Psychophysical relationships among mechanisms sensitive to pattern, motion and flicker

The spatial contrast sensitivity function and its neurophysiological bases

Contrast processing is a fundamental function of the visual system, and contrast sensitivity as a function of spatial frequency (CSF) provides critical information about the integrity of the system. Here, we used a novel iPad-based instrument to collect CSFs and fitted the data with a difference of Gaussians model to investigate the neurophysiological bases of the spatial CSF. The reliability of repeat testing within and across sessions was evaluated in a sample of 22 adults for five spatial frequencies (0.41-13 cycles/degree) and two temporal durations (33 and 500 ms). Results demonstrate that the shape of the CSF, lowpass versus bandpass, depends on the temporal stimulus condition. Comparisons with previous psychophysical studies and with single-cell data from macaques and humans indicate that the major portion of the CSF, spatial frequencies >1.5 cycles/degree regardless of temporal condition, is determined by a 'sustained' mechanism (presumably parvocellular input to primary visual cortex [V1]). Contrast sensitivity to the lowest spatial frequency tested appears to be generated by a 'transient' mechanism (presumably magnocellular input to V1). The model fits support the hypothesis that the high spatial frequency limb of the CSF reflects the receptive field profile of the center mechanism of the smallest cells in the parvocellular pathway. These findings enhance the value of contrast sensitivity testing in general and increase the accessibility of this technique for use by clinicians through implementation on a commercially-available device.

Flicker adaptation or superimposition raises the apparent spatial frequency of coarse test gratings

Independent channels respond to both the spatial and temporal characteristics of visual stimuli. Gratings <3 cycles per degree (cpd) are sensed by transient channels that prefer intermittent stimulation, while gratings >3 cpd are sensed by sustained channels that prefer steady stimulation. From this we predict that adaptation to a spatially uniform flickering field will selectively adapt the transient channels and raise the apparent spatial frequency of coarse sinusoidal gratings. Observers adapted to a spatially uniform field whose upper or lower half was steady and whose other half was flickering. They then adjusted the spatial frequency of a stationary test (matching) grating on the previously unmodulated half field until it matched the apparent spatial frequency of a grating falling on the previously flickering half field. The adapting field flickered at 8 Hz and the spatial frequency of the gratings was varied in octave steps from 0.25 to 16 cpd. As predicted, adapting to flicker raised the apparent spatial frequency of the test gratings. The aftereffect reached a peak of 11% between 0.5 and 1 cpd and disappeared above 4 cpd. We also observed that superimposed 10 Hz luminance flicker raised the apparent spatial frequency of 0.5 cpd test gratings. The effect was not seen with slower flicker or finer test gratings. Altogether, our study suggests that apparent spatial frequency is determined by the balance between transient and sustained channels and that an imbalance between the channels caused by flicker can alter spatial frequency perception.

Spatial properties of flicker adaptation

Prolonged viewing of a flickering region reduces sensitivity to a subsequently flickered test patch of identical extent, but the spatial properties of this adaptation are unknown. What happens to the sensitivity to a smaller flickered test patch completely contained in, but inset from, the adapted region? We show that sensitivity to the inset test patch is only slightly affected by adaptation of the larger region. This suggests that neurons that respond to the edges of the smaller test patch are not adapted by the larger flickering region. We then show that an annulus adapter designed specifically to adapt only those edges only slightly reduces sensitivity, demonstrating that neurons that do not adapt to the flickered edges are also involved in detecting flicker. This gives further evidence that flicker detection depends on at least two mechanisms - one sensitive to flickering edges and one sensitive to local flicker, and shows that these mechanisms can operate in isolation.

The role of the magnocellular and parvocellular pathways in the attentional blink

The attentional blink refers to the transient impairment in perceiving the 2nd of two targets presented in close temporal proximity in a rapid serial visual presentation (RSVP) stream. The purpose of this study was to examine the effect on human attentional-blink performance of disrupting the function of the magnocellular pathway--a major visual-processing pathway specialized in temporal segregation. The study was motivated by recent theories that relate the attentional blink to the limited temporal resolution of attentional responses, and by a number of poorly understood empirical findings, including the effects on the attentional blink of luminance adaptation and distraction. The attentional blink was assessed for stimuli on a red background (Experiment 1), stimuli on an equiluminant background (Experiment 2), and following flicker or motion adaptation (Experiment 3), three psychophysical manipulations known to disrupt magnocellular function. Contrary to our expectations, the attentional blink was not affected by these manipulations, suggesting no specific relationship between the attentional blink and magnocellular and/or parvocellular processing.

Directional motion contrast sensitivity in developmental dyslexia

The present study compared the perception of visual motion in two dyslexia classification schemes; the [Boder, E. (1973). Developmental dyslexia: a diagnostic approach based on three atypical reading-spelling patterns. Developmental Medicine and Child Neurology, 15, 663-687.] dyseidetic, dysphonetic and mixed subgroups and [Williams, M. J., Stuart, G. W., Castles, A., & McAnally, K. I. (2003). Contrast sensitivity in subgroups of developmental dyslexia. Vision Research, 43, 467-477.] surface, phonological and mixed subgroups by measuring the contrast sensitivity for drifting gratings at three spatial frequencies (1.0, 4.0, and 8.0 c/deg) and five drift velocities (0.75, 3.0, 6.0, 12.0, and 18.0 cyc/s) in a sample of 32 children with dyslexia and 32 matched normal readers. The findings show that there were no differences in motion direction perception between normal readers and the group with dyslexia when dyslexia was taken as a homogeneous group. Motion direction perception was found to be intact in the dyseidetic and surface dyslexia subgroups and significantly lowered in both mixed dyslexia subgroups. The one inconsistency in the findings was that motion direction perception was significantly lowered in the [Boder, E. (1973). Developmental dyslexia: a diagnostic approach based on three atypical reading-spelling patterns. Developmental Medicine and Child Neurology, 15, 663-687.] dysphonetic subgroup and intact in the [Williams, M. J., Stuart, G. W., Castles, A., & McAnally, K. I. (2003). Contrast sensitivity in subgroups of developmental dyslexia. Vision Research, 43, 467-477.] phonological subgroup. The findings also provide evidence for the presence of a disorder in sequential and temporal order processing that appears to reflect a difficulty in retaining sequences of non-meaningful auditory and visual stimuli in short-term working memory in children with dyslexia.

The effect of disrupting the human magnocellular pathway on global motion perception

The purpose of this study was to demonstrate the effect of human magnocellular (M)-pathway disruption on global motion perception. Coherence thresholds for global motion direction discrimination in random dot patterns were determined at slow and moderate dot speeds: (1) after adaptation to full-field sinusoidal flicker or a steady gray field, and (2) on a red or a gray background. Adaptation to flicker and a red background increased motion coherence thresholds relative to the gray baseline conditions at both dot speeds. Physiological studies have shown that M cells in the retina and LGN are inhibited by red light and are a main contributor to flicker perception in monkeys. Therefore, our results suggest that interference with processing in the subcortical M pathway disrupts higher-level motion integration.

Visual discomfort: the influence of spatial frequency

The response of different visual discomfort groups to a range of spatial frequencies at threshold and suprathreshold was investigated. In experiment 1, a paired-comparison task was conducted. The high visual discomfort group judged a spatial frequency of 4 cycles deg-1 as the most perceptually distorted and somatically unpleasant to view. The moderate and low visual discomfort groups judged 8 and 12 cycles deg-1 as more perceptually and somatically unpleasant to view than lower spatial frequencies. In experiment 2, the spatial contrast-sensitivity function (CSF) for the high visual discomfort group was depressed for spatial frequencies between 1 and 12 cycles deg-1 in comparison with the moderate and low visual discomfort groups. When these same spatial frequencies were modulated at 6 Hz, CSFs were the same for all groups. These results are discussed in relation to a failure of inhibition across spatial-frequency channels in the high visual discomfort group. This may be explained by a more generalised parvocellular system processing deficit. Possible similarities between some forms of migraine and visual discomfort are highlighted.

Temporal sensitivity of human luminance pattern mechanisms determined by masking with temporally modulated stimuli

Target contrast thresholds were measured using vertical spatial Gabor targets in the presence of full field maskers of the same spatial frequency and orientation. In the first experiment both target and masker were 2 cpd. The target was modulated at a frequency of 1 or 10 Hz and the maskers varied in temporal frequency from 1 to 30 Hz and in contrast from 0.03 to 0.50. In the second experiment both target and masker had a spatial frequency of 1, 5 or 8 cpd. The target was modulated at 7.5 Hz and the same set of maskers was used as in the first experiment. The results are not consistent with a widely used model that is based on mechanisms in which excitation is summed linearly and the sum is transformed by an S-shaped nonlinear excitation-response function. A new model of human pattern vision mechanisms, which has excitatory and divisive inhibitory inputs, describes the results well. Parameters from the best fit of the new model to the results of the first experiment show that the 1 Hz and 10 Hz targets were detected by mechanisms with temporal low-pass and band-pass excitatory sensitivity, respectively. Fits to the second experiment suggest that at 1 cpd, the excitatory tuning of the detecting mechanism is band-pass. At 5 and 8 cpd, the mechanisms are excited by a broad range of temporal frequencies. Mechanism sensitivity to divisive inhibition depends on temporal frequency in the same general way as sensitivity to excitation. Mechanisms are more broadly tuned to divisive inhibition than to excitation, except when the target temporal frequency is high.

Spatio-temporal contrast sensitivity, coherent motion, and visible persistence in developmental dyslexia

Three experiments measured spatio-temporal contrast sensitivity, coherent motion, and visible persistence in a single group of children with developmental dyslexia and a matched control group. The findings were consistent with a transient channel disorder in the dyslexic group which showed a reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. The results were also examined by classifying the dyslexic group into dyseidetic, dysphonetic, and mixed (dysphoneidetic) subgroups. There were no differences between the control and dyseidetic groups in contrast sensitivity, in coherent motion and in visible persistence. In comparison to the control group, the mixed (dysphoneidetic) dyslexic subgroup was found to have a significant reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. In comparison to the control group, the dysphonetic group only showed a reduction in contrast sensitivity at low spatial frequencies. Comparisons between the dyseidetic, dysphonetic and mixed dyslexic subgroups showed that there were no substantive differences in contrast sensitivity, coherent motion, and visible persistence. The results support the proposal and findings by Borsting et al. (Borsting E, Ridder WH, Dudeck K, Kelley C, Matsui L, Motoyama J. Vis Res 1996;36:1047-1053) that a transient channel disorder may only be present in a dysphoneidetic dyslexic subgroup. Psychometric assessment revealed that all the children with dyslexia appear to have a concurrent disorder in phonological coding, temporal order processing, and short-term memory.

Estimating multiple temporal mechanisms in human vision

When studying human ability to perceive temporal changes in luminance it is customary to estimate either temporal impulse response shapes or temporal modulation transfer functions, the representation of the impulse response in the frequency domain. The advantages and limitations of previous methods are summarized. We then describe an approach based on use of an impulse response basis set that resolves some of those limitations. We next present psychophysical results for spatiotemporal signal detection in spatiotemporal noise, together with an economical model of performance. The model is based on accepted notions of psychophysical detection mechanisms and the filter basis set described in the first part of the paper. The best-fitting model requires only eight parameters, as opposed to the 198 parameters required to separately fit each psychometric function, and captures both qualitative and quantitative properties of the psychophysical data. Finally, the best-fitting model indicates that only two temporal filters are necessary to describe the performance of each of three subjects under the specific stimulus conditions employed here.

Effects of spatial frequency, duration, and contrast on discriminating motion directions

The minimum speed required for discriminating the direction of drifting gratings was measured at a variety of spatial frequencies, display durations, and contrasts. As was reported previously, speed thresholds were relatively constant for middle and high spatial frequencies, but speed threshold was found to be almost inversely proportional to spatial frequency in the range of 0.25 to 1.0 c/deg. Speed threshold was also found to be inversely proportional to duration between 73 and 40 ms. These results at low frequencies and short durations are shown to be consistent with limits set by the spread of energy in the stimuli, producing velocity uncertainty. A quantitative model of temporal filtering is presented that largely accounts for results at all spatial frequencies and durations by the inclusion of constant positional noise. A discussion includes the possible roles of magnocellular and parvocellular mechanisms in mediating speed thresholds.

Motion thresholds can be predicted from contrast discrimination

The detection thresholds for oscillatory motion and flicker were compared across a wide range of pedestal contrasts, spatial frequencies, and temporal frequencies in both foveal and peripheral vision. Motion and flicker stimuli were both generated by summation of a counterphase test grating with a static pedestal grating. For the oscillatory motion task the test and the pedestal were presented 90 deg out of phase to each other, whereas for flicker the two gratings were presented in phase. Since detection of both stimuli would depend on the same test stimulus, detection thresholds could be similar even though the tasks differ. Although there was a slight elevation in flicker detection thresholds compared with motion thresholds, our main finding is that oscillatory motion and flicker thresholds for suprathreshold sinusoidal gratings are similar. This finding supports the idea that motion and flicker have a common underlying detection mechanism. Our finding that flicker thresholds are slightly higher than jitter thresholds indicates that the contrast gain control (or saturating energy transducer) has a weak phase dependence. The ability to discriminate motion from flicker was elevated relative to their detection thresholds, particularly at high temporal frequencies. We offer two models to account for this behavior. The discrimination of motion from flicker may require a temporal comparison of the outputs of directionally selective filters tuned to opposite directions or to the population statistics of a bank of separable mechanisms.

Optical blur and the perception of global coherent motion in random dot cinematograms

We evaluated the effect of +3.25 dioptres of optical blur on the discrimination of motion direction in random dot cinematograms. Dot displacement between frames varied from 2.1 to 63' of visual angle while the temporal interval was held constant. Optical blur worsened discrimination in three normal subjects at displacements below 16', but improved discrimination at displacements of 21' or more. In a second experiment, two subjects viewed equivalent velocity stimuli constructed with different combinations of temporal interval and spatial displacement. Results showed that the effect of blur was specific to displacement and not velocity. Furthermore, varying the dot density of the display showed that the effect of blur correlated with dot displacement and not the probability of dot mismatches. Since optical blur attenuates high spatial frequencies, this suggests that high spatial frequencies are important for motion perception when dot displacements are less than 16' to 21', but reduce motion perception at larger dot displacements. The use of random dot cinematograms in populations must take into account stimulus displacement and optical causes of reduced spatial acuity.

Monocular mechanisms determine plaid motion coherence

Although the neural location of the plaid motion coherence process is not precisely known, the middle temporal (MT) cortical area has been proposed as a likely candidate. This claim rests largely on the neurophysiological findings showing that in response to plaid stimuli, a subgroup of cells in area MT responds to the pattern direction, whereas cells in area V1 respond only to the directions of the component gratings. In Experiment 1, we report that the coherent motion of a plaid pattern can be completely abolished following adaptation to a grating which moves in the plaid direction and has the same spatial period as the plaid features (the so-called "blobs"). Interestingly, we find this phenomenon is monocular: monocular adaptation destroys plaid coherence in the exposed eye but leaves it unaffected in the other eye. Experiment 2 demonstrates that adaptation to a purely binocular (dichoptic) grating does not affect perceived plaid coherence. These data suggest several conclusions: (1) that the mechanism determining plaid coherence responds to the motion of plaid features, (2) that the coherence mechanism is monocular, and thus (3), that it is probably located at a relatively low level in the visual system and peripherally to the binocular mechanisms commonly presumed to underlie two-dimensional (2-D) motion perception. Experiment 3 examines the spatial tuning of the monocular coherence mechanism and our results suggest it is broadly tuned with a preference for lower spatial frequencies. In Experiment 4, we examine whether perceived plaid direction is determined by the motion of the grating components or the features. Our data strongly support a feature-based model.

Influence of flicker on perceived size and depth

Previous research (e.g., Wong & Weisstein, 1984a, 1985) has shown that flickering stimuli appear to be more distant than nonflickering stimuli at the same physical distance. Given this relation between flicker and perceived depth, inappropriate constancy scaling theories predict that flickering stimuli should be perceived as larger than nonflickering ones. In contrast, links between flicker and motion perception suggest that flickering stimuli should be perceived as smaller than nonflickering ones. Two experiments tested these contrasting predictions. In Experiment 1, 22 subjects compared flickering and nonflickering vertical lines and reported that the flickering stimulus appeared significantly smaller than the nonflickering one. In Experiment 2, 21 subjects reported that the stimuli used in Experiment 1 produced depth effects similar to those reported in previous experiments: flickering stimuli were perceived as more distant than nonflickering ones. The observed effect of flicker on perceived size was contrary to predictions from inappropriate constancy scaling theory, but consistent with views that motion and flicker are processed by the same pathway.

Development of motion perception in early infancy

This article summarizes some research on the development of motion perception in early infancy. The sensitivity for slow and rapid motion was studied with 1-month-old and 3-month-old babies. The findings suggest that there are different developmental courses for the detection of slow and rapid motion. The ability to detect very slow motion seems to improve gradually with age whereas the sensitivity for very rapid motion seems to be at a level comparable to adults soon after birth. Three-month-old babies do use kinetic visual information in order to perceive object boundaries and form. After having seen a form visible only when moving they are able to "identify" the same form when seeing it under static conditions. Infants and young children do use kinetic visual information for recognizing figures that are never completely in sight only if they have been familiarized with the fully visible form first. Even 4-year-olds have difficulties in perceiving the full form of a figure that moves behind a slit in an opaque occluding surface if there is no familiarity or "priming" with the global form first. In conclusion, infants are able to detect visual motion very early in life and do extract information which leads to the perception of form. However, this ability may be limited to events with uninterrupted, continuous movement of visible elements.

An investigation of some sensory and refractive visual factors in dyslexia

The role of visual factors in dyslexia has been a long-standing source of controversy. Recent research has suggested that there may be a deficit of the transient visual subsystem in dyslexia. The evidence for this hypothesis comes principally from investigations of spatial and temporal contrast sensitivity and visual persistence. This evidence is reviewed and it is noted that previous work has never applied two of these purported "tests of transient function" to the same subject group. The hypothesised transient system deficit in dyslexia was investigated in a study comparing 43 control with 39 dyslexic children who were matched for age, sex, and intelligence. Comprehensive psychometric and optometric data were obtained, including visual acuities and refractive errors. The spatial contrast sensitivity function was determined in such a way as to investigate further the findings of Lovegrove, Martin, Bowling, Blackwood, Badcock and Paxton [(1982) Neuropsychology, 20, 309-315] and Martin and Lovegrove [(1984) Neuropsychologia, 22, 73-77]. It might be expected, from the work of Merigan and Maunsell [(1990) Neuroscience, 5, 347-352], that a better test of magno-cellular function would be to investigate the modulation threshold for a virtually uniform field that was flickering sinusoidally at 10 Hz. This temporal contrast sensitivity was studied in a similar way to Brannan and Williams [(1988) Clinical Vision Sciences, 3, 137-142]. A non-verbal simulated reading visual search task was used to investigate the effect of any visual deficits on a test that was, in its low-level visual requirements, similar to reading. The following factors were found to be significantly associated with dyslexia: reduced visual acuity, impaired flicker detection at 10 Hz, reduced low spatial frequency contrast sensitivity, and slightly slower performance at a simulated reading visual search task. The two alleged "tests of transient function" were only weakly correlated with one another (r = 0.183), suggesting that these variables do not measure the same function. Much of the dyslexic group's slightly slower performance at the simulated reading task could be accounted for by the psychometric variable of visual sequential memory. Like reading, the simulated reading task requires the accurate perception of sequential characters. Hence, it seems unlikely that the low-level visual deficits in the dyslexic group were major causes of their poor reading performance. Alternative explanations for the results are discussed.

Effect of spatial configuration on motion aftereffects

Sensitivity to motion was measured by the percentage of trials on which an observer reported seeing motion of briefly presented high-contrast sinusoidal gratings moving over a range of velocities. The psychometric curve was remeasured following adaptation to a grating moving in one direction for an extended period of time. Adaptation shifted the minimum of the psychometric curve toward the direction of the direction of the adapting stimulus. The shift was smaller when the adapting field was larger than the test. In a second set of experiments we measured the effect of motion adaptation on contrast thresholds for moving gratings of different sizes. Threshold elevation was maximal when adapting and test sizes matched. We present a mechanistic model of the motion aftereffect that consists of independent multiplicative gain controls in motion-sensing mechanisms tuned to different rates of motion. In addition, we discuss a model of size effects in motion adaptation that invokes diffuse inhibitory connections among motion-sensing mechanisms.

The effect of spatial parameters on oscillatory movement displacement thresholds

Earlier work has established that oscillatory movement displacement thresholds (OMDT) are a form of hyperacuity. There is speculation that the mechanism determining OMDT, like motion perception in general, involves direct motion sensing at high temporal frequencies of oscillation and spatial localization processes (from which motion is inferred) at low temporal frequencies, which are both hyperacuities in their own right. OMDT were determined, for three experienced observers, over the temporal frequency range 1-15 Hz, for three stimulus lengths and three stimulus widths. Both decreasing stimulus length and decreasing stimulus width increased OMDT at all temporal frequencies. Furthermore, the resulting functions consistently exhibit a "kink" in the temporal frequency midrange. The results are interpreted as evidence that there are two subsystems involved in the analysis of visual motion with the kink indicating the transition where one system begins to predominate over the other.

Contrast sensitivity of optokinetic nystagmus

To determine the threshold characteristics of optokinetic nystagmus (OKN), contrast thresholds for involuntary OKN were measured for gratings of different spatial frequency to yield an OKN-contrast sensitivity function (OKN-CSF). The OKN-CSF resembled an inverted U-shaped function with temporal-to-nasal and nasal-to-temporal movement yielding similar functions. In addition, when psychophysical CSFs were determined for separate form and movement thresholds, it was discovered that the OKN-CSF approximated the psychophysical-movement CSF rather than the psychophysical-form CSF.

Visual space-time interactions: effects of adapting to spatial frequencies on temporal sensitivity

To study how adaptation to spatial frequency patterns affects temporal sensitivity in vision, observers were selectively adapted for 4 min to either a high- or a low-spatial-frequency sinusoidal grating (12 and 2 cpd, respectively). Their sensitivities to modulation of a blurred patch at high or low temporal frequencies (12 Hz and 2 Hz, respectively) were measured, before and after the adaptation period, by using the yes/no task of signal detection theory. The data consistently indicated that spatial adaptation differentially affected the observers' sensitivities to temporal signals. Specifically, when the observers were adapted to low spatial frequencies, their sensitivity to low temporal frequencies was reduced; when they were adapted to high spatial frequencies, their sensitivity to high temporal frequencies was increased. These results have implications for the psychophysical measurements of temporal and spatial sensitivity, as well as for the issue of the separability of spatial and temporal properties of individual channels.

Age differences in the temporal continuity of gratings as a function of their spatial frequency

This study compared young and elderly observers on the continuity of sinusoidal grating-pairs as a function of interstimulus interval (ISI) and spatial frequency (.5, 1.0, 2.0, 4.0, 8.0, and 12.0 c/deg). Consistent with prior research, the maximum ISI over which pattern continuity was maintained increased with spatial frequency. In addition, among older observers, grating continuity occurred at significantly longer ISI's at the two lowest spatial frequencies; no age difference was observed at the higher spatial frequencies. These results could not be attributed to an age difference in retinal illumination associated with pupillary miosis. However, they do indicate an age-related decline in detecting visual offset and are consistent with the hypothesis of a decline in the effectiveness of the transient visual channels.

Neural contribution to spatiotemporal contrast sensitivity decline in healthy ageing eyes

Contrast sensitivity thresholds were measured over a range of spatial and temporal frequencies in both a group of young and older observers. Results demonstrate a significant reduction in contrast sensitivity of the older age group at all but the lowest combinations of spatial and temporal frequencies investigated. The senile miosis and reduced optical transmission of the older eye was then mimicked using the younger observers as subjects. This combined effect of reducing retinal illumination produced no significant change in sensitivity. These findings are discussed in terms of neural loss within the visual pathways with increasing age.

Uniform-field flicker masking in control and specifically-disabled readers

Possible transient-system deficiencies in subjects with specific reading disabilities (SRDs) were investigated in groups of 13-year-old SRDs and control normal readers. In experiment 1, in which a 6 Hz uniform-field flicker (UFF) mask and a stationary test stimulus were used, it was found that the overall effect of UFF masking was to reduce differences in contrast sensitivity between SRDs and normal readers. In experiments 2a and 2b, with UFF masks of 6 and 20 Hz and a 6 Hz moving (experiment 2a) or flickering (experiment 2b) test stimulus, contrast sensitivity in both groups was decreased in the presence of the 6 Hz UFF mask. Only the control group, however, showed a further decrease in sensitivity with the 20 Hz UFF mask. This indicates that the groups differ in terms of a mechanism sensitive to high temporal frequencies. A 20 Hz counterphase flickering test stimulus was used in experiment 3 in the presence of 6 Hz UFF, and it was found that SRDs are less sensitive than controls to 20 Hz flicker across all spatial frequencies used. The 6 Hz mask, however, did not differentially affect the two groups. These findings provide further evidence for a transient-system deficit in the visual systems of SRDs, but also suggest a more complex situation by showing that the two groups differ in a high-temporal-frequency mechanism.

Flicker masking and developmental dyslexia

Recent studies have provided evidence that dyslexic children tend to show longer visual persistence than control children when presented with low-spatial-frequency grating stimuli. The possibility that this phenomenon might reflect an impairment of inhibitory Y-cell activity in the visual system of dyslexics has been investigated. A flicker masking technique was used to mask Y-cell activity selectively in a group of dyslexic boys and a group of age-matched controls. There were no overall differences in reaction times to the offsets of grating patterns of various spatial frequencies between the groups, and no differences between subgroups defined by age, degree of reading impairment, or any other criterion. The results show no evidence of abnormal Y-cell function in developmental dyslexia.

Some tests of the Marr-Ullman model of movement detection

Several psychophysical experiments are described which test and uphold predictions derived from the Marr-Ullman model of movement detection. First, we demonstrate the existence of adaptation which is specific not merely to the direction of movement of an edge, but also to its contrast polarity. Second, it is shown that adaptation to a spatially homogeneous field whose luminance is modulated according to a temporal sawtooth waveform produces predictable changes in sensitivity to the movement of an edge; these changes, too, are specific to particular conjunctions of direction and edge polarity. Third, similar changes in sensitivity are demonstrated to occur when the luminance of an edge is physically perturbed at the moment of its displacement. Finally, it is shown that, as predicted, the sudden onset of an edge can itself give rise to a momentary impression of movement, the apparent direction of which depends upon the change in luminance that accompanies the onset of the edge.

Spatial and temporal selectivity of the human motion detection system

Measurements were made of spatial frequency, orientation and temporal frequency selectivity of the visual motion system. The results suggest: (1) There exists in the motion system mechanisms selective for spatial frequency. The preferred spatial frequency varies considerably and extends down to at least 0.06 c/deg. (2) At all spatial frequencies (from 0.1 to 10 c/deg) there exist detectors selective for orientation which vary in (directed) orientation tuning to encompass 360 degrees. (3) The bandwidth of both spatial frequency and orientation selectivity vary inversely with spatial frequency: the lower the spatial frequency, the broader the bandwidth. (4) There exist two classes of temporally tuned detectors, one lowpass (sustained) and one bandpass (transient), of preferred temporal frequency of 7-13 Hz (depending on spatial frequency).

Temporal frequency discrimination in human vision: evidence for an additional mechanism in the low spatial and high temporal frequency region

Temporal frequency discrimination at and above the detection threshold has been studied using gratings of low (0.2 c/deg) and medium (2 c/deg) spatial frequencies. At 2 c/deg the results of previous investigators are confirmed: The results being consistent with the existence of two broadly tuned and directionally selective temporal mechanisms (up to 32 Hz). For the lower spatial frequency an additional temporal frequency discrimination at threshold can be made between 4 and 32 Hz and enhanced temporal frequency discrimination at suprathreshold levels occurs above 24 Hz. One interpretation of this result is the existence of one or more additional temporal mechanisms with restricted spatial acuity responding to higher temporal frequencies.

Adaptation to apparent motion

A spot alternating between two positions can produce apparent motion (AM). Following prolonged inspection, the AM degenerates into flicker. This adaptation effect was found to depend on spacing and timing; the probability of seeing motion during a 30-sec inspection period declined linearly with log spatial separation (over a range from 0.1 to 1 deg), and with log alternation rate (over a range from 2 to 4.5 Hz). Cross-adaptation, in which subjects were adapted to one alternation rate and tested at another, showed that low alternation rates gave stronger motion signals than high rates did. Adaptation to real motion (RM) strongly suppressed AM, which suggests that AM must be stimulating the same neural pathways as RM. Flickering spots (i.e. in-phase flicker) produced less adaptation than did a spot alternating between two positions (i.e. counterphase flicker), so the adapting mechanism must be responding to relative temporal phase. Embedding the adapting spots in configurations of other spots, which altered the pattern of perceived adapting motion without altering the local retinal stimulation, minimized the adaption, so the adapting mechanism must be responding to the path of seen motion. Adaptation can be used to measure the strength of AM and shows that AM is strongest for small separations, low alternation rates and high luminance contrast.

Velocity coding: evidence from perceived velocity shifts

Measurements of the perceived velocities of moving patterns were made under a variety of conditions in an attempt to gain information concerning the way in which velocity is encoded in the visual system. Adaptation to a pattern moving in the same or opposite direction reduces the perceived velocity of a moving test pattern, but only if the adaptation pattern moves as fast as or faster than the test pattern. The aftereffect peaks at an adaptation velocity slightly higher than the test velocity and then remains constant at higher velocities. Similar results were obtained for several types of pattern. Perceived velocity reduction also follows adaptation to a flickering homogeneous field. The results can be explained in terms of a theory of velocity coding in which two channels are considered in terms of variations in the velocity sensitivities of the neurones they comprise.

On seeing temporal gaps between gratings: a criterion problem for measurement of visible persistence

Previous estimates of visible persistence based on subjects' ability to see a "blank" between brief flashes of a grating are remarkably long when compared with studies of temporal integration. We show firstly that subjects can detect gaps as short as 5 msec at low spatial frequencies, rising to about 20 msec at high spatial frequencies, and secondly that artefactual luminance transients such as might be found in tachistoscopes do not greatly increase the detection thresholds. Much longer estimates were obtained through the use of a more conservative criterion, which was subjectively ill-defined and very variable across subjects. Studies which use this method may not measure visible persistence very effectively.

The influence of luminance on displacement thresholds for continuous oscillatory movement

Displacement thresholds were determined for a sinusoidally-modulated contrast grating, having a spatial frequency of 2 c/deg, whose mean luminance could be varied. The grating underwent continuous oscillatory simple harmonic motion. Displacement thresholds were determined for frequencies of 1, 3, 5, 7 and 10 Hz using the method of limits for each mean grating luminance of 3.8, 8.9, 26, 62 and 134 cd/m2. With low oscillation frequencies a marked reduction in sensitivity is produced by reducing the mean luminance of the grating. This is not the case with higher oscillation frequencies when mean luminance has little effect upon sensitivity.

Flicker masking of spatial-frequency-dependent visible persistence and specific reading disability

The effect of 6 Hz uniform-field flicker masking of visible persistence at a range of spatial frequencies was investigated in 12-year-old children with specific reading disabilities and a control group of average readers. This reduced differences in visible persistence between the two groups. The results suggest that children with specific reading disabilities experience a deficit in their transient system.

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.

Spatiotemporal conditions which elicit or abolish the oblique effect in man: direct measurement with swept evoked potential

Reversing sine wave gratings were electronically swept in spatial frequency and contrast. The acuity limits and contrast thresholds of 4 observers were inferred from evoked potential stimulus-response functions elicited by these stimuli and retrieved with a quadrature lock-in amplifier. The evoked potential functions, linearized in the case of contrast by increasing contrast logarithmically with time, were extrapolated to the point of zero response. This point provides an electrophysiologically defined threshold value for acuity and for contrast. An oblique effect (superior sensitivity for HV-oriented gratings) could reliably be demonstrated in both acuity and contrast threshold performance. This oblique effect could readily be abolished under low spatial/high temporal frequency conditions. The findings are discussed in terms of shifting relative strengths of X and Y contributions to the steady-state evoked potential.

Rapid motion aftereffect seen within uniform flickering test fields

Prolonged viewing of a moving pattern selectively elevates the threshold for a pattern moving in the same direction and induces the classical motion aftereffect (MAE). The aftereffect is seen as a slow drift in the opposite direction, which is visible even with the eyes shut or while viewing a uniform field. However, as we report here, a strikingly different aftereffect is seen when the test field is uniform and sinusoidally flickered: the field is filled with rapid motion in the direction opposite the adapting motion. This flicker MAE has distinct properties: the adapting grating must be of low spatial frequency; the effect is promoted by high contrast and high temporal frequencies of both adapting and test stimuli; and the aftereffect does not transfer interocularly. In all these respects the flicker MAE differs from the traditional MAE. Motion detectors have been identified in human vision by the threshold detectability and discriminability of moving patterns and by selective adaptation. The flicker MAE selectively taps a class of transient motion mechanisms that are selective for rapid motion and low spatial frequency. Uniform flicker is an effective stimulus for these mechanisms. It thus appears that the human visual system contains at least two distinct classes of mechanisms for sensing motion.

Sensitivity to countermodulating gratings following spatiotemporal adaptation

Contrast sensitivities to countermodulating gratings were measured with a two-alternative temporal forced-choice procedure following adaptation to a static grating of the same spatial frequency, a homogeneous flickering field of the same temporal frequency, or a countermodulating grating of identical spatial and temporal frequencies. At high spatial frequencies, the temporal-frequency content of the adaptation was not critical, that is, a countermodulating adaptation grating was only slightly more effective at raising threshold than was a static adaptation grating. At low spatial frequencies, the sensitivity to countermodulating test gratings could not be reduced by either a high-contrast stimulus matching the test in the spatial domain only or by one matching the test in the temporal domain only. Adapting to a high-contrast stimulus matching the countermodulating test grating in both spatial- and temporal-frequency domains was effective at reducing test sensitivity for one observer but not for another.

Spatial frequency tuning of orientation selective units estimated by oblique masking

Threshold elevations were measured as a function of the spatial frequency of high contrast cosine masks using spatially localized test stimuli with a 1.0 octave bandwidth. The cosine masks were oriented at 14.5 degrees relative to the vertical test patterns in order to average out spatial phase effects. The experiment was repeated for each of 14 test frequencies spanning the range 0.25-22.0 c/deg in 0.5 octave steps. The resulting threshold elevation curves fell into a small number of distinct groups, suggesting the existence of discrete spatial frequency mechanisms in human central vision. The data are shown to be consistent with a model having just six distinct classes of spatial frequency mechanisms in the fovea. Spatial frequency bandwidths of these mechanisms ranged from 2.5 octaves at low frequencies to as narrow as 1.25 octaves at high spatial frequencies. These results require revision of the Wilson and Bergen (1979) [Vision Res. 19, 19-32] model for spatial vision.

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.

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.

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.