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Highlights • Cortical GABA levels correlate with contrast detection thresholds. • GABA appears to reduce contrast response gain and intrinsic noise. • Changes in contrast discrimination can be characterised by the Wilson-Cowan model., High levels of GABA (gamma-aminobutyric acid, the brain's primary inhibitory neurotransmitter) are associated with enhanced cognitive and perceptual performance. It has been proposed that these effects result from GABA reducing neural noise or variability, but the precise mechanisms remain unknown. We have measured how individual differences in GABA concentration in the visual cortex are related to performance on a visual contrast discrimination task. Our results reveal that the facilitatory strength of the typical “dipper” function elicited by this task is strongly correlated with GABA concentration. A simple, biologically plausible, network model comprising excitatory and suppressive neural populations accounts for the data well and indicates that the strength of suppression increases as GABA concentration increases. Inter-individual variations in GABA were correlated both with the inhibition strength of the model (mimicking the effect of GABA) and, inversely, with the magnitude of the response criterion. This enhanced suppression has the dual effect of suppressing noise and reducing the gain of the neural response. Our findings thus suggest that the changes in performance conferred by high GABA concentration are mediated by both a reduction of noise and, paradoxically, a reduction in neural, but not perceptual, sensitivity.

Highlights • Cortical GABA levels correlate with visuo-spatial IQ and surround suppression. • Higher levels predict higher intelligence and stronger surround suppression. • Results provide mechanism linking surround suppression and intelligence. • Results suggest role of cortical GABA levels in determining cognitive performance., Early psychological researchers proposed a link between intelligence and low-level perceptual performance. It was recently suggested that this link is driven by individual variations in the ability to suppress irrelevant information, evidenced by the observation of strong correlations between perceptual surround suppression and cognitive performance. However, the neural mechanisms underlying such a link remain unclear. A candidate mechanism is neural inhibition by gamma-aminobutyric acid (GABA), but direct experimental support for GABA-mediated inhibition underlying suppression is inconsistent. Here we report evidence consistent with a global suppressive mechanism involving GABA underlying the link between sensory performance and intelligence. We measured visual cortical GABA concentration, visuo-spatial intelligence and visual surround suppression in a group of healthy adults. Levels of GABA were strongly predictive of both intelligence and surround suppression, with higher levels of intelligence associated with higher levels of GABA and stronger surround suppression. These results indicate that GABA-mediated neural inhibition may be a key factor determining cognitive performance and suggests a physiological mechanism linking surround suppression and intelligence.

Adaptation to a spatially uniform field that increases or decreases in luminance over time yields a &ldquo, ramp aftereffect&rdquo, whereby a steady, uniform luminance appears to dim or brighten, and an appropriate non-uniform test field appears to move. We measured the duration of this aftereffect of adaptation to ascending and descending luminance for a wide range of temporal frequencies and luminance amplitudes. Three types of luminance ramp profiles were used: linear, logarithmic, and exponential. The duration of the motion aftereffect increased as amplitude increased, regardless of the frequency, slope, or ramp profile of the adapting pattern. At low luminance, this result held for ascending luminance adaptation, but the duration of the aftereffect was significantly reduced for descending luminance adaptation. This reduction in the duration of the aftereffect at low luminance is consistent with differential recruitment of temporally tuned cells of the ON and OFF pathways, but the relative independence of the effect from temporal frequency is not.

The generation of a behaviorally relevant cue to the speed of objects around us is critical to our ability to navigate safely within our environment. However, our perception of speed is often distorted by prevailing conditions. For instance, as luminance is reduced, our perception of the speed of fast-moving patterns can be increased by as much as 30%. To investigate how the cortical representation of speed may vary under such conditions, we have measured the functional MRI blood oxygen level-dependent (BOLD) response of visual cortex to drifting sine gratings at two very different luminances. The average BOLD response in all areas was band-pass with respect to speed (or equivalently, temporal frequency) and thus contained no unambiguous speed information. However, a multivariate classifier was able to predict grating speed successfully in all cortical areas measured. Similarly, we find that a multivariate classifier can predict stimulus luminance. No differences in either the mean BOLD response or the multivariate classifier response with respect to speed were found as luminance changed. However, examination of the spatial distribution of speed preferences in the primary visual cortex revealed that perifoveal locations preferred slower speeds than peripheral locations at low but not high luminance. We conclude that although an explicit representation of perceived speed has yet to be demonstrated in the human brain, multiple visual regions encode both the temporal structure of moving stimuli and luminance implicitly.

We measured the perceived speed and contrast of patterns in peripheral vision relative to foveal patterns for a range of eccentricities at both mesopic and photopic levels. The results indicate that perceived speed varies with eccentricity, speed, and luminance. At high (photopic) luminance, patterns appear slower when viewed peripherally rather than foveally, but at low (mesopic) luminance fast-moving patterns can appear faster when viewed peripherally. When perceived contrast is equated, perceived speed reduces as a function of eccentricity in a speed-independent manner. Peripheral stimuli appear faster or slower than foveal stimuli depending upon luminance-an image parameter known to influence the gain of magno and parvocellular cells. We conclude that speed encoding in the periphery is consistent with a ratio-type speed code that is weighted by ganglion cell density.

Creatine monohydrate is an organic acid that plays a key role in ATP re-synthesis. Creatine levels in the human brain vary considerably and dietary supplementation has been found to enhance cognitive performance in healthy individuals. To explore the possibility that the fMRI Blood Oxygen Level Dependent (BOLD) response is influenced by creatine levels, BOLD responses to visual stimuli were measured in visual cortex before and after a week of creatine administration in healthy human volunteers. The magnitude of the BOLD response decreased by 16% following creatine supplementation of a similar dose to that previously shown to increase cerebral levels of phosphocreatine. We also confirmed that cognitive performance (memory span) is increased. These changes were not found in a placebo group. Possible mechanisms of BOLD change are considered. The results offer potential for insight into the coupling between neural activity and the BOLD response and the more immediate possibility of accounting for an important source of variability during fMRI analysis in clinical studies and other investigations where between-subjects variance is an issue.

It has been shown that the perceived direction of a plaid with components of unequal contrast is biased towards the direction of the higher-contrast component [Stone, L. S., Watson, A. B., & Mulligan, J. B. (1990). Effect of contrast on the perceived direction of a moving plaid. Vision Research 30, 1049–1067]. It was proposed that this effect is due to the influence of contrast on the perceived speed of the plaid components. This led to the conclusion that perceived plaid direction is computed by the intersection of constraints (IOC) of the perceived speed of the components rather than their physical speeds. We tested this proposal at a wider range of component speeds (2–16deg/s) than used previously, across which the effect of contrast on perceived speed is seen to reverse. We find that across this range, perceived plaid direction cannot be predicted either by a model which takes the IOC of physical or perceived component speed. Our results are consistent with an explanation of 2D motion perception proposed by [Bowns, L. (1996). Evidence for a feature tracking explanation of why Type II plaids move in the vector sum direction at short durations. Vision Research, 36, 3685–3694.] in which the motion of the zero-crossing edges of the features in the stimulus contribute to the perceived direction of motion.

Our perception of speed has been shown to be distorted under a number of viewing conditions. Recently the well-known reduction of perceived speed at low contrast has led to Bayesian models of speed perception that account for these distortions with a slow speed ‘prior’. To test the predictive, rather than the descriptive, power of the Bayesian approach we have investigated perceived speed at low luminance. Our results indicate that, for the mesopic and photopic range (0.13–30cdm−2) the perceived speed of lower luminance patterns is virtually unaffected at low speeds (4deg s−1). We show here that the results can be accounted for by an extension to a simple ratio model of speed encoding [Hammett, S. T., Champion, R. A., Morland, A. & Thompson, P. G. (2005). A ratio model of perceived speed in the human visual system. Proceedings of Royal Society B, 262, 2351–2356.] that takes account of known changes in neural responses as a function of luminance, contrast and temporal frequency. The results are not consistent with current Bayesian approaches to modelling speed encoding that postulate a slow speed prior.

The notion that Bayesian processes are fundamental to brain function and sensory processing has recently received much support, and a number of Bayesian accounts of how the brain encodes the speed of moving objects have been proposed that challenge earlier mechanistic models. We measured the perceived speed of low contrast patterns at both low (2.5 cd m(-2)) and high (25 cd m(-2)) luminance in order to assess these competing models of how the human visual system encodes speed. At both luminance levels low contrast stimuli are perceptually biased such that they appear slower at slow (< 8 Hz) speeds but faster at higher (16 Hz) speeds. However, we find that the reversal of the perceptual bias from under- to overestimation occurred at slower speeds at low luminance. We also found that the bias was greater at slow speeds at high luminance but greater at fast speeds at low luminance. Moreover, discrimination thresholds were found to be similar at high and low luminance. These findings can be predicted by models in which speed is encoded by the relative activity within two broadly tuned temporal channels but are inconsistent with Bayesian models of speed encoding. We conclude that Bayesian processes cannot adequately account for speed encoding in the human visual system.

The perceived speed of moving images changes over time. Prolonged viewing of a pattern (adaptation) leads to an exponential decrease in its perceived speed. Similarly, responses of neurones tuned to motion reduce exponentially over time. It is tempting to link these phenomena. However, under certain conditions, perceived speed increases after adaptation and the time course of these perceptual effects varies widely. We propose a model that comprises two temporally tuned mechanisms whose sensitivities reduce exponentially over time. Perceived speed is taken as the ratio of these filters' outputs. The model captures increases and decreases in perceived speed following adaptation and describes our data well with just four free parameters. Whilst the model captures perceptual time courses that vary widely, parameter estimates for the time constants of the underlying filters are in good agreement with estimates of the time course of adaptation of direction selective neurones in the mammalian visual system.

Blurred edges appear sharper in motion than when they are stationary. We proposed a model of this motion sharpening that invokes a local, nonlinear contrast transducer function (Hammett et al, 1998 Vision Research38 2099–2108). Response saturation in the transducer compresses or ‘clips’ the input spatial waveform, rendering the edges as sharper. To explain the increasing distortion of drifting edges at higher speeds, the degree of nonlinearity must increase with speed or temporal frequency. A dynamic contrast gain control before the transducer can account for both the speed dependence and approximate contrast invariance of motION sharpening (Hammett et al, 2003 Vision Research in press). We show here that this model also predicts perceived sharpening of briefly flashed and flickering edges, and we show that the model can account fairly well for experimental data from all three modes of presentation (motion, flash, and flicker). At moderate durations and lower temporal frequencies the gain control attenuates the input signal, thus protecting it from later compression by the transducer. The gain control is somewhat sluggish, and so it suffers both a slow onset, and loss of power at high temporal frequencies. Consequently, brief presentations and high temporal frequencies of drift and flicker are less protected from distortion, and show greater perceptual sharpening.

Blurred edges appear sharper in motion than when they are stationary. We (Vision Research 38 (1998) 2108) have previously shown how such distortions in perceived edge blur may be accounted for by a model which assumes that luminance contrast is encoded by a local contrast transducer whose response becomes progressively more compressive as speed increases. If the form of the transducer is fixed (independent of contrast) for a given speed, then a strong prediction of the model is that motion sharpening should increase with increasing contrast. We measured the sharpening of periodic patterns over a large range of contrasts, blur widths and speeds. The results indicate that whilst sharpening increases with speed it is practically invariant with contrast. The contrast invariance of motion sharpening is not explained by an early, static compressive non-linearity alone. However, several alternative explanations are also inconsistent with these results. We show that if a dynamic contrast gain control precedes the static non-linear transducer then motion sharpening, its speed dependence, and its invariance with contrast, can be predicted with reasonable accuracy.

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 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.

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. Copyright © 1996 Elsevier Science Ltd.

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 om 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.

Slowly moving low contrast patterns appear to drift more slowly than higher contrast patterns. It has been reported that this effect of contrast is reversed for flickering patterns such that they appear to flicker faster than high contrast patterns. This apparent difference in the effect of contrast on perceived speed and flicker may place important constraints upon models of speed encoding in the human visual system. We have measured perceived speed and flicker over a range of spatial and temporal frequencies. The results indicate that contrast has qualitatively (but not quantitatively) similar effects upon perceived speed and flicker. The results also indicate that the effect of contrast upon perceived speed is likely to be inherited from the effect of contrast upon perceived flicker. These findings allow a relaxation of previous constraints upon models of speed encoding.

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.

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.

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 300msec, to minimize fading cues, this improvement disappeared. Similarly, at 4c/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.

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.

It is widely supposed that things tend to look blurred when they are moving fast. Previous work has shown that this is true for sharp edges but, paradoxically, blurred edges look sharper when they are moving than when stationary. This is 'motion sharpening'. We show that blurred edges also look up to 50% sharper when they are presented briefly (8-24 ms) than at longer durations (100-500 ms) without motion. This argues strongly against high-level models of sharpening based specifically on compensation for motion blur. It also argues against a recent, low-level, linear filter model that requires motion to produce sharpening. No linear filter model can explain our finding that sharpening was similar for sinusoidal and non-sinusoidal gratings, since linear filters can never distort sine waves. We also conclude that the idea of a 'default' assumption of sharpness is not supported by experimental evidence. A possible source of sharpening is a nonlinearity in the contrast response of early visual mechanisms to fast or transient temporal changes, perhaps based on the magnocellular (M-cell) pathway. Our finding that sharpening is not diminished at low contrast sets strong constraints on the nature of the nonlinearity.

SENSORY systems can adapt to the conditions imposed on them1. 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 threshold2–4. Most neurons of the striate cortex reduce their responsiveness after being stimulated for some time by a high-contrast pattern5–7. Such an effect may lie behind these psychophysical adaptation phenomena2–4. 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 orientations8. Neurophysiological evidence suggests that neurons with different orientation preferences have inhibitory interconnections9–13. 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.

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 (>10min arc) moving patterns always appeared sharper than their static analogues, but for small blurs (

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.

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.

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 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.

Adaptation, a change in response to a sustained stimulus, can be demonstrated in motion perception by velocity aftereffects—changes in the apparent speed of a moving pattern following adaptation. We measured changes in the apparent speed of sinusoidal gratings drifting at 4 or 7.5 deg/s during 30 s of adaptation followed by 30 s of recovery. The apparent speed of the patterns fell to approximately half the unadapted apparent speed, and the time constants of adaptation were much faster (5 s) than for recovery (22 s). Part of the loss of apparent speed (approximately 12%) was related to a loss of apparent contrast with adaptation. Sensitivity to speed increments and speed decrements increased during adaptation and was well described by a Weber fraction based on apparent speed. The results suggest that adaptation to motion, like light adaptation, may serve to improve an observer’s sensitivity to the prevailing environment.

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.

Perceived speed is modulated by a range of stimulus attributes such as contrast, luminance and adaptation duration. It has been suggested that such changes in perceived speed may influence driving behaviour. In order to evaluate the effect of luminance on driving speed we have measured subjects’ driving speed in a driving simulator for a range of luminance and speed over time. The results indicate that reducing luminance results in a decrease in driving speed for all speeds measured. This reduction in driving speed at low luminance is consistent with previous findings that perceived speed increases at low luminance. However, the results also indicated that driving speed remained stable over a 30 s period. The stability of driving speed over time is inconsistent with previous findings that perceived speed reduces exponentially as a function of adaptation duration. The results are suggestive of a scheme whereby driving speed is consistent with the known effects of luminance upon perceived speed but may also be modulated by higher order processes that serve to maintain a constant speed over time.