The right superior temporal gyrus plays a role in semantic-rule learning: Evidence supporting a reinforcement learning model

In real-life communication, individuals use language that carries evident rewarding and punishing elements, such as praise and criticism. A common trend is to seek more praise while avoiding criticism. Furthermore, semantics is crucial for conveying information, but such semantic access to native and foreign languages is subtly distinct. To investigate how rule learning occurs in different languages and to highlight the importance of semantics in this process, we investigated both verbal and non-verbal rule learning in first (L1) and second (L2) languages using a reinforcement learning framework, including a semantic rule and a color rule. Our computational modeling on behavioral and brain imaging data revealed that individuals may be more motivated to learn and adhere to rules in an L1 compared to L2, with greater striatum activation during the outcome phase in the L1. Additionally, results on the learning rates and inverse temperature in the two rule learning tasks showed that individuals tend to be conservative and are reluctant to change their judgments regarding rule learning of semantic information. Moreover, the greater the prediction errors, the greater activation of the right superior temporal gyrus in the semantic-rule learning condition, demonstrating that such learning has differential neural correlates than symbolic rule learning. Overall, the findings provide insight into the neural mechanisms underlying rule learning in different languages, and indicate that rule learning involving verbal semantics is not a general symbolic learning that resembles a conditioned stimulus-response, but rather has its own specific characteristics.

Contribution of left supramarginal and angular gyri to episodic memory encoding: An intracranial EEG study

The role of the left ventral lateral parietal cortex (VPC) in episodic memory is hypothesized to include bottom-up attentional orienting to recalled items, according to the dual-attention model (Cabeza et al., 2008). However, its role in memory encoding could be further clarified, with studies showing both positive and negative subsequent memory effects (SMEs). Furthermore, few studies have compared the relative contributions of sub-regions in this functionally heterogeneous area, specifically the anterior VPC (supramarginal gyrus/BA40) and the posterior VPC (angular gyrus/BA39), on a within-subject basis. To elucidate the role of the VPC in episodic encoding, we compared SMEs in the intracranial EEG across multiple frequency bands in the supramarginal gyrus (SmG) and angular gyrus (AnG), as twenty-four epilepsy patients with indwelling electrodes performed a free recall task. We found a significant SME of decreased theta power and increased high gamma power in the VPC overall, and specifically in the SmG. Furthermore, SmG exhibited significantly greater spectral tilt SME from 0.5 to 1.6 s post-stimulus, in which power spectra slope differences between recalled and unrecalled words were greater than in the AnG (p = 0.04). These results affirm the contribution of VPC to episodic memory encoding, and suggest an anterior-posterior dissociation within VPC with respect to its electrophysiological underpinnings.

A new semantic list learning task to probe functioning of the Papez circuit

INTRODUCTION

List learning tasks are powerful clinical tools for studying memory, yet have been relatively underutilized within the functional imaging literature. This limits understanding of regions such as the Papez circuit that support memory performance in healthy, nondemented adults.

METHOD

The current study characterized list learning performance in 40 adults who completed a semantic list learning task (SLLT) with a Brown-Peterson manipulation during functional magnetic resonance imaging (fMRI). Cued recall with semantic cues and recognition memory were assessed after imaging. Internal reliability, convergent, and discriminant validity were evaluated.

RESULTS

Subjects averaged 38% accuracy in recall (62% for recognition), with primacy but no recency effects observed. Validity and reliability were demonstrated by showing that the SLLT was correlated with the California Verbal Learning Test (CVLT), but not with executive functioning tests, and by high intraclass correlation coefficient across lists for recall (.91). fMRI measurements during encoding (vs. silent rehearsal) revealed significant activation in bilateral hippocampus, parahippocampus, and bilateral anterior and posterior cingulate cortex. Post hoc analyses showed increased activation in anterior and middle hippocampus, subgenual cingulate, and mammillary bodies specific to encoding. In addition, increasing age was positively associated with increased activation in a diffuse network, particularly frontal cortex and specific Papez regions for correctly recalled words. Gender differences were specific to left inferior and superior frontal cortex.

CONCLUSIONS

This is a clinically relevant list learning task that can be used in studies of groups for which the Papez circuit is damaged or disrupted, in mixed or crossover studies at imaging and clinical sites.

Encoding: the keystone to efficient functioning of verbal short-term memory

Verbal short-term memory (VSTM) is thought to play a critical role in language learning. It is indexed by the nonword repetition task where listeners are asked to repeat meaningless words like 'blonterstaping'. The present study investigated the effect on nonword repetition performance of differences in efficiency of functioning of some part of the neural architecture mediating VSTM. Hypotheses were stated within Baddeley and Hitch's (1974) multicomponent model of VSTM, with respect to regions of the brain known to be active during tasks tapping into VSTM. We were specifically interested in activations associated with the posterior planum temporale (Spt) which emerge during rehearsal since this region is hypothesized to be central to VTSM (Buchsbaum, Olsen, Koch, & Berman, 2005a). Participants performed a delayed reaction time task in the scanner which explicitly mimicked the three main stages of information-processing involved in VSTM (encoding, rehearsal, recall (here recognition)). The data for each stage were then convolved with scores from a separately measured nonword repetition task. Rather than observing a pattern of individual differences located to specific regions specialized for supporting VSTM, a dissociation in direction of correlation in overlapping regions of the brain was observed during encoding and recognition. Larger hemodynamic responses during encoding were associated with better nonword repetition, and vice versa during recognition. There was little evidence for a network of activations specialized for VSTM. Instead, the main correlations were observed in regions also known to be involved in long-term memory. It seems that individuals who are better at nonword repetition and hence at language learning, activate these regions more efficiently than poorer nonword-repeaters early after stimulus input. These observations are discussed with respect to various models proposed for explaining the phenomenon of VSTM.

Neural activity that predicts subsequent memory and forgetting: a meta-analysis of 74 fMRI studies

The present study performed a quantitative meta-analysis of functional MRI studies that used a subsequent memory approach. The meta-analysis considered both subsequent memory (SM; remembered>forgotten) and subsequent forgetting (SF; forgotten>remembered) effects, restricting the data used to that concerning visual information encoding in healthy young adults. The meta-analysis of SM effects indicated that they most consistently associated with five neural regions: left inferior frontal cortex (IFC), bilateral fusiform cortex, bilateral hippocampal formation, bilateral premotor cortex (PMC), and bilateral posterior parietal cortex (PPC). Direct comparisons of the SM effects between the studies using verbal versus pictorial material and item-memory versus associative-memory tasks yielded three main sets of findings. First, the left IFC exhibited greater SM effects during verbal material than pictorial material encoding, whereas the fusiform cortex exhibited greater SM effects during pictorial material rather than verbal material encoding. Second, bilateral hippocampal regions showed greater SM effects during pictorial material encoding compared to verbal material encoding. Furthermore, the left hippocampal region showed greater SM effects during pictorial-associative versus pictorial-item encoding. Third, bilateral PMC and PPC regions, which may support attention during encoding, exhibited greater SM effects during item encoding than during associative encoding. The meta-analysis of SF effects indicated they associated mostly with default-mode network regions, including the anterior and posterior midline cortex, the bilateral temporoparietal junction, and the bilateral superior frontal cortex. Recurrent activity oscillations between the task-positive and task-negative/default-mode networks may account for trial-to-trial variability in participants' encoding performances, which is a fundamental source of both SM and SF effects. Taken together, these findings clarify the neural activity that supports successful encoding, as well as the neural activity that leads to encoding failure.

Individual Differences in Episodic Memory: The Role of Self-initiated Encoding Strategies

Individuals' abilities to form and retrieve episodic memories vary widely. Consistent with this, there are substantial individual differences in brain activity during encoding and retrieval that are associated with individual differences in memory performance. Growing evidence suggests that individual differences in self-initiated encoding strategy use play an important role in individual differences in episodic memory and brain activity during intentional encoding. This review examines the role of individual differences in self-initiated encoding strategy use in individual differences in episodic memory, and outlines the major findings of brain lesion and functional neuroimaging studies that characterize the neural correlates of individual differences in self-initiated encoding strategy use. The relevance of individual differences in self-initiated encoding strategy use to understanding episodic memory impairments and alterations in brain activity in clinical populations such as individuals with schizophrenia is also discussed.