Summary: Prediction errors influence perception during extremely fast visual events. New findings indicate that such errors can persist into later stages of processing and sometimes emerge as brief visual illusions, supporting the view that perception reflects a decision process.
Source: RUB
Neuroscientists at Ruhr-Universität Bochum (RUB), together with colleagues from the University of Freiburg, report evidence that challenges parts of current predictive coding accounts. Their results show that prediction errors are not always immediately “explained away” but can remain available and even appear as visual illusions when viewers observe very rapid image sequences. These findings were published in PLOS One on 4 May 2020.
Our visual system begins predicting within milliseconds
To maintain stable viewing and select relevant details, our eyes make tens of thousands of rapid movements each day, known as saccades. When the eyes fixate, the brain begins forming expectations within a few dozen milliseconds. New input is compared with those expectations, and any mismatch is carried forward as a prediction error. Representing changes as differences instead of reprocessing full image content reduces the amount of information the brain must handle, similar to video compression. Equally important, predicting where content will change helps prioritize locations that are most likely to contain informative differences during the next saccade.
To test whether the brain uses a difference-based strategy, the researchers showed human participants very brief sequences of two images. The first frame contained two superimposed orthogonal gratings; the second frame displayed only one of those gratings. Observers were asked to report the orientation of the final single grating. Most of the time participants correctly identified the actual orientation. Unexpectedly, however, on a small but reliable fraction of trials observers reported an orientation exactly orthogonal to the present grating. In other words, they sometimes perceived the arithmetic difference between the previous combined stimulus and the current single grating. As Robert Staadt from RUB’s Institute of Neural Computation, first author of the study, explains: seeing the difference instead of the current input can be interpreted as directly experiencing a prediction error as a perceptual illusion.
Keeping prediction errors preserves flexibility
“Predictive coding theories typically treat prediction errors as signals used by higher cognitive systems and conscious expectations,” says Dr. Dirk Jancke, head of the Optical Imaging Group at the Institute of Neural Computation. “Our data demonstrate that prediction errors also matter during extremely fast perceptual events that unfold in fractions of a second.” The study suggests the visual system retains parallel information about past, present, and possible future stimulus content, a strategy that supports both stability and adaptability when stimuli change rapidly. By preserving residual error signals instead of discarding them immediately, the brain maintains the flexibility needed for fast perceptual decisions.
Visual perception as a decision-making process
The pattern of results supports models that view perception as the outcome of a rapid decision process operating over multiple competing signals. If residual error-like signals remain accessible at later stages, they can influence which interpretation is selected when the visual input is ambiguous or changing quickly. The authors suggest that not immediately discarding prediction errors allows the system to respond more effectively to unpredictable changes in the environment.
Future work will explore which stimulus and contextual parameters most strongly promote the illusion. In addition to clear physical properties such as stimulus duration, contrast, and brightness, more nuanced psychological variables may also affect when and how the difference percept emerges. Ultimately, the researchers aim to develop sensitive visual assessments that could help detect early deficits in rapid perceptual decision-making associated with certain cognitive disorders.
Funding: The research received partial funding from the Collaborative Research Centre (CRC) 874 at RUB, supported by the German Research Foundation since 2010. The CRC “Integration and representation of sensory processes” examines how sensory signals create neural maps and give rise to behavior and memory.
About this research
Source: RUB
Media contacts: Dirk Jancke – RUB
Image source: The image used is in the public domain.
Original research (open access): “Perception of the difference between past and present stimulus: A rare orientation illusion may indicate incidental access to prediction error-like signals.” Authors: Robert Staadt, Sebastian T. Philipp, Joschka L. Cremers, Jürgen Kornmeier, Dirk Jancke. PLOS One. DOI: 10.1371/journal.pone.0232349
Abstract (summary): Predictive coding frameworks propose that incoming sensory input is compared against top-down predictions so that the resulting mismatch signals are forwarded and minimized across processing stages. The neural implementation and speed limitations of this computation remain unclear, as does the stage at which prediction errors are eliminated, if they are eliminated at all. In the experiment described here, human observers briefly viewed two superimposed orthogonal gratings followed by the abrupt removal of one orientation after either 33 or 200 milliseconds. Rather than always perceiving the remaining orientation, observers sometimes reported an illusory percept corresponding to the difference between the previous and current orientations. Comparable findings in animal studies suggest such difference signals arise early in visual cortical processing. These results are compatible with the idea that residual error signals can persist and influence perceptual decisions, providing flexibility when the visual system faces rapid and ambiguous inputs.
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