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Noise characteristics and prior expectations in human visual speed perception

Abstract

Human visual speed perception is qualitatively consistent with a Bayesian observer that optimally combines noisy measurements with a prior preference for lower speeds. Quantitative validation of this model, however, is difficult because the precise noise characteristics and prior expectations are unknown. Here, we present an augmented observer model that accounts for the variability of subjective responses in a speed discrimination task. This allowed us to infer the shape of the prior probability as well as the internal noise characteristics directly from psychophysical data. For all subjects, we found that the fitted model provides an accurate description of the data across a wide range of stimulus parameters. The inferred prior distribution shows significantly heavier tails than a Gaussian, and the amplitude of the internal noise is approximately proportional to stimulus speed and depends inversely on stimulus contrast. The framework is general and should prove applicable to other experiments and perceptual modalities.

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Figure 1: Illustration of a Bayesian estimator accounting for contrast-induced biases in speed perception.
Figure 2: Bayesian estimation and measurement noise.
Figure 3: Bayesian observer model for 2AFC speed discrimination experiment.
Figure 4: Parameters of the Bayesian observer model fitted to perceptual data of two representative subjects.
Figure 5: Perceived matching speeds as a function of contrast.
Figure 6: Speed discrimination thresholds.
Figure 7: Model comparison: average log-probability of the experimental data.
Figure 8: Model comparison: perceptual bias and discrimination predictions.

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Acknowledgements

The authors thank all subjects for participation in the psychophysical experiments. Thanks to J.A. Movshon and D. Heeger for helpful comments on the manuscript. This work was primarily funded by the Howard Hughes Medical Institute.

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Correspondence to Alan A Stocker.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Fig. 1

Raw psychometric data collected from subject 1 and 2 under all tested conditions. (PDF 1756 kb)

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Stocker, A., Simoncelli, E. Noise characteristics and prior expectations in human visual speed perception. Nat Neurosci 9, 578–585 (2006). https://doi.org/10.1038/nn1669

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