Biomedical subjects

C W Clifford

Publications and source records attributed to C W Clifford.

At least 19 recordsLinked to original sources

PubMed2001

Asynchronous processing in vision: color leads motion.

It has been demonstrated that subjects do not report changes in color and direction of motion as being co-incidental when they occur synchronously. Instead, for the changes to be reported as being synchronous, changes in direction of motion must precede changes in color. To explain this observation, some researchers have suggested that the neural processing of color and motion is asynchronous. This interpretation has been criticized on the basis that processing time may not correlate directly and invariantly with perceived time of occurrence. Here we examine this possibility by making use of the color-contingent motion aftereffect. By correlating color states disproportionately with two directions of motion, we produced and measured color-contingent motion aftereffects as a function of the range of physical correlations. The aftereffects observed are consistent with the perceptual correlation between color and motion being different from the physical correlation. These findings demonstrate asynchronous processing for different stimulus attributes, with color being processed more quickly than motion. This suggests that the time course of perceptual experience correlates directly with that of neural activity.

Color Perception↗

PubMed2001

Orthogonal adaptation improves orientation discrimination.

We investigated the effect of adaptation on orientation discrimination using two experienced observers, then replicated the main effects using a total of 50 naïve subjects. Orientation discrimination around vertical improved after adaptation to either horizontal or vertical gratings, but was impaired by adaptation at 7.5 or 15 degrees from vertical. Improvement was greatest when adapter and test were orthogonal. We show that the results can be understood in terms of a functional model of adaptation in cortical vision.

Adaptation, Physiological↗

PubMed2001

Interaction between first- and second-order orientation channels revealed by the tilt illusion: psychophysics and computational modelling.

This paper examines the interaction between first- and second-order contours in the orientation domain. Using the simultaneous tilt illusion (TI), we show that the apparent rotation of a vertical test grating away from that of a surrounding inducing grating (repulsion effect) occurs when both the inducing and test grating are either first- or second-order. Furthermore, a significant repulsion effect is obtained when a first-order inducing grating surrounds a second-order test. If lateral inhibitory interactions between populations of orientation selective neurons provides a plausible explanation for orientation repulsion effects [Blakemore, C. B. Carpenter, R. H. S. & Georgeson, M. A. (1970) Nature, 228, 37-39], it is likely that the cue-invariant mechanisms that encodes the orientation of first- and second-order contours also exhibit inhibitory interactions. A two-channel computational model of orientation encoding is presented where one channel encodes only first-order stimuli while the second channel encodes both first- and second-order contours. In addition to predicting the orientation repulsion effects we observed, the model also provides a functional account of orientation attraction effects in terms of the responses of populations of orientation-tuned neurons.

Chi-Square Distribution↗

PubMed2001

Characterising temporal delay filters in biological motion detectors.

Motion detection requires the comparison of spatially and temporally displaced samples of the image. Here, we discuss the problems associated with measuring the delay between spatially displaced signals within biological motion detectors. Data are presented from direction-selective neurons in the nucleus of the optic tract of the wallaby, Macropus eugenii. Their motion responses depend on stimulus contrast and the adapted state of the cells. At low contrasts or in an adapted state, it appears that the input to the motion detectors is a temporally low-passed version of the image. At high contrasts or in the unadapted state, the input signals appear to be temporally band-pass-filtered. Contrary to previous claims, we find that neither the response to stimulation with apparent motion nor measurements of temporal frequency response functions provide a direct estimate of the delay filter time constants. Instead, we find that both measures are also dependent on the temporal characteristics of prefiltering stages. A model is proposed to account for the responses of the neurons and their contrast dependence.

Animals↗

PubMed2001

Interactions between ON and OFF signals in directional motion detectors feeding the not of the wallaby.

An apparent motion stimulus is used to probe the interactions between signals representing brightness increments (ON stimuli) and decrements (OFF stimuli) in the directional motion detectors forming the input to the nucleus of the optic tract (NOT) of the wallaby, Macropus eugenii. Direction-selective NOT neurons increase their firing rates during image motion from temporal-to-nasal over the contralateral eye (preferred direction) and their spontaneous activities are inhibited by motion in the opposite, anti-preferred direction. An apparent motion stimulus, consisting of neighboring vertical bars, where the brightness can be manipulated independently, also produces directional responses. Preferred direction sequences of brightness changes of like polarities (ON-ON or OFF-OFF) produce increased firing rates while sequences of opposite polarities (ON-OFF or OFF-ON) in the same direction produce relatively small excitatory responses or inhibit the spontaneous rate. For apparent motion in the anti-preferred direction, these directional properties are reversed, showing that signals for brightness increments and decrements provide inputs to the same motion detectors. There is no evidence for segregation of motion detectors into those receiving only half-wave rectified inputs. Interactions between ON and OFF signals utilize the sign of the incoming signals. An array of Reichardt-type motion detectors receiving inputs represented as positive and negative values for ON and OFF stimuli, respectively, are used to simulate the NOT responses. The brightness signals enter band-pass temporal filters prior to motion detection. By altering the time constants of these prefilters, it was possible to accurately simulate the time courses of each cell's responses.

Animals↗

PubMed2001

Hierarchy of spatial interactions in the processing of contrast-defined contours.

Both psychophysical and neurophysiological evidence suggest that there are two visual cortical processing streams, a linear stream that processes first-order stimuli and a nonlinear stream that also processes second-order stimuli. This evidence also suggests that before the extraction of the second-order signal, the nonlinear pathway broadly but not completely pools signals across initial linear filters that encode the orientation of the carrier of the second-order signal. The evidence suggests that such pooling does not occur across carrier spatial frequencies. We show that similar results are obtained with repulsion tilt illusions but not with attraction effects. Attraction effects exhibit complete orientation crossover (while retaining spatial frequency selectivity), perhaps indicating higher-level processing; an experiment on interocular transfer of the effects supported this conclusion.

Contrast Sensitivity↗

PubMed2000

Recursive implementations of temporal filters for image motion computation.

Efficient algorithms for image motion computation are important for computer vision applications and the modelling of biological vision systems. Intensity-based image motion computation proceeds in two stages: the convolution of linear spatiotemporal filter kernels with the image sequence, followed by the non-linear combination of the filter outputs. If the spatiotemporal extent of the filter kernels is large, then the convolution stage can be very intensive computationally. One effective means of reducing the storage required and computation involved in implementing the temporal convolutions is the introduction of recursive filtering. Non-recursive methods require the number of frames of the image sequence stored at any given time to be equal to the temporal extent of the slowest temporal filter. In contrast, recursive methods encode recent stimulus history implicitly in the values of a small number of variables updated through a series of feedback equations. Recursive filtering reduces the number of values stored in memory during convolution and the number of mathematical operations involved in computing the filters' outputs. This paper extends previous recursive implementations of gradient- and correlation-based motion analysis algorithms [Fleet DJ, Langley K (1995) IEEE PAMI 17: 61-67; Clifford CWG, Ibbotson MR, Langley K (1997) Vis Neurosci 14: 741-749], describing a recursive implementation of causal band-pass temporal filters suitable for use in energy- and phase-based algorithms for image motion computation. It is shown that the filters' temporal frequency tuning curves fit psychophysical estimates of the temporal properties of human visual filters.

Algorithms↗

PubMed2000

Dissociable factors affect speed perception and discrimination.

Factors affecting our judgement of the speed of visual motion were investigated. Two types of judgement were made: perceived speed relative to a standard comparison stimulus, and discrimination between the speeds of similar stimuli. The factors affect ng these two judgements were found to be doubly dissociable, suggesting that they may be constrained by processing at different levels of the visual hierarchy. The results are discussed in terms of the 3-D interpretation of visual image motion, and related to possible neural substrates.

Humans↗

PubMed1999

Anomalous perception of coherence and transparency in moving plaid patterns.

While the low-level processes mediating the detection of primary visual attributes are well understood, much less is known about the way in which these attributes are assigned to objects in the visual world. For example, when a region of the retinal image contains multiple motion signals at a range of spatial scales, how do we know whether these signals come from a single object or multiple objects? Here, we present data from four neurological patients on a psychophysical task requiring them to report whether the two components of a plaid pattern appear to move coherently or transparently. The spatial frequency of one component of the plaid is held constant while that of the other is manipulated. While some of the patients perceive coherent motion over a much smaller range of spatial frequencies than normal controls, others report coherence over almost the entire range tested. We discuss the implications of these findings for computational theories of motion perception and higher-level visual processing.

Adult↗

PubMed1999

A computational model of selective deficits in first and second-order motion processing.

Recent neurological studies of selective impairments in first and second-order motion processing are of considerable relevance in elucidating the mechanisms of motion perception in normal human observers. We examine the stimuli which have been used to assess first and second-order motion processing capabilities in clinical subjects, and discuss the nature of the computations necessary to extract their motion. We find that a simple computational model of first and second-order motion processing is able to account for the data. The model consists of a first-order channel computing motion at coarse and fine scales, and a coarse scale second-order channel. The second-order channel is sensitive to motion information defined by variations in luminance, contrast, spatial frequency and flicker. When elements of the model are disabled, its performance on either first or second-order motion can be selectively impaired in line with the neurological data.

Brain Diseases↗

PubMed1999

The perception and discrimination of speed in complex motion.

Random dot kinematograms were used to simulate radial, rotational and spiral optic flow. The stimuli were designed so that, while dot speed increased linearly with distance from the centre of the display, the density of dots remained uniform throughout their presentation. In two experiments, subjects were required to perform a temporal 2AFC speed discrimination task. Experiment 1 measured the perceived speed of a range of optic flow patterns against a rotational comparison stimulus. Radial motions were found to appear faster than rotations by approximately 10%, with a smaller but significant effect for spirals. Experiment 2 measured discrimination thresholds for pairs of similar optic flow stimuli identical in all respects except mean speed. No consistent differences were observed between the speed discrimination thresholds of radial, rotational and spiral motions and a control stimulus with the same speed profile in which motion followed fixed random trajectories. The perceived speed results are interpreted in terms of a model satisfying constraints on motion-in-depth and object rigidity, while speed discrimination appears to be based upon the pooled responses of elementary motion detectors.

Differential Threshold↗

PubMed1999

Adaptation to temporal modulation can enhance differential speed sensitivity.

During adaptation to a moving pattern, perceived speed decreases. Thus we know that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesised that adaptation to a moving stimulus serves to optimise coding of changes in speed at the expense of maintaining an accurate representation of absolute speed. In this case we would expect discrimination of speeds around the adapted level to be preserved or enhanced by motion adaptation. Speed discrimination thresholds were measured for sinusoidal gratings (1.25 cpd; 12.5 Hz; 40% contrast) with and without prior adaptation to moving, static, and flickering stimuli. After adaptation to motion in the same direction as the test, seven of eight subjects showed a reduction of perceived speed in the adapted region, and seven showed enhanced discrimination. Similar effects were found for adaptation to motion in the opposite direction to the test and to counter-phase flicker, suggesting that adaptation is driven by temporal modulation rather than by motion per se. We conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed.

Accommodation, Ocular↗

PubMed1998

First- and second-order motion perception in Gabor micropattern stimuli: psychophysics and computational modelling.

This paper examines the perception of first- and second-order motion in human vision. In an extension of previous work by Boulton and Baker [J.B. Boulton, C.L. Baker, Motion detection is dependent on spatial frequency not size, Vision Res., 31 (1991) 77-87; J.B. Boulton, C.L. Baker, Different parameters control motion perception above and below a critical density, Vision Res., 33 (1993) 1803-1811], the direction of two-frame apparent motion is measured for stimuli composed of Gabor or Gaussian micropatterns. Three conditions are investigated. Condition 1 is that used by Boulton and Baker, in which motion is defined by the displacement of Gabor micropatterns. In condition 2, motion is defined by the displacement of Gaussian micropatterns. In condition 3, the envelopes of Gabor micropatterns are displaced while their carriers remain static. Using sparsely distributed micropatterns, direction judgements in all three conditions are determined by the spacing of the micropatterns. With a dense stimulus, direction judgements vary as a function of displacement in qualitatively different ways for the three conditions. The psychophysical results are predicted by a two-channel computational model. In one channel, motion is calculated directly from stimulus luminance, while in the other it is preceded by a texture-grabbing operation. The relative activities of the two channels dictates which governs direction judgements for any given stimulus.

Computer Simulation↗

PubMed1998

Adaptation to visual motion in directional neurons of the nucleus of the optic tract.

Extracellular recordings of action potentials were made from directional neurons in the nucleus of the optic tract (NOT) of the wallaby, Macropus eugenii, while stimulating with moving sine-wave gratings. When a grating was moved at a constant velocity in the preferred direction through a neuron's receptive field, the firing rate increased rapidly and then declined exponentially until reaching a steady-state level. The decline in response is called motion adaptation. The rate of adaptation increased as the temporal frequency of the drifting grating increased, up to the frequency that elicited the maximum firing rate. Beyond this frequency, the adaptation rate decreased. When the adapting grating's spatial frequency was varied, such that response magnitudes were significantly different, the maximum adaptation rate occurred at similar temporal frequencies. Hence the temporal frequency of the stimulus is a major parameter controlling the rate of adaptation. In most neurons, the temporal frequency response functions measured after adaptation were shifted to the right when compared with those obtained in the unadapted state. Further insight into the adaptation process was obtained by measuring the responses of the cells to grating displacements within one frame (10.23 ms). Such impulsive stimulus movements of less than a one-quarter cycle elicited a response that rose rapidly to a maximum and then declined exponentially to the spontaneous firing rate in several seconds. The level of adaptation was demonstrated by observing how the time constants of the exponentials varied as a function of the temporal frequency of a previously presented moving grating. When plotted as functions of adapting frequency, time constants formed a U-shaped curve. The shortest time constants occurred at similar temporal frequencies, regardless of changes in spatial frequency, even when the change in spatial frequency resulted in large differences in response magnitude during the adaptation period. The strongest adaptation occurred when the adapting stimulus moved in the neuron's preferred direction. Stimuli that moved in the antipreferred direction or flickered had an adapting influence on the responses to subsequent impulsive movements, but the effect was far smaller than that elicited by preferred direction adaptation. Adaptation in one region of the receptive field did not affect the responses elicited by subsequent stimulation in nonoverlapping regions of the field. Adaptation is a significant property of NOT neurons and probably acts to expand their temporal resolving power.

Action Potentials↗

PubMed1996

Psychophysics of motion adaptation parallels insect electrophysiology.

We investigate the form and time course of motion adaptation, comparing the psychophysical performance of human subjects with existing electrophysiological data on insect vision. In the H1 neuron of the fly, the response to a maintained motion stimulus is known to decrease over time while sensitivity to variations in speed around the maintained level increases. This behaviour can be modelled by modifying a correlation-based motion detector to include adaptable temporal filters (Fig. 1). We find that the form and time course of sensitivity changes in human motion perception are comparable to fly vision. We propose that, in both cases, adaptation serves to improve the transmission of novel motion information along the visual pathways at the expense of maintaining an accurate representation of the unchanging components of the stimulus.

Adaptation, Physiological↗

PubMed1996

A model of temporal adaptation in fly motion vision.

A computational model is proposed to account for the adaptive properties of the fly motion system. The response properties of motion-sensitive neurons in the fly are modelled using an underdamped adaptive scheme to adjust the time constants of delay filters in an array of Reichardt detectors. It is shown that the increase in both temporal resolution and sensitivity to velocity change observed following adaptation to constant motion can be understood as a consequence of local adaptation of the filter time constants on the basis of the outputs of elementary motion detectors.

Adaptation, Ocular↗

PubMed1995

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.

Humans↗

PubMed1995

Perceived motion of contrast-modulated gratings: predictions of the multi-channel gradient model and the role of full-wave rectification.

The paper examines the perception of motion in contrast-modulated sine-wave grating patterns. These non-rigid motion patterns give rise to a spatially-structured motion percept in which perceived speed varies with spatial position. We measured the perceived motion of the low contrast regions of amplitude-modulated gratings as a function of the carrier frequency, the carrier speed, the shape of the modulation signal and the modulation depth. We found that for static carriers perceived speed was greatest in the low contrast regions of the display. The speed of the low contrast regions was underestimated and perceived speed decreased as the spatial frequency of the carrier increased. When the direction of the motion of the carrier was opposite to that of the contrast modulation, the low contrast regions could appear to be stationary. The perceived speed of the contrast modulation increased with modulation depth. The brightness contrast of the carrier grating had little effect on perceived speed of contrast-modulated patterns for average contrasts of over 10%. A motion model which had full-wave rectification as an explicit pre-processing stage followed by low-pass filtering or some other selection criterion, would predict that the motion of contrast-modulated gratings should appear rigid and that the motion of the envelope should be judged correctly. The Multi-channel Gradient Model however predicts both the structured motion field experienced when viewing these second-order motion patterns and the reductions in perceived speed as a function of carrier spatial frequency and carrier speed.

Contrast Sensitivity↗