Many vmPFC neurons changed their activity in relation to appetitive/aversive blocks. Two example neurons are shown in . The first neuron was recorded in the ventral part of the vmPFC (, red dot) and preferred the appetitive block (). In the appetitive block, the neuron was excited by CSs, indicating that an upcoming reward was probable (i.e., 100% CS and 50% CS). It was also excited by the delivery of the reward (US). In the aversive block, the neuron was less active except for the period after TS. The activity of the neuron during ITI (measured during a 3 s period before TS) was higher in the appetitive block than in the aversive block (ITI activity after nonrewarded trials vs aversive block ITI; Wilcoxon's rank-sum test; p < 0.01). Overall, the activity of this neuron was related to the possibility and reception of reward; thus, we term the neuron “positive neuron.”
Figure 2 Two example neurons in the vmPFC. A–C, A neuron that preferred the appetitive block (positive neuron). D–F, A neuron that preferred the aversive block (negative neuron). A, D, Recording sites plotted on coronal MR images (red, positive (more ...)
The second example neuron was recorded in the dorsal part of the vmPFC (, blue dot) and preferred the aversive block (). In the aversive block, the activity of the neuron increased after CSs, indicating that an upcoming punishment was probable (i.e., 100% CS and 50% CS), and the increase in activity was maintained until sometime after the delivery of the punishment. In the appetitive block, the neuron was less active except after the omission of the reward in 50% CS trials (black SDF). Thus, we term this neuron “negative neuron.”
The distinct response properties of these two neurons can also be appreciated in , in which their activity is shown continuously across repeated changes in appetitive/aversive blocks. These graphs confirm that the activity of the positive neuron was higher in the appetitive than aversive block, whereas the activity of the negative neuron was higher in the aversive than appetitive block. Notably, the activity of both neurons changed during one block. This indicates that the activity of these neurons was not solely determined by individual events (TS, CS, US) but was also influenced by the progress of the current context (i.e., appetitive or aversive block) and/or the nearing of the alternate context.
We found that positive and negative neurons were distributed differently in the vmPFC. In two coronal sections in , red dots indicate the locations of neurons preferring the appetitive block (positive neurons) (n = 55), whereas blue dots indicate the locations of neurons preferring the aversive block (negative neurons) (n = 29). The classification was based on ROC analysis that compared activity during the CS period (50% and 100% CSs) between the appetitive and aversive blocks. If the ROC value was significantly larger or smaller than 0.5 (Wilcox-on's rank-sum test; p < 0.05), the neuron was classified as a positive or negative neuron, respectively. Small black dots indicate neurons that were not significantly different from 0.5. A majority of positive neurons were located in the ventral part of the vmPFC, whereas negative neurons were abundant in the dorsal vmPFC.
Figure 3 Regionally differential encoding of appetitive and aversive blocks within vmPFC. A, Single neuronal activity was recorded in the vmPFC (yellow area) from 7 to 15 mm anterior to the anterior commissure (AC). The green line demarcates the dorsal and ventral (more ...)
The dorsal–ventral difference in the appetitive/aversive preference is also illustrated in , separately for three epochs: (1) during ITI and TS epochs (left); (2) for 100 and 50% CS responses (middle); and (3) for US delivery responses (right). The regional difference is summarized by running averages of ROC areas for each epoch (green line). In the dorsal-to-ventral direction, the averaged appetitive/aversive block preference shifted from the negative side to the positive side. For all epochs, the estimated transition point was 3.3 mm from the bottom of the vmPFC.
In the dorsal vmPFC (above the transition point), most appetitive/aversive block-coding neurons were negative (n = 16 of 20). In the ventral vmPFC, a majority of block-coding neurons were positive (n = 51 of 64), but a small number of neurons were negative (n = 13 of 64).
The results of indicate that neurons in vmPFC encode appetitive/aversive blocks persistently during all trial epochs and that the preference for appetitive and aversive blocks was roughly segregated along the dorsal–ventral axis of vmFPC. This is also illustrated in , which compares the distributions of the appetitive/aversive preference magnitudes between the dorsal and ventral vmPFC.
suggests differences in the processing of the possibility (during the ITI and TS), probability (during the CS), and reception of rewards and punishments (after the US) in dorsal and ventral vmPFC. In the following analyses, we examine the details of CS, US, and TS responses in dorsal and ventral vmPFC (see Figs. 4 – 8). We mostly concentrated on the dominant group of modulated neurons in each region: positive neurons in the ventral region [, hereafter denoted as ventral (+) vmPFC neurons] and negative neurons in the dorsal region [, hereafter denoted as dorsal (–) vmPFC neurons].
Figure 4 Dorsal and ventral vmPFC neurons were sensitive to rewards and punishments in different manners. A, Average activity of positive neurons in the ventral vmPFC in the appetitive (left) and aversive (right) block. B, Average activity of negative neurons (more ...)