Role of secondary motor cortex in withholding impulsive action: Inhibition or competition?
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1
Champalimaud Neuroscience Program at IGC , Portugal
In a now-famous set of experiments, Mischel et al. (1968), tested self-control in 4-year-old children by giving each child one marshmallow. The child was told she could either eat the marshmallow now or, if she waited while he stepped out for a few minutes, she could have two when he returned. Surprisingly, the ability to wait for the delayed reward in this task turned out to be a good predictor for academic and social success years later. This and many other studies helped to lead to the current idea that impulse control, as measured in delayed gratification tasks, represents a fundamental cognitive function. According with one view, impulse control results from a general inhibitory mechanism, thought to be localized in the frontal cortex, but whose mechanisms are not well understood. Curiously, however, Mischel et al. observed that children with successful impulse control did not wait passively, but distracted themselves by singing or playing with their hands. Could it be that impulse control arises not from a general inhibitory mechanism but from the ability to generate alternative, competing actions? To investigate these issues, we are studying analogous impulse control behavior in rats. In the first task variant, rats were required to wait at a nose poke. A first tone was presented at a fixed short delay (0.4 s) and a second one at a longer exponentially-distributed delay (~2 s mean). Responses between the two tones received a small reward, while responses after the second tone received a larger reward. Video analysis showed that during waiting subjects engaged in various simple motor actions such as grasping or chewing the waiting port. To study the underlying neural bases, we made single-unit recordings from medial prefrontal cortex (mPFC), an area associated with inhibitory control, and secondary motor cortex (M2), an area involved in motor planning. We tested for the ability of single neurons to predict the amount of time the subject would wait before responding. We found that 20%(109/548) of M2 neurons and only 7%(8/122) of mPFC neurons showed predictive activity, a significantly smaller fraction (P < 0.001, χ2 test). Next, we tested whether waiting-predictive neurons were general to waiting or specific to the action involved in waiting. To do this, we required subjects to alternate in blocks of trials between waiting at a nose poke and pushing a lever. We found that 43/171 M2 neurons were predictive of nose poke waiting time, but only 6 of these were also predictive of lever press waiting time. Neurons predictive in both tasks typically showed distinct temporal activation profiles. These results suggest that the ability of firing of motor cortex neurons to predict waiting time appears to result not only from an involvement in general inhibition, but due to linkage with the planning or initiation of specific actions that occur during waiting. These findings suggest that the ability to generate "self-distractions" - alternative actions that compete with the pursuit or consumption of a tempting reward - may be a general feature of impulse control across species.
Conference:
Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010.
Presentation Type:
Poster Presentation
Topic:
Poster session I
Citation:
Fonseca
A,
Murakami
M,
Vicente
M,
Costa
G and
Mainen
Z
(2010). Role of secondary motor cortex in withholding impulsive action: Inhibition or competition?.
Front. Neurosci.
Conference Abstract:
Computational and Systems Neuroscience 2010.
doi: 10.3389/conf.fnins.2010.03.00032
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Received:
18 Feb 2010;
Published Online:
18 Feb 2010.
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Correspondence:
Ana Fonseca, Champalimaud Neuroscience Program at IGC, Oeiras, Portugal, arsecca@gmail.com