The role of rat medial frontal cortex in effort-based decision making

Abstract

We conducted a series of experiments to elucidate the role of rat medial frontal cortex (MFC) (including prelimbic, infralimbic, and cingulate cortices) in effort-based decision making. Rats were trained on a cost-benefit T-maze task in which they could either choose to climb a barrier to obtain a high reward in one arm (HR arm) or could obtain a small reward in the other with no barrier present (LR arm). Before surgery, all animals were selecting the HR arm on almost every trial. However, after excitotoxic lesions to MFC, the rats shifted to selecting the LR arm on almost every trial. This was not caused by a spatial memory or motor deficit because the same rats returned to selecting the HR arm when the amount of effort needed to be expended to obtain reward in either arm was equalized by putting an identical barrier in the LR arm. Additional experiments demonstrated that the change in effort-based decisions observed in the rats was not caused by a complete insensitivity to reward and effort because they returned to choosing the HR arm if either the cost was reduced (by making the barrier smaller) or the benefit was increased (increasing the food ratio differential). Instead, the MFC lesion shifted the animals' decision criterion, making them more likely to choose the LR arm than the sham-lesioned animals. These results imply that medial frontal cortex is important for allowing the animal to put in more work to obtain greater rewards.

Fig. 1.

a, Schematic top view of the high-walled T-maze apparatus used throughout the test phase of the study. On each choice trial, the animals could either choose the LR arm with two pellets or could climb over a wire mesh barrier to obtain four pellets in the HR arm. The HR arm was to the left for half of the animals and to the right for the others. b, The barriers were constructed out of thick wire mesh in the shape of a right-angled triangle. The rats climbed up the vertical side and descended down the slope to obtain the HR reward. In experiments 1, 2, and 4, the barrier was always 30 cm. In experiment 3, the barrier was either 30 cm (left) or 20 cm (right), depending on the condition.

Fig. 2.

Schematic time line showing the duration and different barrier sizes used during different stages of each experiment. Baseline periods refer to the days with a single 30 cm barrier in the HR arm when the animals were tested until group scores stabilized to within ±4% for 3 consecutive days.

Fig. 3.

Reconstructions of medial frontal cortex lesions. Minimal (left column), representative (center column), and maximal (right column) extent of the lesions in coronal sections between +5.2 mm anterior to bregma and −1.3 mm posterior to bregma are illustrated. Gray shading indicates areas of complete cell loss, whereasstippled shading represents the lesion penumbra in which cells were still present but clearly abnormal compared with those in sham-operated animals.

Fig. 4.

Photomicrographs of coronal sections showing typical cell loss in a representative lesioned rat (left column) compared with a sham-operated animal (right column).

Fig. 5.

Mean ± SE percentage of trials in which sham and lesion group rats chose the HR arm with a single 30 cm barrier in the HR arm. In experiment 1, there was always just a single 30 cm barrier in the HR arm. However, in experiment 2, there could either be a single 30 cm barrier in the HR arm or identical 30 cm barriers in both goal arms. Each testing block consisted of 3 d of 10 choice trials per day.

Fig. 6.

Mean ± SE percentage of trials in which sham and lesion group rats chose the HR arm when the size of barrier was varied in the HR arm (experiment 3). Rats were run with either a single 20 cm barrier in the HR arm or with a 30 cm barrier in both goal arms. The first testing block (test days 16–18) is labeled 4 5 6 because it was a continuation of testing from the last 3 d of experiment 2 to get a consistent baseline measurement with the single 30 cm barrier before attempting any cost–benefit manipulations.

Fig. 7.

Mean ± SE percentage of trials in which sham and lesion group rats chose the HR arm with a single 30 cm barrier in the HR arm in experiment 4. The food differential ratio was changed from 4:2 in the baseline condition (HR/LR arm) to 5:1. The first 3 d of testing (test days 28–30; gray shading) when a stable baseline measure was being obtained have been included for the sake of completeness, although they have not been subjected to statistical analysis.