The detection of multiple global directions: Capacity limits with spatially segregated and transparent-motion signals

Abstract

An important constraint on motion processing is the maximum number of directions that can be perceived at the same time. When transparent-motion stimuli are constructed based solely on direction differences, prior studies demonstrate that no more than two directions are seen simultaneously. However, this limit has been extended to three when signal directions drive independent speed- or disparity-tuned global-motion systems. The present study sought to determine whether this three-direction capacity reflects the specific mechanisms of transparent-motion detection or a more general restriction on global-motion processing. Using both transparent and spatially segregated stimuli, observers indicated which of the two intervals contained the most directions, with simultaneous processing ensured through brief durations and n vs. n + 1 signal comparisons. When spatially segregated directions were interleaved in patches, no more than two were seen, as with direction-defined transparent motion. In contrast, separating these directions into distinct spatial regions allowed the detection of up to three. Signal-detection thresholds did not vary across these signal arrangements, suggesting that the two-direction capacity results from signal-to-noise pooling across the entire stimulus, with the higher capacity for spatially distinct directions arising from independent pooling within each region. Together, these results provide further evidence for an upper capacity of three directions within the global-motion stage.

Temporal synchrony is an effective cue for grouping and segmentation in the absence of form cues

“…However, again, a potential artifact in these studies may have been the present due to the spatial segregation of signal elements (Adelson & Farid, 1999). In contrast, the spatial proximity of signal elements within a mixture of signal and noise elements has no impact on standard global form (Dickinson, Broderick, & Badcock, 2009) or motion processing (Morley & Badcock, 2016), unless the observer has prior knowledge of the location of the subset of signal elements, or signal intensity is at supra threshold levels (Greenwood & Edwards, 2009). Thus, if spatial proximity operates to augment the grouping mechanisms of temporal synchrony, one would expect that when signal elements are arranged in high spatial proximity, fewer elements will be required to extract a global 2D solution, but only in the presence of a synchronous event.…”

Stimulus dependencies of an illusory motion: Investigations of the Motion Bridging Effect

“…In addition, global motion percepts that would otherwise be apparent become imperceptible when the number of motion directions shown becomes too high (e.g., Greenwood & Edwards, 2009). Lee and Lu (2014) demonstrated that such imperceptible global motions can elicit a motion aftereffect.…”

Gradual formation of visual working memory representations of motion directions

“…The memory capacity was reduced by increasing the noise levels of the RDK (Experiment 3a). It has been previously reported that the signal-to-noise level affects the number of motion directions that can be simultaneously perceived (Edwards & Rideaux, 2013;Greenwood & Edwards, 2009). In the current study, however, the perceptual sensitivity to the motion direction of the RDK was not affected by noise levels (Experiment 3b).…”

“…However, there might be an alternative explanation for the difference in results between BM and the RDK. When perceiving multiple motion items, it is known that the processing capacity is greatly affected by the signal-to-noise level of the stimulus-the increase in stimulus noise decreases the number of localized motion signals that can be simultaneously perceived (Edwards & Rideaux, 2013;Greenwood & Edwards, 2009). Since the BM stimuli consist of dots with a distinct spatiotemporal energy, higher internal noise is produced in the motion system, which can induce a low ceiling on motion processing efficiency (Watamaniuk, 1993).…”