The Role of Posterior Parietal Cortex in Episodic Memory Retrieval: Transcranial Direct Current Stimulation Studies (tDCS)

Neuroimaging studies of recognition memory have shown that greater activity in the lateral posterior parietal cortex (PPC) correlates with successful recognition in a variety of paradigms, but experimental techniques that manipulate brain activity are necessary to determine the specific contribution of the PPC in episodic memory retrieval. Transcranial Direct Current Stimulation (tDCS) is a non-invasive technique that can be used to manipulate cortical excitability. The collection of experiments that comprise this dissertation use tDCS to determine: 1) whether or not the lateral PPC is causally involved in episodic retrieval, and 2) whether the lateral PPC has a direct role in memory accuracy for studied information or an indirect role that can influence retrieval judgments during episodic memory retrieval. We applied tDCS during three memory paradigms that have shown correlated activity in the parietal cortex. Experiments in Chapter 1 used a false memory paradigm to test whether the parietal cortex contributes to the perceived oldness of a memory and showed increased false recognition with tDCS over the PPC compared to sham tDCS. The experiment in Chapter 2 tested whether the parietal cortex is involved in item and source accuracy and showed decreased false recognition with tDCS over the parietal cortex compared to sham tDCS. To resolve these discrepant findings, the experiment in Chapter 3 tested whether the parietal cortex is important for integration of contextual cues and mnemonic information. Results showed greater utilization of cues predicting memoranda as new with tDCS over the parietal cortex compared to sham tDCS. Overall, manipulating activity in the parietal cortex with tDCS led to alterations in memory retrieval responses compared to sham stimulation. Collectively, our results causally link the PPC to aspects of memory retrieval, and are consistent with the idea that the parietal cortex indirectly influences retrieval judgments, particularly for new items

Interactions Between Visual Attention and Episodic Retrieval: Dissociable Contributions of Parietal Regions During Gist-Based False Recognition

The interaction between episodic retrieval and visual attention is relatively unexplored. Given that systems mediating attention and episodic memory appear to be segregated, and perhaps even in competition, it is unclear how visual attention is recruited during episodic retrieval. We investigated the recruitment of visual attention during the suppression of gist-based false recognition, the tendency to falsely recognize items that are similar to previously encountered items. Recruitment of visual attention was associated with activity in the dorsal attention network. The inferior parietal lobule, often implicated in episodic retrieval, tracked veridical retrieval of perceptual detail and showed reduced activity during the engagement of visual attention, consistent with a competitive relationship with the dorsal attention network. These findings suggest that the contribution of the parietal cortex to interactions between visual attention and episodic retrieval entails distinct systems that contribute to different components of the task while also suppressing each other.Psycholog

Content reinstatement and source confidence during episodic memory retrieval

Abstract from public.pdf.Episodic retrieval is the process of bringing information about a past experience from memory into conscious awareness. Variation in the retrieval process, in regard to content and quality of the information retrieved, is believed to rely on the reactivation of neural patterns of activity elicited during the original experience -- a process called neural reinstatement. Research in support of this idea has relied on participant reports of retrieval quality, but not content, to assess variation in retrieval. Without measuring the content of retrieval, it is unclear whether reinstatement underlies retrieval per se, or merely the evaluation of retrieval quality. The current study addressed this issue by examining the relationship between the magnitude of neural reinstatement during retrieval, and a direct behavioral measure of both retrieval content and quality. Participants viewed a series of words in the context of three encoding tasks, and then completed a memory test on a series of words in which they first identified the encoding task completed for a given word, and next rated their confidence in that decision. Pattern classification analyses were performed on fMRI data acquired during encoding and retrieval phases to index reinstatement, and reinstatement effects were examined according to the behavioral and neural correlates of source confidence. The findings support a relationship between reinstatement and variation in the content and quality of retrieval, and also suggest a role for regions such as left posterior parietal cortex in monitoring reinstated activity to guide decisions about retrieval quality

Memory accumulation mechanisms in human cortex are independent of motor intentions

Previous studies on perceptual decision-making have often emphasized a tight link between decisions and motor intentions. Human decisions, however, also depend on memories or experiences that are not closely tied to specific motor responses. Recent neuroimaging findings have suggested that, during episodic retrieval, parietal activity reflects the accumulation of evidence for memory decisions. It is currently unknown, however, whether these evidence accumulation signals are functionally linked to signals for motor intentions coded in frontoparietal regions and whether activity in the putative memory accumulator tracks the amount of evidence for only previous experience, as reflected in "old" reports, or for both old and new decisions, as reflected in the accuracy of memory judgments. Here, human participants used saccadic-eye and hand-pointing movements to report recognition judgments on pictures defined by different degrees of evidence for old or new decisions. A set of cortical regions, including the middle intraparietal sulcus, showed a monotonic variation of the fMRI BOLD signal that scaled with perceived memory strength (older > newer), compatible with an asymmetrical memory accumulator. Another set, including the hippocampus and the angular gyrus, showed a nonmonotonic response profile tracking memory accuracy (higher > lower evidence), compatible with a symmetrical accumulator. In contrast, eye and hand effector-specific regions in frontoparietal cortex tracked motor intentions but were not modulated by the amount of evidence for the effector outcome. We conclude that item recognition decisions are supported by a combination of symmetrical and asymmetrical accumulation signals largely segregated from motor intentions

Expectations modulate early but not late event-related potential correlates of successful recognition memory

textRecognizing that an item has been previously encountered may not only depend on the strength of memory for that item, but also the expectation that the item will be remembered. Recent studies by O’Connor, et al (2010) and Jaeger et al (2013) revealed that a significant portion of the functional magnetic resonance imaging (fMRI) “retrieval success” effect (BOLD signal for correctly identified old items > new items) depends upon whether participants expect novel or familiar stimuli. The current study examined how expectancy modulates the event-related potential (ERP) retrieval success effect. We employed a typical recognition memory task with the addition of explicit cues indicating if upcoming memory probes were “likely old”, “likely new” (with 75% validity), or “unknown”. An electrophysiological response to the cue, primarily across frontal electrodes from 700-850ms after cue onset, predicted individual differences in cue- induced bias in memory judgments. Responses to memory probes were examined 300- 400ms and 500-700ms after probe onset, corresponding to time windows previously associated with ERP correlates of memory processing. Differences between old and new items were greatest from 300-400ms when preceded by “likely old” cues, overlapping with a component previously identified as tracking familiarity-based processing. In contrast, the 500-700ms time window, previously associated with recollection, revealed significant differences between old and new items that were not modulated by cue type. Overall this pattern of results shows that cue induced biases influence earlier (300-400ms after onset of a memory probe) but not later retrieval processing (500-700ms).Psycholog

An integrated brain-behavior model for working memory.

Working memory (WM) is a central construct in cognitive neuroscience because it comprises mechanisms of active information maintenance and cognitive control that underpin most complex cognitive behavior. Individual variation in WM has been associated with multiple behavioral and health features including demographic characteristics, cognitive and physical traits and lifestyle choices. In this context, we used sparse canonical correlation analyses (sCCAs) to determine the covariation between brain imaging metrics of WM-network activation and connectivity and nonimaging measures relating to sensorimotor processing, affective and nonaffective cognition, mental health and personality, physical health and lifestyle choices derived from 823 healthy participants derived from the Human Connectome Project. We conducted sCCAs at two levels: a global level, testing the overall association between the entire imaging and behavioral-health data sets; and a modular level, testing associations between subsets of the two data sets. The behavioral-health and neuroimaging data sets showed significant interdependency. Variables with positive correlation to the neuroimaging variate represented higher physical endurance and fluid intelligence as well as better function in multiple higher-order cognitive domains. Negatively correlated variables represented indicators of suboptimal cardiovascular and metabolic control and lifestyle choices such as alcohol and nicotine use. These results underscore the importance of accounting for behavioral-health factors in neuroimaging studies of WM and provide a neuroscience-informed framework for personalized and public health interventions to promote and maintain the integrity of the WM network

Recency Effects in the Inferior Parietal Lobe during Verbal Recognition Memory

The most recently encountered information is often most easily remembered in psychological tests of memory. Recent investigations of the neural basis of such “recency effects” have shown that activation in the lateral inferior parietal cortex (LIPC) tracks the recency of a probe item when subjects make recognition memory judgments. A key question regarding recency effects in the LIPC is whether they fundamentally reflect the storage (and strength) of information in memory, or whether such effects are a consequence of task difficulty or an upswing in resting state network activity. Using functional magnetic resonance imaging we show that recency effects in the LIPC are independent of the difficulty of recognition memory decisions, that they are not a by-product of an increase in resting state network activity, and that they appear to dissociate from regions known to be involved in verbal working memory maintenance. We conclude with a discussion of two alternative explanations – the memory strength and “expectancy” hypotheses, respectively – of the parietal lobe recency effect

The Human Brain Encodes Event Frequencies While Forming Subjective Beliefs

To make adaptive choices, humans need to estimate the probability of future events. Based on a Bayesian approach, it is assumed that probabilities are inferred by combining a priori, potentially subjective, knowledge with factual observations, but the precise neurobiological mechanism remains unknown. Here, we study whether neural encoding centers on subjective posterior probabilities, and data merely lead to updates of posteriors, or whether objective data are encoded separately alongside subjective knowledge. During fMRI, young adults acquired prior knowledge regarding uncertain events, repeatedly observed evidence in the form of stimuli, and estimated event probabilities. Participants combined prior knowledge with factual evidence using Bayesian principles. Expected reward inferred from prior knowledge was encoded in striatum. BOLD response in specific nodes of the default mode network (angular gyri, posterior cingulate, and medial prefrontal cortex) encoded the actual frequency of stimuli, unaffected by prior knowledge. In this network, activity increased with frequencies and thus reflected the accumulation of evidence. In contrast, Bayesian posterior probabilities, computed from prior knowledge and stimulus frequencies, were encoded in bilateral inferior frontal gyrus. Here activity increased for improbable events and thus signaled the violation of Bayesian predictions. Thus, subjective beliefs and stimulus frequencies were encoded in separate cortical regions. The advantage of such a separation is that objective evidence can be recombined with newly acquired knowledge when a reinterpretation of the evidence is called for. Overall this study reveals the coexistence in the brain of an experience-based system of inference and a knowledge-based system of inference

Remember the source: Dissociating frontal and parietal contributions to episodic memory

Event-related fMRI studies reveal that episodic memory retrieval modulates lateral and medial parietal cortices, dorsal middle frontal gyrus (MFG), and anterior pFC. These regions respond more for recognized old than correctly rejected new words, suggesting a neural correlate of retrieval success. Despite significant efforts examining retrieval success regions, their role in retrieval remains largely unknown. Here we asked the question, to what degree are the regions performing memory-specific operations? And if so, are they all equally sensitive to successful retrieval, or are other factors such as error detection also implicated? We investigated this question by testing whether activity in retrieval success regions was associated with task-specific contingencies (i.e., perceived targetness) or mnemonic relevance (e.g., retrieval of source context). To do this, we used a source memory task that required discrimination between remembered targets and remembered nontargets. For a given region, the modulation of neural activity by a situational factor such as target status would suggest a more domain-general role; similarly, modulations of activity linked to error detection would suggest a role inmonitoring and control rather than the accumulation of evidence from memory per se. We found that parietal retrieval success regions exhibited greater activity for items receiving correct than incorrect source responses, whereas frontal retrieval success regions were most active on error trials, suggesting that posterior regions signal successful retrieval whereas frontal regions monitor retrieval outcome. In addition, perceived targetness failed to modulate fMRI activity in any retrieval success region, suggesting that these regions are retrieval specific. We discuss the different functions that these regions may support and propose an accumulator model that captures the different pattern of responses seen in frontal and parietal retrieval success regions