding. The neural mechanisms of crowding are unknown at the moment. Here, I used EEG to study the time course of crowding and the brain areas involved in the processing of crowding. Using event-related potentials (ERPs), I showed that crowding is a late visual process. In particular, the N1 component of ERPs (around 190 ms) was suppressed in crowding while earlier components did not correlate with crowding. The N1 suppression was caused by reduced neural activity in high-level visual areas such as the lateral occipital cortex. In a second study, I attempted to replicate the results with gabor stimuli. In a third study, I used steady-state visually evoked potentials (ssVEPs) to disentangle target and flanker processing in crowding. In this study, I showed that the target is selectively suppressed when it groups with the flankers. In addition to these three projects on crowding, I worked on time-frequency analysis that I performed on backward masking data in schizophrenia patients

Electrophysiological correlates of suppression and facilitation in crowding

In crowding, neighboring elements deteriorate performance on a target. The neural mechanisms of crowding are largely unknown. We have recently shown that the N1 component of the EEG is suppressed during crowding. It is difficult to disentangle the processing of the target and the flankers because they are presented synchronously. Here, we used a frequency-tagging technique to analyze EEG responses separately for the flankers and target. Subjects discriminated the offset direction of a vernier that was slowly increasing in size either to the left or right. Flanking lines were either longer than the vernier or of the same length. Flankers of the same length crowded more strongly than the longer flankers because the former grouped with the vernier. The vernier and the flankers flickered at two different frequencies. EEG responses to the vernier were suppressed and the responses to the flankers were enhanced during crowding (same length flankers) compared to uncrowding (longer flankers). Our results are consistent with the attentional hypothesis of crowding, where attention cannot be focused on the target and spreads to the flankers

Dissociating target from flanker processing in visual crowding by EEG frequency-tagging

In visual crowding, neighboring elements deteriorate performance on a target. The neural mechanisms of crowding are largely unknown. We have recently shown that the N1 component of the EEG is suppressed during crowding. It is difficult to disentangle the processing of the target and the flankers because they are presented synchronously. Here, we used a frequency-tagging technique to analyze EEG responses separately for the flankers and target. Subjects discriminated the offset direction of a vernier that was slowly increasing in size either to the left or right. The vernier and the flankers were either green or red and flickered at two different frequencies. Flankers of the same color as the vernier (green-green or red-red) crowded more strongly than flankers of a different color (green-red or red-green) because the former, as we propose, grouped with the vernier. EEG responses to the vernier were suppressed during crowding (same color flankers) compared to uncrowding (different color flankers). EEG responses to the flankers were slightly larger when the flankers grouped with the target compared to when they ungrouped from the target. Hence, EEG frequency tagging dissociates target and flanker processing. Our results suggest that, in crowding, the target is suppressed when it groups with the flankers while flanker-related activity increases or stays constant

Electrophysiological correlates of suppression and facilitation in crowding

In crowding, neighboring elements deteriorate performance on a target. The neural mechanisms of crowding are largely unknown. We have recently shown that the N1 component of the EEG is suppressed during crowding. It is difficult to disentangle the processing of the target and the flankers because they are presented synchronously. Here, we used a frequency-tagging technique to analyze EEG responses separately for the flankers and target. Subjects discriminated the offset direction of a vernier that was slowly increasing in size either to the left or right. Flanking lines were either longer than the vernier or of the same length. Flankers of the same length crowded more strongly than the longer flankers because the former grouped with the vernier. The vernier and the flankers flickered at two different frequencies. EEG responses to the vernier were suppressed and the responses to the flankers were enhanced during crowding (same length flankers) compared to uncrowding (longer flankers). Our results are consistent with the attentional hypothesis of crowding, where attention cannot be focused on the target and spreads to the flankers

EEG frequency tagging dissociates target and flanker processing in crowding

Flankers can strongly deteriorate performance on a target (crowding). The neural mechanisms of crowding are largely unknown. We have recently shown that the N1 component of the EEG is suppressed during crowding. Because it is difficult to disentangle the neural correlates of target and flanker processing with standard visually evoked potentials, here, we used a frequency-tagging technique to analyze EEG responses separately for flankers and target. Subjects discriminated the offset direction of a vernier that was slowly increasing in size either to the left or right. The vernier and the flankers were either green or red and flickered at two different frequencies. Flankers of the same color as the vernier (green-green or red-red) crowded more strongly than flankers of a different color (green-red or red-green) because the former, as we propose, grouped with the vernier. EEG responses to the vernier were suppressed during crowding (same color flankers) compared to uncrowding (different color flankers). EEG responses to the flankers were slightly larger when the flankers grouped with the target compared to when they ungrouped from the target. Hence, EEG frequency tagging dissociates target and flanker processing. Our results suggest that, in crowding, the target is suppressed when it groups with the flankers while flanker-related activity increases or stays constant

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

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

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