An extension of the transparent-motion detection limit using speed-tuned global-motion systems

The contribution of local and global motion adaptation in the repulsive direction aftereffect

After adapting to a certain motion direction, our perception of a similar direction will be repelled away from the adapting direction, a phenomenon known as the direction aftereffect (DAE). As the motion system consists of local and global processing stages, it remains unclear how the adaptation of the two stages contributes in producing the DAE. The present study addresses this question by independently inducing adaptation at local and global motion-processing levels. Local adaptation was manipulated by presenting test stimuli at either adapted or nonadapted locations. Global adaptation was manipulated by embedding one or five global motion directions in the adapting motion. Repulsive DAE, when measured using a multiple-element test pattern, was stronger when it was produced by global adaptation than when produced by local adaptation. Specifically, the DAE resulting from local adaptation (a) decreased when test orientations differed from adapting orientation, (b) decreased when local directions were disambiguated using plaid stimuli, (c) remained the same even when attention was focused at specific test locations during adaptation, and (d) increased when tested with a single element. Overall, these findings suggest that the strength of repulsive DAE depends on both the motion-processing level at which adaptation occurs and the level at which the DAE was tested. Furthermore, the repulsive DAE arising from local adaptation alone can be explained by the propagation of local speed repulsion instead of local direction repulsion. Findings are discussed in the context of how motion aftereffects arise from the adaptation of a hierarchical motion system.

On the number of perceivable blur levels in naturalistic images

Blur is a useful cue for depth. Natural images contain objects at a range of depths whose depth can be signaled by their perceived blur. Here, to evaluate the usefulness of blur as a depth cue, we estimate the number blur levels that observers can perceive simultaneously. To estimate this value, observers discriminated and classified dead leaves patterns that contained a controlled distribution of blur levels but are more complex or naturalistic than stimuli typically used in blur research. We used a 2-IFC discrimination task, in which observers reported the interval that contained more blur levels and a classification task, in which observers reported the number of perceived blur levels. In both tasks, observers could not discriminate or classify more than four levels of blur in the stimulus reliably. In isolation from other cues, blur may provide only a coarse cue to depth and add limited depth information when present in natural scenes with complex distributions of blur and multiple depth cues.

Information extraction during simultaneous motion processing

When confronted with multiple moving objects the visual system can process them in two stages: an initial stage in which a limited number of signals are processed in parallel (i.e. simultaneously) followed by a sequential stage. We previously demonstrated that during the simultaneous stage, observers could discriminate between presentations containing up to 5 vs. 6 spatially localized motion signals (Edwards & Rideaux, 2013). Here we investigate what information is actually extracted during the simultaneous stage and whether the simultaneous limit varies with the detail of information extracted. This was achieved by measuring the ability of observers to extract varied information from low detail, i.e. the number of signals presented, to high detail, i.e. the actual directions present and the direction of a specific element, during the simultaneous stage. The results indicate that the resolution of simultaneous processing varies as a function of the information which is extracted, i.e. as the information extraction becomes more detailed, from the number of moving elements to the direction of a specific element, the capacity to process multiple signals is reduced. Thus, when assigning a capacity to simultaneous motion processing, this must be qualified by designating the degree of information extraction.

Perceptual separation of transparent motion components: the interaction of motion, luminance and shape cues

Transparency is perceived when two or more objects or surfaces can be separated by the visual system whilst they are presented in the same region of the visual field at the same time. This segmentation of distinct entities on the basis of overlapping local visual cues poses an interesting challenge for the understanding of cortical information processing. In psychophysical experiments, we studied stimuli that contained randomly positioned disc elements, moving at two different speeds in the same direction, to analyse the interaction of cues during the perception of motion transparency. The current work extends findings from previous experiments with sine wave luminance gratings which only vary in one spatial dimension. The reported experiments manipulate low-level cues, like differences in speed or luminance, and what are likely to be higher level cues such as the relative size of the elements or the superposition rules that govern overlapping regions. The mechanism responsible for separation appears to be mediated by combination of the relevant and available cues. Where perceived transparency is stronger, the neural representations of components are inferred to be more distinguishable from each other across what appear to be multiple cue dimensions. The disproportionally large effect on transparency strength of the type of superposition of disc suggests that with this manipulation, there may be enhanced separation above what might be expected from the linear combination of low-level cues in a process we term labelling. A mechanism for transparency perception consistent with the current results would require a minimum of three stages; in addition to the local motion detection and global pooling and separation of motion signals, findings suggest a powerful additional role of higher level separation cues.

How many motion signals can be simultaneously perceived?

Previous research indicates that the maximum number of motion signals that can be simultaneously perceived is 2, if they are defined only by direction differences, or 3 if they also differ in speed or depth (Greenwood & Edwards, 2006b). Those previous studies used transparent, spatially-sparse stimuli. Here we investigate this motion-number perception limit using spatially-localised stimuli that drive either the standard or form-specific motion systems (Edwards, 2009). Each motion signal was defined by four signal-dots that were arranged in either a square pattern (Square Condition), to drive the form-specific system, or a random pattern (Random Condition), to drive the standard motion-system. A temporal 2AFC procedure was used with each interval (150 ms duration) containing n or n+1 signals. The observer had to identify the interval containing the highest number of signals. The total number of dots in each interval was kept constant by varying the number of noise dots (dots that started off in the same spatial arrangement as the signal dots but then each of those dots moved in different directions). A mask was used at the end of each motion sequence to prohibit the use of iconic memory. In the Square Condition, up to five directions could be simultaneously perceived, and only 1 in the Variable condition. Decreasing the number of noise dots improved performance for the Variable condition, and increasing it decreased performance in the Square Condition. These results show that the previously observed limit of 3 is not a universal limit for motion perception and further, that signal-to-noise limits are a fundamental factor in determining the number of directions that can be simultaneously perceived. Hence the greater sensitivity to motion of the form-specific system makes it well suited to extracting the motion of multiple moving objects.

Quantifying "the aperture problem" for judgments of motion direction in natural scenes

The response of motion-selective neurons in primary visual cortex is ambiguous with respect to the two-dimensional (2D) velocity of spatially extensive objects. To investigate how local neural activity is integrated in the computation of global motion, we asked observers to judge the direction of a rigidly translating natural scene viewed through 16 apertures. We report a novel relative oblique effect: local contour orientations parallel or orthogonal to the direction of motion yield more precise and less biased estimates of direction than other orientations. This effect varies inversely with the local orientation variance of the natural scenes. Analysis of contour orientations across aperture pairings extends previous research on plaids and indicates that observers are biased toward the faster moving contour for Type I pairings. Finally, we show that observers' bias and precision as a function of the orientation statistics of natural scenes can be accounted for by an interaction between naturally arising anisotropies in natural scenes and a template model of MT that is optimally tuned for isotropic stimuli.

Perceptual costs for motion transparency evaluated by two performance measures

Transparency perception is recognized as one of the important phenomena to understand the computational mechanism of early visual system. Transparency perception indicates that a simple theory reconstructing a single-valued field of a visual attribute, such as an optical-flow field, cannot model the neural mechanism for the human visual system and raises a fundamental issue of how visual attributes are represented and detected in the brain. It is considered that one of the important cues to reveal the neural encoding mechanism for overlapping surfaces is the perceptual cost in transparency perception. It has been known that the perceptual performance in motion transparency is worse than that expected from single motion perception. This perceptual "cost" would reflect the encoding strategy for transparent motions. Here we present a systematic study comparing the perceptual costs in motion transparency evaluated by two performance measures. The result showed that the properties of the perceptual costs varied with the performance measures. The perceptual cost evaluated by the motion detection threshold became smaller as a directional difference between overlapping motions increased, whereas the cost examined with the precision of directional judgments became worse. A computational analysis suggests that these contradictory results cannot be explained by a simple population coding model for motion directions.

An oblique effect for transparent-motion detection caused by variation in global-motion direction-tuning bandwidths

Despite evidence for the broad direction tuning of global-motion detectors, transparent motion can be detected with comparatively small angular separations. The exact means by which this broad population response is decoded to yield multiple signal directions remains unclear. Consequently, we sought to determine the relationship between angular separation thresholds for transparent motion and the direction-tuning bandwidth of global-motion detectors. Angular separation thresholds were assessed around four axes of motion, with thresholds lower around cardinal axes than the oblique axes. This was also found with lowered signal intensities, despite larger differences between the component directions at threshold, indicating that the transparency oblique effect relies more on the mean direction than the components. Simulations with a model global-motion population suggest this is likely to arise from variation in direction-tuning bandwidths around the cardinal and oblique axes. In a second experiment, adaptation to oblique unidirectional motion produced threshold elevation for a wider range of test directions than adaptation to a cardinal direction. This is consistent with tighter direction tuning around cardinal axes and provides a basis for the transparent-motion oblique effect. Our narrow bandwidth estimates also suggest that transparent-motion detection could rely on bimodal activity within the global-motion stage.

Pushing the limits of transparent-motion detection with binocular disparity

When transparent motion is defined purely by direction differences, observers fail to detect more than two signal directions simultaneously [Edwards, M., & Greenwood, J.A. (2005). The perception of motion transparency: A signal-to-noise limit. Vision Research, 45, 1877-1884]. This limit is strongly related to signal-detection thresholds for transparent motion, which are several times higher than uni-directional thresholds. When the effective signal intensities are elevated by speed differences that drive independent global-motion systems, the transparent-motion limit can be extended to allow detection of three signals [Greenwood, J.A., & Edwards, M. (2006). An extension of transparent-motion detection limit using speed-tuned global-motion systems. Vision Research, 46, 1440-1449]. Because there are independent disparity-tuned global-motion systems, distributing transparent-motion signals across distinct depth planes also allows an increase in their effective signal intensity. In the present study, the addition of depth differences enabled the simultaneous detection of three signals. However, as with the addition of speed differences, observers were not able to detect four signals, which would be predicted if signal intensity were the sole constraint on transparent-motion detection. The combination of depth and speed produced similar results, suggesting that there is a strict higher-order limit, possibly related to attention, restricting the maximum number of signals that can be detected simultaneously to three.