Separating Component Signals of Episodic Simulation Using a Catch Trial Design

Tasks that require mentally simulating events, such as remembering events from one’s past and imagining events from one’s future, have been shown to involve a highly overlapping set of brain regions. Across a growing number of studies, relatively few regions have been found that show differences in activity between remembered and imagined events. However, studies have not disambiguated neural activity related to task orientation: i.e., preparing to remember events from the past or imagine events in the future) from activity related simulating events, per se. The current experiment uses functional MRI and employs a catch trial design to test the hypothesis that by separating orientation and simulation related activity, novel differences might be found between the acts of remembering and imagining events. We find that regions typically shown to activate above baseline in simulation tasks actually deactivate slightly in response to orientation cues, and that by accounting for this activity, regions in bilateral parahippocampal and right retrosplenial cortex show increased activity for the simulation of past events relative to the simulation of future events. This finding suggests that multiple, temporally overlapping processes exist in regions involved in episodic simulation, and that these differences concealed a network of regions sensitive to situations in which information from one’s past is explicitly retrieved

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

The Cognitive Architecture of Spatial Navigation: Hippocampal and Striatal Contributions

Spatial navigation can serve as a model system in cognitive neuroscience, in which specific neural representations, learning rules, and control strategies can be inferred from the vast experimental literature that exists across many species, including humans. Here, we review this literature, focusing on the contributions of hippocampal and striatal systems, and attempt to outline a minimal cognitive architecture that is consistent with the experimental literature and that synthesizes previous related computational modeling. The resulting architecture includes striatal reinforcement learning based on egocentric representations of sensory states and actions, incidental Hebbian association of sensory information with allocentric state representations in the hippocampus, and arbitration of the outputs of both systems based on confidence/uncertainty in medial prefrontal cortex. We discuss the relationship between this architecture and learning in model-free and model-based systems, episodic memory, imagery, and planning, including some open questions and directions for further experiments

Segmentation of experience and episodic memory across species

How continuous ongoing perceptual experience is processed by the brain and mind to form unique episodes in memory is a key scientific question. Recent work in Psychology and Neuroscience has proposed that humans perceptually segment continuous ongoing experience into meaningful units, which allows the successful formation of episodic memories. Despite accumulating work demonstrating that non- human animals also display a capability of episodic-‘like’ memory, whether non-human animals segment continuous ongoing experience into ‘meaningful’ episodic units is a question that has not been fully explored. Hence, the main goal of the research in this thesis aims to address whether a comparable segmentation process (or processes) of continuous ongoing experience occurs for non-human animals in their formation of episodic-like memory, as it does for humans in their formation of episodic memory. Chapter 2 argues that, similarly to humans, rats can use top-down like prediction-error processing in segmenting for subsequent memory to guide behaviour in an episodic-like spontaneous object recognition task. Chapter 3 suggests that mice readily incorporate conspecific-contextual information using episodic-like memory processing, indicating that conspecifics can act as a segmentation cue for non-human animals. Chapter 4 highlights that humans and rodents may similarly segment continuous ongoing experience during turns made around spatial boundaries. Chapter 5 argues that individual place cells can represent content of episodic nature, with the theoretical implication of this being discussed in relation to episodic memory. Thus, the results presented in this thesis, as well as re-interpretation of previous literature, would argue in favour of non-humans segmenting their experience for episodic-like memory. Finally, the evidence is evaluated in the context of whether episodic-like memory in non-human animals is simply just episodic memory as experienced in humans

Forgetting emotional material in working memory

Proactive interference (PI) is the tendency for information learned earlier to interfere with more recently learned information. In the present study, we induced PI by presenting items from the same category over several trials. This results in a build-up of PI and reduces the discriminability of the items in each subsequent trial. We introduced emotional (e.g. disgust) and neutral (e.g. furniture) categories and examined how increasing levels of PI affected performance for both stimulus types. Participants were scanned using functional magnetic resonance imaging (fMRI) performing a 5-item probe recognition task. We modeled responses and corresponding response times with a hierarchical diffusion model. Results showed that PI effects on latent processes (i.e. reduced drift rate) were similar for both stimulus types, but the effect of PI on drift rate was less pronounced PI for emotional compared to neutral stimuli. The decline in the drift rate was accompanied by an increase in neural activation in parahippocampal regions and this relationship was more strongly observed for neutral stimuli compared to emotional stimuli

Content dependence of the neural correlates of recollection: ERP old/new effects for faces, objects and words

As previous research on content-specificity of the neural correlates of recollection is inconclusive, event-related potentials were used to assess old/new effects for faces, objects and words. The data demonstrate temporal differences in ERP old/new effects as a function of item type, supporting the notion that material-dependent processes underlie recollection-related neural activity. The results are discussed in terms of how nameable and non-nameable material elicit different neural representations of mnemonic information, as a consequence of how different item types are encoded and retrieved according to perceptual and contextual content

There is more to memory than recollection and familiarity.

Theoretical models of memory retrieval have focused on processes of recollection and familiarity. Research suggests that there are still other processes involved in memory reconstruction, leading to experiences of knowing and inferring the past. Understanding these experiences, and the cognitive processes that give rise to them, seems likely to further expand our understanding of the neural substrates of memory

Disturbing visual working memory:electrophysiological evidence for a role of the prefrontal cortex in recovery from interference

Single cell recordings in monkeys support the notion that the lateral prefrontal cortex (PFC) controls reactivation of visual working memory representations when rehearsal is disrupted. In contrast, recent fMRI findings yielded a double dissociation for PFC and the medial temporal lobe (MTL) in a letter working memory task. PFC was engaged in interference protection during reactivation while MTL was prominently involved in the retrieval of the letter representations. We present event-related potential data (ERP) that support PFC involvement in the top-down control of reactivation during a visual working memory task with endogenously triggered recovery after visual interference. A differentiating view is proposed for the role of PFC in working memory with respect to endogenous/exogenous control and to stimulus type. General implications for binding and retention mechanisms are discussed

DUAL MECHANISMS OF COGNITIVE CONTROL: AN EYE TRACKING STUDY

The purpose of the present study was to attempt to provide an ocular signature for the dual mechanisms of cognitive control (proactivity and reactivity) by utilizing an eye tracker to record gaze patterns while participants were administered a modified version of the AX-CPT 40. Additionally, we sought to clarify whether context updating or maintenance was responsible for the higher Total Visit Duration (TVD) on the cue location during the ISI that was found in previous studies by providing both a short (1.5 seconds) and long (3 seconds) ISI length. This allowed us to disentangle context updating from maintenance by removing the demand on maintenance with a 1.5 second condition. Our analyses provided conflicting information with the findings from previous studies where TVD on the cue location during the ISI was positively associated with PBI. In the current study, TVD on the cue location during the ISI was negatively associated with PBI. This association is in the opposite direction of what was found in previous experiments. In conclusion, further replication needs to be performed to ascertain the accuracy of these findings

Investigation of Memory Related Cortical Thalamic Circuitry in the Human Brain

This dissertation examined the role of medial prefrontal cortex (mPFC) and the hippocampus (HC) in episodic memory, and provides a novel approach to identify the midline thalamus mediating mPFC-HC interactions in humans. The mPFC and HC are critical to the temporal organization of episodic memory, and these interactions are disrupted in several mental health and neurological disorders. In the first study, I provide evidence that the mPFC is involved in ordinal retrieval, and the HC is active in temporal context retrieval in remembering the order of when events happen. In the second study, I focus on the anatomical basis of the mPFC-HC interactions which is reliant on the midline thalamus. I review in detail the anatomy of the midline thalamus both in location, and connectivity profile with the rest of the brain comparing the extensive anatomical evidence in rodents with the available evidence in monkeys and humans. This section also elaborates on the role of the midline thalamus in memory, stress regulation, wakefulness, and feeding behavior, and how pathological markers along the midline thalamus are a vanguard of several neurological disorders including Alzheimer’s Disease, schizophrenia, depression, and drug addiction. Lastly, I devised a new approach to identify the midline thalamus in humans in vivo using diffusion weighted imaging, capitalizing on known fiber connections gleaned from non-human animals, focusing on connections between the midline thalamus and the mPFC, medial temporal lobe and the nucleus accumbens. The success of this approach is promising for translational imaging. Overall, this dissertation provides new evidence on 1) complementary functional roles of the mPFC and HC in sequence memory, 2) a cross-species anatomical framework for understanding the midline thalamus in humans and neurological disorders, and 3) a new method for non-invasive identification of the midline thalamus in humans in vivo. Thus, this dissertation provides a new fundamental understanding of mPFC-midline thalamic-HC circuit in humans and tools for its non-invasive study in human disease