Distinct relationships of parietal and prefrontal cortices to evidence accumulation
Timothy D. Hanks*, Charles D. Kopec*, Bingni W. Brunton, Chunyu A. Duan, Jeffrey C. Erlich, and Carlos D. Brody (Nature 520:220-223, 9-April-2015)
In this paper on the neural basis of decisions driven by accumulating evidence, Tim and Chuck used our rat gradual accumulation of sensory evidence decision-making task (Brunton et al., Science, 2013), and they
- Show that rat cortical regions PPC and FOF have momentary-evidence-dependent ramping firing rates qualitatively very similar to analogous monkey cortices PPC and FEF. As in the monkey, these firing rate ramps are similar across the two regions.
- Develop a new method to estimate tuning curves (i.e., plots of firing rate as a function of a variable of interest) for accumulated evidence.
- Use their new method to show that despite the similarity in firing rate ramps, PPC and FOF have very distinct encodings of accumulating evidence: the PPC encodes the graded answer to “what is the value of the accumulating evidence”, whereas the FOF appears to have a more categorical encoding that can be roughly described as the categorical answer to “if the GO signal came now, which of the available decision outcomes should I choose?” This suggests that the FOF is more involved in response selection (choosing an available option, based on the accumulated evidence) than in gradual accumulation per se.
- Use halorhodopsin (eNpHR3.0) to test competing predictions: if the FOF is involved in the gradual accumulation process, which occurs throughout sensory evidence accumulation, then perturbing it at any point during the sensory stimulus should affect behavior. In contrast, if the FOF is primarily involved in response selection, perturbing it should have an effect on the behavior only near the end of the sensory stimulus, which is when response selection will occur. The data support the latter interpretation.
Together, the optogenetic and electrophysiological data suggest that despite its ramping firing rates, the FOF is involved in response selection, not graded accumulation. We wonder whether the results could hold for monkey FEF as well, and hope someone will use the methods we developed here to find out.
The results clarify the particular contribution of the FOF to decisions driven by accumulation of evidence. We are excited about applying the methods developed here to other brain regions linked to such decisions, in an effort to elucidate the different contributions of different brain areas, and understand how the whole circuit fits together.
Tim Hanks will become faculty at UC Davis in November 2015