On the relation between intraocular straylight and visual function parameters.

Some patients with lesions in visual cortex lack conscious visual experience but, when tested, exhibit a significant ability, termed 'blindsight', to discriminate visual stimuli. Here we report two different visual displays that induce blindsight in normal observers. Using an objective measure, we show that conscious experience remains defective at presentation times much longer (1 s) than the onset of visual sensitivity (approximately 60 ms). To obtain this effect, we generate a contrast between visual textures and then conceal the contrast by superimposing 'complementary' textures. Complementarity can involve either opposite motion or binocular rivalry and orthogonal orientation. In both cases, observers locate the texture contrast reliably but do not, by either subjective or objective measures, consciously experience it. Taken together with present knowledge of the visual cortical site(s) at which opposite motion and rivalrous orientation interact, this observation bears upon the functional anatomy of conscious visual experience.

Contrast Sensitivity↗

PubMed1995

Two stages of visual processing for radial and circular motion.

As we move through our environment, the flow of the deforming images on our retinae provides rich information about ego motion and about the three-dimensional structure of the external world. Flow-fields comprise five independent components, including radial and circular motion. Here we provide psychophysical evidence for the existence of neural mechanisms in human vision that integrate motion signals along these complex trajectories. Signal-to-noise sensitivity for discriminating the direction of radial, circular and translational motion increased predictably with the number of exposed sectors, implying the existence of specialized detectors that integrate motion signals of different directions from different locations. However, contrast sensitivity for complex motion did not increase greatly with sector number, implying that the specialized detectors are preceded by a first stage of local-motion mechanisms that impose a contrast threshold. These findings fit well with recent electrophysiological evidence in monkey showing that whereas motion-sensitive neurons in primary visual cortex respond best to local translation, many neurons in the medial superior temporal cortex have large receptive fields tuned to radial, circular or spiral motion.

Contrast Sensitivity↗

PubMed1995

A contrast ratio constraint on stereo matching.

Stereopsis employs differences in the location of features in the two eyes to reconstruct their relative depths. Computational models largely ignore the contrast of these features; they simply require them to be visible in each eye and to possess the same contrast polarity. With a competitive matching paradigm we show that only features with a certain ratio of contrasts in the two eyes match. Increasing the contrast in one eye requires proportionally more contrast in the other eye for matching. This contrast relation exactly parallels the relation found for dichoptic masking, which behaves unlike any other form of contrast masking. A strange consequence of this contrast ratio constraint is that a feature may be monocularly visible, yet unmatched because the contrast ratio has not been satisfied. In this case features are seen as faint unmatched 'ghosts' near the plane of fixation. In a competitive matching situation then, stereopsis acts as if it imposes a contrast threshold on matches; features failing to exceed the threshold remain unmatched. This is a simple and biologically plausible way for the system to eliminate false matches and to reduce matching ambiguity.

Contrast Sensitivity↗