Crowding and attention in a framework of neural network model

Crowding expands and is less sensitive to target-flanker differences during a shift of visual attention

Target-flanker similarity and critical spacing control visual crowding when attention is pre-allocated, but these have not been studied when attention shifts. Flanked target Gabors appeared 8° left and right of central fixation throughout each 1.5 s trial. Subjects reported target Gabor tilt. In Expt. 1, target blinks increased accuracy, and flanker blinks decreased it, but only when attention shifted left or right from a central RSVP cue, hardly before it, indicating an exogenous/endogenous synergy. Whether parallel or orthogonal, flankers of the same wavelength as the target crowded substantially. Parallel half-wavelength flankers also crowded, but orthogonal half-wavelength ones did not. In Expt. 2, crowding when attention shifts was the same for targets and flankers within Bouma's bound (2.5° apart) as outside it (5.0° apart.) In Expt. 3, Bouma's bound was restored when attention was focused continuously on the target. We conclude that crowding temporarily expands and becomes less discriminative when attention shifts.

Numerosity depends on normalized contrast energy: Review and square-root law model

The perceived numerosity of many randomly-located items of fixed contrast depends on the integrated contrast energy (CE) of the display. We show here that a model based on √(CE), normalized by contrast amplitude, can fit numerosity judgment data in various tasks and over a wide range of numerosities. The model shows that judged numerosity increases linearly with √(N), where N is the number of displayed items above the subitization range, and can explain: 1) the general underestimation in absolute judgement of numerosity; 2) the contrast independence (constancy) of numerosity judgment in segregated displays, i.e., judged numerosities are not affected by item contrast; 2) a contrast-dependent illusion where the numerosity of higher-contrast items is further underestimated when intermingled with lower-contrast items; and 3) both the threshold and sensitivity of numerosity discrimination between displays of N and M items. The nearly perfect fit of numerosity judgment data by a square-root law over a wide range of numerosities, including the range typically described by Weber's law, but excluding subitization, suggests that normalized contrast energy might be the prevailing sensory code underlying numerosity perception.

Could simplified stimuli change how the brain performs visual search tasks? A deep neural network study

Visual search is a complex behavior influenced by many factors. To control for these factors, many studies use highly simplified stimuli. However, the statistics of these stimuli are very different from the statistics of the natural images that the human visual system is optimized by evolution and experience to perceive. Could this difference change search behavior? If so, simplified stimuli may contribute to effects typically attributed to cognitive processes, such as selective attention. Here we use deep neural networks to test how optimizing models for the statistics of one distribution of images constrains performance on a task using images from a different distribution. We train four deep neural network architectures on one of three source datasets-natural images, faces, and x-ray images-and then adapt them to a visual search task using simplified stimuli. This adaptation produces models that exhibit performance limitations similar to humans, whereas models trained on the search task alone exhibit no such limitations. However, we also find that deep neural networks trained to classify natural images exhibit similar limitations when adapted to a search task that uses a different set of natural images. Therefore, the distribution of data alone cannot explain this effect. We discuss how future work might integrate an optimization-based approach into existing models of visual search behavior.

On the relation between crowding and ensemble perception: Examining the role of attention

Ensemble perception of a crowd of stimuli is very accurate, even when individual stimuli are invisible due to crowding. The ability of high-precision ensemble perception can be an evolved compensatory mechanism for the limited attentional resolution caused by crowding. Thus the relationship of crowding and ensemble coding is like two sides of the same coin wherein attention may play a critical factor for their coexistence. The present study investigated whether crowding and ensemble coding were similarly modulated by attention, which can promote our understanding of their relation. Experiment 1 showed that diverting attention away from the target harmed the performance in both crowding and ensemble perception tasks regardless of stimulus density, but crowding was more severely harmed. Experiment 2 showed that directing attention toward the target bar enhanced the performance of crowding regardless of stimulus density. Ensemble perception of high-density bars was also enhanced but to a lesser extent, while ensemble perception of low-density bars was harmed. Together, our results indicate that crowding is strongly modulated by attention, whereas ensemble perception is only moderately modulated by attention, which conforms to the adaptive view.