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, 21 (13), 4801-8

A Code for Behavioral Inhibition on the Basis of Color, but Not Motion, in Ventrolateral Prefrontal Cortex of Macaque Monkey

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A Code for Behavioral Inhibition on the Basis of Color, but Not Motion, in Ventrolateral Prefrontal Cortex of Macaque Monkey

M Sakagami et al. J Neurosci.

Abstract

To examine the neural mechanism for behavioral inhibition, we recorded single-cell activity in macaque ventrolateral prefrontal cortex, which is known to receive visual information directly from the inferotemporal cortex. In response to a moving random pattern of colored dots, monkeys had to make a go or no-go response. In the color condition, green indicated go, whereas red indicated no-go, regardless of the motion direction; in the motion condition, upward indicated go, whereas downward indicated no-go, regardless of the color. Approximately one-half of the visual cells were go/no-go differential. A majority of these cells (64/73) showed differential activity only in the color condition; they responded nondifferentially in the motion condition, although the same set of stimuli was used. We classified these cells as "go type" (n = 41) and "no-go type" (n = 23) depending on the color for which they showed a stronger response. Interestingly, in both types of cells, the differential effects were observed only for the no-go-indicating color. Compared with the nondifferential responses in the motion condition, go-type cells in the color condition showed weaker responses to the no-go-indicating color, whereas their responses to the go-indicating color were similar; in contrast, no-go type cells showed stronger responses to the no-go-indicating color, whereas their responses to the go-indicating color were similar. Both types of cells did not show any activity change during the actual execution of the go or no-go response. These results suggest that neurons in ventrolateral prefrontal cortex contribute to stimulus-response association in complex task situations by inhibiting behavioral responses on the basis of visual information from the ventral stream.

Figures

Fig. 1.
Fig. 1.
Schematic illustration of the experimental design and target stimuli. A, The trial began when the monkey pressed the lever. The monkey was required to focus his gaze at the fixation spot. A target cue was then presented for 200 msec, followed by a random delay period (0.5–2 sec), until the fixation spot dimmed. If the target indicated a go response, the monkey had to release the lever immediately (within 0.8 sec). If the target indicated a no-go response, the monkey had to continue to press the lever throughout the dim period (1.2 sec), and then the fixation spot was re-illuminated. In no-go trials, the monkey could release the lever at any time after the dim period. Correct responses in both go and no-go trials were rewarded with a drop of orange juice immediately after the lever was released.B, Examples of target cues. The stimulus consisted of a moving random pattern of colored dots, green orred, and upward or downward direction. The color of the fixation spot indicated the attention condition. In the color condition (yellow fixation spot), a green target color indicated a go response, and a red target color indicated a no-go response. In the motion condition (purple fixation spot), upward motion direction indicated a go response, and downward movement indicated a no-go response.
Fig. 2.
Fig. 2.
Activity pattern of the go type C cell.A, Typical example of a go type C cell with the first set of stimuli. Each pair of rasters and histograms illustrates the neuronal response to the target shown on the left. The rasters and histograms are split in two; the left side is aligned on target onset (vertical line; the horizontal bar indicates target duration), and the right side is aligned on lever release (vertical line). Only the data from correct trials were obtained. The left panel represents the neuronal activity in the color condition; the right panel represents that in the motion condition.Arrows in the stimuli indicate motion direction.go, Go trial; ng, no-go trial.Triangles in the rasters indicate fixation dimming or re-illumination (end of dim): fixation dimming in the target-aligned rasters and in the go trials of response-aligned rasters, and re-illumination in the no-go trials of response-aligned rasters. Bin width, 20 msec. B, Activity pattern of the same cell with the second set of stimuli.
Fig. 3.
Fig. 3.
Activity pattern of the no-go type C cell.A, Typical example of a no-go type C cell with the first set of stimuli. B, Activity pattern of the same cell with the second set of stimuli.
Fig. 4.
Fig. 4.
Population average of two types of C cells. A, Population average of 41 go type C cells to the target onset (left panel), onset of the fixation dimming (middle panel), and lever release (right panel). The lineindicated by go (C) shows the response to the go color in the color condition; ng(C), the response to the no-go color in the color condition; go(M), the response to the go color in the motion condition; ng(M), the response to the no-go color in the motion condition (collapsed across motion directions). The curves are based on nonsmoothed data with 10 msec temporal resolution. B, Population average of 23 no-go type C cells.
Fig. 5.
Fig. 5.
Plots of the task relevancy (R) index, presented for go type C cells (A) and no-go type C cells (B). In both panels, plots on theleft show indices for the go color (Rgo), and plots on theright show indices for the no-go color (Rng). Positive values indicate enhancement of the response in the color condition relative to the motion condition; negative values indicate suppression in the color condition. Arrowheads indicate the mean values of the distributions.
Fig. 6.
Fig. 6.
Examples of electrode penetrations in two monkeys (right hemisphere from Monkey #1 and left hemisphere from Monkey #2). Similar distributions were observed in the other hemispheres (data not shown). Circles indicate go type C cells; plus signs indicate no-go type C cells.Small symbols indicate one cell; large symbols indicate two cells. A circlesuperimposed on a plus sign indicates a location in which both types of C cells were found. In penetrations indicated bysmall dots, no C cells were found. PS, Principal sulcus; AS, arcuate sulcus. Theinset is a lateral view of the left hemisphere and shows the location of VLPFC (gray area).

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