Activity in mouse pedunculopontine tegmental nucleus reflects action and outcome in a decision-making task

Abstract

Recent studies across several mammalian species have revealed a distributed network of cortical and subcortical brain regions responsible for sensorimotor decision making. Many of these regions have been shown to be interconnected with the pedunculopontine tegmental nucleus (PPTg), a brain stem structure characterized by neuronal heterogeneity and thought to be involved in several cognitive and behavioral functions. However, whether this structure plays a general functional role in sensorimotor decision making is unclear. We hypothesized that, in the context of a sensorimotor task, activity in the PPTg would reflect task-related variables in a similar manner as do the cortical and subcortical regions with which it is anatomically associated. To examine this hypothesis, we recorded PPTg activity in mice performing an odor-cued spatial choice task requiring a stereotyped leftward or rightward orienting movement to obtain a reward. We studied single-neuron activity during epochs of the task related to movement preparation, execution, and outcome (i.e., whether or not the movement was rewarded). We found that a substantial proportion of neurons in the PPTg exhibited direction-selective activity during one or more of these epochs. In addition, an overlapping population of neurons reflected movement direction and reward outcome. These results suggest that the PPTg should be considered within the network of brain areas responsible for sensorimotor decision making and lay the foundation for future experiments to examine how the PPTg interacts with other regions to control sensory-guided motor output.

Keywords: basal ganglia; decision making; pedunculopontine tegmental nucleus; sensorimotor.

Fig. 1.

Behavioral performance on the odor-cued spatial choice task. A: schematic of trial events and task epochs. In each trial, after entering the odor port, the mouse receives an odor, waits for the go signal, exits the odor port, moves to one of the reward ports, and receives a water reward following correctly performed trials. When (+)-carvone > (−)-carvone in the mixture, reward is available at the right; when (+)-carvone < (−)-carvone, reward is available at the left; and when (+)-carvone = (−)-carvone, the probability of reward at the left and right ports, independently, is 0.5. The delay epoch begins 100 ms after odor valve open. B: choice behavior is unaffected by drive implant. Behavioral performance for each session best fit to p = 1/(1 + e^{−a − bx}), where x is the proportion of (+)-carvone in the mixture ratio, p is the fraction of right choices, and a and b are free parameters. Each line is the average fit across all sessions for either 1 example mouse or all 5 mice. Error bars represent SD. C: gray circles indicate the fraction of completed trials (in which the mouse successfully waited for the go signal before exiting the odor port) as a function of discrimination difficulty, across sessions. Black circles indicate the fraction of completed trials in which the correct reward port was chosen, across sessions. At a mixture contrast of 0, the fraction of rewarded trials is shown (reward was equally likely at left and right on 50/50 trials). Error bars denote SD across sessions. D: solid line shows the distribution of odor sampling durations (from odor valve open until odor port exit) across trials, for all recording sessions (5 mice). In each trial, the delay epoch is delimited by the odor sampling duration. Dotted line shows the distribution of movement durations (from odor port exit to water port entry) across trials, for all recording sessions (5 mice).

Fig. 2.

Confirmation of recording sites and spike clustering. A₁: representative electrolytic lesion administered after final recording session (green dotted polygon), within pedunculopontine tegmental nucleus (PPTg). Coronal section, 4.6–4.9 mm caudal from bregma. A₂: representative anti-nitric oxide synthase (NOS) reaction used to enhance the distinction of the PPTg boundaries (red dotted polygon shows electrolytic lesion administered after final recording session). Coronal section, 4.6–4.9 mm caudal from bregma. A₃: colored boxes show estimated extent of tetrode recording for each experimental animal. B: energy of waveforms from lead 2 plotted against energy of waveforms from lead 3 of one tetrode for a representative recording session. Blue and red points show waveform energies recorded from neurons considered to be distinct; corresponding waveforms (mean ± SD) shown above. Clustering quality was enforced by requiring clusters to meet thresholds for L-ratio and isolation distance (see materials and methods).

Fig. 3.

PPTg neurons exhibit direction preference during each task epoch. A₁, top: rasters grouped by movement choice for an example neuron preferring ipsilateral movement during the delay epoch. Each row shows spikes (black ticks) in 1 trial, aligned to time of odor valve open and sorted by odor sampling duration. Orange ticks, times of odor valve open; green ticks, times of go signal; 50 pseudorandomly selected trials are shown per group. Bottom, perievent histograms show average activity separately for ipsilateral and contralateral trials. Shading indicates ±SE. Histograms are smoothed with a Gaussian filter (σ = 15 ms). A₂: as in A₁, for an example neuron preferring contralateral movement. A₃: histogram showing direction preferences during delay epoch across population of neurons that met criteria for trials and firing rate (materials and methods). Blue and red bars indicate neurons with a significant preference for ipsilateral and contralateral movement, respectively (P < 0.05). Blue and red arrowheads indicate preference for example neurons shown in A₁ and A₂, respectively. B₁–B₃: as in A_1–3, with respect to the movement epoch. C₁–C₃; as in A₁–A₃, with respect to the outcome epoch. Ipsi, ipsilateral; Contra, contralateral.

Fig. 4.

Activity during delay epoch reflects upcoming direction, and not odor identity. A, top: rasters corresponding to 50/50 trials grouped by movement choice for an example neuron. Bottom, perievent histograms showing average activity on 50/50 trials separately for ipsilateral and contralateral trials. Conventions are the same as in Fig. 3A₁. B: direction preference calculated during 50/50 trials, in which the odor does not vary across trials. Arrowhead indicates preference for example neuron shown in A.

Fig. 5.

Movement speed correlates with activity during delay and movement epochs. A₁: speed of movement during each trial plotted against activity of an example neuron during the final 267.5 ms of the delay epoch (the shortest odor sampling duration in our data set) of the corresponding trial, separated by trials in which movement was in the preferred and anti-preferred direction of the neuron. Each circle corresponds to 1 trial. Best-fit lines are shown for each group of trials. A₂: correlation between speed and firing rate for all neurons that were direction selective during the delay epoch (colored bars in Fig. 3A₃) for preferred direction trials. Red bars indicate neurons with a significant correlation on preferred (P < 0.05). Arrowheads indicate correlations for example neuron shown in A₁. A₃: correlation between speed and firing rate for all neurons that were direction selective during the delay epoch (colored bars in Fig. 3A₃) for anti-preferred-direction trials. Blue bars indicate neurons with a significant correlation on anti-preferred-direction trials (P < 0.05). Arrowheads indicate correlations for example neuron shown in A₁. B₁: same example neuron as in A₁, showing activity during the movement epoch. B₂: as in A₂, for activity during the movement epoch. B₃: as in A₃, for activity during the movement epoch.

Fig. 6.

Outcome-epoch activity reflects the presence or absence of reward. A, top: rasters are grouped by outcome (rewarded or nonrewarded) for an example neuron that prefers reward during the outcome epoch, aligned to time of reward port entry (red line). Bottom, perievent histograms showing average activity separately for rewarded and nonrewarded trials. Conventions are the same as in Fig. 3A₁. B: as in A, for an example neuron that prefers no reward. C: histogram showing outcome preferences during outcome epoch across population of neurons that met criteria for trials and firing rate (materials and methods). Black bars indicate neurons with a significant preference for reward or no reward (P < 0.05). Arrowheads indicate preferences for example neurons shown in A and B. D: joint histogram showing outcome and direction preferences during outcome epoch across population of neurons that met criteria for trials and firing rate (materials and methods). Each circle represents 1 neuron. Marginal histograms for outcome and direction preference are shown in C and Fig. 3C₃, respectively. A large fraction of neurons (purple) was selective for both outcome and direction.

Fig. 7.

Dynamics of direction preference throughout trial. A₁: preference curves for all neurons exhibiting significant preference for the ipsilateral, contralateral, or neither direction during the entire delay epoch (blue, red, and gray bars in Fig. 3A₃, respectively), sorted by earliest emergence of preference. Each row corresponds to 1 neuron. Preference curves were calculated by sliding the 200-ms window by 20-ms increments. Trials are aligned to odor port exit. Color scale shows significant preferences (P < 0.05). Black boxes indicate bins with nonsignificant preferences (P > 0.05) or with fewer than 4 ipsilateral or contralateral trials. Activity that occurred before odor valve open or after reward port entry was excluded. A₂: fraction of neurons shown in corresponding panel of A₁ preferring either the ipsilateral (blue) or contralateral (red) direction in each time bin. B₁: direction preference during the movement epoch for all neurons that preferred upcoming contralateral movement during the delay epoch. Here and in B₂, C₁, and C₂, numbers indicate total number of neurons exhibiting a significant preference (red represents contralateral and blue ipsilateral). B₂: direction preference during the movement epoch for all neurons that preferred upcoming ipsilateral movement during the delay epoch. C₁: side preference during the outcome epoch for all neurons that preferred contralateral movement during the movement epoch. C₂: side preference during the outcome epoch for all neurons that preferred ipsilateral movement during the movement epoch.