Two temporal channels or three? A re-evaluation

Mathematical modeling and experimental verification of aging human eyes polarization sensitivity

The polarization perception sensitivity of the human eyes affects the perceived polarized image quality. In this paper, we used polarized spatiotemporal structured images to develop a spatiotemporal age mapping of the polarization perception of human eyes. We built an optical modulation transfer function mathematical model of the aging human eyes with spatiotemporal frequency domains and introduced the Stokes vector to analyze the polarized images. The proposed model provides a testing method based on a set of polarization images with spatiotemporal frequencies varying according to the perception of differently aged viewers. Then, we experimentally validated the proposed model by performing polarization perception tests on a group of volunteers. The test method has the diagnostic potential to confirm the health of human eyes and identify potential age-related macular diseases.

Implicit representations of luminance and the temporal structure of moving stimuli in multiple regions of human visual cortex revealed by multivariate pattern classification analysis

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.

Binocular rivalry produced by temporal frequency differences

When the eyes view images that are sufficiently different to prevent binocular fusion, binocular rivalry occurs and the images are seen sequentially in a stochastic alternation. Here we examine whether temporal frequency differences will trigger binocular rivalry by presenting two dynamic random-pixel arrays that are spatially matched but which modulate temporally at two different rates. We found that binocular rivalry between the two temporal frequencies did indeed occur, provided the frequencies were sufficiently different. Differences greater than two octaves (i.e., a factor of four) produced robust rivalry with clear-cut alternations similar to those experienced with spatial rivalry and with similar alternation rates. This finding indicates that temporal information can produce binocular rivalry in the absence of spatial conflict and is discussed in terms of rivalry requiring conflict between temporal channels.

Distinct perceptual grouping pathways revealed by temporal carriers and envelopes

S. E. Guttman, L. A. Gilroy, and R. Blake (2005) investigated whether observers could perform temporal grouping in multi-element displays where each local element was stochastically modulated over time along one of several potential dimensions--or "messenger types"--such as contrast, position, orientation, or spatial scale. Guttman et al.'s data revealed that grouping discards messenger type and therefore support a single-pathway model that groups elements with similar temporal waveforms. In the current study, we carried out three experiments in which temporal-grouping information resided either in the carrier, the envelope, or the combined carrier and envelope of each messenger's timecourse. Results revealed that grouping is highly specific for messenger type if carrier envelopes lack grouping information but largely messenger nonspecific if carrier envelopes contain grouping information. These imply that temporal grouping is mediated by several messenger-specific carrier pathways as well as by a messenger-nonspecific envelope pathways. Findings also challenge simple temporal-filtering accounts of perceptual grouping (E. H. Adelson & H. Farid, 1999).

The motion aftereffect of transparent motion: Two temporal channels account for perceived direction

Adaptation to orthogonal transparent patterns drifting at the same speed produces a unidirectional motion aftereffect (MAE) whose direction is opposite the average adaptation direction. If the patterns move at different speeds, MAE direction can be predicted by an inverse vector average, using the observer's motion sensitivity to each individual pattern as vector magnitudes. These weights are well approximated by the duration of each pattern's MAE, as measured with static test patterns. However, previous efforts to use the inverse-vector-average rule with dynamic test patterns have failed. Generally, these studies have used spatially and temporally broadband test stimuli. Here, in order to gain insight into the possible contribution of temporal channels, we filtered our test pattern in the temporal domain to produce five ideal, octave-width pass-bands. MAE durations were measured for single-component stimuli drifting at various adaptation speeds and tested at a range of temporal frequencies. Then, two components with orthogonal directions and different speeds were combined and the direction of the resulting MAE was measured. The key findings are that: (i) for a given adaptation speed, the duration of a single component's MAE is dependent on test temporal frequency; (ii) the direction of MAEs produced by transparent motion (i.e., bivectorial adaptation) also varies strongly as a function test temporal frequency (by up to 90 degrees for some speed pairings); and (iii) the inverse-vector-average rule predicts the direction of the transparent MAE provided the MAE durations used to weight the vector combination were obtained from stimuli matched in adaptation speed and test temporal frequency. These results are discussed in terms of the number and shape of temporal channels in our visual system.

Transient lumanopia at high intensities

Transient lumanopia is the loss of sensitivity to a flicker burst presented in early dark adaptation. We earlier reported that lumanopia with 18 Hz flicker was greatest after turning off 400 td adapting fields and progressively disappeared as the adapting field was intensified. We now report that lumanopia can occur with intense adapting fields, but only with faster flickers (e.g. 40 Hz, 5000 td fields). Dimming the field (but not brightening it) can also produce lumanopia. The results illustrate frequency-dependent attenuation by a temporal filter whose parameters are set by light adaptation and which change abruptly when the field is dimmed or extinguished.

Effect of adaptation direction on the motion VEP and perceived speed of drifting gratings

The N200 amplitude of the motion-onset VEP evoked by a parafoveal grating of variable contrast (0.5-64%), constant speed (2 degrees/s), direction (horizontally rightward), and spatial frequency (2 cpd) was studied before and after adaptation to a stationary or drifting grating (1, 2, or 4 degrees/s rightward or leftward). These results are compared to those for the pattern-appearance VEP. Psychophysical measurements were made simultaneously of the perceived speed. While iso-directional (rightward) adaptation leads to a mean amplitude reduction of 39%, the decrease after counter-directional adaptation has a size of 20%. The post-adaptation matches of perceived speed differ in dependence on the iso-directional adapting speed and decrease on average to 98%, 85%, and 69% of the pre-adapt perceived speed after 1, 2, and 4 degrees/s adapting speeds, respectively. The perceived speed is moderately reduced (83% of the pre-adapt value) after counter-directional adaptation nearly independently of the adapting speed. A model of velocity processing is presented, which enables us to predict the trends of the experimental motion VEP and perceived speed data.

A visual motion sensor based on the properties of V1 and MT neurons

The motion response properties of neurons increase in complexity as one moves from primary visual cortex (V1), up to higher cortical areas such as the middle temporal (MT) and the medial superior temporal area (MST). Many of the features of V1 neurons can now be replicated using computational models based on spatiotemporal filters. However until recently, relatively little was known about how the motion analysing properties of MT neurons could originate from the V1 neurons that provide their inputs. This has constrained the development of models of the MT-MST stages which have been linked to higher level motion processing tasks such as self-motion perception and depth estimation. I describe the construction of a motion sensor built up in stages from two spatiotemporal filters with properties based on V1 neurons. The resulting composite sensor is shown to have spatiotemporal frequency response profiles, speed and direction tuning responses that are comparable to MT neurons. The sensor is designed to work with digital images and can therefore be used as a realistic front-end to models of MT and MST neuron processing; it can be probed with the same two-dimensional motion stimuli used to test the neurons and has the potential to act as a building block for more complex models of motion processing.

Adaptation from invisible flicker

Human ability to resolve temporal variation, or flicker, in the luminance (brightness) or chromaticity (color) of an image declines with increasing frequency and is limited, within the central visual field, to a critical flicker frequency of approximately 50 and 25 Hz, respectively. Much remains unknown about the neural filtering that underlies this frequency-dependent attenuation of flicker sensitivity, most notably the number of filtering stages involved and their neural loci. Here we use the process of flicker adaptation, by which an observer's flicker sensitivity is attenuated after prolonged exposure to flickering lights, as a functional landmark. We show that flicker adaptation is more sensitive to high temporal frequencies than is conscious perception and that prolonged exposure to invisible flicker of either luminance or chromaticity, at frequencies above the respective critical flicker frequency, can compromise our visual sensitivity. This suggests that multiple filtering stages, distributed across retinal and cortical loci that straddle the locus for flicker adaptation, are involved in the neural filtering of high temporal frequencies by the human visual system.

On temporal hyperacuity in the human visual system

The spatial grain of the human visual system has always been a central topic for visual sciences, and the optical and physiological basis of perceptual limitations are well described. In particular, we have thorough accounts of spatial hyperacuity, which refers to a precision in the spatial localisation of stimulus contours that is better than the photoreceptor grain that determines spatial resolution. However, although the temporal resolution of the human visual system is comparably well described, we have almost no direct knowledge about the precision of localising visual stimuli in time in the absence of correlated spatial cues. The present study addresses this question by comparing directly the temporal resolution of human observers with their temporal acuity as measured in a temporal bisection task. Despite some improvement with practice, temporal acuity in this task does not fall below 20-30 ms in the best case, which is similar to the temporal resolution limit, and performance does not improve for comparison tasks with multiple stimulus presentations. The absence of visual hyperacuity for purely temporal modulations as tested here contrasts with processing limitations for other types of visual information in comparable tasks, and with other sensory modalities, in particular to those of the auditory system. Such differences can be interpreted in the context of the ecological requirements for organising behaviour, and the functional design of nervous systems.

A model of speed tuning in MT neurons

We have shown previously that neurons in the middle temporal (MT) area of primate cortex have inseparable spatiotemporal receptive fields-their response profiles exhibit a ridge that is oriented in the spatiotemporal frequency domain, and this orientation predicts the neurons' preferred speed. When measured in spatiotemporal frequency space, such MT spectral receptive field (SRF) properties are closely matched to the spectrum generated by a moving edge. In contrast, V1 neurons have SRF properties that are poorly matched to moving edge spectra, indicating that V1 neurons are not tuned to a particular image speed but rather to specific spatial and temporal frequencies. Here we describe a neural mechanism based directly on the properties of V1 neurons that is able to explain the SRF change that occurs between V1 and MT. We outline the theory behind this transformation and posit an explanation for how the visual system extracts true speed (independent of spatial frequency) from retinal image motion. We tested this speed model against our MT neuron data and found that it provides an excellent account of speed tuning in MT.

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.

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.

What covariance mechanisms underlie green/red equiluminance, luminance contrast sensitivity and chromatic (green/red) contrast sensitivity?

In order to investigate the mechanisms underlying green/red equiluminance matches in human observers and their relationship to mechanisms subserving luminance and/or chromatic (green/red) contrast sensitivity, we tested 21 human subjects along these dimensions at 16 different spatial and temporal frequencies (spatial frequency, 0.25-2 c/deg; temporal frequency, 2-16 Hz) and applied factor analysis to extract mechanisms underlying the data set. The results from our factor analysis revealed separate sources of variability for green/red equiluminance, luminance sensitivity and chromatic sensitivity, thus suggesting separate mechanisms underlying each of the three main conditions. When factor analysis was applied separately to green/red equiluminance data, two temporally-tuned factors were revealed (factor 1, 2-4 Hz; factor 2, 8-16 Hz), suggesting the existence of separate mechanisms underlying equiluminance settings at low versus high temporal frequencies. In addition, although the three main conditions remained separate in our factor analysis of the entire data set, our correlation matrix nonetheless revealed systematic correlations between equiluminance settings and luminance sensitivity at high temporal frequencies, and between equiluminance settings and chromatic sensitivity at low temporal frequencies. Taken together, these data suggest that the high temporal frequency factor underlying green/red equiluminance is governed predominantly by luminance mechanisms, while the low temporal frequency factor receives contribution from chromatic mechanisms.

Temporal detection in human vision: dependence on spatial frequency

In the study of perception of temporal changes in luminance, it is customary to model perceptual performance as based on one or more linear filters. The task is then to estimate the temporal impulse responses or the representation of the impulse response in the frequency domain. Previously, temporal masking data have been used to estimate the properties and numbers of these temporal mechanisms (filters) in central vision for 1-cycle-per-degree (cpd) targets [Vision Res. 38, 1023 (1998)]. The same methods have been used to explore how properties of the estimated filters change with stimulus contrast energy [J. Opt. Soc. Am. A 14, 2557 (1997)]. We present estimated properties for temporal mechanisms that detect low spatial-frequency patterns. The results indicate that two filters provide the best model for performance when mask contrast is significant. There are also differences between properties for mechanisms that detect signal spatial frequencies of 1 cpd and 1/3 cpd. The sensitivity of the low-pass mechanism relative to the bandpass mechanism is reduced at 1/3 cpd, consistent with previous findings.

Red-green and achromatic temporal filters: a ratio model predicts contrast-dependent speed perception

We simultaneously measured detection and identification performance by using isoluminant red-green (RG) and achromatic flickering stimuli and fitted these data with a modified line-element model that does not make high-threshold assumptions. The modeling shows that detection and identification data are adequately described by postulating only two underlying temporal filters each for RG and achromatic vision, even when more than two threshold classifications are evident. We use a spatial frequency of 1.5 cycles per degree (c/deg) and compare the derived temporal impulse response functions with those obtained previously with the use of 0.25 c/deg stimuli under otherwise identical conditions [J. Opt. Soc. Am. A 13, 1969 (1996)]. We find that at 1.5 c/deg the luminance impulse response functions peak later and integrate out to longer times compared with those measured at 0.25 c/deg. For RG stimuli, although their relative overall sensitivities change, the impulse response functions are similar across spatial frequency, indicating a constancy of chromatic temporal properties across spatial scales. In a second experiment, we measured RG and achromatic flicker discrimination over a wide range of suprathreshold contrasts. These data suggest a common nonlinear contrast response function operating after initial temporal filtering. Using a ratio model of speed perception in which both RG and achromatic filters are combined at a common motion site, we can predict (1) the perceived slowing of RG stimuli compared with the perceived drift of achromatic drifting stimuli, (2) the contrast dependency of speed perception for RG and achromatic drifting stimuli, and (3) how this dependency changes with base speed. Thus we conclude that there is no need to postulate separate mechanisms for fast and slow motion [Nature (London) 367, 268 (1994)], since a unified ratio model can explain both RG and achromatic contrast-speed dependency.

What is the denominator for contrast normalisation?

The perceived speed of 1 c/deg sinusoidal gratings of contrast 0.02 was measured in the presence of high contrast (0.50) 1 c/deg sinusoidal gratings (called modifiers). The modifiers drifted or were counterphase modulated at various temporal frequencies. The presence of a modifier with temporal frequencies (0 and 3 Hz) lower than the low contrast moving grating decreased its perceived speed while the presence of modifiers with higher temporal frequencies (8, 12 and 16 Hz) increased its perceived speed. A modifier of the same temporal frequency (6 Hz) as the standard grating had no effect upon the perceived speed of the low contrast gratings. Moving modifiers are more effective than counterphase flickering modifiers in biasing the perceived speed of low contrast gratings if they move in the same direction as the test grating and less effective if they move in the opposite direction. Finally, a modifier presented in an annulus surrounding the test grating is more effective than a modifier presented in a circular patch above or below the test grating in raising the perceived speed of low contrast gratings. This suggests that perceived speed depends on the ratio of low and high temporal frequency signals averaged over a significant area of the visual field.

Temporal mechanisms underlying flicker detection and identification for red-green and achromatic stimuli

We have simultaneously measured detection and temporal frequency identification for both red-green isoluminant and achromatic stimuli over a range of temporal frequencies for two observers. Results show that temporal frequency identification can be made along the temporal frequency dimension for both red-green and achromatic stimuli at contrasts close to detection threshold. In general, temporal frequency identification was better for the achromatic than for the red-green stimuli; however, the level of chromatic identification performance was still sufficient to permit us to reject the notion that the red-green mechanism embodies a single temporal filter. We have developed a model based on signal detection theory that assumes that detection and identification both depend on the properties of the temporal filters underlying each mechanism. From this we have derived putative underlying shapes and sensitivities for the temporal filters of the red-green and achromatic mechanisms that comprise a low-pass and a bandpass filter for red-green color vision and two bandpass filters for luminance vision. Finally, we suggest that the relative perceived slowing of isoluminant stimuli may be accounted for by a common motion analysis subserved by different front-end temporal filters for red-green and achromatic motion signals.

Flicker brightness enhancement and visual nonlinearity

The purpose of this study was to investigate the nonlinear mechanism underlying brightness enhancement, in which a flickering stimulus appears brighter than a steady stimulus of equal mean luminance. The flickering and matching stimuli were temporally alternated. Both were cosine windowed to minimize the potential effects of temporal transients. Subjects adjusted the amplitude of the matching stimulus to match it in brightness to the flickering stimulus. The temporal frequency, modulation, and waveform of the flickering stimulus were varied. With sinusoidal flicker, brightness enhancement increased with increasing modulation at all frequencies, peaking at about 16 Hz at full modulation. The results were modeled by a broad temporal filter followed by a single accelerating nonlinearity. The derived temporal sensitivity of the early filter inferred from brightness enhancement decreased more slowly at high frequencies than the filter(s) inferred from flicker modulation thresholds. With low frequency sawtooth flicker, brightness enhancement was phase-dependent at low, but not at high modulations, suggesting that multiple neural mechanisms may also be involved in addition to an early nonlinearity.

Evidence of spatial and temporal channels in the correlational structure of human spatiotemporal contrast sensitivity

1. The statistical correlation of detection thresholds for pairs of stimuli should be higher for stimuli detected by the same mechanism than for stimuli detected by different mechanisms--a property that can be used to probe the visual mechanisms that underlie detection. 2. Correlation of contrast sensitivities for pairs of spatiotemporal stimuli is approximately a linear function of spatial or temporal frequency separation in octaves. Using the slope of this function as an index of neural processing gave results consistent with: more spatial mechanisms than temporal; more spatial mechanisms at low temporal frequencies than at high; and at least two temporal mechanisms active at spatial frequencies up to 22.6 cycles deg-1. 3. This method of analysing sensitivity data is insensitive to experimental conditions and applicable to any sensory detection task mediated by tuned channels. In addition to being applicable to psychophysical sensitivity measurements, it may also be useful in analysing some kinds of electrophysiological measurements that pool the responses from many active mechanisms (such as evoked potentials).

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.

A unified account of three apparent motion illusions

We discuss three motion illusions, the fluted square wave illusion, the reverse phi illusion and the Pantle illusion. In these illusions reversed apparent motion is either induced or eliminated by the introduction of a blank inter-frame-interval between the frames of the apparent motion sequence. In order to simulate these effects with the multi-channel gradient model we had to introduce low-pass spatial filters and second-order temporal differentiating filters. These illusions have been used as evidence of multiple motion mechanisms. Here we demonstrate that they can be considered as emergent properties of a single computational strategy.

The processing of temporal modulation at different levels of retinal illuminance

How does our temporal vision change as the mean illuminance reduces? We have examined the processing of near-threshold temporal information for a range of illuminance values (2850--0.15 phot td). At high illuminance, the modulation transfer function can be shown to be mediated via three underlying temporal filters that vary in sensitivity with spatial frequency. As the mean illuminance decreases these channels appear to change their sensitivity. Even at the lowest (scotopic) illuminance levels we were able to find evidence for at least two channels mediating detection threshold. There are also changes in the tuning properties of these channels such that the processing of high temporal frequencies is differentially compromised, resulting in a reduction in the flicker fusion limit of each channel, and a shift in the peak of the band-pass channel. The slope of the fall-off in sensitivity at high temporal frequencies is unaffected by test spatial frequency at each illuminance level, suggesting its limiting factor is one that is insensitive to spatial frequency. We propose that the changes in the tuning of the temporal filters occur because of an early (e.g. photoreceptor) change in the response dynamics, or by interactions between photoreceptors, rather than changes at or beyond the level of the channel response.

Suprathreshold temporal-frequency discrimination in the fovea and the periphery

To address the question of whether temporal-frequency information in the fovea and the periphery is processed in fundamentally different ways we measured temporal-frequency-discrimination thresholds for spatiotemporally narrow-band stimuli presented at suprathreshold contrast. Temporal-frequency-discrimination thresholds are similar (within a factor of 2) at the fovea and at 30 degrees in the periphery. We use a line-element approach and three spatiotemporally separable temporal mechanisms to model foveal and peripheral data with the same degree of fidelity. These findings suggest that not only are the front-end temporal mechanisms in the fovea and periphery likely to be similar but also the way in which their outputs are combined at more central sites is the same.

Extraction of optical velocity by use of multi-input Reichardt detectors

We study the possibility of a metrical readout of velocity from an ensemble of Reichardt correlators. We show that with a suitable choice of spatial and temporal prefiltering of the correlator input it is possible to devise reliable (nonaliasing) velocity-tuned Reichardt detectors. However, because of the well-known covariance of spatial and velocity tuning of velocity detectors in biological motion vision, an ensemble consisting of these detectors has problems in extracting a pattern-invariant velocity. We find that pattern invariance of the motion estimate can be closely approximated with Reichardt correlators that sample the luminance pattern at more than the minimum number of two locations.

Antagonistic comparison of temporal frequency filter outputs as a basis for speed perception

The prevailing view of motion detection in human vision is that the retinal image is convolved with each of a set of spatiotemporal filters and that perceived speed emerges from a process of pooling the outputs of these filters. Such a system can operate only if multiple filters exist; ideally the filters should also be fairly narrowly tuned in both spatial and temporal frequency. These constraints are met in the case of spatial frequency. But several studies suggest that multiple, finely tuned temporal filters do not exist; instead just two (perhaps three) broadly-tuned temporal mechanisms can be identified. We report some experiments concerning the effects of adaptation to motion on perceived speed. It is shown that perceived speed is increased by adaptation in some circumstances and decreased in others. We then present a computational model in which a temporal frequency code, on which perceived speed is presumed to be based, is derived by a process of antagonistic comparison of the responses of two psychophysically-plausible, broadly-tuned temporal mechanisms. The model, which includes the effects of adaptation to motion upon the sensitivities of the filters and the subsequent comparison of their sensitivities, is shown to give a good fit to the empirical data.

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.

Peripheral temporal frequency channels code frequency and speed inaccurately but allow accurate discrimination

Perceived temporal frequency is vastly underestimated in the peripheral visual field, as all temporal frequencies above 10 Hz are perceived as flickering at 10 Hz even after scaling for acuity. Varying the contrast and spatial frequency of the peripheral pattern four-fold have negligible effects on the perceived flicker rate. Speed and auditory matching experiments also support this finding. Despite the saturation of perceived temporal frequency, frequency discrimination beyond 10 Hz was as accurate as in the fovea. By using a temporal masking paradigm, we obtained threshold elevation data that could be accounted for by three overlapping, broadly tuned temporal channels peaking at 5.5, 12 and 22 Hz. Based on these temporal frequency channels, we proposed that the visual system uses a line-element scheme for mediating temporal frequency discrimination, but adopts a weighted-average method for determining perceived temporal frequency. In the peripheral visual field, the weight assigned to the highest temporal channel is much larger than those assigned to the lower frequency channels.

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.