Visual crowding in driving.

Visual crowding-the deleterious influence of nearby objects on object recognition-is considered to be a major bottleneck for object recognition in cluttered environments. Although crowding has been studied for decades with static and artificial stimuli, it is still unclear how crowding operates when viewing natural dynamic scenes in real-life situations. For example, driving is a frequent and potentially fatal real-life situation where crowding may play a critical role. In order to investigate the role of crowding in this kind of situation, we presented observers with naturalistic driving videos and recorded their eye movements while they performed a simulated driving task. We found that the saccade localization on pedestrians was impacted by visual clutter, in a manner consistent with the diagnostic criteria of crowding (Bouma's rule of thumb, flanker similarity tuning, and the radial-tangential anisotropy). In order to further confirm that altered saccadic localization is a behavioral consequence of crowding, we also showed that crowding occurs in the recognition of cluttered pedestrians in a more conventional crowding paradigm. We asked participants to discriminate the gender of pedestrians in static video frames and found that the altered saccadic localization correlated with the degree of crowding of the saccade targets. Taken together, our results provide strong evidence that crowding impacts both recognition and goal-directed actions in natural driving situations.

Visualizing fMRI BOLD responses to diverse naturalistic scenes using retinotopic projection.

To view brain activity in register with visual stimuli, a technique here referred to as "retinotopic projection," which translates functional measurements into retinotopic space, is employed. Retinotopic projection is here first applied to a previously acquired fMRI dataset in which a large set of grayscale photos of real scenes were presented to three subjects. A simple model of local contrast integration accounts for much of the data in early visual areas (V1 and V2). However, consistent discrepancies were discovered: Human faces tend to evoke stronger responses relative to other scene elements than predicted by the model, whereas periodic patterns evoke weaker responses than predicted by the model. Next, in new fMRI experiments, three subjects directed attention toward various elements of naturalistic scenes (Vermeer paintings). Retinotopic projection applied to these data showed that attending to an object increased activation in cortex corresponding to the location of that object. Together the results suggest that even during passive viewing, the visual system differentially processes natural scenes in a manner consistent with deployment of visual attention to salient elements.

Masking interrupts figure-ground signals in V1.

In a backward masking paradigm, a target stimulus is rapidly (<100 msec) followed by a second stimulus. This typically results in a dramatic decrease in the visibility of the target stimulus. It has been shown that masking reduces responses in V1. It is not known, however, which process in V1 is affected by the mask. In the past, we have shown that in V1, modulations of neural activity that are specifically related to figure-ground segregation can be recorded. Here, we recorded from awake macaque monkeys, engaged in a task where they had to detect figures from background in a pattern backward masking paradigm. We show that the V1 figure-ground signals are selectively and fully suppressed at target-mask intervals that psychophysically result in the target being invisible. Initial response transients, signalling the features that make up the scene, are not affected. As figure-ground modulations depend on feedback from extrastriate areas, these results suggest that masking selectively interrupts the recurrent interactions between V1 and higher visual areas.