The fovea has been proposed as the processing unit of human visual perception 3.
A crowding condition at the fovea is very difficult to create because the resolution in the fovea is very high due to the small size of the perceptive field (PF), which is the psychophysical analog to the classical neuronal receptive field in the visual cortex 20. It was suggested that in foveal vision crowding typically only occurs over very small distances (4–6 arc min) 6 or does not occur at all 19. Two relevant theories are the Attentional Resolution theory, suggesting that peripheral crowding can be reduced by cueing 14, 15 and the Configural Grouping theory (Gestalt principles) 16, which assumes that crowding occurs when a similarity exists between targets and flankers 17, 18. It begins at an early stage of visual processing it occurs when the target and flank overlap within the same neural unit 3, 8, or with pooling 12 top-down effects occur without a clear role of attention 13. According to contemporary models, crowding occurs at multiple stages in the visual hierarchy 10, 11. Visual crowding is the inability to recognize objects in clutter and sets a fundamental limit on conscious visual perception and object recognition throughout most of the visual field 3, 6 it is most pronounced in peripheral vision or in the fovea of people with strabismic amblyopia and can block an ordinarily visible stimulus from visual awareness 3, 6, 7, 8, 9. Interestingly, most crowding and pop-out studies are performed at the periphery (see below). Crowding occurs with grouping it minimizes local information, emphasizing the whole, whereas pop-out breaks up the grouping and enhances local information. Thus, two main phenomena of visual integration, namely, crowding and pop-out, seem to be antagonistic in the processing. However, when some features differ from the others, they stand-out (pop-up) and the grouping process is disrupted. If the local and basic features comply with the Gestalt principles, they can be grouped together. It is suggested that the first step in the feature integration is a preattentive stage 4, 5, which gathers information about basic features in the scene, followed by a second stage that combines the local features (grouping) of an object, to perceive the whole object it requires focused attention. Center-surround, grouping, crowding, and pop-out are considered as a context effect 3. Thus, a contextual effect modifies the appearance of local patterns when they appear between other patterns. This process is context dependent and requires integration between remote image parts obeying the Gestalt theory of perception 1, 2 it is assumed to operate at a very early (pre-attentive) stage of processing, without involving attention. In vision, local features are grouped to produce a meaningful percept. Interestingly, tagging the target with a distinct color can eliminate or reduce the crowding effect and consequently, binocular summation recovers.
We assume that the tagging effect in our experiment improved the subject's abilities (sensitivity and RT) by creating a "segmentation", i.e., a visual simulated separation between the target letter and the background. Our results are consistent with the notion that the crowding effect produces a high processing load on visual processing, which interferes with other processes such as binocular summation.
Interestingly, whereas binocular summation for a single letter was expected to be about 40%, it was significantly reduced and almost absent under crowding conditions. The crowding effect at the fovea was significantly reduced by tagging with a color target. Since a combination of binocular inputs increases the processing load, we investigated whether color tagging the target reduces crowding in the fovea of subjects with normal vision and determined how crowding is combined with binocular vision. When targets and flankers are dissimilar, the crowding is reduced (tagging). However, recently it was shown that crowding occurs in the fovea of people with an abnormal development of functional vision (amblyopia), when the stimulus is presented for a very short time. Models assume that crowding is a hallmark of the periphery but that it is almost absent in the fovea. In perceptual crowding, a letter easily recognized on its own, becomes unrecognizable if it is surrounded by other letters, an effect that confers a limit on the visual processing.
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