Decision-level adaptation in motion perception

Abstract

Prolonged exposure to visual stimuli causes a bias in observers' responses to subsequent stimuli. Such adaptation-induced biases are usually explained in terms of changes in the relative activity of sensory neurons in the visual system which respond selectively to the properties of visual stimuli. However, the bias could also be due to a shift in the observer's criterion for selecting one response rather than the alternative; adaptation at the decision level of processing rather than the sensory level. We investigated whether adaptation to implied motion is best attributed to sensory-level or decision-level bias. Three experiments sought to isolate decision factors by changing the nature of the participants' task while keeping the sensory stimulus unchanged. Results showed that adaptation-induced bias in reported stimulus direction only occurred when the participants' task involved a directional judgement, and disappeared when adaptation was measured using a non-directional task (reporting where motion was present in the display, regardless of its direction). We conclude that adaptation to implied motion is due to decision-level bias, and that a propensity towards such biases may be widespread in sensory decision-making.

Keywords: implied motion; motion adaptation; normalization; response bias.

Figure 1.

Results of Experiment 1. Mean displacement threshold for correctly reporting the direction of leftward and rightward dynamic random dot test stimuli (‘L Test’ and ‘R Test’, respectively), following adaptation to static left-facing or right-facing implied motion images (‘L Adapt’ and ‘R Adapt’, respectively). Vertical bars indicate±1 s.e.m. Test dots were randomly distributed in the entire stimulus aperture.

Figure 2.

Results of Experiment 2. Only one half of the stimulus aperture, either upper or lower, contained coherently moving dots; the other half contained incoherently moving dots. In half of the test presentations, the coherent motion was leftward, and in the remainder it was rightward. Participants were instructed to report which half of the display contained coherent motion, regardless of its direction. The graph plots the mean displacement threshold for correctly reporting the half of the display containing coherent motion, when the motion was leftward or rightward (‘L Test’ and ‘R Test’, respectively), following adaptation to static left-facing or right-facing implied motion images (‘L Adapt’ and ‘R Adapt’, respectively). Vertical bars indicate±1 s.e.m.

Figure 3.

Results of Experiment 3. Stimuli were identical to those in Experiment 2, but the task was the same as that in Experiment 1. Only one half of the test stimulus aperture, either upper or lower, contained coherently moving dots; the other half contained incoherently moving dots. In half of the test presentations, the coherent motion was leftward, and in the remainder it was rightward. Participants were instructed to report the direction of coherent motion in the test pattern, regardless of where in the display it appeared. The graph plots the mean displacement threshold for correctly reporting the direction of coherent motion, when the motion was leftward or rightward (‘L Test’ and ‘R Test’, respectively), following adaptation to static left-facing or right-facing implied motion images (‘L Adapt’ and ‘R Adapt’, respectively). Vertical bars indicate±1 s.e.m.