Knowledge alters visual contrast sensitivity

Research has shown that the visual system’s sensitivity to variations in luminance (visual contrast) within a particular area of the retina is affected in a bottom-up fashion by the ambient contrast levels in nearby regions. Specifically, changes in the ambient contrast in areas surrounding the target area alter the sensitivity to visual contrast within the target area. More recent research has shown that paying attention to the target or target area modulates contrast sensitivity, suggesting a top-down influence over contrast sensitivity that is mediated by attention. Here we report another form of top-down influence over contrast sensitivity that is unlikely to be mediated by attention. In particular, we show that knowledge and/or expectations about the levels of visual contrast that may appear in upcoming targets also affect how sensitive the observer is to the contrast in the target. This sort of knowledge-driven, top-down contrast sensitivity control could be used to preset the visual system’s contrast sensitivity to maximize discriminability and to protect contrast-sensitive processes from a contrast overload. Overall, our results suggest that existing models of contrast sensitivity might benefit from the inclusion of top-down control mechanisms.     

Cortical dynamics of visual persistence and temporal integration

In a temporal integration experiment, subjects must integrate two visual stimuli, presented at separate times, to perform an identification task. Many researchers have assumed that the persistence of the leading stimulus determines the ability to integrate the leading and trailing stimuli. However, recent studies of temporal integration have challenged that hypothesis by demonstrating that several theories of persistence are incompatible with data on temporal integration. This paper shows that an account of visual persistence given by a neural network model of preattentive vision, called the boundary contour system, explains data on temporal integration. Computer simulations of the model explain why temporal integration becomes more difficult when the display elements are separated by longer interstimulus intervals or are of longer duration or of higher luminance, or are spatially closer together. The model suggests that different mechanisms underlie the inverse duration effects for leading and for trailing elements. The model further predicts interactions of spatial separation, duration, and luminance of the trailing display.     

Cortical involvement in visual scan in the monkey

Monkeys performed a visual search task for food reward. Green square targets were embedded in 3 × 3 arrays of colored forms. In distinct-feature arrays, all nontarget stimuli were red diamonds, whereas in shared-feature arrays, some nontarget stimuli shared either form (red square) or color (green diamond) with the target. Reaction time was slower for shared-feature arrays and linearly related to the number of shared-feature distractors. Errors were more common in shared-feature arrays, and shared-feature distractors were mistaken for targets more frequently than distinct-feature distractors. Event-related local field potentials were recorded from implanted transcortical electrodes. Significant task-related differences were obtained from association cortex, but not from projection cortex. Results are discussed in terms of the relative contribution of inferotem-poral,     

A study of visual temporal size contrast

Stimuli exposed successively with an interpolated larger or smaller stimulus (IS) are subject to size-contrast effects to a highly significant degree. While stimuli nearer IS are, in general, more affected than stimuli farther away, there is some indication of paradoxical distance effect (PDE): Stimuli somewhat farther from IS are more affected than those nearest IS. Intermediate IS has a splitting effect in that stimuli above are judged larger, and those below are judged smaller, thus verifying the results with IS below and above the series stimuli. The relation of these results to figural aftereffects is stressed.     

Model of visual adaptation and contrast detection

A three-component model of spatial vision is proposed, consisting of (1) a feedback stage, (2) a feedforward stage, (3) a threshold detector. The components correspond to physiological processes; in particular, the feedforward control signal corresponds to the “surround’s” signal in the receptive fields of retinal ganglion cells. The model makes appropriate qualitative predictions of: (l)a square-root law (Δl ∞ l^1/2) for detection at low luminances, (2) a Weber law (Δl ∞ l) at high luminances, (3) additivity of threshold masking effects at high background luminances, (4) receptive fields that, in the dark, consist only of an excitatory center and that, in the light, also contain inhibitory surrounds, (5) the variation of spatial characteristics of receptive fields depending on the temporal characteristic of the test stimulus used to measure them, (6) the subjective appearance of Mach bands, (7) sine-wave contrast-threshold transfer functions, (8) the frequent failure of disk-detection experiments to demonstrate inhibitory surrounds, and (9) various second-order threshold effects, such as reduced spatial integration for long-duration stimuli, reduced temporal integration for large-area stimuli, and the increased effect of background luminance on the detection of large-area stimuli. Predictions are improved by assuming there exist various sizes of receptive fields that determine thresholds jointly.     

Emotional modulation of visual remapping of touch

The perception of tactile stimuli on the face is modulated if subjects concurrently observe a face being touched; this effect is termed "visual remapping of touch" or the VRT effect. Given the high social value of this mechanism, we investigated whether it might be modulated by specific key information processed in face-to-face interactions: facial emotional expression. In two separate experiments, participants received tactile stimuli, near the perceptual threshold, either on their right, left, or both cheeks. Concurrently, they watched several blocks of movies depicting a face with a neutral, happy, or fearful expression that was touched or just approached by human fingers (Experiment 1). Participants were asked to distinguish between unilateral and bilateral felt tactile stimulation. Tactile perception was enhanced when viewing touch toward a fearful face compared with viewing touch toward the other two expressions. In order to test whether this result can be generalized to other negative emotions or whether it is a fear-specific effect, we ran a second experiment, where participants watched movies of faces-touched or approached by fingers-with either a fearful or an angry expression (Experiment 2). In line with the first experiment, tactile perception was enhanced when subjects viewed touch toward a fearful face and not toward an angry face. Results of the present experiments are interpreted in light of different mechanisms underlying different emotions recognition, with a specific involvement of the somatosensory system when viewing a fearful expression and a resulting fear-specific modulation of the VRT effect. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

The effect of language on visual contrast sensitivity

Embodied cognition and perceptual symbol theories assume that higher cognition interacts with and is grounded in perception and action. Recent experiments have shown that language processing interacts with perceptual processing in various ways, indicating that linguistic representations have a strong perceptual character. In the present study, we have used signal detection theory to investigate whether the comprehension of written sentences, implying either horizontal or vertical orientation, could improve the participants' visual sensitivity for discriminating between horizontal or vertical square-wave gratings and noise. We tested this prediction by conducting one main and one follow-up experiment. Our results indicate that language can, indeed, affect perception at such a low level of the visual process and thus provide further support for the embodied theories of cognition.     

Attentional modulation in perception of visual motion events.

Identical visual targets moving across each other with equal and constant speed can be perceived either to bounce off or to stream through each other. This bistable motion perception has been studied mostly in the context of motion integration. Since the perception of most ambiguous motion is affected by attention, there is the possibility of attentional modulation occurring in this case as well. We investigated whether distraction of attention from the moving targets would alter the relative frequency of each percept. During the observation of the streaming/bouncing motion event in the peripheral visual field, visual attention was disrupted by an abrupt presentation of a visual distractor at various timings and locations (experiment 1; exogenous distraction of attention) or by the demand of an additional discrimination task (experiments 2 and 3; endogenous distraction of attention). Both types of distractions of attention increased the frequency of the bouncing percept and decreased that of the streaming percept. These results suggest that attention may facilitate the perception of object motion as continuing in the same direction as in the past.     

The modulation of visual orienting reflexes across the lifespan

The development of reflexive and voluntary shifts of visual attention, as well as relations between the two forms of shifting, were examined in three groups of children (5, 7, and 9 years old), one group of young adults (24 years old), and two groups of senior adults (young seniors with an average age of 69 years, and old seniors with an average age of 81 years). The task entailed response to the detection of a target (black dot) in one of four possible locations in the visual field. Relations between reflexive and voluntary shifts of attention were gauged by the degree to which flash and arrow facilitation and inhibition were observed in response to the presentation of both arrow and flash cues together in one trial. All age groups oriented reflexively in response to a flash cue and utilized the arrow cue to orient attention strategically. When flash and arrow cues were presented in quick succession and thereby competed for attention, the youngest children and oldest seniors were least efficient and flexible in their approach to the orienting task as they had difficulty modulating visual reflexes.     

The effect of spatial frequency and contrast on visual persistence

The visual persistence of sinusoidal gratings of varying spatial frequency and contrast was measured. It was found that the persistence of low-contrast gratings was longer than that of high-contrast stimuli for all spatial frequencies investigated. At higher contrast levels of 1 and 4 cycles deg-1 gratings, a tendency for persistence to be independent of contrast was observed. For 12 cycles deg-1 gratings, however, persistence continued to decrease with increasing contrast. These results are compared with recently published data on other temporal responses, and are discussed in terms of the different properties of sustained and transient channels.     

Cortical dynamics of visual motion perception: short-range and long-range apparent motion.

This article describes further evidence for a new neural network theory of biological motion perception. The theory clarifies why parallel streams V1----V2, V1----MT, and V1----V2----MT exist for static form and motion form processing among the areas V1, V2, and MT of visual cortex. The theory suggests that the static form system (Static BCS) generates emergent boundary segmentations whose outputs are insensitive to direction-of-contrast and to direction-of-motion, whereas the motion form system (Motion BCS) generates emergent boundary segmentations whose outputs are insensitive to direction-of-contrast but sensitive to direction-of-motion. The theory is used to explain classical and recent data about short-range and long-range apparent motion percepts that have not yet been explained by alternative models. These data include beta motion, split motion, gamma motion and reverse-contrast gamma motion, delta motion, and visual inertia. Also included are the transition from group motion to element motion in response to a Ternus display as the interstimulus interval (ISI) decreases; group motion in response to a reverse-contrast Ternus display even at short ISIs; speed-up of motion velocity as interflash distance increases or flash duration decreases; dependence of the transition from element motion to group motion on stimulus duration and size, various classical dependencies between flash duration, spatial separation, ISI, and motion threshold known as Korte's laws; dependence of motion strength on stimulus orientation and spatial frequency; short-range and long-range form-color interactions; and binocular interactions of flashes to different eyes.     

Visual adaptation to a spatial contrast enhances visual evoked potentials

The effects of adaptation to the visual contrast of a counterphase grating were studied with visual evoked potentials (VEPs). The spatial frequency of the grating was 4 cpd, and its temporal frequency was 4 or 12 Hz. Steady-state VEPs were analyzed through an FFT (fast Fourier transform) algorithm. In the first experiment, contrast thresholds rose strongly just after the end of adaptation and declined regularly over time. The VEPs recorded in the medio-occipital lead showed an initial decrease in amplitude after adaptation, followed by an enhancement well above the preadaptation level and then a return to that level. The paradoxical enhancement of the VEP was found at both high and low contrast in both the medio-occipital lead and the right temporal lead of a right-handed subject. The left temporal leads showed a VEP enhancement at high contrast and a decline at a low value.     

The influence of spatial pattern on visual short-term memory for contrast

Several psychophysical studies of visual short-term memory (VSTM) have shown high-fidelity storage capacity for many properties of visual stimuli. On judgments of the spatial frequency of gratings, for example, discrimination performance does not decrease significantly, even for memory intervals of up to 30 s. For other properties, such as stimulus orientation and contrast, however, such “perfect storage” behavior is not found, although the reasons for this difference remain unresolved. Here, we report two experiments in which we investigated the nature of the representation of stimulus contrast in VSTM using spatially complex, two-dimensional random-noise stimuli. We addressed whether information about contrast per se is retained during the memory interval by using a test stimulus with the same spatial structure but either the same or the opposite local contrast polarity, with respect to the comparison (i.e., remembered) stimulus. We found that discrimination thresholds got steadily worse with increasing duration of the memory interval. Furthermore, performance was better when the test and comparison stimuli had the same local contrast polarity than when they were contrast-reversed. Finally, when a noise mask was introduced during the memory interval, its disruptive effect was maximal when the spatial configuration of its constituent elements was uncorrelated with those of the comparison and test stimuli. These results suggest that VSTM for contrast is closely tied to the spatial configuration of stimuli and is not transformed into a more abstract representation.