Direction Discrimination Performance Research Articles

No effect of spatial phase randomisation on direction discrimination in dense random element patterns

Two computational strategies have been proposed for motion analysis in the human visual system. Energy-based schemes involve detection of spatiotemporal Fourier energy in the frequency components comprising a moving pattern. Edge-based schemes track shifts in the position of local edges in the pattern over time. This paper describes a stimulus manipulation, spatial phase randomisation, that acts as a diagnostic test for the involvement of energy-based processes, and describes the results of two experiments which apply the manipulation to random element patterns. Both experiments compared direction discrimination performance in patterns before and after the spatial phase of their components was randomised in the Fourier domain. For dense patterns, there was no effect of phase randomisation on the maximum displacement supporting reliable direction discrimination, indicating that energy-based responses were dominant. For sparse patterns, a significant effect of phase randomisation was obtained, indicating a greater role for edge-based responses.

Segregation from direction differences in dynamic random-dot stimuli

Previous research has shown that a field of random dots in which each dot alternates between a slow and a fast speed, can give rise to the percept of two superimposed sheets of moving dots when the alternations are out of phase or asynchronous with each other [Vis. Res. 35 (1995) 1691]. Under those conditions, observers can discriminate changes in the slow speed independent of changes in the fast speed. The present study investigated whether such motion-based segregation could result when dots alternated between two different directions. Three observers viewed a variety of displays containing two directions of motion, one upward and one oblique, with the task of discriminating small trial-to-trial changes in the direction of the upward component. The oblique direction component also changed direction from trial-to-trial. The field of dots either alternated synchronously (all dots moved in the same direction and switched to the other direction simultaneously) or asynchronously. Results showed that when the dots alternated synchronously between the directions, observers’ direction discrimination performance was generally poor. However, when dots switched directions asynchronously, direction discrimination was only slightly elevated in comparison to that produced by a field of dots all moving in a single direction. Additional experiments demonstrated that this performance was not due to judging the global direction of the random-dot display. Thus the visual system had to segregate the stimulus into its component directions before integrating to arrive at the motion signal to be discriminated. It is concluded that for displays comprising elements that alternate between different directions, local direction signals can be used by the human visual system to effectively segregate a display so long as both direction signals are present simultaneously.

Active versus passive processing of biological motion.

Johansson's point-light walker figures remain one of the most powerful and convincing examples of the role that motion can play in the perception of form (Johansson, 1973 Perception & Psychophysics 14 201 - 211; 1975 Scientific American 232(6) 76 - 88). In the current work, we use a dual-task paradigm to explore the role of attention in the processing of such stimuli. In two experiments we find striking differences in the degree to which direction-discrimination performance in point-light walker displays appears to rely on attention. Specifically, we find that performance in displays thought to involve top-down processing, either in time (experiment 1) or space (experiment 2) is adversely affected by dividing attention. In contrast, dividing attention has little effect on performance in displays that allow low-level, bottom-up computations to be carried out. We interpret these results using the active/passive motion distinction introduced by Cavanagh (1991 Spatial Vision 5 303-309).

The integration of orientation information in the motion correspondence problem.

We examine how differently oriented components contribute to the discrimination of motion direction along a horizontal axis. Stimuli were two-frame random-dot kinematograms that were narrowband filtered in spatial frequency. On each trial, subjects had to state whether motion was to the left or the right. For each stimulus condition, Dmax (the largest displacement supporting 80% correct direction discrimination performance) was measured. In experiment 1, Dmax was measured for orientationally narrowband stimuli as a function of their mean orientation. Dmax was found to increase as the orientation of the stimuli became closer to the axis of motion. Experiment 2 used isotropic stimuli in which some orientation bands contained a coherent motion signal, and some contained only noise. When the noise band started at vertical orientations and increased until only horizontal orientations contained a coherent motion signal, Dmax increased slightly. This suggests that near-vertical orientations interfere with motion perception at large displacements when they contain a coherent motion signal. When the noise band started at horizontal and increased until only vertical orientations contained the motion signal, Dmax decreased steadily. This implies that Dmax depends at least partly on the most horizontal motion signal in the stimulus. These results were contrasted with two models. In the first, the visual system utilises the most informative orientations (nearest horizontal). In the second, all available orientations are used equally. Results supported an intermediate interpretation, in which all orientations are used but more informative ones are weighted more heavily.

Aliasing for rapidly counterphasing stimuli: a failure to demonstrate an M-cell sampling limit to resolution.

We investigated whether resolution is sampling limited for stimuli optimized for detection by magnocellular mechanisms. We measured peripheral (15 degrees and 30 degrees) spatial detection and resolution thresholds using 50% and 90% contrast flicker-defined gratings (25 Hz) and 90% contrast counterphasing sinusoidal gratings (25 Hz). Direction-discrimination performance for 90% contrast counterphasing sinusoidal gratings (25 Hz) was measured foveally. Our results indicate that resolution of rapidly counterphasing stimuli is sampling limited in peripheral vision but is consistent with limiting of performance by parvocellular mechanisms. Also, undersampling may not be necessary to account for motion reversals observed with gratings that both drift and flicker.

Motion contrast: a new metric for direction discrimination

The Adelson–Bergen energy model (Adelson, E. H., & Bergen, J. R. (1985). Spatiotemporal energy models for the perception of motion. Journal of the Optical Society of America A, 2, 284–299) is a standard framework for understanding first-order motion processing. The opponent energy for a given input is calculated by subtracting one directional energy measure ( E L) from its opposite ( E R), and its sign indicates the direction of motion of the input. Our observers viewed a dynamic sequence of gratings (1 c/deg) equivalent to the sum of two gratings moving in opposite directions with different contrasts. The ratio of contrasts was varied across trials. We found that opponent energy was a very poor predictor of direction discrimination performance. Heeger (1992). Normalization of cell responses in cat striate cortex. Visual Neuroscience, 9, 181–197) has suggested that divisive inhibition amongst striate cells requires a contrast gain control in the energy model. A new metric can be formulated in the spirit of Heeger’s model by normalising the opponent energy ( E L− E R) with flicker energy, the sum of the directional motion energies ( E L+ E R). This new measure, motion contrast ( E L− E R)/( E L+ E R), was found to be a good predictor of direction discrimination performance over a wide range of contrast levels, but opponent energy was not. Discrimination thresholds expressed as motion contrast were around 0.5±0.1 for the sampled drifting gratings used in our experiments. We show that the dependence on motion contrast, and the threshold of about 0.5, can be predicted by a modified opponent energy model based on current knowledge of the response functions and response variance of cortical cells.

Upper displacement limits for spatially broadband patterns containing bandpass noise.

How is the spatial-frequency content of a moving broadband pattern analysed by the visual system? Observers were asked to discriminate the direction of motion in random-noise patterns containing equal energy in each two-dimensional octave band. Uncorrelated noise could be introduced into either low- or high-frequency bands in order to force the visual system to rely on the outputs of putative mechanisms tuned to a narrow frequency range of the stimulus. In two experiments the dependent measure was the magnitude of dmax, the largest discrete displacement whose direction could be discriminated reliably. It was found that dmax was unaffected by the presence of high-frequency noise reaching down to 0.67 c/deg, but that the task became impossible thereafter. In the case of low-frequency noise, dmax fell as the noise was moved up towards about 2 c/deg, at which point the task became impossible at any displacement. This pattern of results would be expected if the system were using information from the lowest signal frequencies in all conditions. In experiment 2, dmax was measured for stimuli in which the spectral position and quantity of high-frequency noise were manipulated. It was found that only noise spectrally-adjacent to the signal band has a detrimental effect on dmax. Three different single-filter models of motion detection each failed to provide a satisfactory account of the spatial-frequency range of good direction discrimination performance. Rather, the modelling shows that the visual system can access the outputs of a low-frequency channel when the noise is high and a high-frequency channel when the noise is low.

Two mechanisms underlie processing of stochastic motion stimuli

We have constructed “limited lifetime” stochastic motion stimuli using Gabor functions instead of dots, thereby controlling the local attributes of spatial frequency and orientation. Human psychophysical data for direction discrimination using these stimuli reveal two qualitatively distinct kinds of processing. For small displacements, direction discrimination performance as a function of displacement is scaled with spatial frequency in a manner consistent with a linear filtering motion mechanism. Motion perception for relatively large displacements is not directly related to the spatial frequency, and is consistent with a nonlinear process which signals motion of contrast envelopes.

Recruitment mechanisms in speed and fine-direction discrimination tasks

The minimum information necessary to specify motion requires a change in position across time. Previous studies have shown that human motion measurements improve with more than two frames of motion. This study clarifies how motion information is integrated to produce the best speed and direction discrimination. Using random-dot kinematograms, fine-direction discrimination thresholds and speed discrimination thresholds are assessed as a function of dot lifetime. Specifically, we ask if performance on both tasks depends on dot lifetime in the same manner. If both speed and direction discrimination performance improve the same way with increasing dot lifetime, this would indicate that both tasks have the same integration limit and both tasks may depend on the same underlying mechanisms. Experiment 1 shows that for both tasks a four-frame dot lifetime is necessary for observers to reach asymptotic threshold levels. The absolute level of performance improves with increasing stimulus duration or signal-to-noise ratio, but the integration limit itself does not vary. Experiment 2 examines whether this integration limit is constrained by the number of frames or by the temporal duration of the dot lifetime. The data in Experiment 2 suggest that both a minimum number of samples and a minimal temporal integration period determine the integration limit for recruitment mechanisms. The results suggest that speed and fine-direction discrimination depend upon the same underlying motion mechanisms. These results are discussed in relation to possible underlying physiological substrates and computational models of motion measurement.

Evidence for an early motion system which integrates information from the two eyes.

In one type of cyclopean motion stimulus one eye views a counterphase flickering grating while the other eye views the same pattern in spatio-temporal quadrature. Algebraic summation of the two image sequences results in a drifting grating. Upon binocular (cyclopean) combination of the two patterns a drifting grating is perceived even though neither monocular pattern is moving. While this appears to support the position that the motion system is binocular, it has been suggested that such demonstrations involve higher level feature tracking rather than early motion system activation. The perceived direction of motion could result from the tracking of features after neural summation of left and right eye images. However, by adding a static, in-phase, pedestal grating to the left and right eye flickering test gratings, the direction information based on feature tracking is removed while leaving the motion energy information unchanged. We have found that when such stimuli are presented for several seconds, direction discrimination performance is significantly better than chance for pedestal grating contrasts several times the test grating contrast. Therefore, in the absence of a feature tracking cue, the direction of motion is identified using a binocular motion energy mechanism. The results do not exclude the existence of a binocular feature tracking system. Both systems are likely to exist.

Motion Contrast: A New Metric for Direction Discrimination

The Adelson - Bergen energy model (1985 Journal of the Optical Society of America A2 284 – 299) is a standard framework for understanding 1st-order motion processing. Its output, the opponent energy for a given input, is calculated by subtracting one directional energy measure ( R) from its opposite ( L), and its sign indicates the direction of motion of the input. Our observers viewed a dynamic sequence of gratings (1 cycle deg−1) equivalent to the sum of two gratings moving in opposite directions with different contrasts, and had to report the dominant direction of motion. The ratio of contrasts was varied across trials. We found that opponent energy was a very poor predictor of direction discrimination performance. Heeger (1992 Visual Neuroscience9 181 – 197) has suggested that divisive inhibition amongst striate cells requires a contrast gain control in the energy model. A new metric can be formulated in the spirit of Heeger's model by normalising the opponent energy ( L- R) with flicker energy, the sum of the directional motion energies ( L+ R). This new measure, motion contrast ( L- R)/( L+ R), was found to be a good predictor of direction discrimination performance over a wide range of contrast levels, but opponent energy was not. Discrimination thresholds expressed as motion contrast were around 0.5 for the sampled drifting gratings used in our experiments, corresponding to an energy ratio ( L: R or R: L) of 3:1 at discrimination threshold. Such high values suggest that the outputs of motion energy filters are very noisy (variable over space and time) or that the use of them is inefficient.

Image segmentation enhances discrimination of motion in visual noise.

The primate visual system uses form cues-such as hue, contrast polarity, luminance, and texture-to segment complex retinal images into the constituent objects of the visual scene. We investigated whether segmentation of dynamic images on the basis of hue, luminance contrast polarity, or luminance contrast amplitude aids discrimination of motion direction. Human subjects viewed dynamic displays of randomly positioned dots, in which a variable proportion of the dots moved in the same direction at the same speed ("signal" dots) while the remaining dots were randomly displaced ("noise" dots). In agreement with previous reports, we observed a reliable relationship between the strength of the motion signal and subjects' ability to discriminate motion direction, enabling the measurement of thresholds for direction discrimination. When signal dots had a different luminance contrast amplitude than noise dots, direction discrimination performance was directly related to the relative contrast of the signal dots, demonstrating the importance of matching the perceived contrast amplitude of signal and noise tokens when testing the effects of segmentation by other cues. When Michelson luminance contrast was matched, distinguishing signal from noise dots by hue or by luminance contrast polarity strongly improved direction discrimination, lowering thresholds by an average factor of five. These results reveal a strong influence of form cues on motion processing in the human visual system, and suggest that segmentation on the basis of form cues occurs prior to motion processing.

Motion Integration with Dot Patterns: Effects of Motion Noise and Structural Information

To better understand how local motion detectors merge their responses so as to permit the global determination of objects' movements in the visual field, direction discrimination performance was measured using a flexible class of moving dots—two sets of dots translating sinusoidally 90 deg out of phase along orthogonal axes. When dots' velocities are combined, a global motion along a circular trajectory emerges, clockwise or counter-clockwise depending on the sign of the phase lag. However, the results of the present experiments indicate that dot patterns are segregated into distinct, but interacting, streams when each dot motion can be accurately determined. In contrast, perceptual coherence of the global motion occurs when each local motion signal is “blurred” by a “motion noise”. Direction discrimination performance then increases regularly with both noise amplitude and noise frequency, i.e., noise speed. Performance also increases when relative motion between dots is added. Testing different dot configurations indicates that performance is better for spatial arrangements that display structural properties (a square shape), as compared to overlapping random distributions. Interestingly, when the delay between stimulus onset and motion onset increases up to 300 msec, performance improves when dot patterns convey some form of structural organization but not when the dots are distributed at random. Relations of these results to existing models of motion integration are considered. Copyright © 1996 Elsevier Science Ltd.

How similar must the Fourier spectra of the frames of a random-dot kinematogram be to support motion perception?

Direction-discrimination performance was measured for two-frame random-dot kinematograms in which one or both frames were spatial frequency filtered with a one octave band-pass filter and the centre frequency of this filter was varied in the range 0.75-9 c/deg independently for each frame. When both frames were filtered so that they contained common (overlapping) spatial frequencies direction discrimination was extremely good but it deteriorated rapidly as the degree of spectral overlap between the two frames decreased. These results are consistent with previous findings that suggest that the mechanisms that mediate the initial stages of motion detection are narrowly tuned for spatial frequency and cannot combine information conveyed at disparate frequencies in order to compute an unambiguous estimate of the direction of local motion. However, when only one of the frames was band-pass filtered and the other was unfiltered (broadband), the correct direction of stimulus motion could be discriminated reliably for a broad range of filter centre frequencies. Performance was best when the centre frequency of the filtered frame was at medium spatial frequencies and tended to deteriorate as the centre frequency approached either extreme of the spatial frequency range examined. This basic pattern of results may be attributed to the visual system's differential sensitivity to the Fourier components present in the unfiltered frame.

Temporal filtering enhances direction discrimination in random-dot patterns

In conventional presentations of random-dot kinematograms, two frames of random dots are presented in temporal sequence, separated by a blank inter-stimulus interval, and a coherent offset in spatial position is added to dots in one frame relative to dots in the other frame. Direction discrimination performance is limited temporally to inter-stimulus intervals below about 100 msec (T max). Experiments are described in which temporal smoothing was applied to the onset and offset of each frame in the kinematogram.T max was found to increase in proportion with the time constant of the temporal smoothing function. An explanation based on contrast-dependent responses in simple motion detectors cannot accommodate the results. Instead, the increase inT max with temporal smoothing, and analogous increase in spatial limit (D max) with spatial blurring, can be related to the spatiotemporal frequency content of the stimulus. Random-dot kinematograms can be viewed as continuously drifting patterns that have been discretely sampled at regular spatiotemporal intervals. Sampling introduces artefacts (alias signals), which become more intrusive as sampling rate declines (i.e. inter-stimulus interval or spatial displacement increases) and consequently limit discrimination performance. Temporal smoothing or spatial blurring extends performance because it removes alias signals generated by high spatiotemporal frequencies in the pattern. Computational modelling to estimate the Fourier energy available in random-dot kinematograms confirmed that the sampling account can predict the proportional increase inT max andD max limits as filter time or space constant increases.

Refraction, aliasing, and the absence of motion reversals in peripheral vision

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd · m −2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7–14 c/deg at 20 deg, 5.5–7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial. The absence of motion reversals may reflect irregularity of sampling, and suggests that reversals are not a simple tool for studying sampling in peripheral vision.

Detection and direction discrimination performance with flicker gratings in peripheral vision

Detection and direction discrimination experiments were conducted with luminance and flicker gratings. The flicker gratings had bars made up of static random pixels interspersed between other bars with flickering random pixels. All experiments were carried out in peripheral vision with grating images centered at 8 deg eccentricity in the superior retina. Detection of flicker gratings (i) was independent of pixel size, (ii) declined with spatial frequency in the range 1–4 c/deg, and (iii) improved with stimulus area (number of grating cycles). Detection performance with a flicker grating was comparable to that obtained with a low-contrast (0.01) luminance grating, and the results suggest that the spatial structure of a flicker-domain stimulus is based upon signals which are weak compared to the maximum signals attainable with a luminance-domain stimulus. With the detectability of flicker and luminance gratings equated, d′ for discriminating the direction of motion of a luminance grating increased with step size (1/12 to 1/4 cycle) whereas direction discrimination performance with a flicker grating remained unchanged and at chance levels. Under the conditions tested, there was no evidence that the motion of a flicker-domain stimulus could be processed peripherally. Constraints on alternative models of motion processing are discussed.

Microstimulation in visual area MT: effects on direction discrimination performance

Physiological and behavioral evidence suggests that the activity of direction selective neurons in visual cortex underlies the perception of moving visual stimuli. We tested this hypothesis by measuring the effects of cortical microstimulation on perceptual judgements of motion direction. To accomplish this, rhesus monkeys were trained to discriminate the direction of motion in a near-threshold, stochastic motion display. For each experiment, we positioned a microelectrode in the middle of a cluster of neurons that shared a common preferred direction of motion. The psychophysical task was then adjusted so that the visual display was presented directly over the neurons' receptive field. The monkeys were required to discriminate between motion shown either in the direction preferred by the neurons or in the opposite direction. On half the trials of an experiment, we applied electrical microstimulation while monkeys viewed the motion display. We hypothesized that enhancing the neurons' discharge rate would introduce a directionally specific signal into the cortex and thereby influence the monkeys' choices on the discrimination task. We compared the monkeys' performance on "stimulated" and "nonstimulated" trials in 139 experiments; all trials within an experiment were presented in random order. Statistically significant effects of microstimulation were obtained in 89 experiments. In 86 of the 89 experiments with significant effects (97%), the monkeys indicated that motion was in the neurons' preferred direction more frequently on stimulated trials than on nonstimulated trials. The data demonstrate a functional link between the activity of direction selective neurons and perceptual judgements of motion direction.

Contrast dependence of short-range apparent motion

The apparent motion of band-pass filtered random dot kinematograms was assessed by measurements of two alternative direction discrimination performance. In the case of two-dimensional (isotropically filtered) stimuli, the results were largely independent of contrast: at Michelson contrasts of 5 and 50%, near-perfect direction discrimination was obtainable for a limited range of displacements. However for one-dimensional (grating) stimuli, apparent motion seems to be highly dependent on contrast. At 5% contrast, performance was comparable to that obtained with the two-dimensional stimuli. At 50% contrast the motion percept broke down to a large extent, with consistently poor direction discrimination being obtained. The breakdown of apparent motion is interpreted in terms of a decreased signal-to-noise ratio in the pooled response of motion detectors that are tending to contrast saturation. Here, “noise” refers to the sampling components present in any apparent motion sequence. Evidence relating to the sampling frequency of the motion sequence is presented to support the hypothesis. It is argued that the discrepancy between the results obtained with one- and two-dimensional stimuli support the idea that motion is initially encoded by orientationally tuned mechanisms.