Our ability to suppress unwanted thoughts and behaviors is thought to be related to a process known as “inhibition,” whereby ventrolateral regions of prefrontal cortex (vlPFC) actively suppress inappropriate representations. A 2001 study by Sakagami et al. recorded firing data from neurons in the vlPFC to determine the exact mechanism by which this might occur.
The authors begin by noting that previous studies of vlPFC have found that a majority of neurons “responded to a stimulus that instructed execution, not suppression, of a behavioral response.” This view is consistent with that of Sharon-Thompson Schill and colleagues, who suggest that vlPFC is involved in the specification or selection of appropriate representations.
The authors therefore recorded from 73 neurons in the vlPFC of two monkeys performing a go/nogo task. In this go/nogo task, monkeys began each trial by pressing a lever; after a variable fixation period on a dot in the center of the screen, a moving, colored image would appear either above, below, or on either side of the center of the screen. Monkeys were required to keep the lever pressed until the dot in the center of the screen dimmed, and to either release that lever within .8s (for go stimuli, which were green moving images if the fixation dot was yellow, or upward moving images if the fixation spot was purple) or to keep it pressed for the next 1.2s (for nogo stimuli, which were red moving images if the fixation dot was yellow, or download moving images if the fixation spot was purple).
The authors found 147 cells in vlPFC which showed firing data that was sensitive to the task; 115 of these cells could be classified as either specific to a color, specific to a direction of motion, specific to the requisite go or nogo response across tasks, or specific to the requisite go or nogo responses only within tasks (i.e., for only the color or motion condition).
Those cells which were selective to the requisite go response in the color task showed an interesting pattern of activity: they became active for both colors, but maintained that activity only for colors indicating a go response. Thus, as the authors note, the only “distinctive input that cells in the next processing stage (e.g., motor preparation) receive” from these cells is a reduction or absence in activity for no-go stimuli. In addition, another 23 cells actually increased their activity to no-go stimuli. But in neither case did the authors observe these cells to do anything different at the time a nogo response was required – i.e., after the fixation dot has dimmed.
This last result indicates that the cells are not conclusively involved in the suppression or inhibition of a response. Alternatively, these cells may simply be evaluating the significance of those stimuli to the task at hand.