Aperture extent and stimulus speed affect the perception of visual acceleration.

Humans are generally poor at detecting the presence of visual acceleration, but it is unclear whether the extent of a field of moving objects through an aperture affects this ability. Hypothetically, the farther a stimulus can accelerate uninterrupted by an aperture's physical constraints, the easier it should be to discern its motion profile. We varied the horizontal extent of the aperture through which continuously accelerating or decelerating random dot arrays were presented at different average speeds, and measured acceleration and deceleration detection thresholds. We also hypothesized that manipulating aperture extent at different speeds would change how observers visually pursue acceleration, which we tested in a control experiment. Results showed that, while there was no difference between the acceleration and deceleration conditions, detection was better in the larger than small aperture conditions. Regardless of aperture size, smaller acceleration and deceleration rates (relative to average speed) were needed to detect changing speed in faster than slower speed ranges. Similarly, observers tracked the stimuli to a greater extent in the larger than small apertures, and smooth pursuit was overall poorer at faster than slower speeds. Notably, the effect of speed on pursuit was greater for the larger than small aperture conditions, suggesting that the small aperture restricted pursuit. Furthermore, there was little difference in psychophysical and eye movement data between the medium and large aperture conditions within each speed range, indicating that it is easier to detect an accelerating profile when the aperture is large enough to encourage a minimum level of pursuit.

A Signal Detection Analysis of the Effects of Alcohol on Visual Contrast Sensitivity.

Numerous studies have shown that acute ethanol consumption can reduce visual contrast sensitivity when measured using traditional psychophysical methods. However, no consideration has been given to whether nonsensory factors may also play a role. The present study used both traditional techniques and signal detection procedures to evaluate this possibility. In three within-subject experiments, 41 observers (19 Females and 22 Males) were presented with faint, contrast-modulated, visual patterns and asked to say if they had seen them. In Experiment 1, contrast thresholds were measured using a randomly interleaved staircase procedure, and the data confirmed an increase in threshold following alcohol. In Experiment 2, using similar stimuli, but applying a signal detection analysis, we found that sensitivity, as reflected in d', did not change following alcohol. However, participants became more conservative in their response criterion. The third experiment was designed to allow thresholds to be measured directly with a conventional psychophysical procedure while permitting a signal detection analysis to be performed on the same data. The conventional psychophysical task showed an increase in contrast threshold, while the signal detection analysis showed no change in sensitivity, but a shift to a more conservative criterion. These data highlight the importance of taking into account alcohol's effects on cognitive processes, even when assessing basic sensory function.

Visual Acceleration Perception for Simple and Complex Motion Patterns.

Humans are able to judge whether a target is accelerating in many viewing contexts, but it is an open question how the motion pattern per se affects visual acceleration perception. We measured acceleration and deceleration detection using patterns of random dots with horizontal (simpler) or radial motion (more visually complex). The results suggest that we detect acceleration better when viewing radial optic flow than horizontal translation. However, the direction within each type of pattern has no effect on performance and observers detect acceleration and deceleration similarly within each condition. We conclude that sensitivity to the presence of acceleration is generally higher for more complex patterns, regardless of the direction within each type of pattern or the sign of acceleration.

Effects of Vertical Direction and Aperture Size on the Perception of Visual Acceleration.

It is not well understood whether the distance over which moving stimuli are visible affects our sensitivity to the presence of acceleration or our ability to track such stimuli. It is also uncertain whether our experience with gravity creates anisotropies in how we detect vertical acceleration and deceleration. To address these questions, we varied the vertical extent of the aperture through which we presented vertically accelerating and decelerating random dot arrays. We hypothesized that observers would better detect and pursue accelerating and decelerating stimuli that extend over larger than smaller distances. In Experiment 1, we tested the effects of vertical direction and aperture size on acceleration and deceleration detection accuracy. Results indicated that detection is better for downward motion and for large apertures, but there is no difference between vertical acceleration and deceleration detection. A control experiment revealed that our manipulation of vertical aperture size affects the ability to track vertical motion. Smooth pursuit is better (i.e., with higher peak velocities) for large apertures than for small apertures. Our findings suggest that the ability to detect vertical acceleration and deceleration varies as a function of the direction and vertical extent over which an observer can track the moving stimulus.

Effects of radial direction and eccentricity on acceleration perception.

Radial optic flow can elicit impressions of self-motion--vection--or of objects moving relative to the observer, but there is disagreement as to whether humans have greater sensitivity to expanding or to contracting optic flow. Although most studies agree there is an anisotropy in sensitivity to radial optic flow, it is unclear whether this asymmetry is a function of eccentricity. The issue is further complicated by the fact that few studies have examined how acceleration sensitivity is affected, even though observers and objects in the environment seldom move at a constant speed. To address these issues, we investigated the effects of direction and eccentricity on the ability to detect acceleration in radial optic flow. Our results indicate that observers are better at detecting acceleration when viewing contraction compared with expansion and that eccentricity has no effect on the ability to detect accelerating radial optic flow. Ecological interpretations are discussed.

Acute alcohol consumption impairs controlled but not automatic processes in a psychophysical pointing paradigm.

Numerous studies have investigated the effects of alcohol consumption on controlled and automatic cognitive processes. Such studies have shown that alcohol impairs performance on tasks requiring conscious, intentional control, while leaving automatic performance relatively intact. Here, we sought to extend these findings to aspects of visuomotor control by investigating the effects of alcohol in a visuomotor pointing paradigm that allowed us to separate the influence of controlled and automatic processes. Six male participants were assigned to an experimental "correction" condition in which they were instructed to point at a visual target as quickly and accurately as possible. On a small percentage of trials, the target "jumped" to a new location. On these trials, the participants' task was to amend their movement such that they pointed to the new target location. A second group of 6 participants were assigned to a "countermanding" condition, in which they were instructed to terminate their movements upon detection of target "jumps". In both the correction and countermanding conditions, participants served as their own controls, taking part in alcohol and no-alcohol conditions on separate days. Alcohol had no effect on participants' ability to correct movements "in flight", but impaired the ability to withhold such automatic corrections. Our data support the notion that alcohol selectively impairs controlled processes in the visuomotor domain.

Alcohol and lateral inhibitory interactions in human vision.

Acute alcohol consumption detrimentally affects many aspects of visual function, but few studies have addressed the neural mechanisms underlying such changes. One candidate mechanism that may be responsible for some alcohol-induced changes in visual function is lateral inhibition. Alcohol has been shown to abolish lateral inhibitory interactions in experimental preparations in which it is applied directly to the retina, but few studies have attempted to link alcohol-induced reductions in lateral inhibitory interactions with psychophysical performance in assessments of visual function dependent on this mechanism. In the present series of studies we addressed this by investigating the effects of alcohol consumption on a perceptual phenomenon mediated in part by lateral inhibition, the Hermann grid illusion. Participants estimated the contrast of the illusory blobs present at the grid intersections using a matching procedure after consumption of a drink containing alcohol or a nonalcoholic drink. The magnitude of the illusion was diminished in the alcohol condition, and this effect was consistent when we parametrically varied the contrast of the grid squares and widths of the grid bars. These data suggest that alcohol reduces lateral inhibitory interactions in human vision.

Effects of acute ethyl alcohol consumption on a psychophysical measure of lateral inhibition in human vision.

Acute consumption of ethyl alcohol affects a variety of visual functions. However, there have been few systematic attempts to investigate the neural mechanisms underlying these effects. Here, we employed the Westheimer paradigm to investigate the hypothesis that alcohol reduces lateral inhibition within human "perceptive fields", the psychophysical analogue of physiological receptive fields. Westheimer functions obtained under alcohol and no-alcohol conditions at photopic, mesopic, and scotopic levels of adaptation showed changes consistent with an alcohol-induced decrease in lateral inhibition. We conclude that this decrease in lateral inhibition may be responsible for some of the changes in visual perception that result from alcohol consumption.

Alcohol does not affect dark adaptation or luminance increment thresholds.

OBJECTIVE: It has been proposed that alcohol might induce within the retina a state akin to dark adaptation. However, the evidence to support this proposal is quite indirect. Another possibility is that alcohol might affect retinal gain control rather than sensitivity. To investigate these proposals psychophysically, we measured dark adaptation functions and increment thresholds with the increment threshold procedure in individuals with moderate blood alcohol concentrations (BACs). METHOD: Individuals were tested under both alcohol and no-alcohol conditions (BAC approximately .08%). In Experiment 1, thresholds for the detection of a parafoveal target were measured over a 25-minute period following a 3-minute bleach in six males. In Experiment 2, the cone dark adaptation function of four males was examined in more detail for a foveal target following bleaching at three different levels. In Experiment 3, we measured the thresholds of nine men for a small target superimposed on a background field that varied over 4 log units in luminance. RESULTS: We found no effects of alcohol on either the rod or the cone portion of the dark adaptation curve or on increment thresholds. CONCLUSIONS: Together, these data indicate that moderate alcohol ingestion does not affect the recovery of visual sensitivity in the dark nor does it affect gain control at the retinal level.

Alcohol slows interhemispheric transmission, increases the flash-lag effect, and prolongs masking: evidence for a slowing of neural processing and transmission.

While the alcohol literature is extensive, relatively little addresses the relationship between physiological effects and behavioural changes. Using the visual system as a model, we examined alcohol's influence on neural temporal processing as a potential means for alcohol's effects. We did this by using tasks that provided a measure of processing speed: Poffenberger paradigm, flash-lag, and backward masking. After moderate alcohol, participants showed longer interhemispheric transmission times, larger flash-lags, and prolonged masking. Our data are consistent with the view that alcohol slows neural processing, and provide support for a reduction in processing efficiency underlying alcohol-induced changes in temporal visual processing.

Effects of ethanol on anti-saccade task performance.

It has been shown that saccade-related neurons in the superior colliculus (SC) display an increased level of prestimulus activity and a higher stimulus-related burst in action potentials preceding direction errors in the anti-saccade task compared with correct anti-saccades. From this, it has been hypothesized that errors occur when the incoming visual signal in the SC passes a threshold and triggers a reflexive saccade. This hypothesis predicts that an attenuated visual signal will reduce the number of direction errors. Since ethanol has been shown to have a suppressive effect on cortical visual event-related potentials (ERPs), the purpose of the present study was to investigate the effects of moderate ethanol consumption on anti-saccade performance. Under both placebo and ethanol conditions, we recorded ERPs and measured eye movements in male subjects during the performance of an anti-saccade task in which the fixation point disappeared 200 ms prior to stimulus presentation. Compared with the placebo condition, we found in the ethanol condition: (1). a decrease in ERP amplitudes during the gap period and after stimulus presentation, (2). an increase in the latencies of anti-saccades, and (3). a decrease in the percentage of direction errors. These data demonstrate the effects of ethanol on anti-saccade task performance and provide further support for the hypothesis that errors in the anti-saccade task are triggered by the incoming visual signal.