Abstract
That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects, has been known for well over a century; however, the processes involved in combining the images from the two eyes are still not fully understood. Importantly, in natural vision, objects are rarely isolated but appear in context. It is well known that nearby contours can either facilitate or suppress detection, depending on their distance from the target and the global configuration. Here we report that at close distances collinear (but not orthogonal) flanking contours suppress detection more under binocular compared to monocular viewing, thus completely abolishing the binocular advantage, both at threshold and suprathreshold levels. In contrast, more distant flankers facilitate both monocular and binocular detection, preserving a binocular advantage up to about four times the detection threshold. Our results for monocular and binocular viewing, for threshold contrast discrimination without nearby flankers, can be explained by a gain control model with uncertainty and internal multiplicative noise adding additional constraints on detection. However, in context with nearby flankers, both contrast detection threshold and suprathreshold contrast appearance matching require the addition of both target-to-target and flank-to-target interactions occurring before the site of binocular combination. To test an alternative model, in which the interactions occur after the site of binocular combination, we performed a dichoptic contrast matching experiment, with the target presented to one eye, and the flanks to the other eye. The two models make very different predictions for abutting flanks under dichoptic conditions. Interactions after the combination site predict that the perceived contrast of the flanked target will be strongly suppressed, while interactions before the site predict the perceived contrast will be more or less veridical. The data are consistent with the latter model, strongly suggesting that the interactions take place before the site of binocular combination.
Highlights
That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects, has been known for well over a century and is u biquitous[1,2]; the processes involved in combining the images from the two eyes are still not fully understood
While the advantage of binocular over monocular vision for detecting isolated low contrast or luminance stimuli has been well documented for over a century, our results show that nearby collinear flanking contours completely abolish the binocular advantage, while surprisingly, more distant flanking contours facilitate both monocular and binocular sensitivity
The failure of binocular summation is largely due to the strong interocular, but weak or absent monocular suppression for abutting stimuli
Summary
That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects, has been known for well over a century; the processes involved in combining the images from the two eyes are still not fully understood. That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects (like Alcor), has been known for well over a century and is u biquitous[1,2]; the processes involved in combining the images from the two eyes are still not fully understood. As Blake and Wilson[7] note in their excellent review, “The evidence moving the field in those directions has come largely from experiments that have measured contrast summation at threshold and suprathreshold levels and contrast masking using dichoptically presented grating patterns...” Most of these studies have involved isolated stimuli; it is well known that nearby contours can modulate detection, depending on their distance from the target and the global configuration. They concluded “that flankers modulate outputs from spatial filters in the monocular processing stage of contrast gain control”
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