ACCUMULATION OF INFORMATION IN THE CORTICAL CIRCUITRY

Even simple (visual) decisions imply higher cognitive functions that integrate noisy sensory stimuli, prior knowledge and the costs-and-benefits related to possible actions in function of their time of occurrence. Accumulation of noisy information is a reliable pattern performed by neural pools in cortical circuitry during decision making (DM) process. This process is time absorbing, especially when the quality of information is poor and there exist many possible alternatives to valuate and compare.

There exists large consensus in the studies of DM toward the conformation of a phase of accumulation of evidence until a decision is made [1]. That is, the decision maker is expected to keep on gathering information until the evidence in favor of one of the alternatives suffices. Thus, the stochastic integration of information up to a certain threshold gives rise to a speed-and-accuracy tradeoff (the performance of the responses increases for slower response times) that is bounded by the costs associated with obtaining more information. In this context the response times (RTs) to the stimuli characterize the speed-accuracy tradeoff because they allow to identify the time when a decision is made (although not yet completed by the motor action) [2]. RT studies have addressed to the implementation of diffusive models for describing decisional behaviors (see par.6.2 for further details on the diffusive model) and to the identification of the neuronal areas related to the decisional activity. 

Neurons in the middle temporal area (MT) are known to encode motion stimulus [3], while the decision process itself occurs downstream of MT (maybe in the posterior parietal cortex and/or prefrontal cortex). Perceptual choice experiments with primates [4,5] enabled to relate the selective activation of neurons in the LIP area  with the perceptual choice and the response time [6], and this activity would persist throughout a delay between the stimulus and the saccadic movement. This implies that the LIP neurons neither can be purely a motor signal, nor simply respond to sensory input [7]. Rather, LIP neurons are also supposed to contribute to the working memory associated with guiding the eye movement [8], that is, they would store information about the target location. Neurons in the prefrontal cortex display similar properties during visual motion discrimination tasks [9]. Further studies of human neuroimaging and monkey single-neuron physiology have supported the hypothesis that the parietal and frontal cortices form a system for temporal accruing of data and categorical decision-making. These areas would exert executive control on sensory neurons by providing top-down signals that convey information on semantic categorization derived by the stimulus-response association [10,11]. 

Hence, so far as it has been yet reported, the eye movement decision tasks involves the neural activity from the areas MT, LIP, FEF and SC. The association between the neural firing rates and the DM process is by now an accepted fact and, by the way, some points are worthy to be mentioned. The ramping of the firing rates does not merely anticipate the eye movements, but would also relate to the target selection. Rather, the rate of growth of the neural activity is proportional to the response times, and so it may predict the decision time. In fact, it triggers the spiking burst of the downstream neurons (in SC and CD) and the occurrence of their crossing of a defined threshold level marks the decision time. Therefore, the ramping of the firing rates is also proportional to the prior probabilities of the alternatives and to their probabilities of being rewarded (as before shown). These findings can be interpreted using mathematical and statistical approaches to model the behavioral decisions [12].

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