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Instrumental task condition. In the instrumental task condition, the monkey obtained reward immediately after contact with the target (Fig. 1). This condition will be referred to as the "immediate condition." A variant of the task, called the "fixed delay condition," was designed to investigate the effect of delaying the delivery of reward after target contact by a fixed interval of 1 sec. In another version of the task, called the "variable delay condition," the time of reward was randomly varied relative to target contact (0.5, 1, and 1.5 sec). Both monkeys had achieved a consistent correct performance rate of >95% in the immediate condition before the neuronal recording started, and we used the delayed reinforcement procedures only during neuronal data collection. All three task conditions were run in separate blocks of 30-40 trials throughout the course of recording sessions, the order of the conditions being counterbalanced.
The Journal of Neuroscience, August 1, 2001, 21(15):5730-5739
Reward Unpredictability inside and outside of a Task Context as a Determinant of the Responses of Tonically Active Neurons in the Monkey Striatum
Sabrina Ravel1, Pierangelo Sardo2, Eric Legallet1, and Paul Apicella1 1 Laboratoire de Neurobiologie Cellulaire et Fonctionnelle, Centre National de la Recherche Scientifique, 13402 Marseille Cedex 20, France, and 2 Istituto di Fisiologia Umana, Università di Palermo, 90134 Palermo, Italy
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
Tonically active neurons (TANs) in the monkey striatum are involved in detecting motivationally relevant stimuli. We recently provided evidence that the timing of conditioned stimuli strongly influences the responsiveness of TANs, the source of which is likely to be the monkey's previous experience with particular temporal regularities in sequential task events. To extend these findings, we investigated the relationship of TAN responses to a primary liquid reward, the timing of which is more or less predictable to the monkey either outside of a task or during instrumental task performance. Reward predictability was indexed by the timing characteristics of the mouth movements. The responsiveness of TANs to reward increased with the range and variability of time periods before reward, notably when the liquid was delivered outside of a task. A change in the temporal order of events in a task context produced an increase of response to reward, suggesting an influence of the predicted nature of the event in addition to its time of occurrence. By contrast, we observed no substantial changes in neuronal activity at the expected time of reward when this event failed to occur, suggesting that these neurons do not appear to carry information about an error in reward prediction. These results demonstrate that TANs constitute a neuronal system that is involved in detecting unpredicted reward events, irrespective of the specific behavioral situation in which such events occur. The responses influenced by stimulus prediction may constitute a neuronal basis for the notion that striatal processing is crucial for habit learning.
Key words: striatum; basal ganglia; TANs; prediction; reward; learning
INTRODUCTION
Several lines of evidence suggest that the striatum, the main input structure of the basal ganglia, and the ascending dopamine (DA) system from the midbrain are involved in the acquisition and maintenance of reward-mediated behaviors (Graybiel, 1995
; Houk et al., 1995
In recent work, we found that the majority of TANs respond to the unsignaled delivery of a liquid reward outside the context of a behavioral task, whereas these responses are reduced considerably when the same reward is delivered on correct instrumental responding (Apicella et al., 1997
MATERIALS AND METHODS
Data were collected from two male macaque monkeys (Macaca fascicularis, monkeys A and B, 5-6 kg) that were trained in an instrumental task in which they were required to perform visually triggered arm-reaching movements to obtain a liquid reward. We also recorded from the striatum of a third monkey performing another version of the instrumental task that we used with monkeys A and B. The experimental protocol was performed according to the National Institutes of Health Guide for the Care and Use of Laboratory Animals and the French laws on animal experimentation.
The monkeys were seated in a restraining box that was described in an earlier publication (Apicella et al., 1997
Behavioral procedures
In the present experiments, the monkey's ability to predict the time of reward delivery was manipulated under two behavioral situations: (1) an instrumental task condition in which the monkey performed an arm-reaching movement leading to delivery of reward, and (2) a free reward condition in which the same liquid was delivered outside of a task and in the absence of any external reward-predicting signal. Animals were informed of the change of situation by the experimenter entering the recording room to open (instrumental task condition) or close (free reward condition) the sliding door of the restraining box.
After recording from monkeys A and B, one additional monkey was trained and then tested in another version of the reaching task that we called the "surprising reward condition." For this condition, exactly the same procedure was used as that used for the immediate condition, except that both the specific times at which stimuli occurred and the nature of stimuli were variable from trial to trial. This condition was designed to compare neuronal activity for trials with reward normally occurring on correct target reaching, as compared with trials on which an unexpected premature reward was automatically delivered at the start of a trial. Each trial began with the monkey maintaining its hand on the bar for a random period of 0.5-2 sec, after which the trigger stimulus was presented 70% of the time (usual trials) and reward occurred 30% of the time (surprising trials) during the same session. Thus, even if one event was more frequent than the other, the monkey could not be sure which event would occur first on any given trial. When the monkey received the surprising reward, it had to wait for a variable time period while resting its hand on the bar until the total duration of the trial had elapsed (7 sec). The surprising reward condition comprised 80 trials. In a variant of this condition, the monkey did not know exactly what would be the first event of each trial, but it could know when this event would occur because an external cue signaled its onset time at the end of a constant interval. A first visual stimulus (green light, 500 msec duration), serving as a temporal cue, appeared in the center of the screen at the location at which the trigger stimulus (two-colored LED) was presented . This cue began a waiting interval of 1.5 sec that lasted when the next event occurred. At the end of this interval, the trigger stimulus was presented 70% of the time, and reward occurred 30% of the time during the same session. This design combined cueing of the moment of the upcoming event and uncertainty about the nature of the event. This condition comprised 80 trials. Free reward condition. Monkeys A and B were also subjected to a testing procedure in which the fruit juice was delivered without engaging the animal in any specific task. Delivery of reward was tested by administering the liquid in the absence of any external stimulus predicting its time of delivery (Fig. 1). The time interval between successive liquid deliveries was randomly varied from trial to trial and ranged from 5.5 to 8.5 sec to make it impossible for animals to have reliable temporal information during a run of trials. We refer to this situation as the "irregular condition." To investigate the influence of the rate of reward delivery, we introduced regularities in inter-reward intervals so that each reward might serve the animal as a time marker from which it could predict the timing of the next reward. In the first condition, the liquid was delivered once every 4 sec. In the second condition, the constant interval was shortened to 2 sec. We refer to these as the "regular 4 sec condition" and the "regular 2 sec condition." In each condition, the trials were run in separate blocks of 30-40 trials, and the order of the conditions was counterbalanced across recording sessions to ensure that the changes in neuronal responsiveness in these conditions were not caused by order effects. Classically conditioned task. Monkeys A and B were also trained on a typical Pavlovian conditioning procedure in which a visual stimulus is repeatedly paired with a primary reward. In this condition, the sliding door of the restraining box was closed, and a red light was illuminated at unpredicted times as an external signal preceding the delivery of reward by a fixed interval of 1 sec. The visual stimulus duration was 0.3 sec, and its onset was initiated by the experimenter. Both monkeys received numerous training sessions with this same time interval, and on some occasions, the duration was prolonged to 2 sec to assess the effects of delivering reward beyond the time of its usual occurrence. The monkeys practiced the condition using the long signal-reward interval very infrequently.
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Figure 1. Description of the sequence of events in the testing conditions. Instrumental Task Condition, This is a task state in which the monkey awaited the visual stimulus with its hand on the bar. Illumination of the light signaled the monkey to release the bar and reach for the target, which it had to touch to receive a liquid reward. Free Reward Condition, This is a no-task state in which the monkey remained motionless and received the same liquid reward in the absence of any external predictive signal. The present study used three versions of each behavioral situation. In the task state, the delivery of reward occurred immediately after contact with the target (Immediate) or was delayed after target contact by a constant 1 sec interval (Fixed Delay) or a randomly varying interval of 0.5-1.5 sec (Variable Delay). In the no-task state, liquid was delivered at irregular time intervals of 5.5-8.5 sec (Irregular) or at a constant interval of 4 sec (Regular 4-s) or 2 sec (Regular 2-s). In each of these two behavioral states, the various conditions were tested in separate blocks of trials.
Surgical procedures
After completing training on the immediate condition of the instrumental task, each monkey was surgically prepared for chronic single-neuron recording experiments. The surgery was performed in aseptic conditions and under pentobarbital sodium anesthesia (35 mg/kg, i.v.; Sanofi, Libourne, France). A stainless steel chamber (outer diameter, 25 mm) and a head-restraining device were implanted on the skull at the following coordinates according to the Szabo and Cowan (1984)Neuronal recordings
The experimental methods that were used to record single-neuron activity have been described elsewhere (Apicella et al., 1997Data analysis
Performance in the instrumental task condition was measured by using the reaction time, which was the time between the onset of the trigger stimulus and release of the bar, and the movement time, which was the time taken to move from the bar to the target. Upper limits of both behavioral parameters were specified (1 sec for each). Behavioral performance was assessed by calculating the median (50th percentile) of reaction and movement times of correct responses for each session. The data for each condition were taken from 30 and 20 sessions in monkeys A and B, respectively, with the exception of the variable condition in monkey A, which comprised 14 sessions. The Mann-Whitney U test was used for comparison between task conditions. Signals from the strain gauge circuit (mouth movements) were digitized at 100 Hz and stored into an analog file continuously during each block of trials. The timing characteristics of the mouth movements that animals performed in the different conditions were assessed off-line by single-trial analysis. Significant changes in neuronal activity were detected on the basis of a Wilcoxon signed rank test (Apicella et al., 1997Histology
Toward the end of the experiment in monkeys A and B, neuronal recording sites were marked with small electrolytic lesions by passing negative currents through the microelectrode (20 µA for 15-20 sec). After the completion of all testing, the monkeys were killed with an overdose of sodium pentobarbital and perfused transcardially with 0.9% saline followed by a fixative (4% paraformaldehyde, pH 7.4 phosphate buffer). Frozen coronal sections (50 µm thick) were cut through the region of the recordings and stained with cresyl violet. The monkey striatum was divided into two territories based on topographic projections from different cortical regions: the associative striatum, which includes the caudate nucleus and anterior putamen, and the sensorimotor striatum, which includes more posterior regions of the putamen. These two territories are innervated, respectively, by associative cortical areas and by the primary motor and sensory cortical areas (Künzle, 19752 test. The recording sites were not localized in the third monkey, which is still used in neurophysiological experiments.
RESULTS
Behavior
Monkeys A and B consistently showed >95% correct task performance in every condition. Reaction and movement time analyses failed to reveal significant differences between conditions in the two monkeys (p > 0.01, Mann-Whitney U test), indicating that arm movements were performed with equal speed, regardless of the moment of reward delivery after target contact. As illustrated in Figure 2, the timing characteristics of the mouth movements showed clear differences between conditions. In the instrumental task condition (Fig. 2, left), both monkeys systematically licked the spout immediately before the receipt of reward when a fixed or variable delay period was introduced between the target contact and reward, whereas the licking activity started on correct reaching when the reward event coincided with target contact. In the free reward condition (Fig. 2, right), both monkeys made licking movements continuously before the delivery of reward in the irregular and the regular 4 sec conditions. This differed from the regular 2 sec condition in which licking movements occurred over a short period centered around the time at which the reward was delivered. In this latter condition, visual inspection of the monkey's face by video monitoring showed rhythmic tongue protrusions that were well synchronized with the rate of reward delivery. The restriction of mouth movements to a narrow time range in the immediate condition and the regular 2 sec condition suggests that the temporal structure of these testing conditions provided more exact information as to the time at which the liquid would be delivered, thus making it more predictable.
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Figure 2. Oral behavior in the testing conditions. Mouth contacts with the spout were detected by means of force transducers (strain gauges). Data from the different variants of each testing condition are ordered from top to bottom. For each condition are shown consecutive and superimposed traces of mouth movement records aligned on the delivery of reward. Data in each condition are shown from 15-20 consecutive trials, the chronological order of trials being preserved from top to bottom. The monkey started to lick the spout on the delivery of reward in the immediate condition and maintained oral activity thereafter, whereas licking movements began before the reception of liquid in the two delayed reinforcement procedures. Mouth movements appeared similar in the irregular and regular 4 sec conditions and consisted of frequent, randomly timed licking movements before the delivery of reward. By contrast, in the regular 2 sec condition, licking movements were restricted to a short time period centered on the receipt of reward so that the movements were performed in synchrony with reward delivery. These mouth movements were recorded simultaneously with neuronal activity.
Neuronal data
In monkeys A and B, we recorded the activity of 179 striatal neurons that were categorized as TANs on the basis of their relatively continuous spontaneous activity as well as their typical extracellularly recorded action potential waveform (see Materials and Methods). Of the 179 neurons studied, 152 responded to reward in at least one of our testing conditions. The responses of TANs consisted of a phasic depression of the tonic firing, followed in some instances by a transient increase in discharge rate. These response patterns agree with those previously reported and distinguish TANs from other neurons in the striatum (Kimura et al., 1984
Influence of the timing of reward inside and outside of a task context
As it has been previously reported (Apicella et al., 1997
A high percentage of TANs responded to the delivery of liquid at irregular time intervals outside of a task, consistent with previous findings (Apicella et al., 1997
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Figure 3. Influence of delaying the delivery of reward in the instrumental task condition in one tonically active neuron. Each dot in the raster displays represents one neuronal impulse, and each line of dots represents the neuronal activity occurring during a single trial. Dot displays and perievent time histograms are aligned on the onset of the trigger stimulus or the delivery of reward, which are marked by vertical lines. The sequence of trials is shown chronologically from top to bottom in each raster display. The change of condition occurred over three successive blocks of trials. This neuron showed a weak response to reward in the immediate condition but had a clear-cut response to the same event in both the fixed delay and variable delay conditions. This neuron also showed a response to the trigger stimulus that persisted in all instances. Vertical calibration on histograms is in impulses per bin. Bin width for histograms is 10 msec.
Table 1 shows the response parameters that were recorded in the various testing conditions. No significant differences were observed in the latency, duration, and magnitude of reward responses among instrumental task conditions (one-way ANOVA followed by Fisher's test, p > 0.05). Responses to reward varied insignificantly (p > 0.05) in all three free reward conditions in terms of latency and duration, whereas the magnitude of change was significantly greater for both the irregular (F(1,157) = 6.79; p < 0.01) and regular 4 sec (F(1,155) = 6.84; p < 0.01) conditions than for the regular 2 sec condition. Thus, both the incidence of responsive neurons and the magnitude of the response decreased when repetitive deliveries of liquid occurred at the same 2 sec intervals.
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Figure 4. Influence of shortening the interval between consecutive deliveries of liquid in the free reward condition in one tonically active neuron. Conventions are similar to those in Figure 3. The change of condition occurred over three successive blocks of trials. Response to reward occurred in both the irregular and regular 4 sec conditions and disappeared almost immediately when the monkey was switched from the regular 4 sec condition to the regular 2 sec condition.
To determine whether the responsiveness of TANs to reward was influenced by the particular behavioral state in which the liquid was delivered, we compared responses under the variable delay condition to responses under the irregular and regular 4 sec conditions, namely conditions that were supposed to minimize the temporal predictability of reward. No significant differences were observed in the fraction of neurons responding to reward between the variable delay condition and the irregular and regular 4 sec conditions (p > 0.05;
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Table 1. Latencies, durations, and magnitudes of reward responses in the two behavioral situations
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Figure 5. Population responses of tonically active neurons to events occurring in the two behavioral situations. Histograms of the activity of TANs for a sample included responsive and nonresponsive neurons recorded. Because of randomly varying intervals between target contact and reward (1 ± 0.5 sec), the time base in the variable delay condition was split, and histograms were separately referenced to target contact and reward. N, Number of neurons included for each population histogram.
Influence of the temporal order of task events
In the experiments reported above, we have tried to impair the monkey's ability to predict the timing of reward during task performance by delaying reward after target contact. However, the monkey could predict that an upcoming reward would follow correct instrumental responses in any case at some point of time because the temporal order of task events remained the same on every trial and every condition. It is possible that the test of the influence of temporal predictability of reward would be made more stringent by changing the event sequence itself. To this aim, we used, for a third monkey, the surprising reward condition in which reward was delivered unexpectedly soon at the beginning of some trials. This situation was quite different from the immediate condition in that the monkey was not able to predict precisely which event would be presented first on any given trial. Consistent with the notion of a reduced stimulus prediction, behavioral data showed that reaction times in the usual trials of the surprising reward condition were significantly longer (293 ± 41 msec, mean ± SD) than those in the immediate condition (268 ± 52 msec) (p < 0.01, Mann-Whitney U test ). Of the 52 neurons tested in this condition, 20 (38%) responded to reward that normally occurred on target contact, whereas 39 (75%) responded to the surprising reward. An example of a neuron showing a strong response to surprising rewards with no response to usual rewards is shown in Figure 6. As can be seen, this same neuron was also tested in the free reward condition with irregular inter-reward intervals, and the response was strikingly similar to the response elicited by the surprising reward. A sample of 12 neurons was tested in the surprising reward condition and the free reward condition, all of them being responsive to reward in both conditions. Magnitudes of reward responses were79 ± 24% and
Because the surprising reward condition combined the effect of time uncertainty with the effect of event uncertainty, we added another condition in which a temporal cue pointed to either the trigger stimulus or a reward occurring 1.5 sec later. Among the 52 neurons tested in the surprising reward condition, 12 were tested in the cued one. All of these neurons responded to the surprising reward, regardless of the condition. Quantitative analysis in these neurons revealed that magnitudes of reward responses were significantly higher (p < 0.01) in the uncued surprising reward condition (
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Figure 6. Influence of delivering reward unexpectedly early in the instrumental task condition in one tonically active neuron. Same conventions as in Figure 3. Data were collected during the same test session in which reward was delivered normally after target reaching (usual trials) or unexpectedly soon at the beginning of a trial (surprising trials). The two types of trials occurred pseudorandomly within the same block of trials and were separated off-line for analysis. The raster and histogram display in the lower right panel is for a separate block of trials in which the liquid was given outside of the task at unpredictable intervals (free reward condition). This neuron responded in a similar way to reward, both in the surprising and free reward trials.
Influence of the expected time of reward delivery
Because the responses of TANs appeared to depend critically on the temporal predictability of reward, we were interested in the possibility that these neurons could carry information about the time of the usual occurrence of reward. In such a case, the response properties of TANs would be similar to those reported for the midbrain DA neurons, which are thought to provide a signal reflecting the omission of an expected reward at a particular point in time (Hollerman and Schultz, 1998
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Figure 7. Influence of changing the time of reward delivery in the Pavlovian conditioning procedure in one tonically active neuron. Same conventions as in Figure 3. The neuron was first tested with the usual interval of 1 sec between the visual signal and reward. Then, the neuron was tested with the 2 sec delay. Delivering the liquid at the end of the well practiced 1 sec interval after the visual signal did not elicit a reward response, although such a response appeared when liquid delivery was delayed. Note that no change in firing rate was detectable at the usual time of reward delivery, i.e., 1 sec after the visual signal, when the event occurred later than expected.
Locations of neurons with reward responses
Histological reconstructions of recording sites of all neurons tested in monkeys A and B are shown in Figure 8. Most of the neurons sampled were located in the dorsomedial regions of the striatum and distributed throughout the mediolateral extent of both the caudate nucleus and putamen. According to anatomical criteria for defining functionally distinct territories (see Materials and Methods), 99 neurons were located in the associative striatum and 73 in the sensorimotor striatum. Only a small number of neurons (n = 7) were recorded in the ventral striatum, which is composed of the nucleus accumbens and adjacent ventromedial caudate and ventral putamen. Reward responses were found in 86 neurons of the associative striatum (87%) and 59 neurons of the sensorimotor striatum (81%). Incidences of reward responses did not vary significantly between these two striatal territories (p > 0.05;
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Figure 8. Locations of tonically active neurons with reward responses. Data from monkeys A and B were drawn at corresponding positions on coronal sections of the striatum. Anteroposterior levels are shown according to the atlas of Cowan and Szabo (1984)
DISCUSSION
Our initial study had shown that the responses of TANs to reward were reduced or absent in an instrumental task condition in which reinforcement followed a movement, although stronger responding was observed in relation to the same reward given without external cues (Apicella et al., 1997
Reward unpredictability in the context of an instrumental task
Although the proportions of neurons responding to reward tended to increase when monkeys were exposed to delayed reinforcement, as compared with the condition in which reward delivery immediately followed behavioral responses, none of these increases reached statistical significance. However, it appears that the average response of all neurons that were recorded in the delayed reinforcement procedures was sufficiently strong to result in a net population response to reward, notably when the timing of reward was randomly varied relative to target reaching. The finding that TANs are relatively poorly related to delayed reinforcement, in terms of fraction of neurons responding, is probably attributable to the fact that performance levels and circumstances remain well organized and may allow for a more general level of reward predictability, at least with the range of target-reward intervals used in this report. This interpretation is strengthened by the finding that TANs were particularly responsive to reward in the framework of the instrumental task for trials in which reward was delivered sooner than expected. In the surprising reward condition, we attempted to specify whether the increase in responsiveness reflects a lack of expectation for a probable event or a probable time. We found that the surprising presence of rewards produced an increase of the responses to reward, even when the monkey was cued to attend to the upcoming event at a particular time. This finding expands on our earlier findings by demonstrating that the timing of reward alone may not be the sole basis for modulation of TAN responses. It remains an open question as to how the effects of time uncertainty and event uncertainty might act together.
Another explanation for the effects of the timing of reward is that a difference in the monkey's level of attention to the relation between successive events may bring about the difference in the neuronal responsiveness. Because attention and prediction are difficult to untangle in the present experiments, further experimental work is required to examine the influence of each of the single factors separately.
Influence of the range of time intervals between reward events
Considerations of temporal range may provide an important clue for explaining the changes in the responsiveness of TANs to reward; the closer the stimuli are in time during testing, the weaker the responses to these stimuli are presumed to be. We found that the responses to reward delivered at a constant rate outside of a task were much more prevalent and of greater magnitude when the liquid was delivered at an interval of 4 sec than when it was given at a 2 sec interval on successive trials. As evidenced by the monkey's overt oral behavior, animals could only synchronize mouth movements with the rate of reward delivery in the regular 2 sec condition, suggesting that they appeared to take advantage of this short interval to predict the moment of the expected reception of reward. In this condition, the monkey used the delivery of reward as a temporal cue indicating that another reward would be delivered at a particular time point, whereas the animal was not able to retain this information to influence the timing of mouth movements when the inter-reward interval was longer than 2 sec. Analysis of the effect of the duration of constant intervals between a predictive cue and a movement-triggering stimulus on reaction time performance and the responses of TANs to this stimulus in our previous experiments (Sardo et al., 2000
It must be emphasized that the responsiveness of TANs to primary liquid rewards has varied between different reports, according to the nature of the experimental designs used and to the monkey's experience with temporal regularities in sequential events. At first glance, our results seem to be contradictory to the results of Kimura and coworkers, claiming that TANs do not respond to a liquid given outside of a task (Kimura et al., 1990
Influence of the time of expected reward
Another new finding reported here is that TANs showed little or no differences in activity at the moment when the reward was expected, suggesting that they do not emit a signal that reports errors in prediction of reward. Although there is evidence of a role of DA inputs in the expression of the TAN responses to reward-related stimuli (Aosaki et al., 1994a
To summarize, we have shown that the responsiveness of TANs recorded in wide areas of the primate striatum is dependent on the temporal structure of a series of events, including primary rewards. Moreover, TANs do not appear to encode an error in reward prediction, suggesting that their response properties are not similar to those described on DA neurons. There has been considerable interest in possible roles for TANs in the acquisition and maintenance of reward-mediated behaviors (Graybiel, 1995
FOOTNOTES Received March 19, 2001; revised April 25, 2001; accepted May 10, 2001.
This research was supported by Centre National de la Recherche Scientifique, the European Human Capital and Mobility Program (Grant CHRX-CT94-0463), and the Biomed II Program of the European Commission (Grant BMH4-CT95-0608). We thank R. Massarino for expert mechanical work and C. Wirig for designing the electronic device.
Correspondence should be addressed to Paul Apicella, Laboratoire de Neurobiologie Cellulaire et Fonctionnelle, Centre National de la Recherche Scientifique, 31 chemin Joseph Aiguier, 13402 Marseille Cedex 20, France. E-mail: apicella{at}lncf.cnrs-mrs.fr.
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