Stephen T. Hammett

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.

Speed can go up as well as down at low contrast : Implications for models of motion perception

It is well-known that reducing the contrast of a slow moving stimulus reduces its apparent speed. [Thompson, P. (1982). Perceived rate of movement depends on contrast. Vision Research, 22, 377-380.] report of this finding also suggested that at speeds above 8 cycles/s reducing contrast increased perceived speed. However in a later report, Stone and Thompson (1992), using a more rigorous, forced-choice procedure, failed to collect reliable data at these higher speeds. Here, we confirm that faster moving stimuli can appear to move faster than their true speed at low contrasts and we propose a physiologically plausible ratio model that unlike recent Bayesian models (e.g. Weiss, Y., Simoncelli, E. P., & Adelson, E. H. (2002). Motion illusions as optimal percepts. Nature Neuroscience, 5, 598-604) can account well for the results.

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.

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.

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.

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.

The dynamics of velocity adaptation in human vision

Abstract Since Barlow and Hills 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.

Dietary supplementation of creatine monohydrate reduces the human fMRI BOLD signal

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.

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.

Seeing blur:'Motion sharpening' without motion

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 sharpenin’. 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 ‘defaul’ 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.