Tuesday, January 21, 2014

Anterior Cingulate Neurons and Postdecisional Variables in a Foraging Task (Or: How to Get Laid Just by Staring at Somebody!)

A couple of my colleagues at the University of Rochester, Tommy Blanchard and Ben Hayden, recently published a single-cell recording study in the prestigious Journal of Neuroscience. Publishing in that journal is a big deal which everyone in my area aspires to, since it contains cutting-edge science that is read by several leading authorities in the field. (I, on the other hand, having neither the talent nor the motivation to do anything nearly that impressive but still craving attention, recently posted a video where I stuffed fourteen marshmallows into my mouth - and was still able to say "Chubby Bunny". Not that I'm bragging or anything.)

Blanchard and Hayden pin their colors to the mast at once. While some theories posit that the dorsal anterior cingulate cortex (dACC) represents the value of a choice - in other words, that the dACC encodes information about options before a decision is made - the authors argue in the present study that the dACC monitors specific variables about the chosen option and about its outcome, in effect encoding information after a decision is made. In addition, this implies that the observed dACC signals will be affected by not only the type of choice made, but also by variables about the foregone (not chosen) option.

To test this, the researchers tested  rhesus macaque monkeys used a paradigm known as a "diet selection task." In this task, the monkeys looked at a bar descending across a screen. The length of the bar determined how long the monkeys needed to fixate on the bar to receive a reward, while the color of the bar represented the size of the reward. If the monkeys fixated on the bar long enough, after a certain amount of time they would get the reward. The paradigm, I presume, was based off of the observation that young men at nightclubs and bars apparently believe that if they stare long enough at a female across the room, eventually she will become so overwhelmed with passion that she will tear off all of her clothes, even if the male who is staring happens to possess the sex appeal of a deceased gerbil. The fact that this rarely occurs, they think, is probably because they are not given enough time to stare; with a sufficiently long period of ogling, success would be virtually guaranteed.

Figure 1 reproduced from Blanchard & Hayden (2014). A: Monkey either does not fixate and does not get a reward (i.e., does not choose the option), or fixates on the bar, which progressively shrinks until reward is obtained. B: Reward sizes and fixation times for bar lengths. C: Recording site in dACC.

The recordings from single cells within the dACC showed a pattern of increased firing rate when an option was presented, along with a period of ramping-up in activity right before the reward was expected to appear (as shown in panel A of figure 3). Within this same cortical region, relatively high percentages of the neurons showed high correlations between their neural firing and reward, between the neural firing and the delay when they would receive the reward, or between the neural firing and both the size of the reward and the time it would take to receive the reward (panel B of figure 3).

Figure 3 of Blanchard & Hayden (2014)

A crucial test between the competing hypotheses, therefore, would be to examine whether the firing patterns of the dACC were qualitatively different depending on whether the option was accepted or not, and furthermore whether certain properties of the option (such as its reward size and the delay time) would be preferentially encoded depending on whether the option was accepted or not. It was found that on accept trials, more neurons tended to signal the delay of the reward rather than the size of the reward, while during reject trials, more neurons tended to signal the size of the reward than the delay time for the reward (Figure 4, panels C and D). Encoding the reward of the option that was not chosen is also known as a foregone option, since it was not selected but still apparently exerted an effect on neuronal firing.

Figure 4 of Blanchard & Hayden (2014)

Finally, the researchers observed that profitability - the ratio of reward size to delay - was significantly different depending on whether the monkeys decided to accept or reject the given option. Both this and the previous observations can all be described as postdecisional; the variables studied here show significant differences based on whether an option is chosen or rejected, and only specific aspects of that option are preferentially encoded by neurons in the dACC once the decision is made. This is in contrast to a predecisional framework of the dACC, which should encode aspects about the presented option, such as reward size and delay, regardless of whether the option is selected or not.

Link to paper: http://www.jneurosci.org/cgi/content/abstract/34/2/646

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