Adaptive filtering in spatial vision: evidence from feature marking in plaids

Gain control in the response of human visual cortex to plaids

A recent intrinsic signal optical imaging study in tree shrew showed, surprisingly, that the population response of V1 to plaid patterns comprising grating components of equal contrast is predicted by the average of the responses to the individual components (MacEvoy SP, Tucker TR, Fitzpatrick D. Nat Neurosci 12: 637-645, 2009). This prompted us to compare responses to plaids and gratings in human visual cortex as a function of contrast and orientation. We found that the functional MRI (fMRI) blood oxygenation level-dependent (BOLD) responses of areas V1-V3 to a plaid comprising superposed grating components of equal contrast are significantly higher than the responses to a single component. Furthermore, the orientation response profile of a plaid is poorly predicted from a linear combination of the responses to its components. Together, these results indicate that the model of MacEvoy et al. (2009) cannot, without modification, account for the fMRI BOLD response to plaids in human visual cortex.

No masking between test and mask components in perceptually different depth planes

2-D cues to perceived depth organization have been used to segregate test and mask stimulus components in a discrimination task. Observers made either spatial-frequency or orientation judgments on a rectangular test component by itself or in the presence of constant rectangular masks. There were two basic masking conditions: same-plane or different-plane. In the same-plane conditions, the test components and masks are perceived as existing in the same depth plane. In the different-plane conditions, the test and mask components are perceived to exist in different depth planes. The perception of different depth planes was achieved by using perceived occlusion, which could place either component closer or further from the observer. The results suggest that when test and mask components are separated into different depth planes they no longer influence one another. This effect could be observed in either depth organization, test components in front of the masks or mask components in front of the test. These results indicate that the figure-ground organization of components is not important. Only the designation as existing in the same or different depth planes affects whether or not a mask is effective.

Disruptive coloration, crypsis and edge detection in early visual processing

Many animals use concealing markings to reduce the risk of predation. These include background pattern matching (crypsis), where the coloration matches a random sample of the background and disruptive patterns, whose effectiveness has been hypothesized to lie in breaking up the body into a series of apparently unrelated objects. We have previously established the effectiveness of disruptive coloration against avian predators, using artificial moth-like stimuli with colours designed to match natural backgrounds as perceived by birds. Here, we investigate the mechanism by which disruptive patterns reduce detectability, using a computational vision model of edge detection applied to photographs of our experimental stimuli, calibrated for bird colour vision. We show that, disruptive coloration is effective by exploiting edge detection algorithms that we use to model early visual processing. Thus, 'false' edges are detected within the body rather than at its periphery, so inhibiting successful detection of the animal's body outline.