Low-contrast Stimuli Research Articles

Visual search in cynomolgus monkeys: stimulus parameters affecting two stages of visual search.

Monkeys searched for a target pattern presented at varying eccentricities (from 11 to 57 deg, along the horizontal meridian) with varying numbers of distractor patterns and at two contrast levels. The intercepts of the RT-target eccentricity functions were increasingly elevated as the number of distractors increased within the range presented in training, suggesting that increasing the number of distractors lengthened the time required to confirm the target’s identity following each scanning phase of search. The additional increase of the intercepts when the number of distractors increased even further without benefit of training may be interpreted similarly. Decreasing the pattern contrast increased the slopes of the functions, as did increasing the number of distractors when low-contrast stimuli were presented, implying that these stimulus changes slowed the rate of scanning by increasing the number of fixations, their duration, or both. The slopes of the functions relating RTs to number of distractors (between 0 and 9) were .030 and .035 sec/distractor for the high- and low-contrast conditions, respectively

Orientational anisotropy in the human visual system: an evoked potential and psychophysical study.

Orientational anisotropy in the human visual system was investigated using electrophysiological and psychophysical techniques. Grating patterns of various orientations were presented and contrast detection thresholds were measured using a two alternative forced choice procedure. Using the same observers, visual evoked cortical potentials (VECPs) were elicited by identical gratings set at various contrasts ranging from subthreshold to high levels. Plots of contrast sensitivity versus grating orientation were derived using the amplitude of a negative wave of the VECP for low contrast data and the amplitude of a positive wave for moderate to high contrast data. These plots were compared with plots obtained during the psychophysical experiment. The results indicate that the low contrast VECP functions exhibit an orientational anisotropy that is qualitatively and quantitatively similar to that observed in the psychophysical data. The high contrast VECP functions, however, exhibit an orientational effect that is qualitatively similar but greater in magnitude than that found in the low contrast VECP or psychophysical experiments. These findings suggest that the amplitudes of VECPs elicited by low contrast stimuli are highly correlated with psychophysical detection performance, and that an additional physiological mechanism is reflected in the VECPs elicited by moderate to high contrast stimuli.

The effects of contrast and length of gratings on the visual evoked potential

Visual evoked potentials (VEP's) were obtained to grating stimuli modulated in phase at 8 Hz. The amplitude of the VEP was linearly related to the length of a centrally-fixated grating up to a critical length value equivalent to 9–16 grating cycles, for a range of spatial frequencies; thereafter amplitude first decreased then increased with increasing length. VEP amplitude was proportional to log. contrast as well as to length, and the intercept of the regression line with the contrast axis coincided with the psychophysical threshold for gratings of different lengths. More sensitive measurements with lowcontrast stimuli around threshold indicated that in this range VEP amplitude is linearly related to contrast, and there is no threshold nonlinearity. The effects of visual field location and adaptation on the VEP to gratings of different lengths and spatial frequencies were also studied. The relationship of these findings to psychophysical, single-cell and transient VEP data is discussed.

Visuo-oculomotor properties of cells in the superior colliculus of the alert cat.

Visual responses and eye movement (EM) -related activities were studied in single units of the superior colliculus (SC) of alert cats. Spontaneous EMs were encouraged by training. Throughout the SC (i.e., in intermediate and deep layers as well as in superficial layers), units were found to respond well to visual stimuli. Strong and consistent responses could be elicited by very dim, low-contrast stationary stimuli. Visual responses varied from phasic to tonic; some units responded tonically to stationary stimuli in the center of the receptive field, and phasically to peripheral stimuli. Many cells responded more vigorously to moving than to stationary stimuli, but very few responded exclusively to stimulus movement. The vast majority of cells were directionally selective. A small number of units were sensitive to the absolute, as well as the retinal, position of visual stimuli. These cells were activated by visual stimuli which fell in the receptive field only if the cat's gaze was fixated on one half of the screen. It seems that these cells must receive information about both eye position and the retinal (receptive field) position of the stimulus. It is possible that they reflect coding of target location within a head (or body) frame of reference. EM-related units were of two types: (1) about 20% of the sample responded prior to spontaneous or visually-triggered EMs, and (2) another 10% (or more) responded with, but not before, EMs. Some cells in the second group discharge almost synchronously with EMs and, thus, cannot plausibly be said to respond to the movement of images across the retina. All cells in the first group were directionally selective. The percentage of EM-related cells in the deep layers of SC is lower in cat than in monkey. Possible reasons for such differences are discussed.

Suprathreshold Binocular Interactions

A simple reaction time paradigm was used to study binocular interactions for suprathreshold grating patterns before and after 8 days of constant monocular occlusion. Prior to patching, subjects demonstrated binocular summation for near threshold contrast stimuli and binocular summation and facilitation for high contrast stimuli. After the period of occlusion, the subjects showed binocular inhibition for high contrast stimuli but still demonstrated binocular summation for low contrast stimuli. The effects of occlusion were reversible and appear to be related to an oculomotor imbalance.

Abnormalities of Central Contrast Sensitivity in Glaucoma

The detectability of foveally presented low-contrast flickering stimuli was determined for glaucoma patients, ocular hypertensives, and normal control subjects. Two types of stimuli, a homogeneous flickering field, and a counterphase flickering grating of low spatial frequency, were presented on a screen subtending 4 degrees of visual angle. The average of the contrast sensitivities to these two simuli (defined as the dynamic response coefficient) was consistently lower in glaucomatous than in normotensive eyes. The dynamic response coefficient was also below normal in half the ocular hypertensive eyes.

Proceedings: Intrauterine Candida albicans infection.

Visually responsive neurons typically exhibit a monotonic-saturating increase of firing with luminance contrast of the stimulus and are able to adapt to the current spatiotemporal context by shifting their selectivity, therefore being perfectly suited for optimal contrast encoding and discrimination. Here we report the first evidence of the existence of neurons showing selective tuning for contrast in area V4d of the behaving macaque (<i>Macaca mulatta</i>), i.e., narrow bandpass filter neurons with peak activity encompassing the whole range of visible contrasts and pronounced attenuation at contrasts higher than the peak. Crucially, we found that contrast tuning emerges after a considerable delay from stimulus onset, likely reflecting the contribution of inhibitory mechanisms. Selective tuning for luminance contrast might support multiple functions, including contrast identification and the attentive selection of low contrast stimuli.

A vector-magnitude model of contrast detection.

A model of contrast detection is proposed in which the visibility of a low-contrast stimulus is determined by a non-Euclidean magnitude of the vector composed of the responses of a large number of independent channels. Although the vector-magnitude model is quite different from the probability-summation model which has been suggested previously, the contrast thresholds and psychometric functions predicted by the two models can be in agreement within 10% for up to 105 channels in the system. Presently available experimental evidence is insufficient to establish the correctness of either model, but the computational simplicity of the vector-magnitude model makes it interesting, if only as a useful approximation to the probability-summation model.