Sabina Orso and Andrea Pavan asked twenty-eight volunteers to watch a classic laboratory illusion called a random-dot kinematogram: a circle of 200 white dots, each drifting coherently in one direction at about six degrees per second for half a second. On every trial, the direction changed unpredictably across the full 360-degree range. Participants then indicated the direction they had just seen by moving a mouse cursor along a circular ring, a setup that allows errors to be measured to the fraction of a degree. The critical twist was timing. On different trials, participants waited 0, 1, 3 or 6 seconds after the dots vanished before giving their answer. If the pull of the previous motion only appears once memory gets involved, the immediate-report condition should look clean, while the longer delays should accumulate bias. If instead the bias is baked into perception itself, it should be there from the very first moment.

The answer the experiment produced was neither of the two predicted patterns. Using a derivative-of-Gaussian model, a standard mathematical tool in serial-dependence research that fits a peaked curve to how response errors depend on the angular difference between consecutive directions, the researchers found a significant attractive bias at every single delay, including the supposedly pristine zero-second condition. Participants did not wait for a retention interval to start being influenced by history; the previous motion direction was already pulling their reports the instant they responded. Peak bias estimates ranged from 0.80 degrees at 0 seconds to 2.04 degrees at 6 seconds, and all of them were statistically greater than zero after correction for multiple comparisons.

Yet the delay effect was real, and it was strange. The strength of the bias did not climb steadily with time as a purely memory-based account would predict. Instead it followed a non-monotonic trajectory: moderate immediately after the stimulus, larger at 1 second, dipping at 3 seconds, and peaking at 6 seconds. Statistical modeling confirmed that the interaction between delay and the serial-dependence term significantly improved the fit of the model, meaning the magnitude of the history effect genuinely shifted across retention intervals. A tidy theory in which immediate responses are dominated by repulsion, an aftereffect that pushes perception away from the previous stimulus to sharpen sensitivity to change, while delayed responses reveal attraction, simply did not survive contact with the data. Attraction was present from the start; delay modulated it rather than creating it.

There is an important caveat here, one the authors are careful to underline. Even the zero-second condition was not a pure window into perception, because responding inevitably involves post-perceptual decision and motor stages. So the study cannot cleanly isolate a perceptual mechanism. What it can say is that a simple dissociation between immediate sensory repulsion and delayed mnemonic attraction is wrong, and that recent motion history is already shaping reports when no additional retention interval exists at all. Between those two poles lies a messier, more interesting picture in which multiple processes with different time courses combine to produce the final bias.

A complementary analysis drove that point home even harder. The parametric Gaussian-based model imposes a smooth shape on the data, so the researchers also performed a folded-bias analysis that collapses positive and negative direction changes and examines bias at each absolute angular distance separately. This revealed that attraction was not spread evenly across the angular range. It was strongest at small-to-intermediate differences, particularly around 10 to 20 degrees, and at some intermediate and larger distances the errors actually flipped into local repulsive deflections. In other words, the fitted attraction curve is best read as a compact summary of a composite phenomenon, not as evidence of a single uniform attractive mechanism stretching across all direction changes.

The study also contained an uncomfortable self-check. When the researchers added a predictor based on the direction participants had responded on the previous trial, rather than the stimulus they had seen, the response history explained a large share of the variance in current errors, and the delay-dependent stimulus effect lost its statistical significance. Because stimulus history and response history were moderately correlated in the design, the two cannot be fully pulled apart. The honest conclusion is that part of the measured serial dependence may reflect decisional, motor or response-based carryover, and future experiments will need designs that deliberately decorrelate what people saw from what they said to determine how much of the effect is truly stimulus-driven.

Two exploratory analyses hinted at further layers of complexity. One suggested that motion history may bias judgments not only toward the exact previous direction but also toward the opposite direction, consistent with the idea that the visual system encodes motion along a broader axis rather than as a single vector, and with neuroimaging work showing that motion direction can be represented as a bimodal probability distribution in visual cortex. Because the stimulus sequence was optimized for the main direction-based question, with small transitions overrepresented and exact 180-degree reversals excluded, the authors treat this axis effect as hypothesis-generating rather than confirmed. A second exploratory analysis of response times found no simple relationship between how fast people answered and how similar consecutive directions were, offering no easy converging support for a similarity-based decisional account.

The Bologna team was also candid about an inescapable confound in serial-dependence research: because small direction changes were intentionally overrepresented, participants could in principle have learned the transition statistics and used them to guide responses, a strategy resembling regression to the mean. Control checks showed that target directions were broadly distributed and that positive and negative transitions were balanced, which limits but does not eliminate this concern. The authors note too that serial dependence is not automatically helpful; recent large-scale evidence suggests it can even worsen perceptual decisions in some contexts. Whether the bias is adaptive depends on the task, not on some benevolent design feature of the brain.

What the study ultimately delivers is a more honest map of a phenomenon that has often been drawn too simply. Recent motion history biases our judgments of motion direction from the very first moment we report them, the strength of that bias shifts in a non-monotonic way as the retention interval stretches from zero to six seconds, and the overall effect appears to be a composite of perceptual, memory-related and response-related influences whose relative weights change over time. Raw data, analysis code and experiment scripts are publicly available, inviting the field to build on the result. For now, one thing is clear: the continuous, stable visual world you experience is stitched together, in part, from what you saw just before, and the thread connecting those moments is woven over seconds, not switched on by a clock.

Subject of Research: Serial dependence in visual motion direction perception and its modulation by retention interval

Article Title: Retention interval modulates motion-history biases in visual motion perception

Article References: Retention interval modulates motion-history biases in visual motion perception. (n.d.). https://doi.org/10.3758/s13414-026-03345-1

Image Credits: AI Generated

DOI: 10.3758/s13414-026-03345-1

Keywords: serial dependence, motion perception, visual psychophysics, random-dot kinematograms, visual short-term memory, retention interval, perceptual stability, derivative-of-Gaussian, motion axis, adaptation, response bias, decision-making