Neurons in the orbitofrontal cortex encode economic value

Abstract

Economic choice is the behaviour observed when individuals select one among many available options. There is no intrinsically 'correct' answer: economic choice depends on subjective preferences. This behaviour is traditionally the object of economic analysis and is also of primary interest in psychology. However, the underlying mental processes and neuronal mechanisms are not well understood. Theories of human and animal choice have a cornerstone in the concept of 'value'. Consider, for example, a monkey offered one raisin versus one piece of apple: behavioural evidence suggests that the animal chooses by assigning values to the two options. But where and how values are represented in the brain is unclear. Here we show that, during economic choice, neurons in the orbitofrontal cortex (OFC) encode the value of offered and chosen goods. Notably, OFC neurons encode value independently of visuospatial factors and motor responses. If a monkey chooses between A and B, neurons in the OFC encode the value of the two goods independently of whether A is presented on the right and B on the left, or vice versa. This trait distinguishes the OFC from other brain areas in which value modulates activity related to sensory or motor processes. Our results have broad implications for possible psychological models, suggesting that economic choice is essentially choice between goods rather than choice between actions. In this framework, neurons in the OFC seem to be a good candidate network for value assignment underlying economic choice.

Figure 1

Experimental design. a. Trial structure (see Methods). b. Example of behavioral choice pattern. The plot shows the percentage of trials in which the monkeys chose juice B (y-axis) for various offer types (x-axis). A sigmoid fit provides the measure of the relative value n*=4.1.

Figure 2

Activity of one neuron. a. Rasters. Each line represents one trial and each small dot represents one spike. Trials, arranged by offer type, are aligned at the offer (left) and at the juice (right). The blue highlight marks the post-juice time window. b. Activity profiles shown separately for trials in which juice A is offered on the left (red) or on the right (green). The cell activity does not depend on the spatial configuration of the visual stimulus. c. Activity profiles shown separately for trials in which the monkey chooses the juice offered on the left (red) or on the right (green). The cell activity does not depend on the direction of the eye movement. d. Top: choice pattern recorded in this session (n*=1.9). Bottom: activity of the cell (±s.e.m.) recorded in the pre-offer (light gray, control) and post-juice time window (black). Note that the response does not reflect simple physical properties of the visual stimulus, such as the number of squares displayed on the monitor. For example, offer types 1B:3A and 3B:1A, which are visually identical except for the color of the squares, elicit very different activation.

Figure 3

Activity of six neurons. For each cell, the top panel shows the choice pattern, with the relative value indicated on the top left. a–c. Responses encoding the chosen value. The response in c is negatively correlated with the chosen value (high activity for low value). d–e. Responses encoding the value of juice A offered (d) and the value of juice B offered (e). We refer to these responses as related to the offer value. d. Response encoding the juice taste. Here we separate trials in which the monkey chose juice A (diamonds) or juice B (circles). The response reflects the chosen juice type independently of the amount. Responses were recorded in the post-offer (a d e, blue), pre-juice (b, cyan), and post-juice (c f, black) time windows. For each cell, the curves in light gray show the activity in the pre-offer time window. Error bars represent the s.e.m.

Figure 4

Time course. We assign each neuron to one variable of the three variables if the regression slope is significantly different from zero (p<0.01), and we include all 931 neurons in the analysis. The dotted line indicates chance level (9.31).