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A neural network trained for prediction mimics diverse features of biological neurons and perception

A preprint version of the article is available at arXiv.

Abstract

Recent work has shown that convolutional neural networks (CNNs) trained on image recognition tasks can serve as valuable models for predicting neural responses in primate visual cortex. However, these models typically require biologically infeasible levels of labelled training data, so this similarity must at least arise via different paths. In addition, most popular CNNs are solely feedforward, lacking a notion of time and recurrence, whereas neurons in visual cortex produce complex time-varying responses, even to static inputs. Towards addressing these inconsistencies with biology, here we study the emergent properties of a recurrent generative network that is trained to predict future video frames in a self-supervised manner. Remarkably, the resulting model is able to capture a wide variety of seemingly disparate phenomena observed in visual cortex, ranging from single-unit response dynamics to complex perceptual motion illusions, even when subjected to highly impoverished stimuli. These results suggest potentially deep connections between recurrent predictive neural network models and computations in the brain, providing new leads that can enrich both fields.

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Fig. 1: Deep predictive coding networks (PredNets).
Fig. 2: Length suppression.
Fig. 3: On/off temporal dynamics.
Fig. 4: Image sequence learning effects.
Fig. 5: Illusory contours.
Fig. 6: The flash-lag effect.

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Data availability

The primary dataset used in this work is the KITTI Dataset13, which can be obtained at http://www.cvlibs.net/datasets/kitti/raw_data.php. All other data may be obtained upon request to the authors.

Code availability

Code for the PredNet model is available at https://github.com/coxlab/prednet. All other code may be obtained upon request to the authors.

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Acknowledgements

This work was supported by IARPA (contract no. D16PC00002), the National Science Foundation (NSF IIS 1409097) and the Center for Brains, Minds and Machines (CBMM, NSF STC award CCF-1231216).

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W.L. and D.C. conceived the study. W.L. conceived the model and implemented the experiments and analysis. G.K. and D.C. supervised the study. All authors contributed to interpreting the results. All authors contributed to writing the manuscript.

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Correspondence to William Lotter.

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Extended data

Extended Data Fig. 1 Length suppression analysis for A1 and R1 units.

The average (± s.e.m) response of A1 and R1 units and exemplars are shown (expanding upon Fig. 2 in main text). Red: Original network. Blue: Feedback weights from R2 to R1 set to zero. The average A1 response demonstrates length suppression, whereas the average R1 response does not show a strong effect, with some units overall showing length suppression (for example, unit 15 - bottom right panel) and other units showing an opposite effect (for example, unit 33 - bottom middle panel). The removal of feedback led to a significant decrease in length suppression in A1, with a mean (± s.e.m) decrease in percent length suppression (\({\mathrm{\% }}LS = 100 \ast \frac{r_{\mathrm{max}} - {r_{\mathrm{longest}\;\mathrm{bar}}}}{r_{\mathrm{max}}}\)) of 31±7% (p = 0.0004, Wilcoxon signed rank test, one-sided, z = 3.3). The R1 units exhibited a mean %LS decrease of 5±6% upon removal of feedback, which was not statistically significant (p = 0.18, z = 0.93).

Extended Data Fig. 2 Temporal dynamics in the A and R units in the PredNet.

The average response of A and R units to a set of naturalistic objects on a gray background, after training on the KITTI car-mounted camera dataset13 is shown (expanding upon Fig. 3 in the main text). The A and R layers seem to generally exhibit on/off dynamics, similar to the E layers. R1 also seems to have another mode in its response, specifically a ramp up between time steps 3 and 5 post image onset. The responses are grouped per layer and consist of an average across all the units (all filters and spatial locations) in a layer. The mean responses were then normalized between 0 and 1. Given the large number of units in each layer, the s.e.m. is O(1%) of the mean. Responses for layer 0, the pixel layer, are omitted because of their heavy dependence on the input pixels for the A and R layers. Note that, by notation in the network’s update rules, the input image reaches the R layers at a time step after the E and A layers.

Extended Data Fig. 3 Response differential between predicted and unpredicted sequences in the sequence learning experiment.

The percent increase of population peak response between predicted and unpredicted sequences is quantified for each PredNet layer. Positive values indicate a higher response for unpredicted sequences. *p < 0.05, **p < 0.005 (paired t-test, one-sided).

Extended Data Fig. 4 Illusory contours responses for A and R units in the PredNet.

The mean ± s.e.m. is shown (expanding upon Fig. 5 in the main text). Averages are computed across filter channels at the central receptive field.

Extended Data Fig. 5 Quantification of illusory responsiveness in the illusory contours experiment.

Units in the monkey recordings of Lee and Nguyen57 are compared to units in the PredNet. We follow Lee and Nguyen57 in calculating the following two measures for each unit: \(IC_a = \frac{{R_i - R_a}}{{R_i + R_a}}\) and \(IC_r = \frac{{R_i - R_r}}{{R_i + R_r}}\), where Ri is the response to the illusory contour (sum over stimulus duration), Ra is the response to amodal stimuli, and Rr is the response to the rotated image. For the PredNet, these indices were calculated separately for each unit (at the central receptive field) with a non-uniform response. Positive values, indicating preferences to the illusion, were observed for all subgroups. Mean ± s.e.m.; *p < 0.05 (t-test, one-sided).

Extended Data Fig. 6 Additional predictions by the PredNet model in the flash lag experiment.

The images shown consist of next-frame predictions by the PredNet model after four consecutive appearances of the outer bar. The model was trained on the KITTI car-mounted camera dataset13.

Extended Data Fig. 7 Comparison of the PredNet to prior models.

The models under comparison are a (non-exhaustive) list of prior models that have been used to probe the phenomena explored here. The top section indicates if a given model (column) exhibits each phenomenon (row). The bottom section considers various learning aspects of the models. From left to right, the models considered correspond to the works of Rao and Ballard (1999)9, Adelson and Bergen (1985)78, McIntosh et al. (2016)79, Spratling (2010)45, Jehee and Ballard (2009)46, and Dura-Bernal et al. (2012)80. Additionally, traditional deep CNNs are considered (for example AlexNet81, VGGNet82, ResNet83). The PredNet control (second column from right) refers to the model in Extended Data Figures 9 and 10.

Extended Data Fig. 8 Comparison of PredNet predictions in the flash-lag illusion experiment to psychophysical estimates.

The psychophysical estimates come from Nijhawan, 199458. With the frame rate of 10 Hz used to train the PredNet as a reference, the average angular difference between the inner and outer bars in the PredNet predictions was quantified for various rotation speeds. The results are compared to the perceptual estimates obtained using two human subjects by Nijhawan58. Mean and standard deviation are shown. For rotation speeds up to and including 25 rotations per minute (RPM), the PredNet estimates align well with the psychophysical results. At 35 RPM, the PredNet predictions become noisy and inconsistent, as evidenced by the high standard deviation.

Extended Data Fig. 9 PredNet control model lacking explicit penalization of activity in “error units”.

An additional convolutional block (\(\hat A_0^{frame}\)) is added that generates the next-frame prediction given input from R0. The predicted frame is used in direct L1 loss, with the removal of the activity of the E units from the training loss altogether. Thus, in this control model, the E units are unconstrained and there is no explicit encouragement of activity minimization in the network.

Extended Data Fig. 10 Results of control model with the removal of explicit minimization of “error” activity in the PredNet.

Overall, the control model less faithfully reproduces the neural phenomena presented here. a) The control network E1 units exhibit enhanced length suppression when feedback is removed (opposite of the effect in biology and the original PredNet). b) The responses in the control network still peak upon image onset and offset, however the decay in activity after peak is non-monotonic in several layers and less dramatic overall than the results shown in Fig. 3. As opposed to the 20–49% decrease in response after image onset peak in the original PredNet and the 44% decrease in the Hung et al.53 macaque IT data, the control network exhibited a 5% (E1) to 30% (E2) decrease. c) Response of the control network E3 layer in the sequence pairing experiment. The unpredicted images actually elicit a higher response than the first image in the sequence and the predicted images hardly elicit any response, both effects which are qualitatively different than the macaque IT data from Meyer and Olson54 and the original PredNet. d,e) The average E1 response in the control network demonstrates a decrease in activity upon presentation of the illusory contour.

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Lotter, W., Kreiman, G. & Cox, D. A neural network trained for prediction mimics diverse features of biological neurons and perception. Nat Mach Intell 2, 210–219 (2020). https://doi.org/10.1038/s42256-020-0170-9

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