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Revealing the multidimensional mental representations of natural objects underlying human similarity judgements


Objects can be characterized according to a vast number of possible criteria (such as animacy, shape, colour and function), but some dimensions are more useful than others for making sense of the objects around us. To identify these core dimensions of object representations, we developed a data-driven computational model of similarity judgements for real-world images of 1,854 objects. The model captured most explainable variance in similarity judgements and produced 49 highly reproducible and meaningful object dimensions that reflect various conceptual and perceptual properties of those objects. These dimensions predicted external categorization behaviour and reflected typicality judgements of those categories. Furthermore, humans can accurately rate objects along these dimensions, highlighting their interpretability and opening up a way to generate similarity estimates from object dimensions alone. Collectively, these results demonstrate that human similarity judgements can be captured by a fairly low-dimensional, interpretable embedding that generalizes to external behaviour.

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Fig. 1: Task and modelling procedure for the large-scale identification of mental object representations.
Fig. 2: Predictiveness of the computational model for single-trial behavioural judgements and similarity.
Fig. 3: Example object dimensions illustrating their interpretability.
Fig. 4: Illustration of example objects with their respective dimensions, using rose plots.
Fig. 5: Two-dimensional visualization of the similarity embedding, combining dimensionality reduction (multidimensional-scaling-initialized t-distributed stochastic neighbourhood embedding; dual perplexity, 5 and 30; 1,000 iterations) with rose plots for each object (see Fig. 4).
Fig. 6: How many dimensions are required to capture behavioural judgements and object similarity?
Fig. 7: The relationships between seemingly categorical dimensions and typicality ratings of those categories.
Fig. 8: Task and results of direct ratings of dimensions.

Data availability

The learned embedding, triplet odd-one-out behavioural data for testing model performance, typicality scores, participant-generated dimension labels and dimension ratings are available at The behavioural data used for training the model are available from the corresponding author upon request.

Code availability

To reproduce the relevant analyses and figures, the relevant MATLAB scripts and functions are available at The computational modelling code to create an embedding is available from the corresponding author upon request.


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We thank A. Corriveau for help with the data collection in the laboratory experiment, L. Stoinksi and J. Perkuhn for help with finding public-domain images for this publication, and I. Charest, B. Love, A. Martin and P. McClure for helpful discussions and/or comments on the manuscript. This research was supported by the Intramural Research Program of the National Institutes of Health (grant nos ZIA-MH-002909 and ZIC-MH002968), under National Institute of Mental Health Clinical Study Protocol 93-M-1070 (NCT00001360), and by a research group grant awarded to M.N.H. by the Max Planck Society. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

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Authors and Affiliations



M.N.H. and C.I.B. conceived and designed the study. M.N.H. collected the data. C.Y.Z., M.N.H. and F.P. designed the computational model. M.N.H., C.Y.Z. and F.P. analysed the data. M.N.H., C.I.B., F.P. and C.Y.Z. wrote the manuscript and provided critical revisions.

Corresponding author

Correspondence to Martin N. Hebart.

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The authors declare no competing interests.

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Peer review information Primary Handling Editor: Marike Schiffer.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Reproducibility of dimensions.

Reproducibility of dimensions in the chosen 49-dimensional embedding across 20 random initializations (see Extended Data Fig. 2 for a list of all dimension labels). Shaded areas reflect 95% confidence intervals.

Extended Data Fig. 2 Labels and word clouds for all 49 model dimensions.

Labels for all 49 dimensions, with respective word clouds reflecting the naming frequency across 20 participants. The dimensions appear to reflect both perceptual and conceptual properties of objects. A visual comparison between labels and word clouds indicates a generally good agreement between participant naming and the labels we provided for the dimensions.

Extended Data Fig. 3 Category-typicality correlations.

Detailed results of inferential statistical analyses correlating category-related dimensions with typicality of their category. One-sided p-values were generated using randomization tests and were controlled for false discovery rate (FDR) across multiple tests. 90% confidence intervals were used to complement one-sided tests.

Extended Data Fig. 4 Model performance and dimensionality as a function of training data size.

Model performance and dimensionality varied as a function of the amount of data used for training the model. Models were trained in steps of 100,000 trials. Six models with random initialization and random subsets of data were trained per step and all models applied to the same test data as in the main text, making it a total of 78 trained models. For each step, computation of up to two models did not complete on the compute server for technical reasons, making the total between 4 and 6 models per step. Results for each individual model and the average for each step are shown in the Figure. a. Model performance was already high for 100,000 trials as training data but increased with more data, saturating around the final model performance. b. Dimensionality increased steadily with the amount of training data.

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Hebart, M.N., Zheng, C.Y., Pereira, F. et al. Revealing the multidimensional mental representations of natural objects underlying human similarity judgements. Nat Hum Behav 4, 1173–1185 (2020).

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