Visual masking by flickering surrounds

Observers detected drifting sine-wave gratings presented in a circular 3 dia test field which was surrounded by a 3.25 degrees wide annulus. Forced choice contrast thresholds were measured with surrounds consisting of a steady field of light or uniform sinusoidal flicker. The flickering surround raised detection thresholds only for gratings with spatial frequencies below 2-4 c/deg. Variations on the basic experiment revealed that: (1) low spatial frequency gratings drifting through the surround masked detection of uniform flicker presented to the center; (2) masking did not depend greatly on the drift rate of the test grating but could not be obtained with stationary targets; (3) flicker restricted to either the top or side borders of the test field was a sufficient condition to produce masking; (4) the size of the masking effect decreased with center-surround separation. These results suggest a destructive interaction between transient mechanisms subserving neighboring regions of the visual field.     

Two-criterion threshold techniques: Evidence for separate spatial and temporal mechanisms?

Contrast thresholds were determined for counterphase flickering and drifting spatial gratings using pattern and flicker/motion criteria. In contrast to previous reports, the two criteria yielded contrast sensitivity functions (CSF) of similar form in the counterphase condition. However, moving gratings yielded CSF's of different form for the two criteria. These differences are probably due to eye movements.     

Mechanisms isolated by frequency-doubling technology perimetry

PURPOSE: The frequency-doubling (FD) phenomenon describes the increase in apparent spatial frequency occurring when low-spatial-frequency sine wave gratings undergo rapid counterphase flicker. It is unclear whether the visual mechanisms isolated when pattern appearance is used as a threshold criterion are the same as when a simple detection criterion (as in FD perimetry) is used. It is also unclear whether the FD stimulus isolates mechanisms that differ from those isolated by spatially uniform flicker. In the current study, adaptation and spatiotemporal tuning functions were determined, using the FD stimulus, uniform flicker, and static (nonflickering) grating targets, to establish whether distinct mechanisms are isolated by the FD stimulus. METHODS: Spatiotemporal tuning functions were determined in six observers, using an FD stimulus under conditions of detection and resolution and using spatially uniform flickering stimuli and static grating stimuli. The effect of light adaptation on these stimulus classes was also assessed. All stimuli were 10 degrees-wide squares. RESULTS: Spatiotemporal tuning functions and adaptation characteristics were identical for both the FD detection and resolution paradigms. Spatially uniform flicker gave indistinguishable tuning functions and adaptation characteristics to the FD stimulus at 25 Hz and higher, but differed below this frequency. Static grating stimuli differed from FD stimuli in both tuning functions and adaptation characteristics. CONCLUSIONS: Absolute detection of the FD stimulus involves mechanisms that are indistinguishable from those involved when a criterion based on spatial form (i.e., resolution of a pattern) is used, indicating that a simple detection criterion can be used in FD perimetry. The FD stimulus isolates similar mechanisms to spatially uniform flickering stimuli at high temporal frequencies.     

Sustained and transient mechanisms in human vision: Temporal and spatial properties

In two experiments, properties ofsustained andtransient mechanisms were studied psychophysically. In the first contrast thresholds were measured for 6sinewave gratings ranging from 0.375 to 12.0c/deg at 10 durations ranging from 18 to 3000msec. Thresholds were measured in the presence and absence of high contrast 20msec gratings whichmasked the onsets and offsets of the signals. At 1.5 c/deg and above, the unmasked thresholds decreased as power functions of duration in two stages, reaching an asymptotic level near 1000msec. Below l.5c/deg, the unmasked threshold became independent of duration beyond 100 msec. At all frequencies, the masked thresholds decreased as power functions of duration to 1000msec or more, but the curves for 0.375 and 0.75c/deg never reached the unmasked asymptotic level. In the second experiment,spatial frequency bandwidths were obtained for sinewave gratings ranging from 0.375 to 12.0c/deg. by measuring threshold elevation as a function of the spatial frequency of masking gratings. At 3.0, 6.0 and 12.0 c/deg, the bandwidth functions peaked at the signal frequencies and showed medium bandwidth frequency selectivity. Below 1.5 c/deg, the bandwidth functions exhibited broader spatial frequency tuning, were of higher magnitude, and there was a shift in peak masking to frequencies near 1.0–1.5 c/deg which were above the signal frequencies. The results of both experiments are discussed in terms of the sustained/transient dichotomy.     

Temporal and Spatial Frequency Tuning of the Flicker Motion Aftereffect

The motion aftereffect (MAE) was used to study the temporal and spatial frequency selectivity of the visual system at supra-threshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the MAE induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest MAE was found with slowly counterphasing test gratings (at approximately 0.125–0.25 Hz). The largest MAEs were also found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 c/deg). These data suggest that MAEs are dominated by a single, low-pass temporal frequency mechanism and by a series of band-pass spatial frequency mechanisms. The band-pass spatial frequency tuning even at low spatial frequencies suggests that the “lowest adaptable channel” concept Cameron et al. (1992). Vision Research, 32, 561–568) may be an artifact of disadvantaged low spatial frequencies using static test patterns. Copyright © 1996 Elsevier Science Ltd.     

Macaque vision after magnocellular lateral geniculate lesions

Ibotenic-acid lesions of the magnocellular portion of the macaque lateral geniculate nucleus were used to examine the role of the M-cell pathway in spatio-temporal contrast sensitivity. A lesion was placed in layer 1 of the lateral geniculate of each of two monkeys. Physiological mapping in one animal demonstrated that the visual-field locus of the lesion was on the horizontal meridian, approximately 6 deg in the temporal field. Visual thresholds were tested monocularly in the contralateral eye, and fixation locus was monitored with a scleral search coil to control the retinal location of the test target. Three threshold measures were clearly disrupted by the magnocellular lesions. Contrast sensitivity for a 1 cycle/deg grating that drifted at 10 Hz was reduced from about twofold greater than, to about the same as, that for 10-Hz counterphase modulated gratings. Sensitivity for a very low spatial frequency (Gaussian blob), 10-Hz flickering stimulus was reduced so severely that no threshold could be measured. In addition, flicker resolution was greatly reduced at lower modulation depths (0.22), but not at higher depths (1.0). Two of the measured thresholds were unaffected by the lesions. Contrast sensitivity for 2 cycle/deg stationary gratings remained intact, and little or no effect on sensitivity was found for 1 cycle/deg, 10-Hz counterphase modulated gratings. Together, these results suggest that the magnocellular pathway makes little contribution to visual sensitivity at low to moderate temporal frequencies. On the other hand, some contribution to detection sensitivity is evident at lower spatial and high temporal frequencies, especially for drifting stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

Frequency dependence in scotopic flicker sensitivity

Sensitivity to rod-mediated (scotopic) flicker was parametrically studied in the parafoveal retina of human observers. Confirming prior studies, the present results show that sensitivity to scotopic flicker has many similarities to that at photopic levels. Specifically, our results show that the frequency response function for scotopic flicker is characterized by both low- and high-frequency cutoffs and that sensitivity to low frequencies is described by Weber's law. Overall, however, scotopic flicker sensitivity is characterized by higher increment thresholds and lower frequency tuning than photopic flicker. The influences of spatial factors and the prevailing level of illuminance on sensitivity is sufficiently different for relatively low (less than 3 Hz) and relatively high (greater than 5 Hz) temporal frequencies to suggest that they may be mediated by different channels. This possibility is also suggested by selective adaptation experiments. These show that adaptation to flicker frequencies of 3, 5, and 7 Hz have a similar influence on sensitivity to subsequent flicker which is different from the influence on 1 Hz flicker adaptation. Results are compared with prior evidence for channeling within both the scotopic and photopic visual systems.     

Spatial and temporal tuning of motion in depth

We used the Pulfrich effect to investigate perception of motion in depth. Independent manipulation of spatial and temporal frequency content in stereoscopic motion stimuli revealed the tuning characteristics of motion-in-depth perception. Sensitivity to interocular phase difference between sinusoidally oscillating sine-wave gratings was measured in four observers who judged direction of motion in depth. Discrimination thresholds in terms of interocular phase difference were determined to investigate spatial and temporal tuning characteristics of a system that is based on interocular phase difference, interocular delay, binocular disparity and velocity difference. Temporal frequency tuning of interocular phase difference thresholds was band pass and relatively dependent on spatial frequency variation. These results together with evidence from two control experiments support the idea that sensitivity to direction of motion in depth is limited by a stereo-motion system that monitors binocular horizontal disparity and motion rather than interocular phase difference, interocular delay, or interocular velocity difference.     

Flicker masking in spatial vision

The effects of uniform field masking at a 6 Hz temporal rate were studied on visual reaction times, response persistence and contrast sensitivity for sinusoidal gratings. The differences in performance between unmasked and masked conditions allowed an analysis of the temporal response properties of spatial frequency channels in human vision. The results of the experiments showed that (a) the temporal sensitivity of high and low spatial frequency channels differs markedly, so that rapid flicker selectively masks low spatial frequencies with little effect on high spatial frequencies; (b) the shift in the distribution of reaction times from a biphasic to a monotonie distribution during masking may be explained by assuming a dichotomy in the response characteristics of transient and sustained mechanisms; (c) transient channels' reponse persistence is shorter than that of sustained channels, and (d) whereas sustained channels preferably respond to the pattern component of gratings, transient channels preferably respond to the transient (on, off) or flicker component; (e) there appears to be considerable overlap in the spatial frequency response characteristics of sustained and transient channels.     

Velocity specificity of the flicker to pattern sensitivity ratio in human vision

Flicker and pattern sensitivities were compared for sine-wave gratings drifting at different rates. Relative sensitivity was found to be constant at each velocity, irrespective of spatial and temporal frequencies, and to vary monotonically with velocity. At velocities below 1 deg/sec, pattern sensitivity exceeded flicker sensitivity. At higher velocities the reverse was true.     

Contrast detection and direction discrimination of drifting gratings

Observers performed simple detection and left right discrimination of drifting sinusoidal gratings. Ratio of detection to discrimination sensitivities was measured under variations in several experimental parameters. In the first experiment, it was found that combinations of spatial and temporal frequency which resulted in the same velocity produced similar detection discrimination ratios. At an exposure duration of 800 msec, the relationship between the ratio and velocity described a power function with the intercept at 0.6 sec^?1. Decreasing duration shifted the curve to higher velocities. I examined the effect of grating orientation in a second experiment. Visual sensitivity was poorer for oblique than for vertical gratings with detection and discrimination exhibiting similar size anisotropies. In a third experiment, observers viewed gratings presented to different retinal loci, Visual performance in both detection and discrimination fell with greater eccentricity. However, motion discrimination fell more steeply resulting in an increase in the ratio. The results demonstrate that form and motion analyzing mechanisms cannot be distinguished by their response to changes of spatial frequency, orientation or retinal locus.     

Influence of exposure duration on the tuning of spatial channels.

Differences between the spatial frequency tuning curves of psychophysical mechanisms detecting temporally modulated and unmodulated sinusoidal gratings have been found in previous work with frequency specific adaptation. Using a subthreshold additivity technique, which avoids problems associated with the adaptation technique, we found no differences between curves for briefly flashed and steadily presented gratings at center frequencies of 1, 3, and 13.3 c/deg. Under both conditions the channels were narrow and symmetrical about the center frequency. Our results do not support simple analogies between the psychophysical mechanisms and electrophysiologically defined “sustained” and “transient” neurons.     

Contrast sensitivity for oscillating sine wave gratings during ocular fixation and pursuit

The influence of pursuit eye movements on the visibility of sine wave gratings was investigated by the simultaneous measurement of eye movements and contrast thresholds under different viewing conditions. It was concluded that: 1. with moving gratings contrast sensitivity during ocular pursuit was equal to contrast sensitivity during maintained fixation on a stationary target, provided that the magnitude of retinal image motion (0-20 deg/sec) was equal in both cases; 2. accurate pursuit eye movements led to a paradoxical suppression of the visibility of low spatial frequency gratings; 3. the visibility of high spatial frequency gratings remained unchanged during accurate pursuit eye movements; 4. iso-contrast-sensitivity resulting from both pursuit and non-pursuit conditions could be used to quantify the ocular pursuit performance.     

Interocular transfer of spatial adaptation is weak at low spatial frequencies

Adapting one eye to a high contrast grating reduces sensitivity to similar target gratings shown to the same eye, and also to those shown to the opposite eye. According to the textbook account, interocular transfer (IOT) of adaptation is around 60% of the within-eye effect. However, most previous studies on this were limited to using high spatial frequencies, sustained presentation, and criterion-dependent methods for assessing threshold. Here, we measure IOT across a wide range of spatiotemporal frequencies, using a criterion-free 2AFC method. We find little or no IOT at low spatial frequencies, consistent with other recent observations. At higher spatial frequencies, IOT was present, but weaker than previously reported (around 35%, on average, at 8 c/deg). Across all conditions, monocular adaptation raised thresholds by around a factor of 2, and observers showed normal binocular summation, demonstrating that they were not binocularly compromised. These findings prompt a reassessment of our understanding of the binocular architecture implied by interocular adaptation. In particular, the output of monocular channels may be available to perceptual decision making at low spatial frequencies.     

Visual evoked potentials to luminance and chromatic contrast in rhesus monkeys

The spatial and temporal tuning of the luminance and chromatic systems were investigated using visual evoked potentials (VEPs) recorded from rhesus monkeys. VEPs were recorded from eight bipolar electrodes which were chronically implanted in the foveal projection region of area 17 in two monkeys. They were elicited by luminance (yellow-black) and chromatic (red-green) since-wave gratings which varied in spatial frequency (1, 4, 8 and 12 c/deg) and temporal frequency (3, 6, 10 and 15 Hz). The findings showed that chromatic VEPs exhibit low-pass tuning in both the spatial and temporal domains, whereas luminance VEPs exhibit bandpass tuning in the spatial domain and low-pass tuning in the temporal domain. The spatial tuning findings are consistent with the psychophysical literature, while the low-pass temporal tuning shown by the luminance VEPs was attributed to various cancellations and distortions in the suprathreshold VEP at high flicker rates and to the effects of the anesthesia.     

Radial keratotomy and glare effects on contrast sensitivity

After radial keratotomy (RK) to correct myopia, some patients complain of ‘glare’. Effects of a glare source on contrast sensitivity were measured in fifteen patients after unilateral RK. With each eye, determinations were made of the contrast required for detection of steady gratings (spatial frequencies of 0.7 and 2.9 cycles/deg), and for detection of flicker (unpatterned field flickering at rates of 8, 16, and 32 Hz). Grating or flicker was presented on a centrally fixated 4 deg test target (34 cd/m² mean luminance), surrounded by a diffuse glare source (1700 cd/m² mean luminance). For each stimulus, contrast thresholds were determined with glare-sourceoff and with glaresourceon. ‘Glare loss’ was defined as the decrement in contrast sensitivity measured with the glare source on. Significant findings were: (1) Both eyes showed glare losses for detection of gratings and for detection of flicker; (2) Spectacle lenses increased glare losses both for gratings and for flicker; (3) The RK eye showed a larger glare loss for flicker than the unoperated eye, but a smaller glare-loss for gratings; (4) For both flicker and gratings, glare loss tended to be greater in the RK eye, compared to the unoperated eye, in subjects who hadlarger pupil diameters in the testing situation; (5) The psychophysical measurements obtained in this study were not significantly correlated either with a questionnaire index of glare complaints or with the score obtained with the Miller-Nadler GlareTester.     

Perceptual bistability with counterphase gratings

Suprathreshold counterphase modulated gratings induce a bistable percept of drift or flicker. It is argued that these perceptual alternations might provide a new means for the investigation of directional selective mechanisms. The prevalance of either of the two perceptions has been studied as a function of the spatio-temporal characteristics of the stimulus and compared with: (1) the spatio-temporal contrast sensitivity surface for counterphase modulated gratings; (2) the motion/counterphase sensitivity ratio. Drift perception elicited by suprathreshold counterphase gratings attains a maximum for 8 c/deg, 12 Hz stimuli and decreases for any other experimental condition. For spatial frequencies below 1 c/deg, or temporal frequencies below 2 Hz, only flicker perception is reported. These phenomenal experiences do not show any systematic dependence on the involuntary eye movements of the observer. Comparison with the threshold measurements does not support their explanation in terms of the transient-sustained dichotomy, nor does it allow for a straightforward equivalance between the spatio-temporal characteristics of direction-selective mechanisms at threshold and at suprathreshold levels. It is suggested that the balance between flicker and motion is the perceptual outcome of the competition between lower and higher order motion detectors.     

Low spatial-frequency channels in human vision: Adaptation and masking

Previous work showed that adapting to low spatial frequency gratings (below 1.5 cycles/ degree) may cause maximal spatial adaptation at a significantly higher spatial frequency. It has been suggested that there are no adaptable spatial-frequency channels tuned to below 1.5 c/deg. Contrary to this view, we found that adaptation and masking with low spatial frequencies (0.12–1.0 c/deg) produced maximal threshold elevations when the test patterns were thesame spatial frequency as the adapting or masking pattern. These results were obtained using test patterns that turned on and off gradually or sharply. The results suggest that there are form mechanisms optimally sensitive to very low spatial frequencies. Adaptation was selective to position (phase) and orientation at low spatial frequencies; masking was observed to be selective to orientation at a spatial frequency as low as 0.2 c/deg. A clear dichotomy between transient, motion channels and sustained, form channels at low spatial and temporal frequencies may represent an unrealistic simplification. There may exist directionally-selective motion mechanisms sensitive to very slow motion, and these may play a role in the discrimination of form. The discussion considers the bandwidths of the low spatial frequency mechanisms.     

Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration"

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.     

Spatial frequency and temporal frequency selectivity of single cells in the pigeon optic tectum

The spatial and the temporal tuning characteristics of cells in the pigeon optic tectum to drifting sine-wave gratings were analysed quantitatively. Most cells could be identified as having temporal frequency tuning curves with shallow cut to both low and high temporal frequencies. The results indicate that the cells' spatial frequency selectivity to moving sine-wave gratings is independent of the temporal modulation of the stimulus. All the cells but one were tuned to velocity rather than to temporal frequency.