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Letter perception emerges from unsupervised deep learning and recycling of natural image features

A Correction to this article was published on 02 November 2017

This article has been updated


The use of written symbols is a major achievement of human cultural evolution. However, how abstract letter representations might be learned from vision is still an unsolved problem1,2. Here, we present a large-scale computational model of letter recognition based on deep neural networks3,4, which develops a hierarchy of increasingly more complex internal representations in a completely unsupervised way by fitting a probabilistic, generative model to the visual input5,6. In line with the hypothesis that learning written symbols partially recycles pre-existing neuronal circuits for object recognition7, earlier processing levels in the model exploit domain-general visual features learned from natural images, while domain-specific features emerge in upstream neurons following exposure to printed letters. We show that these high-level representations can be easily mapped to letter identities even for noise-degraded images, producing accurate simulations of a broad range of empirical findings on letter perception in human observers. Our model shows that by reusing natural visual primitives, learning written symbols only requires limited, domain-specific tuning, supporting the hypothesis that their shape has been culturally selected to match the statistical structure of natural environments8.

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Fig. 1: Deep learning architecture and examples of natural image and printed letter data.
Fig. 2: Emergent neuronal receptive fields, representational selectivity, and letter identification accuracy in the model.
Fig. 3: Simulations of human psychophysical studies.
Fig. 4: Spatial-frequency analysis of perceptual channel mediating letter identification.

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  • 02 November 2017

    In the version of this Letter originally published, in the sentence beginning “Written symbols are culture specific...”, in the second example, ‘Φ’ was used instead of ‘F’; it should have read ‘(for example, versus F)’. This has now been corrected in all versions of the Letter.


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This work was supported by grants from the European Research Council (no. 210922) and University of Padova (Strategic Grant NEURAT) to M.Z., I.S. was supported by a Marie Curie Intra European Fellowship PIEF-GA-2013-622882 within the 7th Framework Programme. We thank J. McClelland for useful discussions and K. Friston for suggestions on the simulation of the neuroimaging data. No funders had any role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

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A.T., M.Z. and I.S. conceived the experiments, discussed the results and wrote the paper. A.T. wrote the code and ran the simulations. A.T. and I.S. analysed the data.

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Correspondence to Marco Zorzi.

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Testolin, A., Stoianov, I. & Zorzi, M. Letter perception emerges from unsupervised deep learning and recycling of natural image features. Nat Hum Behav 1, 657–664 (2017).

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