Shared attentional resources for global and local motion processing

Abstract

One of the most important aspects of visual attention is its flexibility; our attentional "window" can be tuned to different spatial scales, allowing us to perceive large-scale global patterns and local features effortlessly. We investigated whether the perception of global and local motion competes for a common attentional resource. Subjects viewed arrays of individual moving Gabors that group to produce a global motion percept when subjects attended globally. When subjects attended locally, on the other hand, they could identify the direction of individual uncrowded Gabors. Subjects were required to devote their attention toward either scale of motion or divide it between global and local scales. We measured direction discrimination as a function of the validity of a precue, which was varied in opposite directions for global and local motion such that when the precue was valid for global motion, it was invalid for local motion and vice versa. There was a trade-off between global and local motion thresholds, such that increasing the validity of precues at one spatial scale simultaneously reduced thresholds at that spatial scale but increased thresholds at the other spatial scale. In a second experiment, we found a similar pattern of results for static-oriented Gabors: Attending to local orientation information impaired the subjects' ability to perceive globally defined orientation and vice versa. Thresholds were higher for orientation compared to motion, however, suggesting that motion discrimination in the first experiment was not driven by orientation information alone but by motion-specific processing. The results of these experiments demonstrate that a shared attentional resource flexibly moves between different spatial scales and allows for the perception of both local and global image features, whether these features are defined by motion or orientation.

Figure 1

Sample stimulus for local and global judgments. For global discriminations (red), subjects were required to judge which entire pattern was more upward moving (left, in this example). For local trials, subjects judged the relative motion of two elements (circled in blue) in the display (left, in this example).

Figure 2

Experimental task overview. (A) A color-coded attentional precue specified the spatial scale of the to-be-made motion discrimination. (B) Bilateral test arrays of drifting Gabors could drift more or less upward and contained both local and global motion. (C) Depending on the experimental condition, a postcue indicated whether subjects should judge local or global motion. The postcue either matched or did not match the precue (cue-valid and cue-invalid trials, respectively).

Figure 3

Psychometric function showing 83% direction discrimination thresholds for a single subject in the 100% cue-valid condition. A logistic function was fit to the data for both local (top panel) and global (bottom panel) data.

Figure 4

(A) Direction discrimination thresholds as a function of precue validity for four individual subjects. Each subject displayed a characteristic trade-off in direction discrimination thresholds: Increasing cue validity at one spatial scale reduced thresholds at that spatial scale while simultaneously elevating thresholds at the other spatial scale. The data presented at 50% cue validity in the graphs include both valid and invalid trials (Figure 5 separates the valid and invalid trials, for comparison). (B) Group results. There was a significant effect of cue validity on direction discrimination thresholds, F(2) = 91.38, p < .01, η = .968. Error bars denote ±SEM.

Figure 5

Results of a control experiment for two subjects. The discrimination thresholds for the no-precue control condition fell between the cue-valid and cue-invalid conditions (from Experiment 1) for both local and global scales, as expected. This pattern of results for the control condition indicates that subjects can simultaneously encode multiple scales of motion information regardless of the precue.

Figure 6

Results of a follow-up experiment, using static Gabors, for two subjects. With increasing cue validity, thresholds declined, consistent with the first experiment. Threshold orientation discrimination at both local and global scales was higher than it was for motion direction discrimination (cf. Figure 4). Subject D.B. was unable to discriminate global orientation at any cue validity (dotted line).