Neural population code for fine perceptual decisions in area MT

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In the middle temporal (MT) area of primates, many motion-sensitive neurons with a wide range of preferred directions respond to a stimulus moving in a single direction. These neurons are involved in direction perception, but it is not clear how perceptual decisions are related to the population response. We recorded the activities of MT neurons in rhesus monkeys while they discriminated closely related directions, and examined the relationship between the activities of neurons tuned to different directions and the monkeys' choices. Perceptual decisions were significantly correlated with the activities of the highest-precision neurons but not with those of the lowest-precision neurons. The combined performance of the high-precision neurons matched the monkeys' behavior, whereas the ability to predict behavior based on the entire active population was poor. These results suggest that fine discrimination decisions are crucially dependent on the activities of the most informative neurons.

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Figure 1: Fine direction-discrimination task and the monkeys' psychophysical performance.
Figure 2: Neurometric functions.
Figure 3: Relationship between neural precision and preferred direction.
Figure 4: Covariation between neural responses and the monkeys' choices.
Figure 5: Mutual information between neuron's firing rates and the monkeys' decisions.
Figure 6: Fine discrimination performance of various neural coding schemes analyzed using a discrimination model.


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Supported by US National Institutes of Health grant R01 EY013138. We thank H. Bedell, R. Born, W. Bosking, K. Britten, G. DeAngelis, M. Fukushima, W. Newsome, H. Ogmen, S. Patel, B. Scott, M. Shadlen, P. Wallisch and S. Watamaniuk for suggestions and comments.

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Correspondence to David C Bradley.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Fig. 1

Neural precision is plotted as a function of the slope of the direction-tuning curve near the stimuli directions. (GIF 9 kb)

Supplementary Fig. 2

Psychometric function for reference stimulus directions of 85° and 95°, pooled from data over several recording sessions. If the monkey had ignored the reference, then the responses would have been at the 100% for all direction differences about 85° reference and at 0% for all direction differences about the 95° reference, resulting in a flat line near the 50% level. (GIF 16 kb)

Supplementary Fig. 3

Threshold ratio of neuron to behaviour calculated using the firing rates from the test interval (for the trials in which the test direction = reference direction) as the reference histogram. These ratios are not statistically different from those in Fig. 3a (t-test, p>0.8). (GIF 15 kb)

Supplementary Fig. 4

Thresholds are shown as a function of integration time. The top figure shows the psychophysical data and the bottom, the neural data. (GIF 9 kb)

Supplementary Fig. 5

Psychophysical and neurometric functions for three neurons are shown comparatively for two ranges of direction-differences. The left row shows psychometric and neurometric functions for the smaller range of direction differences and the right row for the larger range. Each row of data is for one neuron. “N Thresh” is the neurometric threshold and “P Thresh” is the psychophysical threshold. (GIF 20 kb)

Supplementary Fig. 6

Psychophysical thresholds are compared for the two ranges of direction-differences. (GIF 9 kb)

Supplementary Fig. 7

Ratio of the neurometric threshold estimated with the wider range of direction differences (−16°,+16°) to that estimated with the shorter range (−3°,+3°) is shown for 132 neurons. (GIF 13 kb)

Supplementary Notes (PDF 154 kb)

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Purushothaman, G., Bradley, D. Neural population code for fine perceptual decisions in area MT. Nat Neurosci 8, 99–106 (2005) doi:10.1038/nn1373

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