Ann. N.Y. Acad. Sci. ISSN 0077-8923 A N N A L S O F T H E N E W Y O R K A C A D E M Y O F SC I E N C E S Issue: Critical Contributions of the Orbitofrontal Cortex to Behavior Orbitofrontal contributions to value-based decision making: evidence from humans with frontal lobe damage Lesley K. Fellows Department of Neurology and Neurosurgery, McGill University, Montreal, Canada Address for correspondence: Lesley K. Fellows, MD CM, DPhil, Department of Neurology and Neurosurgery, McGill University, 3801 University St., Montreal, QC Canada. [email protected] The work described here aims to isolate the component processes of decision making that rely critically on particular subregions of the human prefrontal cortex, with a particular focus on the orbitofrontal cortex. Here, experiments isolating specific aspects of decision making, using very simple preference judgment and reinforcement learning paradigms, were carried out in patients with focal frontal damage. The orbitofrontal cortex and the adjacent ventromedial prefrontal cortex play a critical role in decisions based on subjective value, across many categories of stimuli, and in learning to choose between stimuli based on value feedback. However, these regions are not required for learning to choose between actions based on feedback, which instead seems to rely critically on the dorsomedial prefrontal cortex. These results point to a potentially common role for the orbitofrontal cortex in representing the context-sensitive, subjective value of stimuli to allow consistent choices between them. They also argue for multiple, parallel, value-based processes that influence behavior through dissociable mechanisms. Keywords: executive function; neuropsychology; lesion study; reversal learning; reward Decision making: a novel window on the human brain Life is full of choices, large and small. Making the best of our environment involves determining the value of the possibilities that the environment presents, and acting to obtain those with more value. Effective decision making will confer an adaptive advantage, and it is thus likely that the brain is organized to ensure optimal decisions. This may be particularly true for decisions about primary goods like food, water, or mating opportunities but probably applies quite generally to the more abstract choices that are also part of everyday human experience. Viewing human behavior through the lens of decision making provides an interesting perspective for cognitive neuroscience that has led to novel insights about the functional organization of the brain. This vantage point leads to a series of basic questions: how is value abstracted from environmental information, how is it represented in the brain, and how do these representations influence behavior? Over the last several years, progress in answering these questions has come from two directions: the first has been to build on an extensive literature on reinforcement learning in animals, and the second to combine analytic models and behavioral paradigms drawn from economics and decision psychology with cognitive neuroscience methods. This volume as a whole showcases these different approaches. Here, I will review work based on both of these frameworks applied in human subjects with focal brain injury. Figure 1 provides a general schematic of the processes that may be engaged during decision making, showing how the economic and learning frameworks connect, at least on a conceptual level.1,2 Is economic value represented in the brain? The economic framework to date has focused on the concept of value: This is a subjective property that can be assigned to stimuli (i.e., goods) and perhaps also to actions. Value is a potentially useful construct; if it exists as a biologically definable signal, then it provides an elegant way to understand doi: 10.1111/j.1749-6632.2011.06229.x c 2011 New York Academy of Sciences. Ann. N.Y. Acad. Sci. 1239 (2011) 51–58 51 The human frontal lobes in choice and learning Fellows Figure 1. Schematic overview of the steps leading to a decision, that is, any nonarbitrary choice between two or more options. These putative stages may be highly interlinked and are likely to have bidirectional influences on each other. The last two stages suppose that the outcome is experienced, and this experience will influence future value and choice events: that is, that learning will occur. how the brain supports adaptive decision making. Experimental work in economics and decision psychology supports the usefulness of subjective value in analyzing human (and indeed, animal) behavior.3 Is value information encoded in the brain, and if so, where and for what purpose? There is ample evidence that signals correlating with subjective value are indeed present in the brain. This has been shown in a variety of organisms, including humans, with both neurophysiological methods and functional neuroimaging.2,4–6 The presence of value signals in many regions of the brain, notably including the ventral striatum, and various regions within prefrontal and parietal cortex, is cause for both celebration and caution. Value may be a very useful construct for understanding particular aspects of brain function. On the other hand, it may be so generic that it correlates with or subsumes more specific processes important in motivated behavior, in which case it would be unhelpful or even misleading as a basis for interpreting brain signals.7 This makes it especially important to be as clear as possible in defining and operationalizing value-related processes. That said, the concept of value has added to the study of these brain regions, both by providing a fresh conceptual perspective, and by opening the door to a rich array of behavioral tasks and analytic methods that can be useful tools for both probing and understanding brain function. Evidence from loss of function studies can address whether value-related processes are distinct from other aspects of motivated behavior.8 Manipulations that disrupt the function of a brain region that carries value signals should have predictable effects on decision making if those signals reflect a neurobiologically relevant aspect of value. In humans, there are currently only two loss-of-function methods: transcranial magnetic stimulation and studies 52 of patients with naturally occurring brain damage, typically due to a stroke or other local injury. This review will focus on the latter, which, for practical reasons, has been the most instructive to date. There is no doubt that brain damage can affect decision making. Impairments in judgment and decision making are well recognized in various neurological conditions but have historically been particularly linked to frontal lobe damage. Experimental work aimed at understanding the mechanisms of these impairments began only relatively recently, and has largely continued that focus on the frontal lobes. Influential early studies suggested that damage to orbitofrontal and adjacent ventromedial prefrontal cortex might lead to specific deficits in decision making,9,10 leading to a further focus on these two regions, which I will refer to here together as the ventromedial frontal lobe (VMF). Much of the work to date in patients with VMF damage has used relatively complex tasks, beginning with the Iowa Gambling Task. While VMF damage clearly disrupts task performance, task complexity leaves room for debate about which processes are, in fact, disrupted.11,12 Simple preference judgments Preference judgment is a simpler alternative task that would seem to focus quite tightly on subjective value. When choosing between fully known options, such as “chocolate or vanilla?,” in principle, one is somehow comparing the anticipated relative value in a task that would seem to require few other processes. If VMF representations of relative value are necessary for choice, then damage to the VMF should disrupt decision making even under these very simple conditions. The challenge here is to know how to detect that disruption, since preferences of this kind are subjective and obviously vary normally across individuals. c 2011 New York Academy of Sciences. Ann. N.Y. Acad. Sci. 1239 (2011) 51–58 Fellows One solution is to consider how consistently an individual is in his or her choices. We took this approach in an initial study, asking subjects to choose between all possible pairs of stimuli within several categories, and looking at the consistency of these choices as the dependent measure. While subjective values may vary normally even within an individual, they tend not to vary dramatically for a given set of choices over a short time frame. As an example, if you choose an apple over a banana, and a banana over an orange, when you are offered the choice of apple or orange, you should choose the apple if you are basing the choice on some consistent subjective value metric. Choosing the orange instead would constitute a violation of transitivity of preferences. We have carried out two studies using this basic approach. The first involved 10 subjects with VMF damage, compared to nine subjects with frontal damage sparing the VMF, and 19 healthy, demographically matched control participants. Participants made pairwise choices between stimuli printed on cue cards across three categories: foods, color swatches, famous people. They were asked to treat each choice as independent, to pick the option they “liked best” and were not prompted to be consistent. The choices were entirely hypothetical. A control task with similar demands asked them to make perceptual, rather than value-based, judgments: which of two color swatches (along a spectrum from red to blue) looked bluer, and which of two lines was longer. Patients with VMF damage performed as well as either control group in the perceptual tasks but, as predicted, showed significantly more inconsistency in their preference judgments. Patients with frontal damage sparing the VMF showed no deficit in preference judgments.13 This finding is strong support for the claim that the VMF (relative) value representations, demonstrated in a range of functional magnetic resonance imaging (fMRI) paradigms, are indeed necessary for choosing between goods based on subjective value. There is mixed support for the same claim from lesion studies in nonhuman primates with variably selective lesions to homologous frontal areas.14,15 This issue is so central to current models of VMF function, and of the neural substrates of decision making, that we pursued it further in two more recent experiments. One used the identical experimental structure, but a new version of the preference task The human frontal lobes in choice and learning that had more categories of stimuli (to assess more fully the generality of any effects) and more stimuli within each category, to guard against the possibility that controls, but not patients, used explicit strategies to stay consistent. This version of the task was computerized to allow accurate reaction time (RT) measurement and to probe whether VMF damage prolongs decision time as well as disrupts choice consistency. In a sample of patients with VMF damage (n = 15), non-VMF frontal damage (n = 11), and 23 healthy controls, we replicated the findings from the previous study. Again, subjects with VMF damage showed significantly more inconsistent choices, collapsed across categories, while those with nonVMF frontal damage were comparable to healthy subjects. Three of the patients in the VMF group had also participated in the 2007 study; when their data are excluded from the analysis, the same pattern remains, thus constituting an independent replication of preference deficits after VMF damage.16 The inclusion of a non-VMF control group supports the claim that impaired preference judgment is the result of VMF damage specifically, and not frontal damage in general. However, this particular study is not powered to provide strong evidence about the potential role of other specific prefrontal regions. Further work will be needed to definitively establish that dorsomedial or dorsolateral prefrontal cortex (PFC), for example, are not critical for such decisions. The five categories examined in this version of the task were color, fruits, vegetables, landscapes, and puppies. There were no discernible differences in the basic pattern of inconsistencies across categories, in keeping with a very general role for the VMF in preference judgments regardless of the specific class of stimuli. Interestingly, there were no consistent effects of lesion on decision time. Although VMF damage led to increased inconsistency, this was not associated with prolonged RT (i.e., equivocation) or with impulsive, overly quick choices. Healthy controls showed the expected orderly relationship between subjective value distance and RT, taking longer to decide between options that were closer together on their subjective value ordering. This phenomenon has been reported as far back as the 1930s.17 Frontal lobe damage, whether affecting the VMF or elsewhere, did not alter this relationship in any substantial way. This observation has c 2011 New York Academy of Sciences. Ann. N.Y. Acad. Sci. 1239 (2011) 51–58 53 The human frontal lobes in choice and learning Fellows implications for mechanistic models of the role of the VMF in decision making. Damage seems to affect value-based decision “accuracy” without a substantial effect on speed. This argues that there is little monitoring of the quality of the putative relative value signal arising from the VMF as it leads to choice: a degraded or noisy value signal does not lead to longer processing, at least for these hypothetical choices, made in the absence of substantial time pressure. If decisions follow the roughly linear sequence suggested by Figure 1, then whatever checks and balances may be built in to value processing would seem to occur prior to the VMF. Alternatively, the VMF may play some modulatory or fine-tuning role in value-based choice that is not (strongly) monitored. Value maximizing choice A third recent study probed subjective value using a different choice paradigm more amenable to formal analysis. In this study,18 we borrowed a task from experimental economics to test whether VMF damage disrupts the ability to consistently choose between bundles of goods so as to maximize subjective value. Participants chose between different bundles of chocolate bars and fruit juice (e.g., between two bars of chocolate bundled with two boxes of juice, or four bars of chocolate with no juice). Following the same logic as was applied to the preference tasks, above, here expressed as the Generalized Axiom of Revealed Preferences (GARP),19 we evaluated the degree to which bundle choices were rational, that is, maximized subjective value as indicated by the overall pattern of choices. Patients with VMF damage (n = 9) had more GARP violations, and these violations were more severe, than healthy subjects of the same age and education. These findings again support a necessary role for the human VMF in value-based decisions, here in a task without memory requirements, and involving choices with real outcomes. (Subjects actually received one of their chosen bundles at the end of the task.) Learning about value The experiments described up to this point have focused on “one-off” decisions taken without feedback, based on prior knowledge, and without uncertainty, in the formal sense of that term. A quite different literature that is also relevant to deci- 54 sion making instead focuses on the accumulation of value information through repeated choices with feedback. Learning the value of stimuli An extensive animal literature has established the neural circuits involved in learning by trial and error from reward and from punishment. Much of this work has been carried out in the rat, and has focused on striatum and related subcortical nuclei, and on the role of dopamine. Research building on this foundation addresses rodent frontal cortex contributions to learning and choice. A related literature has established that the orbitofrontal cortex (OFC) in nonhuman primates plays a key role in learning from feedback, particularly under conditions that require flexible responding and/or interpretation of the meaning of feedback within a particular context.20,21 A canonical example of such conditions is that of reversal learning. In this classic reinforcement learning task, subjects learn through trial and error to prefer one (rewarded) stimulus over another (that leads to nonreward or punishment, depending on the paradigm). Once a learning criterion has been met, the contingencies are reversed, and the subject must flexibly adapt and learn to prefer the previously punished, now rewarded stimulus. Lesion studies in nonhuman primates extending back several decades established that the OFC is necessary for reversal learning.20 Several studies have confirmed that the same is true in humans. Damage to the VMF, and perhaps specifically to the OFC, but not to dorsolateral frontal areas, disrupts stimulus-based reversal learning.22–24 These studies used fairly simple tasks, generally with deterministic feedback, leading to the impression that the reinforcement learning deficits were present only in reversal, and not in initial learning. This in turn led to the deficits being framed as “perseverative,” in that subjects tended to continue to choose the originally rewarded stimulus after reversal. Subsequent work with more challenging reinforcement schedules requires that this interpretation be revised. In a recent study of prefrontal contributions to stimulus-based reversal learning, this time with probabilistic feedback, we found that VMF damage disrupts both initial and reversal learning.25 This study involved 39 subjects with focal damage affecting the frontal lobes, of whom 11 had damage to the VMF. The sample size c 2011 New York Academy of Sciences. Ann. N.Y. Acad. Sci. 1239 (2011) 51–58 Fellows permitted the use of voxel-based lesion-symptom mapping (VLSM) to supplement the conventional region-of-interest analysis. VLSM provides a statistical test of structure–function relationships at the voxel-by-voxel level, and so can identify the specific areas contributing to a given functional deficit, within the constraints of the variability and overlap of lesions in a given sample.26 This analysis identified damage to the bilateral medial OFC (as well as a region with the left lateral OFC) as being most strongly related to poor performance on the task as a whole. Trial-by-trial analysis sheds further light on the deficits of these patients in a probabilistic feedback environment. OFC damage seems to disrupt the ability to interpret a given instance of feedback within the larger task context. That is, healthy participants typically “stay” after a win, and “shift” after a loss, particularly when that loss is in keeping with a larger pattern, suggesting that that stimulus is a poor choice. In contrast, those with OFC damage were more likely to shift after a win. They also shifted after probabilistic losses from the overall better choice, suggesting that they were struggling to interpret the feedback in relation to the stream of preceding feedback. Interestingly, they showed no tendency to perseverate in this noisy, rapidly changing feedback environment. Learning the value of actions Models of both decision making and reinforcement learning make distinctions between value attached to stimuli (i.e., goods) and the value of an action, that is, the behavioral event that expresses the choice. One plausible model has subjective value of stimuli encoded in VMF, in turn biasing the motor system to make the choice of the most valuable option. There is evidence from fMRI and monkey electrophysiology that action-related value information is encoded in the dorsal medial frontal lobe (DMF), a region that might be a candidate for this goods-to-action value transformation. This view supposes sequential processing of value information. In general, the brain tends toward parallel processing, and there is evidence from lesion studies in macaques that the linking of value information to stimuli, and to actions, may be dissociable.27,28 We addressed the same question in human subjects, using very similar methods. Subjects who had The human frontal lobes in choice and learning completed the probabilistic stimulus-value reversal learning task described, and who had focal damage affecting either VMF (and not DMF; N = 5) or DMF (sparing VMF; N = 4) participated in a second experiment to assess action-value learning. This task used exactly the same probabilistic reinforcement with reversals, but related to two distinct wrist actions, rather than to two stimuli. Subjects held a joystick, and on a generic “go” signal, chose between supinating or pronating their wrist, twisting the joystick in one of two directions. This was followed by feedback in the form of a $50 play money win or loss, exactly as in the stimulus-based task. Also as in that task, the reinforcement contingencies reversed once a learning criterion was met. Compared to healthy controls, patients with OFC damage were impaired on the stimulus-value learning task. They made significantly fewer reversals, and also made more errors in the initial learning phase. In contrast, their performance on the actionvalue task was comparable to controls. The patients with DMF damage showed the opposite pattern. They were impaired on action-value learning, making more errors in the initial phase of the task. They performed within normal on the stimulusvalue task. This double dissociation was most starkly revealed in the trial-by-trial analysis. Patients with OFC damage were significantly more likely to shift after a win in the stimulus-value task, which is a highly maladaptive response in this task, and so rarely produced by controls. In contrast, they did not differ from controls in how often they shifted after a win in the action-value task. Patients with DMF damage showed the opposite pattern of impairment, shifting more often after a win in the action-, but not stimulus-value task, compared to controls.29 This pattern is very similar to the effects of focal lesions to the dorsal anterior cingulate and OFC in macaques. The human lesion data relies on too few subjects to be very confident about the localization within the DMF, but dACC was the area with the most shared damage within that patient group. Our prior, larger study, described earlier, argues that the medial OFC is the key region responsible for the stimulus-value learning deficit. This double dissociation argues against a sequential model of valuebased decisions, instead supporting multiple routes, and perhaps multiple timecourses, by which value may influence choice. c 2011 New York Academy of Sciences. Ann. N.Y. Acad. Sci. 1239 (2011) 51–58 55 The human frontal lobes in choice and learning Fellows The DMF has been implicated in several other aspects of response control, including conflict monitoring,30,31 error-related processes,32,33 and selfgenerated action.34 It remains to be seen whether and how these relate to the observation that this region is also critical for selecting between actions on the basis of value. There have been efforts to unite these possibilities from a modeling point of view;35 our data underline the potential usefulness of reinforcement learning models in understanding the processes that are supported by this brain region. damage for whom we have both measures, there is an overall trend for performance on the two tasks to be correlated, but this relationship is not obligatory: one patient is an extremely poor performer on the preference task but is near the healthy control average for reversal learning. If this outlier is excluded, the remaining seven VMF subjects show a weak trend toward a relationship between reversal learning and preference impairments (Spearman = .54, P = .19). Further work will be needed to definitively answer this question. Value and learning: two sides of the same coin? Conclusions and future directions This article summarizes recent studies of the component processes of decision making subserved by specific regions of human prefrontal cortex. I initially showed that the VMF is necessary for even very simple decisions that rely on subjective value comparisons. I then reviewed the evidence that VMF also plays a critical and specific role in learning to choose between stimuli on the basis of valenced feedback (wins and losses). In the opening paragraphs of this review, I proposed a simple model of decision making that included a learning mechanism to update value estimates based on feedback, when available. A natural question at this point is whether the effects of VMF damage on value-based choice, and value-based learning, reflect the disruption of a single process, or of two processes that happen to be anatomically proximate. Both decisions and learning rely on anticipating the value of an outcome: imagining the pleasures of chocolate compared to vanilla, and anticipating that the choice of a given deck of cards is more likely to yield a win. It is the difference between anticipated and actual outcomes that drives learning,36 and the same anticipation could inform stimulus-based choices even if no learning subsequently occurs, as in the hypothetical preference tasks described.37 In principle, lesion studies should be able to provide a clear answer to this question, by determining whether the two abilities are dissociable. In practice, this requires that the measures of the two abilities are similarly difficult, among other factors, which is by no means certain in this case. Only a subset of the participants in the studies just reviewed completed both the more sensitive reinforcement learning measures and the more sensitive preference judgment task. In the eight subjects with VMF 56 The work reviewed here argues that information about subjective value is not only represented in specific regions of prefrontal cortex, but that this information is required for flexibly learning about value in rapidly changing environments, and for value-based decision making. The VMF in particular, including the OFC and the adjacent vmPFC, is critical both for learning about and choosing stimuli based on subjective value. It remains to be seen whether these two functions rest on a common process. DMF plays a role in choosing actions on the basis of feedback, and this role does not depend on an intact VMF, arguing for parallel value influences on choice between stimuli, and between actions in prefrontal cortex. Overall, these results from loss-of-function experiments provide reassuringly convergent evidence to support current views of the human VMF as playing a critical role in decision making. Importantly, the findings reviewed here argue that the VMF is specifically important in representing subjective value information that is called upon in stimulusbased decision making: damage here disrupts even very simple forms of decision making, in the absence of risk, delay, or ambiguity. Also importantly, these results largely agree with findings using very similar paradigms in macaques with selective prefrontal lesions, suggesting that the neural circuits supporting value-based learning and choice are conserved across primate species. The growing evidence that value-based feedback can influence choice through dissociable mechanisms, depending on the nature of the task, emphasizes the importance of continuing to draw on convergent forms of evidence in studying decision making. Value, even when defined in formal economic terms, remains a broad construct that is likely c 2011 New York Academy of Sciences. Ann. N.Y. Acad. Sci. 1239 (2011) 51–58 Fellows to be linked to behavior in many ways, and via multiple neural circuits. As this volume illustrates, there is still much to understand about these processes. However, it is clear that even complex human decision making builds on processes conserved across species, likely developed in the pursuit of primary rewards, and the avoidance of simple punishments, meaning that the whole range of systems neuroscience methods can be drawn on to help us develop that full understanding. Acknowledgments Ami Tsuchida, Nathalie Camille, and Joe Kable contributed importantly to the work described here. Salary support from the FRSQ and the Killam Trust, operating support from the CIHR (MOP-77583), and the patient databases of McGill University and the University of Pennsylvania made this work possible. 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