Perceptual grouping and visual awareness
We generally have the impression that our eyes give us direct access to the outside world that we are viewing. However, the way we perceive the world is the end product of complex processing of the visual input by our brain. Where most of the input is coded into visible units or objects, some of the information that is visible to the eye does not reach visual awareness, but is instead so-called perceptually suppressed. Inhibitory cortical mechanisms are thought to play an import role in this suppression. In two of my studies we have looked at the specificity of the mechanisms responsible for perceptual suppression, focusing on two types of suppression: flash-induced perceptual fading and binocular rivalry suppression.
Flash-induced perceptual fadingWhen your eyes fixate on one specific location for a prolonged period of time, objects in the periphery of your vision may disappear or 'fade out' of your perception. This effect is commonly explained by the thought that retinal receptors and sensory neurons adapt to the unchanging retinal input and hence reduce in sensitivity. This loss of sensitivity eventually causes peripheral objects to perceptually disappear. In a variant of this effect, a white ring is briefly flashed around a peripherally presented object. As a consequence, the object immediately perceptually disappears. This so-called 'flash-induced perceptual fading' is demonstrated here on the left. If you fixate on the small plus on the top, then you will notice that (sometimes) the red disk below disappears when a white ring is flashed around it. We performed an experiment in which we presented a second element next to the central element. We found that the two disks disappear more often simultaneously when they share the same shape and color compared to when they are different. This effect of perceptual grouping indicates that the flash-induced perceptual fading effect involves a mechanism that is selective for the properties of the suppressed element.
Published as Vergeer, M. & van Lier, R. (2007). Grouping effects in flash-induced perceptual fading. Perception, 36, 1036-1042. |
Binocular rivalry suppression
In normal vision both eyes receive highly similar input that the brain can combine in a coherent representation of the outside world. However, when both eyes receive different, conflicting input, the brain may not be able to integrate these images and this conflict of input will need to be resolved. This results in competition of the two images, so-called binocular rivalry, where one of the images will become visible while the other image will be perceptually suppressed. This phenomenon has been studied extensively to answer questions regarding perceptual selection and visual awareness. In our study we were interested in the mechanisms responsible for the suppression on the unseen stimulus. We presented a colored grating at the center of the screen to one eye flanked by two perceptually suppressed colored gratings, one similar and one dissimilar to the central grating. We found that when the central grating was visible, the similar flanking grating became visible before the dissimilar grating. However, when the central grating was perceptually suppressed, the opposite effect occurred, where the similar grating became visible after the dissimilar grating. These effects indicate that selective mechanisms are involved both in enhancing and in suppressing visual information in binocular rivalry, consistent with the above mentioned findings on flash-induced perceptual fading.
Published as Vergeer, M. & van Lier, R. (2010). Feature-based activation and suppression during binocular rivalry. Vision Research, 50, 743-749.
Published as Vergeer, M. & van Lier, R. (2010). Feature-based activation and suppression during binocular rivalry. Vision Research, 50, 743-749.