Spatial frequency selective mechanisms underlying the motion aftereffect.

The motion aftereffect (MAE) was used to study the spatial frequency selectivity of suprathreshold motion perception. Observers were adapted to drifting sine-wave gratings confined to a retinal eccentricity of approx. 4 deg. The magnitude of the subsequent MAE was measured while viewing a stationary sine-wave grating test surface of one of a number of spatial frequencies. The largest MAE was found when the spatial frequency of the test stimulus was the same as that of the adapting stimulus. This phenomenon held for spatial frequencies between 0.5 and 4 c/deg, and was robust with changes in contrast of either adapting or test gratings. However, at an adapting spatial frequency of 0.25 c/deg, the peak MAE was observed at 0.5 c/deg. Control experiments indicated that this peak shift was not the result of the reduced number of cycles in the stimulus, nor the temporal frequency. There was no measurable MAE at spatial frequencies lower than 0.25 c/deg. These results suggest the existence of a "lowest adaptable channel" for the motion aftereffect.     

Orientation bandwidth: the effect of spatial and temporal frequency.

The orientation bandwidths of psychophysically defined channels of human vision were estimated by two techniques for a wide range of spatial and temporal frequencies. The first technique was an adaptation paradigm, where the subjects' ability to see patterns of various orientations was measured before and after adapting to a high contrast pattern. The second technique evaluated subjects' ability to discriminate between two gratings of different orientations in relation to their ability to detect the patterns. Bandwidths were unaffected by temporal frequency at high spatial frequencies but increased with temporal frequency at low spatial frequencies. Bandwidths increased modestly with decreasing spatial frequency at low temporal frequencies but more drastically at high temporal frequencies. Both techniques gave similar results except for patterns with very low spatial and high temporal frequencies. In this region the stimulus appears "spatial-frequency doubled" which may be used as a cue for the orientation discrimination task.     

The sequential processing of visual motion in the human electroretinogram and visual evoked potential

Mechanisms of motion vision in the human have been studied extensively by psychophysical methods but less frequently by electrophysiological techniques. It is the purpose of the present investigation to study electrical potentials of the eye (electroretinogram, ERG) and of the brain (visual evoked potential, VEP) in response to moving regular square-wave stripe patterns spanning a wide range of contrasts, spatial frequencies, and speeds. The results show that ERG amplitudes increase linearly with contrast while VEPs, in agreement with the literature, show an amplitude saturation at low contrast. Furthermore, retinal responses oscillate with the fundamental temporal stimulus frequency of the moving pattern while brain responses do not. In both the retina and the brain, the response amplitudes are tuned to certain speeds which is in agreement with the nonlinear correlation-type motion detector. Along the ascending slopes (which means increasing amplitudes) of the tuning functions, the ERG curves overlap at all spatial frequencies if plotted as a function of temporal stimulation frequency. The ascending slopes of the tuning functions of the VEP overlap if plotted as a function of speed. The descending slopes (which means decreasing amplitudes) of the tuning functions show little (ERG) or no (VEP) overlap and the waveforms at high speeds approach pattern-offset-onset responses. These observations suggest that in the retina motion processing along the ascending slopes of the tuning curves takes place by coding the temporal stimulation frequency which depends on the spatial frequency of the moving pattern. In the brain, however, motion processing is by speed independent of spatial frequency. Simple calculations show that the VEP information is decoded from the ERG signal into a speed signal.     

Velocity discrimination in scotopic vision

To characterize scotopic motion mechanisms, we examined how variation in average luminance affects the ability to discriminate velocity. Stimuli were drifting horizontal sine-wave gratings (0.25, 1.0 and 2.0 c/deg) viewed through a 2 mm artificial pupil and neutral density filters to produce mean adapting levels from 2.5 to -1.5 log photopic trolands. Drift temporal frequency varied from 0.5 to 36.0 Hz. Grating contrasts were either three or five times direction discrimination threshold contrasts at each adaptation level. Following 30 min adaptation, two drifting gratings were presented sequentially at the fovea. Subjects were asked to indicate which interval contained the faster moving stimulus. The Weber fraction for each base temporal frequency was determined using a staircase method. As previously reported, velocity discrimination performance was most acute at temporal frequencies of about 8.0 Hz and greater than 20.0 Hz (though there are individual differences), and fell off at both higher and lower temporal frequencies under photopic conditions. As adaptation level decreased, discrimination of high temporal frequencies in the central retina became increasingly worse, while discrimination of low temporal frequencies remained largely unaltered. The overall scotopic discrimination performance was best at about 3.0 Hz. These results can be explained by a motion mechanism comprising both low-pass and band-pass temporal filters whose peak and temporal cut-off shifts to lower temporal frequencies under scotopic conditions.     

Detection of radial motion depends on spatial displacement

Nakayama and Tyler (1981) disentangled the use of pure motion (speed) information from spatial displacement information for the detection of lateral motion. They showed that when positional cues were removed the contribution of motion or spatial information was dependent on the temporal frequency: for temporal frequencies lower than 1 Hz the mechanism used to detect motion relied on speed information while for higher temporal frequencies a mechanism based on displacement information was used. Here we test whether the same dependency is also revealed in radial motion. In order to do so, we adapted the paradigm previously used by Nakayama and Tyler to obtain detection thresholds for lateral and radial motion by using a 2-IFC procedure. Subjects had to report which of the intervals contained the signal stimulus (33% coherent motion). We replicated the temporal frequency dependency for lateral motion but results indicate, however, that the detection of radial is always consistent with detecting a spatial displacement amplitude.     

Failure of direction identification for briefly presented second-order motion stimuli: evidence for weak direction selectivity of the mechanisms encoding motion

We sought to investigate why the direction of second-order motion, unlike first-order motion, cannot be identified when the stimulus exposure duration is brief (<200 ms). In a series of experiments observers identified both the orientation (vertical or horizontal) and the direction (left, right, down or up) of a drifting sinusoidal modulation (0.93 c/ degrees ) in either the luminance (first order) or the contrast (second order) of a two-dimensional noise carrier. All motion stimuli were equated for visibility, and the duration was varied using the method of constant stimuli. Performance was measured for second-order motion over a range of drift temporal frequencies (0.63-5.04 Hz) and for first-order motion stimuli composed of two, opposite drifting modulations in luminance of unequal modulation depth. Orientation-identification performance was nearly 100% correct for both first-order and second-order motion stimuli, even at the briefest stimulus duration tested (26.49 ms). Direction identification for first-order motion was also typically good with brief presentations, but was poor for second-order motion when the exposure duration was < approximately 200 ms. Importantly increasing either the drift temporal frequency of second-order motion or the bidirectional nature of the first-order motion patterns produced comparable levels of performance for the two varieties of motion (i.e. the minimum duration required for reliable direction identification could be equated). As orientation-identification performance for the first-order and second-order motion stimuli was comparably good and minimally affected by duration, the marked differences on the direction-identification task must be specific to mechanisms that encode drift direction, rather than spatial structure. We propose that second-order motion detectors are much less selective for stimulus direction than first-order motion sensors, and thus are more susceptible to the deleterious effects of limiting stimulus duration (which introduces spurious motion in the opposite direction, particularly at low drift rates). Alternative explanations based on the delayed propagation of second-order motion signals or the temporal characteristics of the underlying motion mechanisms are not supported by our findings.     

Attentional modulation of threshold sensitivity to first-order motion and second-order motion patterns

Previous studies [e.g. Vision Research 40 (2000) 173] have shown that when observers are required to selectively attend to one of two, spatially-adjacent patches containing either first-order (luminance-defined) or second-order (contrast-defined) motion, threshold sensitivity for identifying the direction of second-order motion, but not first-order motion, is enhanced for the attended stimuli. The processing of second-order motion, unlike first-order motion, may, therefore, require attention. However, other studies have found little evidence for differential effects of attention on the processing of first-order and second-order motion [Investigative Ophthalmology and Visual Science 42(4) (2001) 5061]. We investigated the effects of attention instructions on the ability of observers to identify the directions and spatial orientations of luminance-defined and contrast-defined motion stimuli. Pairs of motion stimuli were presented simultaneously and threshold performance was measured over a wide range of drift temporal frequencies and stimulus durations. We found: (1) direction discrimination thresholds for attended motion stimuli were lower than those for unattended stimuli for both types of motion. The magnitude of this effect was reduced when the observers were not given prior knowledge of which patch of motion (attended or unattended) they had to judge first. (2) Direction discrimination for first-order motion was similarly affected at all temporal frequencies and durations examined, but for second-order motion the effects of attention depended critically on the drift temporal frequency and stimulus duration used. (3) Orientation discrimination showed little or no influence of attention instructions. Thus, whether or not attention influences the processing of second-order motion depends crucially on the precise stimulus parameters tested. Furthermore under appropriate conditions the processing of first-order motion is also influenced by attention, albeit to a lesser extent than second-order motion.     

The Comparative Development of Movement Hyperacuity and Visual Acuity in Children

Oscillatory movement displacement thresholds (OMDT) have been suggested as a test of neural integrity of the visual system, detecting deficit even in the presence of normal resolution. Both OMDT and visual resolution were measured in 153 normal children between the ages of 2.2 and 13 yr (mean = 6.2 ± 2.8 yr). OMDT were obtained using a computer-generated vertical bar stimulus oscillating at 4 Hz. Visual acuity was determined using a Polymetric Vision Assessment technique (PVA) where a single letter optotype is presented at increasing viewing distance to determine threshold.OMDT is a hyperacuity at all ages, exhibiting improving thresholds up to around 8 yr of age when results are typical of adults. PVA thresholds improve less markedly over the range measured, appearing mature by 6–7 yr. Correlation between OMDT hyperacuity and visual acuity is poor, illustrating the complexity of the relationship between hyperacuity and resolution functions. Copyright © 1996 Elsevier Science Ltd     

Temporal phase discrimination depends critically on separation

Temporal phase discrimination was measured as a function of spatial separation of the stimulus components. In contrast to many previous studies, phase discrimination thresholds were measured directly, rather than inferred from the ability to discriminate synchronous from antiphase stimuli, or from segregation or shape tasks. For abutting bars, relative phase thresholds were closely proportional to temporal frequency. The proportionality corresponded to a threshold temporal offset of 2.5-9.5 ms, across subjects. Introduction of a small gap (0.125 degrees or greater) led to a dramatic (3- to 7-fold) increase in thresholds for temporal phase discrimination, and thresholds were no longer proportional to temporal frequency. Insertion of a third bar filling the gap resulted in a recovery of the low thresholds, provided that its modulation was consistent with apparent motion across the three bars. Below 8 Hz, phase discrimination thresholds across three bars were equivalent to thresholds for two abutting bars. Above 8 Hz, phase discrimination thresholds for the three bar combination were lower than thresholds for two adjacent bars, implying that phase information was integrated across all three bars.Phase discrimination thresholds do not appear to reflect the properties of a single mechanism. Especially at high temporal frequencies, low thresholds for phase discrimination are closely tied to the presence of apparent motion. Temporal phase discrimination is markedly impaired by a small separation of stimulus components. Moreover, the inability to detect phase differences across gaps corresponds to the loss of phase-dependence of vernier acuity thresholds across gaps.     

Adaptation and the temporal delay filter of fly motion detectors.

Recent accounts attribute motion adaptation to a shortening of the delay filter in elementary motion detectors (EMDs). Using computer modelling and recordings from HS neurons in the drone-fly Eristalis tenax, we present evidence that challenges this theory. (i) Previous evidence for a change in the delay filter comes from 'image step' (or 'velocity impulse') experiments. We note a large discrepancy between the temporal frequency tuning predicted from these experiments and the observed tuning of motion sensitive cells. (ii) The results of image step experiments are highly sensitive to the experimental method used. (iii) An apparent motion stimulus reveals a much shorter EMD delay than suggested by previous 'image step' experiments. This short delay agrees with the observed temporal frequency sensitivity of the unadapted cell. (iv) A key prediction of a shortening delay filter is that the temporal frequency optimum of the cell should show a large shift to higher temporal frequencies after motion adaptation. We show little change in the temporal or spatial frequency (and hence velocity) optima following adaptation.     

The effects of aging on the pattern electroretinogram and visual evoked potential in humans.

We have recorded patterns electroretinograms (PERGs) and visual evoked potentials (VEPs) from 14 elderly subjects (mean age 72 yr) and 12 young subjects (mean age 21 yr) in response to stimulation by high contrast sinusoidal grating patterns of variable spatial frequency (at 9 Hz) and temporal frequency (at 1.7 c/deg). The major effect of aging on the PERG was an aspecific reduction in amplitude (of about 40%) at most spatial and temporal frequencies, together with a small but systematic phase lag. Control measurements suggest that senile miosis may be responsible for the phase lag, but not for the reduction in amplitude. The effects of aging on the VEP were more dramatic and depended on the spatial and temporal properties of the stimulus. VEP amplitudes (at 1.7 c/deg) were significantly lower for the aged at low temporal frequencies (below about 6 Hz), but were similar at high temporal frequencies. At 9 Hz, there was no effect of spatial frequency on VEP amplitude. At high temporal frequencies (above 10 Hz), the latencies of VEPs (estimated from the rate at which phase varied with temporal frequency) were similar for old and young (94 and 99 msec respectively). Below 10 Hz, however, the latencies of the old observers was much greater (153 compared with 108 msec). The second-harmonic phase of VEPs of the old but not the young decreased considerably with spatial frequency, by about 1.9 pi radians (52 msec) over the range from 0.5 to 11 c/deg. The selective reduction in amplitude at low temporal frequencies, the longer latencies at low temporal frequencies and the phase lag at high spatial frequencies are consistent with the hypothesis that mechanisms sensitive to high spatial and low temporal frequencies are selectively degraded by aging.     

Nonlinearity and oscillations in X-type ganglion cells of the cat retina

Intracellularly recorded light-responses of X-type ganglion cells in the cat retina were separated, with the help of a wavelet method, into “slow” membrane (“G”)-potentials and the corresponding spike trains. In response to sinusoidally modulated high intensity light spots with different sizes and frequencies, X-type ganglion cells show both oscillations correlated with the stimulus frequency and other, faster, oscillations that were not always locked to the stimulus. A forced van der Pol oscillator model with stimulus-dependent coefficients proved to describe the empirical findings quite well. A linearity-coefficient of the equations indicates strong nonlinearity at a temporal frequency of 8 Hz with spot sizes on the order of 0.5–0.7 deg and decreasing nonlinearity at lower temporal frequencies or smaller spot sizes, while the faster oscillations become more prominent. We could not determine whether the oscillations are intrinsic to the cell-membrane or generated by (or in interaction with) the preganglionic retinal meshwork. The results show that X-cell spike-trains can contain oscillations that are not phase-locked to the stimulus and that are therefore virtually invisible after stimulus synchronous averaging. It is not likely that these retinal oscillations directly induce the well described oscillations in cat visual cortex, since they usually fall in a different frequency range.     

The coding of velocity of movement in the human visual system

A very simple model of velocity perception which requires only 17 channels is outlined. The important points of the model are that: (1) in each direction of movement just two temporal frequency channels are necessary at any spatial frequency, (2) at low temporal frequencies the spatial frequency domain is encoded by many channels, but only those at low spatial frequencies are direction-specific. Using a detection/discrimination technique the supposition that channels which detect high spatial, low temporal frequencies are not direction specific is investigated. Possible reasons for the apparent nondirectional behaviour of these channels are investigated: the notion that non-directionality reflects a failure of the stimulus to travel some threshold distance across the retina is rejected, but the proposal that a velocity threshold must be exceeded before the direction of a grating may be identified at detection threshold remains a possibility.     

Two-frequency analysis of interactions elicited by Vernier stimuli

In five subjects, we measured visual evoked potentials (VEPs) elicited by Vernier targets in which the contrast of the two components of the stimuli were modulated by sinusoids at distinct frequencies fl and f2. This approach allows for the extraction of VEP signatures of spatial interactions, namely, responses at intermodulation frequencies n1f1 + n2f2, without the need to introduce motion into the stimulus. The most prominent interactions were at the sum frequency f1 + f2, and, for frequency pairs that were sufficiently separated, the difference frequency f1- f2. These responses had a systematic dependence on the temporal parameters of the stimulus, corresponding to an effective latency of 145 to 165 ms. Fourth-order interactions were also detected, particularly at the frequencies 2f1 +/- 2f2. These VEP signatures of interaction were similar to interactions seen for colinear line segments separated by a gap. Thus, for Vernier stimuli devoid of motion, VEP signatures of interaction are readily detected but are not specific to hyperacuity displacements. The distribution of interactions across harmonic orders is consistent with local rectification preceding the spatial interactions. Their effective latencies and dependence on spatial parameters are consistent with interactions within V1 receptive fields or mediated by horizontal connections between cells with a similar orientation tuning within V1.     

Vertical-size disparities are temporally integrated for slant perception

We investigated temporal properties of vertical-size and horizontal-size disparity processing for slant perception. Subjects indicated perceived slants for a stereoscopic stimulus in which the two magnitudes of vertical-size or horizontal-size disparities were oscillated stepwise with various frequencies (from 0.2 to 10 Hz). For the stimulus with vertical-size disparity oscillation, two slants corresponding to the two magnitudes of disparity were perceived for low-frequency conditions, whereas only a static mean slant of the two slants was perceived for high frequencies (5 and 10 Hz). For the stimulus with horizontal-size disparity oscillation, two slants were perceived for all the temporal frequency conditions. These results indicate that temporal properties of vertical- and horizontal-size disparity processing are clearly different and vertical-size disparities are temporally integrated over a period of around 500 ms for slant perception.     

Movement hyperacuity in childhood amblyopia

BACKGROUND—Amblyopia results in deficits in a number of visual functions in both the amblyopic and dominant eye. The present work describes oscillatory movement displacement thresholds (OMDT) in childhood amblyopia.
METHODS—The OMDT from the dominant and amblyopic eyes of 50 orthoptic patients (aged 74 (SD 16) months) were compared with those from a group of 24 controls (79 (21) months). OMDT were measured using a forced choice staircase procedure. Subjects were asked to identify which of the computer controlled monitors displayed the oscillating stimulus. Visual acuity and stereoscopic responses were noted from clinical records.
RESULTS—Amblyopic children demonstrating stereopsis showed no significant OMDT deficit in the amblyopic eye. Those children having no stereopsis had elevated OMDT in the amblyopic eye (p<0.05). Results suggest that the dominant eye of children with amblyopia may also have a pattern of visual development which is anomalous (difference in correlation coefficient with age; p <0.05).
CONCLUSION—OMDT deficits demonstrated in some amblyopic eyes indicate that amblyopia is incompletely described by its "clinical" definition. Results suggest that the dominant eye in those with unilateral amblyopia may not be "normal".

Keywords: amblyopia; children; vision; movement hyperacuity; stereopsis     

Spatio-temporal frequency characteristics of the optomotor response in zebrafish

The optomotor response (OMR) is a simple experimental paradigm that is widely used in the study of visual system functions. In the current paper we investigated how spatial and temporal properties of repetitive stimuli determine the OMR in zebrafish. The experiments showed that the OMR has the temporal characteristic of a low-pass filter when the spatial frequencies are low and of a band-pass filter when the spatial frequencies are high. These findings are discussed on the basis of inherent sampling constraints of any motion detector. We found some indications that the strength and direction of the OMR vary with the spatio-temporal frequency of the stimulus pattern as has previously been described for other species.     

Smooth and sampled motion

Stroboscopic or sampled motion is indistinguishable from smooth motion if the frequency of sampling is sufficiently high. We report measurements of the minimum sample frequency for smooth motion of drifting sinusoidal gratings (extended and truncated) which varied in spatial frequency from 0.06 to 24 c/deg, in temporal frequency from 1.5 to 24 Hz and in contrast from 3 to 100 times detection threshold. Threshold sampling frequency for smoothness increased with temporal frequency and contrast, and inversely with spatial frequency. The threshold step size associated with the sampling frequency ranged from 20" arc (for gratings of 24 c/deg) to 6 deg (for gratings of 0.06 c/deg). Calculations of the spurious frequencies introduced by sampling lead us to conclude that motion appears smooth provided that sampling is above the Nyquist limit and that the amplitude of the spurious components is below their independent threshold.     

Short latency ocular-following responses in man

The ocular-following responses elicited by brief unexpected movements of the visual scene were studied in human subjects. Response latencies varied with the type of stimulus and decreased systematically with increasing stimulus speed but, unlike those of monkeys, were not solely determined by the temporal frequency generated by sine-wave stimuli. Minimum latencies (70-75 ms) were considerably shorter than those reported for other visually driven eye movements. The magnitude of the responses to sine-wave stimuli changed markedly with stimulus speed and only slightly with spatial frequency over the ranges used. When normalized with respect to spatial frequency, all responses shared the same dependence on temporal frequency (band-pass characteristics with a peak at 16 Hz), indicating that temporal frequency, rather than speed per se, was the limiting factor over the entire range examined. This suggests that the underlying motion detectors respond to the local changes in luminance associated with the motion of the scene. Movements of the scene in the immediate wake of a saccadic eye movement were on average twice as effective as movements 600 ms later: post-saccadic enhancement. Less enhancement was seen in the wake of saccade-like shifts of the scene, which themselves elicited weak ocular following, something not seen in the wake of real saccades. We suggest that there are central mechanisms that, on the one hand, prevent the ocular-following system from tracking the visual disturbances created by saccades but, on the other, promote tracking of any subsequent disturbance and thereby help to suppress post-saccadic drift. Partitioning the visual scene into central and peripheral regions revealed that motion in the periphery can exert a weak modulatory influence on ocular-following responses resulting from motion at the center. We suggest that this may help the moving observer to stabilize his/her eyes on nearby stationary objects.     

Rod- versus cone-driven ERGs at different stimulus sizes in normal subjects and retinitis pigmentosa patients

Purpose

To study how rod- and cone-driven responses depend on stimulus size in normal subjects and patients with retinitis pigmentosa (RP), and to show that comparisons between responses to full-field (FF) and smaller stimuli can be useful in diagnosing and monitoring disorders of the peripheral retina without the need for lengthy dark adaptation periods.

Method

The triple silent substitution technique was used to isolate L-cone-, M-cone- and rod-driven ERGs with 19, 18 and 33% photoreceptor contrasts, respectively, under identical mean luminance conditions. Experiments were conducted on five normal subjects and three RP patients. ERGs on control subjects were recorded at nine different temporal frequencies (between 2 and 60 Hz) for five different stimulus sizes: FF, 70°, 60°, 50° and 40° diameter circular stimuli. Experiments on RP patients involved rod- and L-cone-driven ERG measurements with FF and 40° stimuli at 8 and 48 Hz. Response amplitudes were defined as those of the first harmonic component after Fourier analysis.

Results

In normal subjects, rod-driven responses displayed a fundamentally different behavior than cone-driven responses, particularly at low temporal frequencies. At low and intermediate temporal frequencies (≤ 12 Hz), rod-driven signals increased by a factor of about four when measured with smaller stimuli. In contrast, L- and M-cone-driven responses in this frequency region did not change substantially with stimulus size. At high temporal frequencies (≥ 24 Hz), both rod- and cone-driven response amplitudes decreased with decreasing stimulus size. Signals obtained from rod-isolating stimuli under these conditions are likely artefactual. Interestingly, in RP patients, both rod-driven and L-cone-driven ERGs were similar using 40° and FF stimuli.

Conclusion

The increased responses with smaller stimuli in normal subjects to rod-isolating stimuli indicate that a fundamentally different mechanism drives the ERGs in comparison with the cone-driven responses. We propose that the increased responses are caused by stray light stimulating the peripheral retina, thereby allowing peripheral rod-driven function to be studied using the triple silent substitution technique at photopic luminances. The method is effective in studying impaired peripheral rod- and cone- function in RP patients.