Serial dependence in visual perception

Abstract

Visual input often arrives in a noisy and discontinuous stream, owing to head and eye movements, occlusion, lighting changes, and many other factors. Yet the physical world is generally stable; objects and physical characteristics rarely change spontaneously. How then does the human visual system capitalize on continuity in the physical environment over time? We found that visual perception in humans is serially dependent, using both prior and present input to inform perception at the present moment. Using an orientation judgment task, we found that, even when visual input changed randomly over time, perceived orientation was strongly and systematically biased toward recently seen stimuli. Furthermore, the strength of this bias was modulated by attention and tuned to the spatial and temporal proximity of successive stimuli. These results reveal a serial dependence in perception characterized by a spatiotemporally tuned, orientation-selective operator-which we call a continuity field-that may promote visual stability over time.

Figure 1

Experiment 1 event sequence. Participants viewed high contrast, suprathreshold Gabor patches presented to either the left or right of fixation (on separate, interleaved runs) and reported the perceived orientation of each Gabor by adjusting the orientation of a response bar.

Figure 2

Orientation perception is serially dependent. a) Error plot from Experiment 1 for one subject. Positive values on the abscissa indicate that the previous trial was more clockwise than the present trial, and positive errors indicate that the reported orientation was more clockwise than the true stimulus orientation. Gray line is average error; black line shows a DoG curve fit to the data. The peak of the DoG fit gives the amplitude of serial dependence. Each individual subject showed significant serial dependence in orientation perception (all P < 0.01) b) Serial dependence amplitude computed for stimuli presented 1, 2, and 3 trials back from the present trial; significant serial dependence was observed in each case. Error bars are 1 s.d. of the bootstrapped distribution. Data in each bar are based on 4 subjects and 260 data points per subject. c) Experiment 3 results. Thin lines are psychometric curve fits to individual subjects’ 2 AFC data and thick lines are fits to group data. PSE was significantly shifted by the presence of an inducer Gabor at the location of one of the stimuli (mean PSE shift of 3.44°; P = 0.0004 for the group; all subjects P < 0.05; based on three subjects, 448 data points each).

Figure 3

Attentional modulation of serial dependence. a) Event sequence for one trial in Experiment 4; subjects reported the orientation of the Gabor at the cued location. b) Serial dependence was strong when the same location was attended on successive trials (red data). There was no serial dependence within a location when the location was unattended on the previous trial (light blue data), but there was significant transfer of serial dependence from one location to another when the two locations were attended on successive trials (dark blue data). Thus, attention is necessary for serial dependence and can carry serial dependence across spatial locations. Error bars are 1 s.d. of the bootstrapped distribution. Significance testing was conducted with permutation tests based on 104 data points from each of three subjects per test.

Figure 4

The spatial tuning of serial dependence. a) Experimental design for Experiment 5: stimuli and timing were the same as in Experiment 1, but the location of the Gabor was randomized on each trial. b) Serial dependence computed within a rolling window over the spatial distance between the current and previous trials; 1-back data in blue, 2-back data in red. The amplitude of serial dependence fell off with increasing spatial separation between successive trials. Shaded regions show ±1 s.d. of the bootstrapped amplitude. The width of a Gaussian curve fit to the 2-back data was significantly smaller than the width of a Gaussian curve fit to the 1-back data (P = 2x10⁻⁷; bootstrap test based on curves fit to the means in 61 bins in each the 1-back and 2-back data). c) Serial dependence computed within a two-dimensional rolling window over the relative positions of the previous and current stimuli, with the current stimulus location plotted at the origin. The results reveal a spatial field within which a prior stimulus attracts the perceived orientation of the present stimulus, which we term a perceptual continuity field (CF).