Unconscious local motion alters global image speed

Abstract

Accurate motion perception of self and object speed is crucial for successful interaction in the world. The context in which we make such speed judgments has a profound effect on their accuracy. Misperceptions of motion speed caused by the context can have drastic consequences in real world situations, but they also reveal much about the underlying mechanisms of motion perception. Here we show that motion signals suppressed from awareness can warp simultaneous conscious speed perception. In Experiment 1, we measured global speed discrimination thresholds using an annulus of 8 local Gabor elements. We show that physically removing local elements from the array attenuated global speed discrimination. However, removing awareness of the local elements only had a small effect on speed discrimination. That is, unconscious local motion elements contributed to global conscious speed perception. In Experiment 2 we measured the global speed of the moving Gabor patterns, when half the elements moved at different speeds. We show that global speed averaging occurred regardless of whether local elements were removed from awareness, such that the speed of invisible elements continued to be averaged together with the visible elements to determine the global speed. These data suggest that contextual motion signals outside of awareness can both boost and affect our experience of motion speed, and suggest that such pooling of motion signals occurs before the conscious extraction of the surround motion speed.

Figure 1. A schematic of the stimulus used in the present study.

The moving Gabor stimuli undergoing rotational local motion was presented to the left eye while a dynamic Mondrian mask was presented to the right eye. When viewed binocularly perception was of a Gabor stimulus with half of its elements masked by the Mondrian stimulus.

Figure 2. A schematic diagram of the two interval stimulus presentation sequence.

In one interval was the test stimulus which consisted of Gabor elements all moving at a speed between 1–5°/s. In the other interval a reference stimulus was presented that consisted of Gabor elements moving at 3°/s. Both intervals were separated by an inter stimulus interval of 500 ms in which the screen was mid gray set to the background luminance. The task of the observer was to indicate the interval containing the fastest motion. After the presentation of both intervals, dynamic white noise was presented.

Figure 3. Speed discrimination thresholds for translational motion (A) and rotational motion (B).

Speed discrimination thresholds as Weber fractions plotted as a function of the number of elements visible in the stimulus. Circles depict data for conditions in which the actual number of elements in the stimulus was changed; diamonds - removed elements were replaced by a dynamic Mondrian patch; and circles elements are present but suppressed from awareness using CFS. Error bars represent 95% confidence intervals.

Figure 4. The ability to judge the speed of suppressed elements.

The proportion of times in which observers were able to correctly identify the faster moving stimulus comprising of a number of different elements (different symbols) suppressed from awareness plotted as a function of their speed. Error bars signify 95% confidence intervals.

Figure 5. Perceived speed judgments of patterns with visible and invisible elements.

The proportion of times in which the test stimulus was judged faster than the reference stimulus is plotted in against the speed of the test stimulus in Fig. 4A; Average observer data is plotted for conditions in which Gabor elements moved at the same speed (circles), when half the elements moved at a slower speed and were visible (triangles) or invisible suppressed by CFS (squares). In Fig. 4B, the PSE for the function fits in Fig. 4A are plotted as bar graphs. In each plot error bars represent 95% confidence intervals.