Differential modulation of visual responses by distractor or target expectations.

Discriminating relevant from irrelevant information in a busy visual scene is supported by statistical regularities in the environment. However, it is unclear to what extent immediate stimulus repetitions and higher order expectations (whether a repetition is statistically probable or not) are supported by the same neural mechanisms. Moreover, it is also unclear whether target and distractor-related processing are mediated by the same or different underlying neural mechanisms. Using a speeded target discrimination task, the present study implicitly cued subjects to the location of the target or the distractor via manipulations in the underlying stimulus predictability. In separate studies, we collected EEG and MEG alongside behavioural data. Results showed that reaction times were reduced with increased expectations for both types of stimuli and that these effects were driven by expected repetitions in both cases. Despite the similar behavioural pattern across target and distractors, neurophysiological measures distinguished the two stimuli. Specifically, the amplitude of the P1 was modulated by stimulus relevance, being reduced for repeated distractors and increased for repeated targets. The P1 was not, however, modulated by higher order stimulus expectations. These expectations were instead reflected in modulations in ERP amplitude and theta power in frontocentral electrodes. Finally, we observed that a single repetition of a distractor was sufficient to reduce decodability of stimulus spatial location and was also accompanied by diminished representation of stimulus features. Our results highlight the unique mechanisms involved in distractor expectation and suppression and underline the importance of studying these processes distinctly from target-related attentional control.

The structural and functional brain networks that support human social networks.

Social skills rely on a specific set of cognitive processes, raising the possibility that individual differences in social networks are related to differences in specific brain structural and functional networks. Here, we tested this hypothesis with multimodality neuroimaging. With diffusion MRI (DMRI), we showed that differences in structural integrity of particular white matter (WM) tracts, including cingulum bundle, extreme capsule and arcuate fasciculus were associated with an individual's social network size (SNS). A voxel-based morphology analysis demonstrated correlations between gray matter (GM) volume and SNS in limbic and temporal lobe regions. These structural changes co-occured with functional network differences. As a function of SNS, dorsomedial and dorsolateral prefrontal cortex showed altered resting-state functional connectivity with the default mode network (DMN). Finally, we integrated these three complementary methods, interrogating the relationship between social GM clusters and specific WM and resting-state networks (RSNs). Probabilistic tractography seeded in these GM nodes utilized the SNS-related WM pathways. Further, the spatial and functional overlap between the social GM clusters and the DMN was significantly closer than other control RSNs. These integrative analyses provide convergent evidence of the role of specific circuits in SNS, likely supporting the adaptive behavior necessary for success in extensive social environments.

Re-evaluating the role of the orbitofrontal cortex in reward and reinforcement.

The orbitofrontal cortex and adjacent ventromedial prefrontal cortex carry reward representations and mediate flexible behaviour when circumstances change. Here we review how recent experiments in humans and macaques have confirmed the existence of a major difference between the functions of the ventromedial prefrontal cortex and adjacent medial orbitofrontal cortex (mOFC) on the one hand and the lateral orbitofrontal cortex (lOFC) on the other. These differences, however, may not be best accounted for in terms of specializations for reward and error/punishment processing as is commonly assumed. Instead we argue that both lesion and functional magnetic resonance imaging studies reveal that the lOFC is concerned with the assignment of credit for both reward and error outcomes to the choice of specific stimuli and with the linking of specific stimulus representations to representations of specific types of reward outcome. By contrast, we argue that the ventromedial prefrontal cortex/mOFC is concerned with evaluation, value-guided decision-making and maintenance of a choice over successive decisions. Despite the popular view that they cause perseveration of behaviour and inability to inhibit repetition of a previously made choice, we found that lesions in neither orbitofrontal subdivision caused perseveration. On the contrary, lesions in the lOFC made animals switch more rapidly between choices when they were finding it difficult to assign reward values to choices. Lesions in the mOFC caused animals to lose their normal predisposition to repeat previously successful choices, suggesting that the mOFC does not just mediate value comparison in choice but also facilitates maintenance of the same choice if it has been successful.

Social network size affects neural circuits in macaques.

It has been suggested that variation in brain structure correlates with the sizes of individuals' social networks. Whether variation in social network size causes variation in brain structure, however, is unknown. To address this question, we neuroimaged 23 monkeys that had been living in social groups set to different sizes. Subject comparison revealed that living in larger groups caused increases in gray matter in mid-superior temporal sulcus and rostral prefrontal cortex and increased coupling of activity in frontal and temporal cortex. Social network size, therefore, contributes to changes both in brain structure and function. The changes have potential implications for an animal's success in a social context; gray matter differences in similar areas were also correlated with each animal's dominance within its social network.

Distinct roles of three frontal cortical areas in reward-guided behavior.

Functional magnetic resonance imaging was used to measure activity in three frontal cortical areas, the lateral orbitofrontal cortex (lOFC), medial orbitofrontal cortex (mOFC)/ventromedial frontal cortex (vmPFC), and anterior cingulate cortex (ACC), when expectations about type of reward, and not just reward presence or absence, could be learned. Two groups of human subjects learned 12 stimulus-response pairings. In one group (Consistent), correct performances of a given pairing were always reinforced with a specific reward outcome, whereas in the other group (Inconsistent), correct performances were reinforced with randomly selected rewards. The mOFC/vmPFC and lOFC were not distinguished by simple differences in relative preference for positive and negative outcomes. Instead lOFC activity reflected updating of reward-related associations specific to reward type; lOFC was active whenever informative outcomes allowed updating of reward-related associations, regardless of whether the outcomes were positive or negative, and the effects were greater when consistent stimulus-outcome and response-outcome mappings were present. A psychophysiological interaction analysis demonstrated changed coupling between lOFC and brain areas for visual object representation, such as perirhinal cortex, and reward-guided learning, such as the amygdala, ventral striatum, and habenula/mediodorsal thalamus. In contrast, mOFC/vmPFC activity reflected expected values of outcomes and occurrence of positive outcomes, regardless of consistency of outcome mappings. The third frontal cortical region, the ACC, reflected the use of reward type information to guide response selection. ACC activity reflected the probability of selecting the correct response, was greater when consistent outcome mappings were present, and was related to individual differences in propensity to select the correct response.

Separate value comparison and learning mechanisms in macaque medial and lateral orbitofrontal cortex.

Uncertainty about the function of orbitofrontal cortex (OFC) in guiding decision-making may be a result of its medial (mOFC) and lateral (lOFC) divisions having distinct functions. Here we test the hypothesis that the mOFC is more concerned with reward-guided decision making, in contrast with the lOFC's role in reward-guided learning. Macaques performed three-armed bandit tasks and the effects of selective mOFC lesions were contrasted against lOFC lesions. First, we present analyses that make it possible to measure reward-credit assignment--a crucial component of reward-value learning--independently of the decisions animals make. The mOFC lesions do not lead to impairments in reward-credit assignment that are seen after lOFC lesions. Second, we examined how the reward values of choice options were compared. We present three analyses, one of which examines reward-guided decision making independently of reward-value learning. Lesions of the mOFC, but not the lOFC, disrupted reward-guided decision making. Impairments after mOFC lesions were a function of the multiple option contexts in which decisions were made. Contrary to axiomatic assumptions of decision theory, the mOFC-lesioned animals' value comparisons were no longer independent of irrelevant alternatives.

Does the medial orbitofrontal cortex have a role in social valuation?

It has been claimed that social behaviour changes after lesions of the ventromedial prefrontal cortex (vmPFC). However, lesions in humans are rarely restricted to a well defined cortical area. Although vmPFC lesions usually include medial orbitofrontal cortex (mOFC), they typically also affect subgenual and/or perigenual anterior cingulate cortex. The purpose of the current study is to investigate the role of mOFC in social valuation and decision-making. We tested four macaque monkeys prior to and after focal lesions of mOFC. Comparison of the animals' pre- and postoperative performance revealed that, unlike lesions of anterior cingulate gyrus (ACCg), lesions of mOFC did not induce alterations in social valuation. MOFC lesions did, however, induce mild impairments in a probabilistic two-choice decision task, which were not seen after ACCg lesions. In summary, the double dissociation between the patterns of impairment suggest that vmPFC involvement in both decision-making and social valuation may be mediated by distinct subregions centred on mOFC and ACCg respectively.