The effects of temporal waveform upon temporal darkness enhancement

Temporal darkness enhancement refers to the finding that decremental flashes of 50–140 msec appear darker than longer flash decrements. The present experiment determined the effects of temporal waveform upon darkness enhancement by obtaining darkness judgments for flashes that had abrupt onset/abrupt offset, abrupt onset/gradual offset, gradual onset/abrupt offset, and gradual onset/gradual offset. Temporal darkness enhancement was found only for flashes that had abrupt onsets regardless of offset waveform. These results are discussed in terms of the role of transients in the coding of perceived darkness.     

The age deficit on photopic counterphase flicker: contrast, spatial frequency, and luminance effects.

This study evaluated the contribution of reduced contrast sensitivity and retinal illuminance to the age-related deficit on the temporal resolution of suprathreshold spatial stimuli. The discrimination of counterphase flicker was measured in optimally refracted young and elderly observers for sinusoidal gratings of three spatial frequencies (1, 4, and 8 cycles per degree) at three contrast levels (0.11, 0.33, and 0.66). Age deficits in flicker discrimination at the two higher contrast levels and at the two lower spatial frequencies were unrelated to observer contrast sensitivity. Flicker discrimination of young observers who carried out the task through .5 ND filters to simulate a two-thirds reduction of retinal illuminance in the older eye, was similar to that of the elderly observers. An age-related reduction in retinal luminance appears to be a major determinant of the age-related spatiotemporal deficit at suprathreshold contrast levels, although neural factors may also be involved.     

Area and contrast effects upon perceptual and imagery acuity

We measured the size of visual fields within which actual and imagined circular patterns could be resolved, as the patterns varied in both area and relative contrast. As pattern area increased, imagery fields increased in size at the same rate as perceptual fields. However, as the relative contrast between parts of the patterns was reduced, perceptual fields diminished in size, while imagery fields did not. for all variations of pattern area and contrast, fields in imagery were roughly the same shape as fields in perception. A control experiment provided no support for the possible criticism that these results might be explained on the basis of subjects' expectations for the size and shape of the imagery fields, especially since control subjects falsely believed that reductions in contrast would reduce considerably the size of these fields. Although these findings suggest that images and physical objects are functionally equivalent at pattern-processing levels of the visual system, they demonstrate that images and objects are not functionally equivalent at certain levels of the visual system where information about relative contrast is processed.     

Decrease in metacontrast masking following adaptation to flicker

Selective adaptation was used to explore the characterisitcs of a metacontrast masking stimulus which contribute to its effectiveness in masking the test stimulus. Subjects adapted for 10 s to a configuration like the masking stimulus that was either continuously on or flickering. Following this they viewed a metacontrast presentation and estimated the brightness of the test stimulus. Prior adaptation to a continuously present stimulus did not appreciably affect metacontrast masking; however, masking was greatly reduced following adaptation to flickering stimuli. These results are consistent with recent models of metacontrast masking based on transient and sustained visual channels.     

The effects of flicker on eye movement control

Groups of typists with extensive experience of screen-based editing and groups of students with no such experience carried out a reading task under three conditions of illumination (50-Hz flicker, 100-Hz flicker, and steady illumination). Subjects read a sentence, which was followed by the presentation of a single stimulus word on the same line to the right-hand side of the display. The task was to decide whether or not the stimulus was present in the sentence. Subjects were free to re-inspect the sentence when making the decision. Eye movements were measured as subjects completed the task. In comparison with students, typists adopted a more cautious reading style, making more right-to-left saccades, shorter saccades, and more corrective eye movements. Flicker affected the performance of both groups of subjects in the first pass, leading to shorter saccades. In the second pass, its effect for students was to shorten the extent of large saccades made to check the presence of the stimulus word. In the group of typists, flicker led to an increase in the variability of saccade extent and a doubling in the number of small corrective saccades. The results are consistent with the view that flicker has two distinct effects on reading, both of which are potentially disruptive. The first relates to an increase in the number of prematurely triggered saccades, which are, as a result, less accurate. The second is an increase in the number of saccades perturbed in flight, which land short of their intended target. These two mechanisms may have different consequences for readers, depending on their reading style.     

The demonstration of dichoptic flicker effects by use of polaroid

A simple way of setting up dichoptic flicker with variable phase relationship is described. Sine wave flashes result, and for these, as for square waves, Sherrington's hypothesis of independent testing for intermittence of inputs to each eye is approximately borne out. Observations of the attributes of these two modes of stimulation at sub-fusion frequencies suggest that no such independence holds for computation of luminous flux over time.     

Perceptual enhancement of contrast by attention

Allocating spatial attention to a visual stimulus or increasing stimulus contrast both enhance neuronal responses. In a recent study Carrasco et al. demonstrated that attention itself changes perceived contrast. Using an elegant experimental manipulation, they showed that the contrast of an attended stimulus was perceived to be higher than when the same stimulus was unattended. This provides evidence that the enhancement of stimulus salience observed in electrophysiological studies creates an enhanced perceptual representation of attended stimuli.     

The perception of flicker: Theoretical note on the relation between brightness and darkness enhancement

A reexamination of experiments on the brightness and darkness enhancement of flickering lights suggests that the mechanism responsible for the asymmetry of the effects (darkness enhancement being a stronger effect than brightness enhancement) is independent and central to the mechanism responsible for the frequency-dependent brightness and darkness variations.     

Some temporal properties of behavioral contrast

Pigeons were rewarded on a variable time interval for pecking a translucent key illuminated with either a 45 degrees or a vertical line. The key illumination changed every 2 min during daily 1-hr sessions. When the rates of pecking were stable, reinforcement was omitted in the presence of the 45 degrees line. Responding in the presence of the vertical line increased. This increase did not disappear when responses to the 45 degrees line were once more reinforced, but when reinforcements for responses in the presence of the 45 degrees line were again omitted, responding to the vertical line increased again. After the second alternation of these two procedures, the increased responding to the vertical line appeared when responses were not reinforced in the presence of the 45 degrees line, and disappeared when reinforcement was available during both stimuli. In a second experiment, the key illumination changed between sessions only, so that 1-hr sessions of reinforcement and non-reinforcement occurred on alternate days. Responding to the vertical line still increased when responding to the 45 degrees line was not reinforced, but the increase tended to disappear during the session.     

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.     

The effect of temporal separation and prior habituation upon preconditioning

The effect of temporal separation and prior habituation was tested by measuring response transfer in a shuttle box following different initial treatments. Thirty hooded rats were split into three main groups and six sub-groups. One main group was preconditioned with concurrent light and sound stimuli, another with temporally spaced stimuli, and a control group had no preconditioning. Half of each main group was permitted to explore the apparatus before preconditioning on the supposition that this might reduce emotionality and hence reduce startle or attention responses.

It was found that concurrent stimulation produced significantly greater response transfer than in controls, whereas preconditioning with spaced stimuli did not. Prior habituation produced no significant effect on the amount of response transfer.     

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.     

The selective adaptation effects of burst-cued stops

One of the most compelling arguments that selective adaptation affects a phonetic level of processing is the demonstration that adaptation with burst-cued stimuli (which have no formant transitions) affects the perception of transition-cued stimuli (which have formant transitions but no bursts). Experiment 1 showed that adaptation with burst-cued [pi] and [ti] affects the perception of a (transition-cued) [mi-ni] test series, as well as a [bi-di] test series. Inasmuch as nasals never contain bursts, this demonstrates that the adaptation effect of burst-cued stops is not limited to those stimuli which normally contain bursts. If adaptation with burst-cued stops affects the perception of transition-cued stops at a more central, phonetic level of processing, their adaptation effect should transfer interaurally completely. Experiment 2 showed that the adaptation effect of both burst and transition-cued stops transfers interaurally only about one-third. These results suggest that the adaptation effect of burstcued stops is mediated by the fatigue of peripheral auditory detectors which are sensitive to both bursts and formant transitions.