Neural Correlates of Hierarchical Reinforcement Learning
Neural Correlates of Hierarchical Reinforcement Learning
Abstract and Keywords
This chapter discusses the relevance of reinforcement learning (RL) to its hierarchical structure. It first reviews the fundamentals of RL, with a focus on temporal-difference learning in actor-critic models. Next, it discusses the scaling problem and the computational issues that stimulated the development of hierarchical reinforcement learning (HRL). The potential neuroscientific correlates of HRL are also described. The chapter also presents the results of some initial empirical tests and ends with directions for further research.
Over the past two decades, ideas from computational reinforcement learning (RL) have had an important and growing effect on neuroscience and psychology. The impact of RL was initially felt in research on classical and instrumental conditioning.13,104,112 Soon thereafter, its reach extended to research on midbrain dopaminergic function, where the temporal-difference learning paradigm provided a framework for interpreting temporal profiles of dopaminergic activity.10,51,67,93 Subsequently, actor-critic architectures for RL have inspired new interpretations of functional divisions of labor within the basal ganglia and cerebral cortex52 (see also chapters 17 and 18, this volume), and RL-based accounts have been advanced to address issues as diverse as motor control,66 working memory,74 performance monitoring,48 and the distinction between habitual and goal-directed behavior.31
Despite this widespread absorption of ideas from RL into neurobiology and cognitive science, important questions remain concerning the scope of its relevance. In particular, RL-inspired research has generally focused on highly simplified decision-making situations involving choice among a small set of elementary actions (e.g., left vs. right saccades), or on Pavlovian settings involving no action selection at all. It thus remains uncertain whether RL principles can help us understand learning and action selection in more complex behavioral settings, akin to those arising in everyday life.30
In the present chapter, we consider the potential relevance of RL to one part icular aspect of complex behavior, namely, its hierarchical structure. Since the inception of cognitive psychology, it has been noted that naturalistic behavior displays a stratified or layered organization.58,64,89 As stated by Fuster,38 “Successive units with limited short-term goals make larger and longer units with longer-term objectives. … Thus we have a pyramidal hierarchy of structural units of increasing duration and complexity serving a corresponding hierarchy of purposes” (p. 159). A concern with hierarchical action structure has continued to inform behavioral research to the present day,19,25,91,117 and has figured importantly in neuroscientific research bearing on the prefrontal cortex, where evidence has arisen (p.286) that representations of successive levels of task structure may map topographically onto the cortical surface.7,20,27,38,57,114
Can hierarchical behavior be understood in terms provided by RL, or does it involve fundamentally different computational principles? One lead in pursuing this question can be gleaned from recent RL research. As it turns out, a great deal of recent work in computational RL has focused precisely on the question of how RL methods might be elaborated to accommodate hierarchical behavioral structure. This has given rise to a general framework referred to, aptly enough, as hierarchical reinforcement learning (HRL).11,34,106 In considering whether RL might be relevant to hierarchical behavior in animals and humans, a natural approach is to evaluate whether the brain might implement anything like the mechanisms stipulated in computational HRL. In recent work, we have undertaken this project.21,26,82 Our objective in the present chapter is to review the results obtained so far, and to offer an interim evaluation of the HRL hypothesis.
Before getting to neuroscientific data, a number of preliminaries are in order. We begin by reviewing the basics of RL, and in particular temporal-difference learning within the actor-critic architecture. Next, we discuss the computational issues that stimulated the development of HRL, and introduce the fundamental elements of HRL itself. With this foundation in place, we consider potential neuroscientific correlates of HRL, describe results of some initial empirical tests, and finally chart out some directions for further research.
Fundamentals of RL: Temporal Difference Learning in Actor-Critic Models
RL problems comprise four elements: a set of world states; a set of actions available to the agent in each state; a transition function, which specifies the probability of transitioning from one state to another when performing each action; and a reward function, which indicates the amount of reward (or cost) associated with each such transition. Given these elements, the objective for learning is to discover a policy, that is, a mapping from states to actions, that maximizes cumulative discounted long-term reward.
There are a variety of specific algorithmic approaches to solving RL problems.16,105 We focus here on the approach that has arguably had the most direct influence on neuroscientific translations of RL, referred to as the actor-critic paradigm. 10,52 In actor-critic implementations of RL,14,51,52,103 the learning agent is divided into two parts, an actor and a critic (see figure 16.1a). The actor selects actions according to a modifiable policy, π(s), which is based on a set of weighted associations from states to actions, often called action strengths. The critic maintains a value function, V(s), associating each state with an estimate of the cumulative, long-term reward that can be expected subsequent to visiting that state. Importantly, both the action strengths (p.287)
The prediction error is used to update both the value attached to the previous state and the strength of the action that was selected in that state. A positive prediction error leads to an increase in the value of the previous state and the propensity to perform the chosen action at that state. A negative error leads to a reduction in these values. After the appropriate adjustments, the agent selects a new action, a new state is reached, a new prediction error is computed, and so forth. As the agent explores its environment and this procedure is repeated, the critic’s value function becomes progressively more accurate, and the actor’s action strengths change so as to yield progressive improvements in behavior, in terms of the amount of reward obtained.
The actor-critic architecture, and the temporal-difference learning procedure it implements, have provided a very useful framework for decoding the neural substrates of learning and decision making. Although accounts relating the actor-critic architecture to neural structures do vary,52 one influential approach has been to identify the actor with the dorsolateral striatum (DLS), and the critic with the ventral striatum (VS) and the mesolimbic dopaminergic system32,71 (figure 16.1c). Dopamine (DA), in particular, has been associated with the function of conveying reward prediction errors to both actor and critic.10,67,93 This set of correspondences provides an important backdrop for our later discussion of HRL and its neural correlates.
The Scaling Problem in RL
Even as excitement initially grew concerning potential applications of RL within neuroscience, concerns were already arising in computer science over the limitations of RL. In particular, it became clear very early on in the history of RL research that RL algorithms face a scaling problem: They do not cope well with tasks involving a large space of environmental states or possible actions. It is this scaling problem (p.289) that immediately stimulated the development of HRL, and it is therefore worth characterizing the problem before turning to the details of HRL itself.
A key source of the scaling problem is the fact that an RL agent can learn to behave adaptively only by exploring its environment, trying out different courses of action in different situations or states of the environment, and sampling their consequences. As a result of this requirement, the time needed to arrive at a stable behavioral policy increases with both the number of different states in the environment and the number of available actions. In most contexts, the relationship between training time and the number of environmental states or actions is a positively accelerating function. Thus, as problem size increases, standard RL eventually becomes infeasible.
Several computational maneuvers have been proposed to address the scaling problem. For example, one important approach is to simplify the state space by treating subsets of environmental states as behaviorally equivalent, a measure referred to as state abstraction.60 Another approach aims at optimizing the search for an optimal behavioral policy by balancing judiciously between exploration and exploitation of established knowledge.55
HRL methods arose as another way of addressing the scaling problem in RL. The key to HRL is the use of temporal abstraction.11,34,77,106 Here, the basic RL framework is expanded to include “temporally abstract” actions, representations that group together a set of interrelated actions (for example, grasping a spoon, using it to scoop up some sugar, moving the spoon into position over a cup, and depositing the sugar), casting them as a single higher-level action or skill (“add sugar”). These new representations are described as temporal abstractions because they abstract over temporally extended, and potentially variable, sequences of lower-level steps. A number of other terms have been used as well, including “skills,” “operators,” “macro-operators,” and “macro-actions.” In what follows, we often refer to temporally abstract actions as options.106
In most versions of RL that use temporal abstraction, it is assumed that options can be assembled into higher-level skills in a hierarchical arrangement. Thus, for example, an option for adding sugar might form part of other options for making coffee and tea. It is the importance of such hierarchical structures in work using temporal abstraction that gave rise to the moniker HRL.
Adding temporal abstraction to RL can ease the scaling problem in two ways. The first way is through its impact on the exploration process. In order to see how this works, it is useful to picture the agent as searching a tree structure (figure 16.2a). At the apex is a node representing the state occupied by the agent at the outset of exploration. Branching out from this node are links representing primitive actions, each leading to a node representing the state (and, possibly, reward) consequent on that action. Further action links project from each of these nodes, leading to their (p.290)
Temporally abstract actions can alleviate this problem by introducing structure into the exploration process. Specifically, the policies associated with temporally abstract actions can guide exploration down specific partial paths through the search tree, potentially allowing earlier discovery of high-value traversals. The principle is illustrated in figure 16.2. Discovering the pathway illustrated in figure 16.2a,using only primitive, one-step actions, would require a specific sequence of seven independent choices. This changes if the agent has acquired—say, through prior experience with related problems—two options corresponding to the differently colored subsequences in figure 16.2b. Equipped with these, the agent would only need to make two independent decisions to discover the overall trajectory, namely, select the two options. Here, options reduce the effective size of the search space, making it easier for the agent to discover an optimal trajectory.
The second, and closely related, way in which temporally abstract actions can ease the scaling problem is by allowing the agent to learn more efficiently from its experiences. Without temporal abstraction, learning to follow the trajectory illustrated in figure 16.2a would involve adjusting parameters at seven separate decision points. With predefined options (figure 16.2c), policy learning is required at only two decision points, the points at which the two options are to be selected. Thus, temporally abstract actions allow the agent not only to explore more efficiently, but also to make better use of its experiences.
Hierarchical Reinforcement Learning
In order to frame specific hypotheses considering neural correlates of HRL, it is necessary to get into the specifics of how HRL works. In this section we introduce the essentials of HRL, focusing on the options framework106 as adapted to the actor-critic framework.21 We focus on aspects of HRL that we believe are potentially most relevant to neuroscience (see ref. 11 for a detailed and comparative discussion of HRL algorithms).
The options framework supplements the set of single-step, primitive actions with a set of temporally abstract actions or options. An option is, in a sense, a “mini-policy.” It is defined by an initiation set, indicating the states in which the option can be selected; a termination function, which specifies a set of states that will trigger (p.292) termination of the option; and an option-specific policy, mapping from states to actions (which now include other options).
Like primitive actions, options are associated with strengths, and on any time step the actor may select either a primitive action or an option. Once an option is selected, actions are selected based on that option’s policy until the option terminates. At that point, a prediction error for the option is computed (figure 16.2d). This error is defined as the difference between the value of the state where the option terminated and the value of the state where the option was initiated, plus whatever rewards were accrued during execution of the option. A positive prediction error indicates that things went better than expected since leaving the initiation state, and a negative prediction error means that things went worse. As in the case of primitive actions, the prediction error is used to update the value associated with the initiation state, as well as the action strength associating the option with that state.
Implementing this new functionality requires several extensions to the actor-critic architecture, as illustrated in figure 16.1b. First, the actor must maintain a representation of which option is currently in control of behavior (o) or, in case of options calling other options, of the entire set of active options and their calling relations. Second, because the agent’s policy now varies depending on which option is in control, the actor must maintain a separate set of action strengths for each option, πo(s), together with option-dependent reward functions, Ro(s), and value functions, Vo(s). Important changes are also required in the critic. Because prediction errors are computed when options terminate, the critic must receive input from the actor, telling it when such terminations occur (the arrow from o to δ). Finally, to be able to compute the prediction error at these points, the critic must also keep track of the amount of reward accumulated during each option’s execution and the identity of the state in which the option was initiated.
Learning Option Policies
The description provided so far explains how the agent learns a top- or root-level policy, which determines what action or option to select when no option is currently in control of behavior. We turn now to the question of how option-specific policies are learned.
In versions of the options framework that address such learning, it is often assumed that options are initially defined in terms of specific subgoal states. The question of where these subgoals come from is an important one, to which we will return later. It is further assumed that when an active option reaches its subgoal, the actions leading up to the subgoal are reinforced. To distinguish this reinforcing effect from the one associated with external rewards, subgoal attainment is said to yield pseudo-reward.34
(p.293) In order for subgoals and pseudo-reward to shape option policies, the critic in HRL must maintain not only its usual value function, but also a set of option-specific value functions, Vo(s)(see figure 16.1b). As in ordinary RL, these value functions predict the cumulative long-term reward that will be received subsequent to occupation of a particular state. However, they are option-specific in the sense that they take into account the pseudo-reward that is associated with each option’s subgoal state. A second reason that option-specific value functions are needed is that the reward (and pseudo-reward) that the agent will receive following any given state depends on the actions it will select. These depend, by definition, on the agent’s policy, and under HRL the policy depends on which option is currently in control of behavior. Thus, only an option-specific value function can accurately predict future rewards.
Despite the additions discussed here, option-specific policies are learned in quite the usual way: On each step of an option’s execution, a prediction error is computed based on the (option-specific) values of the states visited and the reward received (including pseudo-reward). This prediction error is then used to update the option’s action strengths and the values attached to each state visited during the option (figure 16.2d). With repeated cycles through this procedure, the option’s policy evolves so as to guide behavior, with increasing directness, toward the option’s subgoals.
Potential Neural Correlates
Having laid out the basic mechanisms of HRL, we are now in a position to consider its potential implications for understanding neural function. To make these concrete, we will leverage the actor-critic formulation of HRL21 presented earlier. As previously noted, existing research has proposed parallels between the elements of the actor-critic framework and specific neuroanatomical structures. Situating HRL within the actor-critic framework thus facilitates the formation of hypotheses concerning how HRL might map onto functional neuroanatomy.
As figure 16.1 makes evident, elaborating the actor-critic architecture for HRL requires only insertion of a very few new elements. The most obvious of these is the component labeled“o” in figure 16.1b. As established previously, the role of this component is to represent the identity of the option currently in control of behavior. From a neuroscientific point of view, this function seems very closely related to those commonly ascribed to the dorsolateral prefrontal cortex (DLPFC). The DLPFC has long been considered to house representations that guide temporally integrated, goal-directed behavior.38,40,41,78,96,114 Recent work has refined this idea by demonstrating that DLPFC neurons play a direct role in representing task sets. Here, a single pattern of DLPFC activation serves to represent an entire mapping from stimuli to (p.294) responses, that is, a policy.5,23,50,53,85,99,108,110 According to the guided activation theory,63 prefrontal representations do not implement policies directly, but instead select among stimulus-response pathways implemented outside the prefrontal cortex. This division of labor fits well with the distinction in HRL between an option’s identifier and the policy with which it is associated.
There is evidence that, in addition to the DLPFC, other frontal areas may also carry representations of task set, including presupplementary motor area (pre-SMA)86 and premotor cortex (PMC).69,109 Furthermore, like options in HRL, neurons in several frontal areas including DLPFC, pre-SMA, and supplementary motor area (SMA) have been shown to code for particular sequences of low-level actions.6,17,97,98 Research on frontal cortex also accords well with the stipulation in HRL that temporally abstract actions may organize into hierarchies, with the policy for one option (say, an option for making coffee) calling other, lower-level options (say, options for adding sugar or cream). This fits with numerous accounts suggesting that the frontal cortex serves to represent action at multiple, nested levels of temporal structure,41,102,114,118 possibly in such a way that higher levels of structure are represented more anteriorly.20,39,40,46,57
As reviewed earlier, neuroscientific interpretations of the basic actor-critic architecture generally place policy representations within the DLS. It is thus relevant that such regions as the DLPFC, SMA, pre-SMA, and PMC—areas potentially representing options—all project heavily to the DLS.4,76 Frank, O’ Reilly, and colleagues36,74,85 (see also chapter 17, this volume) have put forth detailed computational models that show how frontal inputs to the striatum could switch among different stimulus-response pathways. Here, as in guided activation theory, temporally abstract action representations in frontal cortex select among alternative (i.e., option-specific) policies.
In order to support option-specific policies, the DLS would need to integrate information about the currently controlling option with information about the current environmental state, as is indicated by the arrows converging on the policy module in figure 16.1b. This is consistent with neurophysiological data showing that some DLS neurons respond to stimuli in a way that varies with task context.81,88 Other studies have shown that action representations within the DLS can also be task dependent.2,42,43,59 For example, in rats, different DLS neurons fire in conjunction with simple grooming movements, depending on whether those actions are performed in isolation or as part of a grooming sequence.1 This is consistent with the idea that option-specific policies (action strengths) might be implemented in the DLS, since this would imply that a particular motor behavior, when performed in different task contexts, would be selected via different neural pathways.
Unlike the selection of primitive actions, the selection of options in HRL involves initiation, maintenance, and termination phases. At the neural level, the (p.295) maintenance phase would be naturally supported within DLPFC, which has been extensively implicated in working memory function.27,28,80 With regard to initiation and termination, it is intriguing that phasic activity has been observed, both within the DLS and in several areas of frontal cortex, at the boundaries of temporally extended action sequences.37,68,116 Since these boundaries correspond to points where new options would be selected, boundary-aligned activity in the DLS and frontal cortex is also consistent with a proposed role of the DLS in gating information into prefrontal working memory circuits.74,85
The points considered so far all relate to control, that is, the guidance of action selection. Also critical to HRL is the machinery that drives learning, centering on the temporal-difference prediction error. Here, too, HRL gives us some very specific things to look for in terms of neural correlates. In particular, moving from RL to HRL brings about important alterations in the way the prediction error is computed. One important change is that HRL widens the scope of the events that the prediction error addresses. In standard RL, the prediction error indicates whether things went better or worse than expected since the immediately preceding single-step action. HRL, in addition, evaluates at the completion of an option whether things have gone better or worse than expected since the option was initiated (see figure 16.2d). Thus, unlike standard RL, the prediction errors associated with options in HRL are framed around temporally extended events. Formally speaking, the HRL setting is no longer a Markov decision process, but rather a semi-Markov decision process (SMDP).
The widened scope of the prediction error computation in HRL resonates with work on midbrain DA function. In particular, Daw29 suggested, based on midbrain responses to delayed rewards, that dopaminergic function is driven by representations that divide event sequences into temporally extended segments. In articulating this account, Daw provided a formal analysis of DA function that draws on precisely the same principles of temporal abstraction that also provide the foundation for HRL, namely, an SMDP framework.
Note that in HRL, in order to compute a prediction error when an option terminates, certain information is needed. In particular, the critic needs access to the reward prediction it made when the option was initially selected, and for purposes of temporal discounting it also needs to know how much time has passed since that prediction was made. These requirements of HRL resonate with data concerning the orbitofrontal cortex (OFC). Neurophysiologic data have shown that within OFC, unlike some other areas, reward-predictive activity tends to be sustained, spanning temporally extended segments of task structure.94 In addition, in line with the integration of reward and delay information in HRL, the response of OFC neurons to the receipt of primary rewards has been shown to vary depending on the wait time leading up to the reward.83
(p.296) Another difference between HRL and ordinary temporal-difference learning is that prediction errors in HRL occur at all levels of task structure (see figure 16.2d). At the top-most or root level, prediction errors signal unanticipated changes in the prospects for primary reward. However, in addition, once the HRL agent enters a subroutine, separate prediction error signals indicate the degree to which each action has carried the agent toward the currently relevant subgoal and its associated pseudo-reward. Note that these subroutine-specific prediction errors are unique to HRL. In what follows, we refer to them as pseudo-reward prediction errors (PPE), reserving reward prediction error (RPE) for prediction errors relating to primary reward.
Because the PPE is not found in ordinary RL, it can be considered a functional signature of HRL. If the neural mechanisms underlying hierarchical behavior are related to those found in HRL, it should be possible to uncover a neural correlate of the PPE. On grounds of parsimony, one would expect to find PPE signals in the same structures that have been shown to carry RPE-related signals, in particular targets of midbrain dopaminergic projections including VS45,71,75 and anterior cingulate cortex,48,49 as well as other structures including the habenula62,87,107 and amygdala.22,115 Unlike some of the other predictions from HRL that we have discussed, for which at least circumstantial evidence can be drawn from the literature, we are aware of no previous work that sheds light on whether anything like the PPE is computed in the brain. Given this, we undertook a set of experiments assaying for neural correlates of the PPE. In the following section, we present an overview of these experiments and their results. (For full details, see ref. 82.)
Testing for the Pseudo-Reward Prediction Error
To ground the distinction between the RPE and PPE, and to set the scene for our experiments, consider the video game illustrated in figure 16.3, which is based on a benchmark task from the computational HRL literature.33 Only the icon elements in the figure (truck, house, and package) appear in the task display. The overall objective of the game is to complete a “delivery” as quickly as possible, using joystick movements to guide the truck first to the package and from there to the house. The task has a transparent hierarchical structure, with delivery serving as the (externally rewarded) top-level goal and acquisition of the package as an obvious subgoal. For an HRL agent, delivery would be associated with primary reward, and acquisition of the package with pseudo-reward.
Consider now a version of the task in which the package sometimes unexpectedly jumps to a new location before the truck reaches it. According to RL, a jump to point A in the figure, or any location within the ellipse shown, should trigger a positive RPE, because the total distance that must be covered to deliver the package has decreased. We assume temporal discounting or effort costs that imply attaining (p.297)
These points translate directly into neuroscientific predictions. As noted earlier, previous research has revealed neural correlates of the RPE in numerous structures.15,22,45,48,49,73,75,107,115 HRL predicts that neural correlates should also exist for the PPE. To test this, we had normal undergraduate participants perform the delivery task from figure 16.3 while undergoing electroencephalography (EEG) and, in two further experiments, functional magnetic resonance imaging (fMRI).
Inourfirst experiment, participants performed the delivery task while undergoing EEG recording. Over the course of the recording session, one third of trials involved a jump event of type D from figure 16.3; these events were intended to elicit a negative pseudo-reward prediction error. Earlier EEG research indicates that ordinary negative reward prediction errors trigger a midline negativity, commonly referred to as the feedback error-related negativity, or fERN48,49,65 (see also chapters 17 and 18, this volume). Based on HRL, we predicted that a similar negativity would occur following the critical changes in pseudo-reward. To provide a baseline for comparison, another third of trials included jump events of type E (following figure 16.3).
Stimulus-aligned EEG averages indicated that class-D jump events, which should induce negative pseudo-reward prediction errors, triggered a phasic negativity in the EEG as shown in figure 16.4a. Like the fERN, this negativity was largest in the midline leads, and the time course was consistent with the fERN, as observed in studies where information about the outcome and the appropriate stimulus-response mapping are shown simultaneously.8
In a second experiment, we examined neural correlates of the PPE using fMRI. A new group of normal participants underwent fMRI while performing a slightly different version of the delivery task. The task was again designed to elicit negative pseudo-reward prediction errors. As in the EEG experiment, one third of trials included a jump of type D (as in figure 16.3) and another third included a jump of type E. In contrast to the EEG task, the increase in subgoal distance in this experiment varied in size across trials. By this means, type D jumps were intended to induce PPEs that varied in magnitude. Our analyses tested for regions that showed phasic activation correlating with predicted PPE size.
A whole-brain general linear model analysis revealed such a correlation, negative in sign, in the dorsal anterior cingulate cortex (ACC; figure 16.4b). This region is believed to contain the generator of the fERN,48 and the fMRI result is thus consistent with the result of our EEG experiment. The same parametric fMRI effect was also observed bilaterally in the anterior insula, a region often coactivated with (p.299) ACC in the setting of unanticipated negative events.79 The only other region displaying the same effect is a small focus within the lingual gyrus.
A set of region-of-interest (ROI) analyses focused in on additional neural structures that, like the ACC, were previously proposed to encode negative reward prediction errors: the habenular complex,87,107 nucleus accumbens,95 and amygdala.22,115 The habenular complex was found to display greater activity following type D than type E jumps, consistent with the idea that this structure is also engaged by negative pseudo-reward prediction errors. A comparable effect was also observed in the right, though not the left, amygdala. In the nucleus accumbens (NAcc), where some studies have observed deactivation accompanying negative reward prediction errors,56 no significant pseudo-reward prediction error effect was observed in this first fMRI study. However, it should be noted that NAcc deactivation with negative reward prediction errors has been an inconsistent finding in previous work.24,72 More robust is the association between NAcc activation and positive reward prediction errors.15,45,71,75 With this in mind, we ran a second, smaller ROI fMRI study specifically looking for NAcc activation within a region of interest, with positive pseudo-reward prediction errors. Fourteen participants performed the delivery task, with jumps of type C (in figure 16.3) occurring on one third of trials, and jumps of type E on another third. As described earlier, a positive pseudo-reward prediction error is predicted to occur in association with type C jumps, and in this setting significant activation was observed in the right NAcc, scaling with predicted pseudo-reward prediction error magnitude.
Directions for Further Investigation
Our initial experiments, together with the evidence we have pieced together from the existing literature, suggest that HRL may provide a useful framework for investigating the neural basis of hierarchical behavior. As detailed in our recent work,21 HRL also gives rise to further testable predictions, each of which presents an opportunity for further research.
To take just one example, HRL predicts that neural correlates should exist for option-specific state-value representations. As explained earlier, in addition to the top-level state-value function, the critic in HRL must also maintain a set of option-specific value functions. This is because the value function indicates how well things are expected to go following arrival at a given state, which obviously depends on which actions the agent will select. Under HRL, the option that is currently in control of behavior determines action selection, and also determines which actions will yield pseudo-reward. Thus, whenever an option is guiding behavior, the value attached to a state must take the identity of that option into account. (p.300) If there is a neural structure that computes something like option-specific state values, this structure would be expected to communicate closely with the VS, the region typically identified with the locus of state or state-action values in RL. However, the structure would also be expected to receive inputs from the portions of frontal cortex that we have identified as representing options. One brain region that meets both of these criteria is the orbitofrontal cortex (OFC), an area that has strong connections with both VS and DLPFC.3,84 The idea that the OFC might participate in computing option-specific state values also fits well with the behavior of individual neurons within this cortical region. OFC neurons have been extensively implicated in representing the reward value associated with environmental states.84,94 However, other data suggest that OFC neurons can also be sensitive to shifts in response policy or task set.70 Critically, OFC representations of event value have been observed to change in parallel with shifts in strategy,92 a finding that fits precisely with the idea that the OFC might represent option-specific state values. Although these findings are consistent with an HRL interpretation, further research on OFC directly guided by the relevant predictions from HRL could be quite informative.
Discovering Hierarchical Structure
Another avenue for further research arises from a question we have so far avoided: Where do options come from? Throughout this chapter, we have assumed that the HRL agent simply has a toolbox full of options available for selection. The same assumption is adopted in much purely computational work in HRL. The question inevitably arises, however, of how this toolbox of options is initially assembled. This question, sometimes referred to as the option discovery problem, is obviously relevant to human learning.21 Indeed, influential behavioral work has characterized childhood development as involving a process of building up a hierarchical set of skills.35,111 Characterizing this building-up process in specific computational terms turns out to be a challenging task. Indeed, option discovery stands as an open problem in computational HRL.12,101
One interesting recent machine-learning proposal for how an HRL agent might discover useful options centers on the notion of bottleneck states.100 These are states that give access to a wide range of subsequent states. As an example, consider a stairwell connecting two floors in a house. To reach any location on one floor from any location on the other, one must pass through the stairwell. The stairwell is, in this sense, a bottleneck location in the house. Both intuitively and formally, such bottleneck locations make good subgoals, around which useful subroutines can be built.
One may well ask, however, whether human learners identify bottleneck states, and if so whether they use such states as a basis for parsing tasks. Some affirmation of this is provided by Cordova and colleagues.26 In this study, participants were presented with a set of “landmarks” (e.g., post office, school), and learned their adjacency relations within a fictive town. These adjacencies were based on simple graphs like the one in figure 16.5, each of which included a clear bottleneck (although the graph itself was never shown to the participants). Once the adjacency relations among the town’s landmarks had been learned, participants were then asked to make “deliveries” in the town, navigating each time from a specified point of origin to a specified goal, and receiving a reward that varied inversely with the number of steps taken to complete the delivery. Before beginning these deliveries, however, the participant was asked to select one landmark as a location for a “bus stop,” understanding that he or she could “jump” to this location from any other during the deliveries, potentially reducing the number of steps taken. Without knowledge of the specific upcoming delivery assignments, the optimal choice for the bus stop location corresponds to the bottleneck location, and participants overwhelmingly selected this location.
In a related experiment, we have shown that human learners not only identify bottleneck states, but also use these as a basis for segmentation.90 Here, a distinctive visual stimulus was assigned to each vertex of a bottleneck graph (which was itself not shown), and participants viewed these stimuli in sequences generated based on a random walk through the underlying graph. Subjects were then asked to parse a further set of stimulus sequences, generated in the same way, pressing a button when they perceived a transition between subsequences. Participants showed a significant (p.302) tendency to parse at junctures where the sequence traversed a bottleneck in the underlying graph, consistent with the idea that events or tasks can be decomposed based on a structural analysis of their underlying topological structure. Together, these initial findings suggest that ideas from computational HRL may help answer the question of how humans discover hierarchical structure in the task environment, developing action hierarchies tailored to this structure.
Dual Modes of Control
We believe that further HRL-related research, and indeed all research drawing on RL, must attend to an important but often neglected distinction between two forms of learning or decision making. Work on animal and human behavior suggests that instrumental actions arise from two modes of control, one built on established stimulus-response links or “habits,” and the other on prospective planning.9 Recent work has mapped these modes of control onto RL constructs,31 characterizing the former as relying on cached action values or strengths and model-free RL, and the latter as looking ahead based on an internal model relating actions to their likely effects, that is, model-based RL.18 Here, we have cast HRL in terms of the cache-based system, both because this is most representative of existing work on HRL and because the principles of model-based search have not yet been as fully explored, either at the computational level or in terms of neural correlates. However, incorporating temporal abstraction into model-based, prospective control is straightforward. This is accomplished by assuming that each option is associated with an option model, a knowledge structure indicating the ultimate outcomes likely to result from selecting the option, the reward or cost likely to be accrued during its execution, and the amount of time this execution is likely to take.106 Equipped with models of this kind, the agent can use them to look ahead, evaluating potential courses of action. Importantly, the search process can now “skip over” potentially large sequences of primitive actions, effectively reducing the size of the search tree.47,54,61 This kind of saltatory search process seems to fit well with everyday planning, which introspectively seems to operate at the level of temporally abstract actions (“Perhaps I should buy one of those new cell phones. … Well, that would cost me a few hundred dollars. … But if I bought one, I could use it to check my email …”). The idea of action models, in general, also fits well with work on motor control, which strongly suggests the involvement of predictive models in guiding bodily movements.113 Because option models encode the consequences of interventions, it is interesting to note that recent neuroimaging work has mapped representations of action outcome information in part to prefrontal cortex,44 a region whose potential links with HRL we have already considered. Further investigating the potential relevance of model-based HRL to human planning and decision making offers an inviting area for further research.
Computational RL has proved extremely useful in research on behavior and brain function. Our aim here has been to explore whether HRL might prove similarly applicable. An initial motivation for considering this question derives from the fact that HRL addresses an inherent limitation of RL, the scaling problem, which would clearly be relevant to any organism relying on RL-like learning mechanisms. Implementing HRL along the lines of the actor-critic framework, thereby bringing it into alignment with existing mappings between RL and neuroscience, reveals direct parallels between components of HRL and specific functional neuroanatomic structures, including the DLPFC and OFC. HRL suggests new ways of interpreting neural activity in these as well as several other regions. We have reported results from initial experiments prospectively testing predictions from HRL, which provide evidence for a novel form of prediction-error signal. All things considered, HRL appears to offer a potentially useful set of tools for further investigating the computational and neural basis of hierarchical structured behavior.
The present work was completed with support from Fundação para a Ciência e Tecnologia (SFRH/BD/33273/2007, J. R-F.), the National Institute of Mental Health (P50 MH062196, M.M.B.), and the James S. McDonnell Foundation (M.M.B.).
• How is hierarchically structured behavior represented at the neural level? Are levels of behavioral structure represented discretely, maximizing compositional flexibility, or in a continuous distributed fashion, maximizing generalization and information sharing? Does the answer to this question differ across decision-making systems (habitual versus goal-directed, for example)?
• How are internal representations of hierarchical behavior learned? How does the brain establish and refine a toolbox of subroutines, skills, subtasks or options, which can be exploited across a wide range of potential future activities? How are useful subgoals discovered, when they are not associated with primary reward?
• Given a set of options, how does learning discover the particular combinations and sequences that solve new task challenges?
• Does the framework of computational reinforcement learning provide a useful heuristic for pursuing the answers to the above questions? In particular, does the (p.304) resemblance between dopaminergic function and temporal-difference learning extend to the hierarchical case, and does recent evidence concerning hierarchical representation in prefrontal cortex bear any logical relation to hierarchical representation in HRL?
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