In order to study the encoding of motivational salience and to disambiguate it from the processing of hedonic valence, we employed a Go/Nogo task because sensory cues in this task can be motivationally salient and at the same time they can be associated with either reward or punishment. Specifically, we trained rats to associate three previously neutral and clearly perceptible sensory cues with either an appetitive sucrose solution (0.3M) or an aversive quinine solution (3mM), delivered through the same licking spout. Sucrose delivery was signaled by a light cue (LS) or an auditory cue (TS), while quinine delivery was signaled by a different auditory cue (TQ) (). By design, the cues differed in terms of their sensory modalities (visual vs. auditory), associated motor responses (Go vs. Nogo), and the hedonic valence of predicted outcomes (reward vs. punishment). The only common feature among the three cues was their motivational salience. As expected, rats learned to lick for sucrose after TS and LS (Go response, 88.1±2.5% and 87.6±2.5%, respectively, mean±s.e.m.), while correctly avoided licking after TQ (Nogo response, 67.3±4.3%) (). Even when rats incorrectly licked during TQ trials, their response latency (1.63±0.08 sec) was significantly longer than the response latency in correct sucrose trials (1.10±0.09 to TS and 1.04±0.05 sec to LS, paired t-test, p<10−5). These results indicate that, after training, all cues in the Go/Nogo task were motivationally salient and rats responded to each cue according to its hedonic valence.
We first assessed whether BF neurons showed bursting responses to all three motivationally salient cues in the Go/Nogo task. A total of 210 BF neurons were recorded with movable multi-electrode bundles (Supplemental Figure S1
) while rats performed the task (4 rats, 19 sessions). The most common type of BF neuronal response to cues was a short latency burst (see Supplemental Figure S2
for responses of all BF neurons and Supplemental Figure S3
for definition of bursting response). When rats correctly responded to the cues, half of BF neurons (105/210) responded to the onset of all three cues with a robust and remarkably similar short spike burst, illustrated in and . This early bursting response of BF neurons stood in clear contrast with the subsequent sustained firing modulation, which was mostly excitatory for Go responses and inhibitory for Nogo responses (, see also Supplemental Figure S4
), indicating that the two phases of responses are dissociable. In addition, the bursting latency and amplitude of the early response to different cues were highly correlated, i.e. BF neurons with earlier and stronger responses to one cue likely showed earlier and stronger responses to other cues (). These strong correlations suggest that bursting responses toward all cues were qualitatively similar. Thus, while the subsequent sustained response was correlated with the rats' behavioral output (Go vs. Nogo) and the hedonic valence of the expected reinforcement (reward vs. punishment), the early bursting response was consistent only with the encoding of motivational salience that is common to all three cues.
Motivationally salient cues elicit bursting responses of BF neurons
If BF bursting indeed encoded the abstract motivational salience of the cues, the same BF neurons should not respond to novel sensory cues that have not yet acquired motivational salience through learning. To investigate this issue, two separate groups of rats were trained on subsets of cues — one group was trained with TS
but never experienced LS
(novel-L group, 3 rats, 6 sessions) and the other group was trained with LS
but never experienced tone cues (novel-T group, 2 rats, 4 sessions). During the recording sessions, these rats were presented with all three cues of the Go/Nogo task, as well as the same behavioral contingency. Overall, rats performed well with learned (salient) cues but rarely responded to novel (non-salient) cues (10.0±2.9% Go response) (). In novel-L rats, 54/100 BF neurons showed bursting responses to learned cues – TS
– similar to those in . Yet, none of these 54 neurons showed any response to non-salient LS
(). In novel-T rats, 58/75 BF neurons showed bursting responses to LS
. Yet, only 3/58 neurons showed bursting responses to non-salient TS
(). BF bursting responses were also absent even when the analysis was restricted to the first 10 trials rats encountered a novel cue (Supplemental Figure S5
). To further demonstrate that BF bursting responses to cues was acquired through learning, one novel-L rat was subsequently trained with the LS
-sucrose association. In this rat, the proportion of BF neurons with bursting responses to both TS
that also responded to LS
increased from 0/37 before learning to 43/44 after learning LS
(). These results clearly show that BF bursting response to sensory cues was acquired through learning and paralleled the acquired motivational salience of the cues. The absence of responses to non-salient cues indicates that these BF neurons do not respond to sensory properties per se.
Absence of BF bursting responses to novel cues before learning cue-reinforcement associations
In contrast to acquiring motivational salience through learning, sensory cues can lose their motivational salience via extinction training. We therefore investigated whether BF bursting responses to sensory cues would diminish or even disappear after extinction. Extinction training was carried out in a subset of rats that had mastered all three cue-reinforcement associations of the Go/Nogo task (3 rats, 15 sessions). In extinction sessions, rats were presented with only sucrose-predicting cues (TS and LS), but sucrose delivery was withheld following either LS (extinct-L, 6 sessions) or TS (extinct-T, 9 session). Rats maintained a high level of Go response toward rewarded cues but quickly stopped responding to cues that no longer delivered reward (23.1±4.2% Go response) (). Despite the fact that many BF neurons continued to burst robustly toward rewarded cues (51/62 neurons in extinct-L rats, 54/67 neurons in extinct-T rats), many of these neurons did not show bursting responses toward extinguished cues (23/51 in extinct-L rats, 41/54 in extinct-T rats) (). As a result, the population bursting amplitude to extinguished cues decreased significantly. The bursting amplitude to extinguished cues further decreased when rats consolidated their Nogo behavior (paired t-test, p<0.001) (). Thus, even though BF bursting to extinguished cues may persist for some time after rats stopped responding to these cues, the bursting amplitude was significantly reduced. This suggests that the amplitude of BF bursting responses tracked the motivational salience of the cue.
Diminished BF bursting responses to cues after extinction
Because both primary reward and punishment were motivationally salient to rats, we next investigated whether BF neurons would show similar bursting responses toward sucrose and quinine. We focused this analysis on BF neurons that encoded motivational salience of cues in the Go/Nogo task (). Single neuron and population responses to sucrose and quinine are shown in , aligned to the first delivery of sucrose and quinine in each trial. Indeed, 55% (54/99) and 69% (68/99) of BF neurons that showed bursting responses toward motivationally salient cues in the Go/Nogo task also showed significant bursting responses following the first delivery of sucrose or quinine, respectively (). This BF bursting response, however, was not present (or much diminished) for the first two unreinforced licks or the 2nd-5th sucrose deliveries in each trial. The bursting amplitude of individual neurons toward sucrose and quinine were highly correlated with each other, and also were correlated with burst amplitude toward sensory cues (). These results indicate that BF neurons also encode the motivational salience of primary reward and punishment using similar bursting responses.
Bursting responses of BF neurons to sucrose and quinine
To further investigate whether BF bursting responses to sucrose and quinine were innate or learned, we analyzed how BF neurons in the novel-T group () responded to unexpected deliveries of sucrose and quinine, on those rare occasions that rats licked within 5-sec following onsets of novel cues, TS and TQ. Despite low trial numbers, BF bursting responses to unexpected reinforcement were quite robust and their bursting amplitudes were similar (, paired t-test, p=0.41). This indicates that BF bursting responses to primary reward and punishment were not the result of learning but reflected the innate response property of these BF neurons. The similar bursting amplitudes toward sucrose and quinine further support the conclusion that BF encoding of motivational salience occurs independent of hedonic valence.
It is important to note that, despite the physiological heterogeneity of BF neuronal populations in vivo (Lee et al., 2004
), salience-encoding BF neurons represented a homogeneous subset of cells that shared similar baseline firing rates (2-8 Hz) and peak bursting rates (30-80 Hz) (Supplemental Figure S2
). These firing properties are consistent with in vitro characterizations of non-cholinergic BF neurons (Alonso et al., 1996
). Moreover, unlike ACh BF neurons, which show characteristic higher firing rates during waking (WK) and rapid-eye-movement sleep (REM) than during slow-wave sleep (SWS) (Lee et al., 2005
), salience-encoding BF neurons maintained similar average firing rates (2-8 Hz) between WK and SWS states (paired t-test, p>0.05) (). On the other hand, none of the putative ACh neurons (8/143) overlapped with salience-encoding BF neurons (Supplemental Figure S6
). These observations suggest that salience-encoding BF neurons are non-ACh neurons with similar firing properties to those described in our previous report (Lin et al., 2006
Salience-encoding BF neurons do not change average firing rates between waking and slow-wave sleep
To determine whether BF encoding of motivational salience has any impact on behavioral performance, we trained a separate group of three rats in a near-threshold tone-detection task. During training, rats were rewarded for making a Go response following an 80 dB tone. Importantly, during the recording phase (11 sessions), we manipulated the difficulty of tone detection by randomizing the target tone level between 56-80 dB against an additional 61 dB constant background noise. While it is possible that the quality of sound presented at different levels might differ, this manipulation created conditions in which successful tone detection could only occur on a fraction of trials when the tone sound level was near the detection threshold (). Thus, we were able to investigate whether successful tone detection was correlated with the presence of BF bursting response. Indeed, BF neurons displayed a clear all-or-none response pattern corresponding to successful detection — showing a clear bursting response when rats successfully detected the tone, and no response when rats failed to detect the tone (). Even when tones were presented near detection threshold (≤ 65 dB), the bursting response of single BF neurons was able to predict trials in which successful tone detection was achieved (). In other words, given the same near-threshold stimuli, successful tone detection strongly correlated with the presence of BF burst firing. Such a strong correlation suggests that the encoding of motivational salience by ensemble bursting of BF neurons may enhance behavioral performance toward attended stimuli.
BF bursting responses predict successful detection of near-threshold tones
Within successful detection trials, we also found that the target sound level modulated the latency and amplitude of BF bursting responses (repeated measure ANOVA, p<0.001) (). This graded bursting response likely reflected the varying levels of motivational salience for different tone levels, with louder tones associated with higher motivational salience. Consistent with this interpretation, we found that in trials with stronger BF bursting response to the target, and hence higher motivational salience, the behavioral response latency was shorter (). Intriguingly, BF response amplitudes to the target tone and the water reward were inversely correlated: stronger BF bursting responses to the target lead to weaker BF responses to the reward, and vise versa (). This pattern was also evident at the single neuron level (). Overall, this anti-correlated response pattern supports the notion that expected rewards are less salient, while surprising ones are more salient.
Graded BF bursting responses in the detection task