Summary: Prediction errors play a role in the context of dynamic perceptual events that take place within fractions of a second. Findings support the hypothesis that visual perception occurs as a result of a decision process.
Neuroscientists at the Ruhr-Universität Bochum (RUB) together with colleagues at the Freiburg University show that this is not strictly the case. Instead, they show that prediction errors can occasionally appear as visual illusion when viewing rapid image sequences. Thus, rather than being explained away prediction errors remain in fact accessible at final processing stages forming perception. Previous theories of predictive coding need therefore to be revised. The study is reported in Plos One on 4. May 2020.
Our visual system starts making predictions within a few milliseconds
To fixate objects in the outside world, our eyes perform far more than one hundred thousand of rapid movements per day called saccades. However, as soon as our eyes rest about 100 milliseconds, the brain starts making predictions. Differences between previous and current image contents are then forwarded to subsequent processing stages as prediction errors. The advantage to deal with differences instead of complete image information is obvious: similar to video compression techniques the data volume is drastically reduced. Another advantage turns up literally only at second sight: statistically, there is a high probability that the next saccade lands on locations where differences to previous image contents are largest. Thus, calculating potential changes of image content as the differences to previous contents prepares the visual system early on for new input.
To test whether the brain uses indeed such a strategy, the authors presented rapid sequences of two images to human volunteers. In the first image two gratings were superimposed, in the second image only one of the gratings was present. The task was to report the orientation of the last seen single grating. In most cases, the participants correctly reported the orientation of the present orientation, as expected. However, surprisingly, in some cases an orientation was perceived that was exactly orthogonal to the present orientation. That is, participants saw sometimes the difference between the previous superimposed gratings and the present single grating. „Seeing the difference instead of the real current input is here a visual illusion that can be interpreted as directly seeing the prediction error,” says Robert Staadt from the Institute of Neural Computation of the RUB, first author of the study.
Avoiding the pigeonhole benefits flexibility
“Within the framework of the predictive coding theory, prediction errors are mostly conceived in the context of higher cognitive functions that are coupled to conscious expectations. However, we demonstrate that prediction errors also play a role in the context of highly dynamic perceptual events that take place within fractions of a second,” explains Dr. Dirk Jancke, head of the Optical Imaging Group at the Institute of Neural Computation. The present study reveals that the visual system simultaneously keeps up information about past, current, and possible future image contents. Such a strategy allows both stability and flexibility when viewing rapid image sequences. “Altogether, our results support hypotheses that consider perception as a result of a decision process,” says Jancke. Hence, prediction errors should not be sorted out too early, as they might become relevant for following events.
Visual perception underlies decision making
In next studies the scientists will scrutinize the sets of parameters that drive the perceptual illusion most effectively. Besides straightforward physical parameters like stimulus duration, brightness, and contrast, other, more elusive factors that characterize psychological features might be involved. The authors’ long-term perspective is the development of practical visual tests that can be used for an early diagnosis of cognitive disorders connected to rapid perceptual decision processes.
Funding: The study was partly financed through grants of the Collaborative Research Centre (CRC) 874 at RUB, which is supported by the German Research Foundation since 2010. The CRC “Integration and representation of sensory processes” investigates how sensory signals generate neuronal maps, and result in complex behavior and memory formation.
Perception of the difference between past and present stimulus: A rare orientation illusion may indicate incidental access to prediction error-like signals
A popular model for sensory processing, known as predictive coding, proposes that incoming signals are iteratively compared with top-down predictions along a hierarchical processing scheme. At each step, error signals arising from differences between actual input and prediction are forwarded and recurrently minimized by updating internal models to finally be “explained away”. However, the neuronal mechanisms underlying such computations and their limitations in processing speed are largely unknown. Further, it remains unclear at which step of cortical processing prediction errors are explained away, if at all. In the present study, human subjects briefly viewed the superposition of two orthogonally oriented gratings followed by abrupt removal of one orientation after either 33 or 200 milliseconds. Instead of strictly seeing the remaining orientation, observers report rarely but highly significantly an illusory percept of the arithmetic difference between previous and actual orientations. Previous findings in cats using the identical paradigm suggest that such difference signals are inherited from first steps of visual cortical processing. In light of early modeling accounts of predictive coding, in which visual neurons were interpreted as residual error detectors signaling the difference between actual input and its temporal prediction based on past input, our data may indicate continued access to residual errors. Such strategy permits time-critical perceptual decision making across a spectrum of competing internal signals up to the highest levels of processing. Thus, the occasional appearance of a prediction error-like illusory percept may uncover maintained flexibility at perceptual decision stages when subjects cope with highly dynamic and ambiguous visual stimuli.