Abstract
Evidence accumulation models of simple decision-making have long assumed that the brain estimates a scalar decision variable corresponding to the log likelihood ratio of the two alternatives. Typical neural implementations of this algorithmic cognitive model assume that large numbers of neurons are each noisy exemplars of the scalar decision variable. Here, we propose a neural implementation of the diffusion model in which many neurons construct and maintain the Laplace transform of the distance to each of the decision bounds. As in classic findings from brain regions including LIP, the firing rate of neurons coding for the Laplace transform of net accumulated evidence grows to a bound during random dot motion tasks. However, rather than noisy exemplars of a single mean value, this approach makes the novel prediction that firing rates grow to the bound exponentially; across neurons, there should be a distribution of different rates. A second set of neurons records an approximate inversion of the Laplace transform; these neurons directly estimate net accumulated evidence. In analogy to time cells and place cells observed in the hippocampus and other brain regions, the neurons in this second set have receptive fields along a “decision axis.” This finding is consistent with recent findings from rodent recordings. This theoretical approach places simple evidence accumulation models in the same mathematical language as recent proposals for representing time and space in cognitive models for memory.
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Notes
Zhang and Maloney (2012) provide an outstanding discussion of the centrality of log likelihood to understanding cognitive psychology.
When z = a/2, the prior is uninformative.
In many experiments, such as the random dot motion task discussed extensively below, it may be difficult to make a connection to the normative sequential sampling model.
For our purposes, it is sufficient to consider real values of s.
To see this, set \(\alpha (t) = \frac {dx}{dt}\) and multiply both sides of Eq. 9 by \(\frac {dt}{dx}\).
This implies the ordinal variable \(n \propto \log \overset {*}{x}\).
An increase in receptive field width would appear as increase in the width of the central ridge in Fig. 2b. This spread is not visible due to the properties of the recording method used in that study; an increase in receptive field width with peak time is observed in time cell studies using other recording techniques (Jin et al. 2009; Salz et al. 2016; Mello et al. 2015; Tiganj et al. 2018).
More precisely, we implemented \(X_{t+{\Delta }} = X_{t} + A {\Delta } + c \sqrt {\Delta } \mathcal {N}(0,1)\). The value of Δ was set to .001, A was 1.25, .83, and .625 across conditions. X(0) = 0 and we terminated the decision when X = 1.
Because the growth/decay of the units is not at the same rate for each neuron, we would expect additional principal components to capture this residual.
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Acknowledgements
We acknowledge helpful discussions with Bing Brunton, Josh Gold, Chandramouli Chandrasekaran, Chris Harvey, Ben Scott, and Karthik Shankar.
Funding
This work was supported by NIBIB R01EB022864, ONR MURI N00014-16-1-2832, and NIMH R01MH112169.
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Howard, M.W., Luzardo, A. & Tiganj, Z. Evidence Accumulation in a Laplace Domain Decision Space. Comput Brain Behav 1, 237–251 (2018). https://doi.org/10.1007/s42113-018-0016-2
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DOI: https://doi.org/10.1007/s42113-018-0016-2