Contrast (vision)

About: Contrast (vision) is a research topic. Over the lifetime, 10379 publications have been published within this topic receiving 221480 citations.

Spatial contrast sensitivity and grating acuity of barn owls.

Abstract: The eyes of barn owls (Tyto alba pratincola) display very little aberrations, and have thus excellent optical quality. In a series of behavioral experiments, we tested whether this presumably beneficial feature is also reflected at a perceptual level in this species. As fundamental indicators for visual performance, the spatial contrast sensitivity function (CSF) and grating acuity were measured in two barn owls with psychophysical techniques. Stimulus luminance was 2.7 cd/m(2). The CSF found here renders the typical band-limited, inverted U-shaped function, with a low maximum contrast sensitivity of 8-19 at a spatial frequency of 1 cyc/deg. Grating acuity was estimated from the CSF high frequency cut-off and yielded 3.0-3.7 cyc/deg. In a second experiment, in which contrast was held constant and spatial frequency was varied, grating acuity was measured directly (2.6-4.0 cyc/deg). These results put barn owls at the very low end of the visual acuity spectrum of birds, and demonstrate that visual resolution and sensitivity cannot be predicted by optical considerations alone.

Physiological mechanisms underlying psychophysical sensitivity to combined luminance and chromatic modulation

Abstract: If psychophysical detection thresholds are plotted in a middle-wavelength-sensitive (M) and long-wavelength-sensitive (L) cone coordinate system, the shape of the contour can be used to infer underlying detection mechanisms. We measured responses of macaque ganglion cells to combine chromatic and luminance modulation and expressed our results in such an M,L-cone space. Our aim was to test whether, with the use of this space, readily separable luminance and chromatic psychophysical mechanisms might be expected from physiological data. For parvocellular pathway cells, detection contours approximated elongated ellipses with maximum responsivity to chromatic modulation. The degree of elongation decreased as temporal frequency increased. Responses could be well described by linear subtraction of M- and L-cone signals, with a phase delay of 1–3 deg/Hz. For cells of the magnocellular pathway, detection contours were more complex. Orientation was variable between cells and temporal frequency dependent, and a frequency-doubled component was evoked by chromatic modulation. In relation to psychophysical detection thresholds plotted in such a space, the properties of parvocellular-pathway cells were sufficiently linear and homogeneous to make it plausible that this pathway might form the substrate for a linear chromatic mechanism. The properties of magnocellular-pathway cells, however, indicate that , insofar as a psychophysical luminance mechanism is based on their activity, its signature in the M,L-cone contrast space would be more difficult to identify.

Bayesian Model of Dynamic Image Stabilization in the Visual System

Abstract: Humans can resolve the fine details of visual stimuli although the image projected on the retina is constantly drifting relative to the photoreceptor array. Here we demonstrate that the brain must take this drift into account when performing high acuity visual tasks. Further, we propose a decoding strategy for interpreting the spikes emitted by the retina, which takes into account the ambiguity caused by retinal noise and the unknown trajectory of the projected image on the retina. A main difficulty, addressed in our proposal, is the exponentially large number of possible stimuli, which renders the ideal Bayesian solution to the problem computationally intractable. In contrast, the strategy that we propose suggests a realistic implementation in the visual cortex. The implementation involves two populations of cells, one that tracks the position of the image and another that represents a stabilized estimate of the image itself. Spikes from the retina are dynamically routed to the two populations and are interpreted in a probabilistic manner. We consider the architecture of neural circuitry that could implement this strategy and its performance under measured statistics of human fixational eye motion. A salient prediction is that in high acuity tasks, fixed features within the visual scene are beneficial because they provide information about the drifting position of the image. Therefore, complete elimination of peripheral features in the visual scene should degrade performance on high acuity tasks involving very small stimuli.