The dynamics of velocity adaptation in human vision.

Since Barlow and Hill's classic study of the adaptation of the rabbit ganglion cell to movement [1], there have been several reports that motion adaptation is accompanied by an exponential reduction in spike rate, and similar estimates of the time course of velocity adaptation have been found across species [2-4]. Psychophysical studies in humans have shown that perceived velocity may reduce exponentially with adaptation [5,6]. It has been suggested that the reduction in firing of single cells may constitute the neural substrate of the reduction in perceived speed in humans [1,5-7]. Although a model of velocity coding in which the firing rate directly encodes speed may have the advantage of simplicity, it is not supported by psychophysical research. Furthermore, psychophysical estimates of the time course of perceived speed adaptation are not entirely consistent with physiological estimates. This discrepancy between psychophysical and physiological estimates may be due to the unrealistic assumption that speed is coded in the gross spike rate of neurons in the primary visual cortex. The psychophysical data on motion processing are, however, generally consistent with a model in which perceived velocity is derived from the ratio of two temporal channels [8-14]. We have examined the time course of speed adaptation and recovery to determine whether the observed rates can be better related to the established physiology if a ratio model of velocity processing is assumed. Our results indicate that such a model describes the data well and can accommodate the observed difference in the time courses of physiological and psychophysical processes.

The effects of surround contrast on contrast thresholds, perceived contrast and contrast discrimination.

Perceived contrast, contrast detection thresholds and contrast discrimination thresholds were measured in the presence and absence of surrounding patterns of a similar spatio-temporal makeup. In the foveal retina we found that the perceived contrast of the central pattern was reduced by the presence of the contrast surrounds with the effect being greatest at low test contrast. Detection thresholds were not affected and contrast discrimination thresholds were only affected over a small range of low test contrasts. However if the test pattern was made smaller, or if its central part was occluded detection thresholds were raised. In the peripheral retina detection thresholds were raised and discrimination thresholds were affected over most of the range of contrasts. We argue that the pattern of results resembles those produced in masking paradigms where the test and mask are coextensive if the spatial range of interactions is taken into account and hence the effects of the contrast surround may be merely a manifestation of normal masking processes.

Motion blur and motion sharpening: temporal smear and local contrast non-linearity.

Blurred images may appear sharper when drifting than when stationary. But, paradoxically, moving sharp edges may appear more blurred. To resolve this paradox, the perceived sharpness of drifting, blurred, square wave gratings was compared with that of their static analogues over a range of speeds, blurs and spatial frequencies. Both motion blur and motion sharpening occurred, depending upon the physical blur of the patterns. For large extents of blur (> 10 min arc) moving patterns always appeared sharper than their static analogues, but for small blurs (< 10 min arc) moving edges appeared more blurred than stationary ones. We present a quantitative model for the distortion of waveforms in motion based on two factors: (i) visual temporal integration that smears moving images, and (ii) a local contrast non-linearity that increasingly sharpens the effective profile of edges as speed and contrast increase. We suggest that a plausible account of the speed-dependent non-linearity is the differential recruitment of M and P cells at different speeds.

Spatio-temporal vernier acuity.

The study of space-time vernier (STV) provides information on the spatio-temporal structure of the visual system in the same way that the classical spatio-spatial vernier (SSV) provides information on its spatial structure. The transposition of a SSV task into a STV one yields the following experimental format: an object (in the present case a Gaussian Blob) drifts with a constant velocity, V, disappears at x0, t0 and reappears after a variable duration delta t at a position x1 +/- delta x with x1 the correct position (given a constant V) and delta x the minimum (positive and negative) spatial offset discriminable from x0, i.e. the STV threshold. Observer's task is to specify whether the reappearance position is ahead of, or behind x1. The STV functions of delta t measured for 1, 5 and 10 deg/s reference velocities are linear with non-zero spatial and temporal intercepts at the origin. We refer to these x and t intercepts as dynamic dmin and tmin. Dynamic dmin is the smallest instantaneous displacement (infinite velocity) discriminable from a continuous drift, V. Dynamic tmin is the shortest 'motion stop' discriminable from the same continuous drift, V. To our knowledge these quantities have not yet been assessed. Estimated dynamic dmin increases with V. whereas tmin is more or less V independent suggesting that the motion sensors presumably involved in the STV task have peak spatial frequencies inversely proportional with V and a temporal frequency characteristic independent of V (at least within the studied range). The observed STV linearity with the spatio-temporal separation implies that the STV task is equivalent to a velocity discrimination. Two additional observations yield support to this conclusion. (i) The slopes of these functions yield velocities very similar to those discriminable from the reference V in a standard V-discrimination experiment. (ii) The predicted STV performances based on a decomposition of the task into two velocity discrimination tasks run as independent experiments are reasonably accurate.

Motion blur and motion sharpening in the human visual system.

The effect of motion sharpening upon blur discrimination thresholds was examined for a range of speeds and blur widths. Blur discrimination thresholds were measured for drifting edges whose blur was either physically or perceptually constant. Under conditions where edges were kept at a constant physical blur width, discrimination thresholds rose as a function of speed as previously reported. However, when the perceived blur of edges was held constant, discrimination performance was more-or-less constant for speeds up to at least 6.3 deg sec-1. The results indicate that the deterioration of blur discrimination performance with speed may be due to motion sharpening and not motion blur as has previously been suggested. The results are discussed in terms of a scheme whereby a non-linearity in motion processing serves to sharpen moving edges, whilst the finite integration time of the system tends to smear them.

Adaptation to motion of a second-order pattern: the motion aftereffect is not a general result.

It has become apparent from recent work that the spatial frequency and orientation content of the first-order (luminance) carrier is very important in determining the properties of a second-order (contrast) modulation of that carrier. In light of this we examined whether there was any evidence for a motion aftereffect in one-dimensional second-order patterns containing only two sinusoidal luminance components: a spatial beat. The stimuli were either 1 cpd luminance sinusoids or 1 cpd luminance beats modulating a carrier sinusoid of 5 cpd. The magnitude of any motion aftereffect, or any directionally specific effect of adaptation, was measured for all combinations of first and second-order test and adapting patterns. Both flickering and non-flickering stimuli were used. The results indicate that a motion aftereffect is only induced by first-order adapting stimuli, and likewise, is only measurable in first-order test stimuli. We find no evidence for any directionally specific effect of adaptation in second-order stimuli, whether the test is counterphased or otherwise. These results apparently conflict with recent reports of a second-order induced motion aftereffect, but are consistent with many other findings which show differences between the detection of motion for first and second-order stimuli. We conclude that the induction of a motion aftereffect for second-order stimuli is not a general result and is critically dependent upon (amongst other things) the local properties of the stimulus, including the spatial frequency and orientation content of the first-order carrier.

Motion sharpening: evidence for the addition of high spatial frequencies to the effective neural image.

The perceived blur of drifting sinusoidal gratings was compared to that of static, blurred "square wave" gratings before and after adaptation to a missing fundamental (MF) pattern. The results indicate that the perceived blur of a drifting sine grating is inversely related to its drift speed. However, after adaptation to a MF pattern, this effect is reduced. The adaptation effect is most profound for low contrast gratings. The results provide tentative evidence for a non-linear stage in motion processing which serves to introduce higher frequencies into the neural image which are not present in the original signal.

Spatial frequency adaptation: threshold elevation and perceived contrast.

We have measured the spread of contrast adaptation across the dimension of spatial frequency. Threshold elevation was tightly tuned to the adapting spatial frequency but became much broader as test contrast was increased. This means that, for a given test frequency, there are some frequencies which do not raise threshold but do result in a loss of perceived contrast. The contrast dependence, retinal specificity and interocular transfer of adaptation effects elicited from same-and remote-frequency adaptation were compared. While we were able to show some distinct differences between threshold and suprathreshold tests, we were unable to demonstrate any reliable differences in the retinal specificity and interocular transfer between same- and remote-frequency adaptation.

The effect of contrast adaptation on briefly presented stimuli.

Wilson and Humanski (1993) have recently reported evidence that adapting to low temporal frequency sinewave gratings yields little threshold elevation for briefly presented test stimuli. We postulated that brief stimuli may be detected by a transient channel which would be minimally affected by a low temporal frequency adapting pattern. We therefore measured the effect of adaptation on briefly presented test stimuli for a wider range of adapting temporal frequencies. The results indicate that adaptation may yield threshold elevation for briefly presented stimuli and that threshold elevation is greater for high than low temporal frequency adapting patterns. These results are consistent with the hypothesis that briefly presented stimuli are detected by a transient channel.

Temporal beats in the human visual system.

The human visual system is generally assumed to comprise multiple independent channels tuned to limited ranges of spatial and temporal frequency. However, it is known that spatial beats effectively mask a sinusoidal grating whose spatial frequency coincides with the beat frequency of the mask, even though no Fourier energy exists in the mask at that frequency. We investigated whether analogous masking effects occur in the temporal domain and whether adaptation to either spatial or temporal beats results in threshold elevation. Our results show that temporal beat patterns do not mask a grating temporally modulated at the beat frequency. Adaptation to neither spatial nor temporal beats results in measurable threshold elevation at the beat frequency. Since a luminance non-linearity would lead to temporal as well as spatial distortion products, we conclude that sensitivity to spatial beats does not rely on the presence of distortion products but instead reflects subsequent processing that occurs in the spatial domain only. The most likely site of such processing is cortical.

Perceived contrast as a function of adaptation duration.

We measured how the perceived contrast of a sinusoidal grating fades as a function of time. Measurements were made for a range of temporal and spatial frequencies and eccentricities. Patterns of high temporal and low spatial frequency exhibited a greater and more rapid loss of apparent contrast (fade) than those of medium frequencies. The rate and amount of fading for a subgroup of moderate frequencies increased when presented peripherally rather than foveally. Further measurements revealed that gratings of disparate spatial frequencies, but with the same threshold sensitivity, exhibit very different fading characteristics but equal threshold elevation. We conclude that the differential loss of apparent contrast is not an artefact of differing proximities to threshold, nor can it be accounted for by differences in the adaptability of underlying spatio-temporal mechanisms at threshold. The differences in fading may thus reflect either a difference in the adaptability of underlying channels above threshold or a differential contribution of such channels to perceived contrast.

Transparent motion from feature- and luminance-based processes.

The apparent motion of a discretely displaced complex waveform with a periodic contrast modulation or "beat" of frequency f and sinusoidal components of frequencies 3f and 4f was examined at various interstimulus intervals (ISIs). At short ISIs perception of motion of both the beat and an aliased component of the waveform results in transparent motion. At longer ISIs motion is perceived only in the direction of the features of the waveform. The transparent motion observed at short ISIs indicates that, under certain conditions, "short-range" motion sensors do not constrain "long-range" feature processing and both may be active simultaneously.

Two temporal channels or three? A re-evaluation.

The initial filtering of the image by the human visual system involves only a small number of temporal filters. Several studies suggest there are in fact only two, but some suggest that a third filter, sensitive to high frequencies, exists, at least at low spatial frequencies. This conclusion is derived in part from the observation that temporal frequency discrimination performance is better at very high (30-40 Hz) than at medium (20 Hz) temporal frequencies. We show that this apparent improvement at high frequencies is not real but is an artifact of differences in the rate of perceptual fading as a function of temporal frequency. Using suprathreshold counterphase gratings and a stimulus duration of 1.5 sec we replicated the finding of an improvement at high frequencies at a low (0.5 c/deg) spatial frequency. But when duration was reduced to 300 msec, to minimize fading cues, this improvement disappeared. Similarly, at 4 c/deg, the improvement was present at 3 sec duration but absent at 1.5 sec or less. Direct evidence that this effect of duration reflects differences in the ability to use fading cues was obtained in an experiment in which naive subjects were instructed to discriminate on the basis of fading: at high temporal frequencies and long durations performance was as good or better than for subjects instructed to use frequency; at short durations performance on this task was poor. Thus, the claim that a third temporal channel exists may need to be re-evaluated.

Subtractive and divisive adaptation in the human visual system.

Sensory systems can adapt to the conditions imposed on them. In the visual system, adapting to a pattern increases the threshold of the ability to see that pattern, and reduces the perceived contrast of the pattern above threshold. Most neurons of the striate cortex reduce their responsiveness after being stimulated for some time by a high-contrast pattern. Such an effect may lie behind these psychophysical adaptation phenomena. These adaptation effects have been reported to be confined to patterns of similar orientation, which is understandable in that the visual neurons that adapt are only excited by a small range of orientations. Neurophysiological evidence suggests that neurons with different orientation preferences have inhibitory interconnections. It is therefore of interest to explore the possible effects of these connections on perception. Here we show that adapting to a horizontal pattern can reduce the perceived contrast of a vertical test pattern more than a horizontal test pattern. These 'cross-orientation' effects are modelled by a division-like process, whereas the more normal 'similar-orientation' effects are modelled by a subtractive process.

Flicker adaptation in the periphery at constant perceived modulation depth.

At constant physical flicker modulation depth, the time taken to adapt to flicker in the periphery varies inversely with temporal frequency. It has recently been suggested that this effect may indicate differential susceptibility to adaptation of the underlying temporal mechanisms. Using suprathreshold gratings, temporally modulated in contrast at constant perceived, rather than physical, modulation depth, we found the opposite result: the time required to adapt increased with temporal frequency. Given some uncertainty concerning the appropriateness of employing apparent or physically constant modulation depths, we conclude that rate of adaptation does not, at present, provide clear evidence as to the nature of the underlying temporal mechanisms.