Serial dependence in the perception of faces

Abstract

From moment to moment, we perceive objects in the world as continuous despite fluctuations in their image properties due to factors like occlusion, visual noise, and eye movements. The mechanism by which the visual system accomplishes this object continuity remains elusive. Recent results have demonstrated that the perception of low-level stimulus features such as orientation and numerosity is systematically biased (i.e., pulled) toward visual input from the recent past. The spatial region over which current orientations are pulled by previous orientations is known as the continuity field, which is temporally tuned for the past 10-15 s. This perceptual pull could contribute to the visual stability of low-level features over short time periods, but it does not address how visual stability occurs at the level of object identity. Here, we tested whether the visual system facilitates stable perception by biasing current perception of a face, a complex and behaviorally relevant object, toward recently seen faces. We found that perception of face identity is systematically biased toward identities seen up to several seconds prior, even across changes in viewpoint. This effect did not depend on subjects' prior responses or on the method used to measure identity perception. Although this bias in perceived identity manifests as a misperception, it is adaptive: visual processing echoes the stability of objects in the world to create perceptual continuity. The serial dependence of identity perception promotes object identity invariance over time and provides the clearest evidence for the existence of an object-selective perceptual continuity field.

Figure 1. Experiment 1 Trial Sequence and Results

(A) Trial sequence for the method of adjustment (MOA) task in experiment 1. On each trial, a randomly selected target face was presented for 750 ms, followed by a 1,000 ms noise mask of black and white pixels to reduce afterimages and a 250 ms fixation cross. Subjects then saw a test screen containing a random adjustment face, which they modified by scrolling through the continuous identity wheel to match the target face (see Figure S1A). (B) Example data from subject 4, with each data point showing performance on one trial. In units of face morph steps, the×axis is the shortest distance along the morph wheel between the current and one-back target face (one-back target face − current target face), and the y axis is the shortest distance along the morph wheel between the selected match face and target face (match face – current target face). Positive×axis values indicate that the one-back target face was clockwise on the face morph wheel relative to the current target face, and positive y axis values indicate that the current match face was also clockwise relative to the current target face (Figure S1A). The running average (dashed line) reveals a clear trend in the data, which followed a derivative-of-Gaussian (DoG) shape (model fit depicted as solid line). (C) Half-amplitude of the serial dependence for each subject in experiment 1A. Error bars are bootstrapped 95% confidence intervals, and p value is based on group permuted null distribution. Additional experiments show that memory confusions cannot fully explain our pattern of results (see Figure S2). (D) Half-amplitude of the serial dependence for each subject in experiment 1B for trials with no one-back response. Sequential effects have been known to influence subjects’ responses by introducing dependencies between current and previous trial decisions [–6]. However, these results are not due entirely to sequential decision biases, since we observed serially dependent perception without a one-back response. Error bars are bootstrapped 95% confidence intervals, and p value is based on group permuted null distribution.

Figure 2. Experiment 2 Trial Sequence and Results

(A) Trial sequence for 2IFC task in experiment 2. For each trial, the first face was presented for 1,000 ms, followed by a 1,000 ms noise mask and 250 ms fixation cross. Subjects saw the second face for 500 ms and judged whether the first face (press “1”) or second face (press “2”) looked more like original face A. Trial type was determined by comparing the position in the morph continuum of the current trial first face to that of the one-back trial first face. If the one-back first face was closer to original face A along the morph continuum, the trial was labeled an “A-previous” trial. Faces are shown here without added noise. (B) Example psychometric functions for subject 3. The abscissa shows the difference in the identity of the first face relative to the second face in the current trial. Trials that fell in bins −12 and −6 had a first face that was more B-like relative to the second face, trials in the 0 bin had identical first and second faces, and trials in the +6 and +12 bin had a first face that was more A-like relative to the second face. The ordinate shows the proportion of first faces on the current trial that were chosen as being more A-like. The black curve consists of all trials with one-back first faces that were more “A”-like, and the gray dashed curve consists of all trials with one-back first faces that were more “B”-like. If the one-back first face positively pulled subjects’ perception of face identity, then there should be a leftward horizontal displacement of the black curve relative to the gray dashed curve, which is what we found for all subjects (see Figure S3A). (C) Point of subject equality (PSE) difference between the black and gray dashed curve for each subject. Error bars are bootstrapped 95% confidence intervals, and p value is based on group permuted null distribution.

Figure 3. Experiment 3 Trial Sequence and Results

A) Trial sequence for the 2IFC task in experiment 3. We used grayscale image morphs based on two original neutral male faces across three different viewpoints (frontal, left, right), cropped by an oval to remove hairline (see Figure S1C). The trial sequence was identical to that of experiment 2, except both one-back trial faces were always of a different viewpoint relative to current trial faces. Faces in the figure are shown without added noise. B) Example data from subject 3. The black curve consists of all trials with one-back first faces that were more “A”-like, and the gray dashed curve consists of all trials with one-back first faces that were more “B”-like. If the one-back first face positively pulls subjects’ perception of face identity, then there should be a leftward horizontal displacement of the black curve relative to the gray dashed curve, which is what we found for all subjects (see Figure S3B). C) PSE difference for each subject. Error bars are bootstrapped 95% confidence intervals, and p value is based on group permuted null distribution.

Figure 4. Experiment 4 Trial Sequence and Results

(A) Trial sequence for the MOA task in experiment 4. A circular morphed continuum of female face identities was created with two possible viewpoints (right- or left-facing profiles) (see Figure S1D). The identities shown here are similar to those in the experiment, with permission obtained for reproduction purposes. Randomly selected target faces were presented for 750 ms, followed by a 1,000 ms mask of black and white pixels. Subjects responded by matching the adjustment face to the target face. The one-back trial target face was always from a different viewpoint relative to the current trial target face, but the target and adjustment face had the same viewpoint. (B) Example data from subject 2, with each dot showing performance on one trial. In units of face morph steps, the×axis is the shortest distance along the morph wheel between the current and one-back target face, and the y axis is the shortest distance along the morph wheel between the selected match face and target face. The DoG model fit is depicted as a solid line, and the running average is depicted as a dashed line. (C) Half-amplitude of the serial dependence for each subject in experiment 4. Error bars are bootstrapped 95% confidence intervals, and p value is based on group permuted null distribution.