We provide the first evidence of serial dependence in color working memory. Serial dependence had been characterized with great detail for other visual features2, and in particular in spatial working memory3,21,22. Several common features of color and spatial working memory suggest that serial dependence could also be similar in color: simultaneously memorized stimuli interfere attractively when presented at close distances43,44, and memory precision decreases with memory period duration3,45,46. These commonalities are in contrast with the differences of neural representations. While spatial representations consolidate early in the visual pathway47, complex transformations in color representations occur as color information travels from the photoreceptors in the retina, to visual cortex, and into association cortex48. The fact that serial dependence is similar for color and spatial working memory thus suggests that it depends on inter-trial interferences that occur at processing stages with representational maps equally distant from the corresponding perceptual map, and this points at higher color processing stages. A candidate region for this is the inferotemporal cortex, where continuous neuronal representations of color of circular shape on the two perceptual cardinal axes (yellowish-bluish and greenish-reddish axis) have been found49,50. Recent evidence suggests that similar color representations, maintained through working memory delays, may reach up to the monkey prefrontal cortex51.
Color Impact 4 Serial
The analogy of color and angular location neural representations motivated us to simulate color working memory similarly to spatial working memory of angular locations52. We simulated the angular memory trace in the memory period as a diffusion process39 with a drift toward the previous trial memory trace that introduces serial dependence53. We used this model to test the concerns about the impact of systematic biases in the estimation of serial dependence. This is a general concern that has been raised for other visual features3,9,21,22, but in the case of color it may be particularly important for the marked perceptual systematic biases that have been reported32. We therefore incorporated strong systematic biases in the reports of our model simulations, and we developed new analysis strategies avoid their impact on the estimation of serial dependence (Supplementary Fig. S4). One typical strategy for systematic bias removal is to low-pass filter the responses as a function of stimulus feature3,9,21,22. This approach depends on parameters that are often subjectively decided (e.g. size of sliding window). In addition, removing systematic biases incorrectly, for example when subjects do not have systematic biases, can introduce extra biases in otherwise clean data (Supplementary Fig. S4). We showed that by folding the serial dependence function, one can reduce the impact of systematic biases on serial dependence without adding biases in unbiased data and without specifying arbitrary parameter values. We therefore conclude that this analysis allows a more robust estimation of serial dependence in behavioral studies.
As in previous studies (e.g. ref.11), serial dependence was measured by calculating the mean error as a function of distance between current and previous target (\(\theta _d\)) in sliding windows with size \(\pi /2\) and in steps of \(\pi /30\). Most of the studies, except for Foster et al.38, were multi-item working memory. On trials with more than one stimulus, we use the target stimulus as reference.
The reported perception of a visual stimulus on one trial can be biased by the stimulus that was presented on the previous trial. In the present study we asked whether encoding the previous-trial stimulus is sufficient to produce this serial dependence effect, or whether the effect also depends on postencoding processes. To distinguish between these possibilities, we designed a task in which participants reported either the color or the direction of a set of colored moving dots on each trial. The to-be-reported dimension was indicated by a postcue after stimulus offset, so participants were required to encode both features of every stimulus. We assessed serial dependence for motion perception as a function of which feature dimension had been reported on the previous trial. In Experiment 1, we found a serial dependence effect for motion only when participants had reported the direction of motion on the previous trial, and not when they had encoded the direction of motion but reported the color of the stimulus. Experiment 2 confirmed that this pattern of results was not driven by the difficulty of the color task. When we used the same response modality for both motion and color reports in Experiment 3, we found significant serial dependence effects following both color-report and motion-report trials, but the effect was significantly weaker following color-report trials. Together, these findings indicate that postperceptual processes play a critical role in serial dependence and that the mere encoding of the previous-trial target is not sufficient to produce the serial dependence effect.
Most previous studies of serial dependence required participants to report a single target feature dimension on every trial, which confounds the perception of this feature with the report of the feature. Some studies, however, have included a condition in which no explicit response was required on some trials, which could potentially unconfound the perception and report of the target feature (Czoschke, Fischer, Beitner, Kaiser, & Bledowski, 2019; Fischer & Whitney, 2014; Suárez-Pinilla, Seth, & Roseboom, 2018). These studies showed that serial dependence was present even when the target feature had not been reported on the previous trial, suggesting that serial dependence can be produced by the mere encoding of the previous-trial stimulus. However, because participants were frequently required to report the target feature, it is possible that participants prepared a response even if none was required on the current trial, and that some aspect of response preparation produced the serial dependence effect.
A recent study used a stimulus with two feature dimensions and tested whether serial dependence was present for one feature dimension when the other feature dimension had been reported on the previous trial (Suárez-Pinilla et al., 2018). The participants in this study saw a random-dot kinematogram (RDK) and reported either the mean or the variance of the dot directions on a given trial. A core finding was that, even though participants saw both the variance and the direction, serial dependence in visual variance occurred only when participants had reported visual variance on the previous trial. However, it should be noted that the study used a precue paradigm in which the relevant feature dimension on a given trial was known to participants prior to stimulus onset. Consequently, feature-based attention might have attenuated the perceptual or memory encoding of the unattended dimension. That is, the lack of serial dependence might have reflected a reduction in the encoding of the other feature dimension, rather than a lack of the report of this dimension.
In the present study, participants had to perceive and remember both features of a two-dimensional stimulus on every trial, but they were asked to report only one of the two features on a given trial. Specifically, participants saw an RDK with colored dots for a short period of time and were then cued to report either the direction or the color of the dots (Fig. 1a). Crucially, color-report and direction-report trials were randomly intermixed, and the relevant dimension was postcued after stimulus offset. Consequently, participants had to perceive and remember both the direction and the color of the dots on each trial to perform the task accurately. This made it possible to test whether stimulus information that had been encodedFootnote 1 but remained unused on the previous trial would still bias the processing of the next stimulus.
If encoding the stimulus is sufficient to produce serial dependence, then serial dependence for motion direction should be observed even when participants reported color on the previous trial. However, if additional report-related processes are necessary for serial dependence to occur, then serial dependence for motion direction should be reduced or eliminated when color was reported on the previous trial. In a series of three experiments, we found that serial dependence for motion direction was largely absent following color-report trials. These results demonstrate that encoding a feature is not sufficient for serial dependence and that postencoding processes play a key role.
Twenty-four college students between the ages of 18 and 30 participated (13 female, 11 male). The sample size was determined a priori on the basis of published studies of serial dependence (Bae & Luck, 2019; Fischer & Whitney, 2014). The study was approved by the UC Davis Institutional Review Board, and the participants gave informed consent.
We used a standard RDK algorithm to generate the motion stimulus (Roitman & Shadlen, 2002). The stimuli consisted of three groups of dots (dot diameter = 0.3) that were randomly distributed within the black disk (16.7 dots per square degree per second). We used a larger-than-typical dot size in order to increase the discriminability of the dot colors. Each group of dots was presented for one video frame (16.67 ms/group), and the dots were replotted in new locations after a two-frame delay. Thus, the coherent motion was created by the correspondence between the dots in frame n and in frame n+3. When the new location of a given dot was outside the black disk, that dot was replotted at a random location on the circumference of the black disk, to maintain dot density. The coherence level was set to 100%, and the speed of motion was set to 6/s. 2ff7e9595c
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