Rodent mnemonic similarity task performance requires the prefrontal cortex

The effects of moderate prenatal alcohol exposure on performance in object and spatial discrimination tasks by adult male rats

Exposure to alcohol during pregnancy produces Fetal Alcohol Spectrum Disorders, which in its most severe form is characterized by physical dysmorphology and neurobehavioral alterations. Moderate prenatal alcohol exposure (mPAE) is known to produce deficits in discrimination of spatial locations in adulthood. However, the impact of mPAE on higher-order sensory representations, such as discrimination of perceptually similar stimuli, is currently unknown. In the present study, we tested the hypothesis that mPAE would disrupt performance on hippocampal-sensitive tasks that require discrimination between perceptually similar objects or discrimination between spatial locations in a radial arm maze. Here we report that male mPAE rats exhibited intact performance on three types of object discrimination tasks: one in which rats discriminated between distinct toy objects, a second in which discrimination was made between distinct and similar LEGO objects, and a mnemonic similarity task in which rats discriminated between randomly presented LEGO objects that varied in similarity with a learned object. Although adult male mPAE rats performed similarly to control rats on all three object discrimination tasks, they showed deficits when tested in a radial arm maze spatial discrimination task. Specifically, male mPAE rats expressed a significantly higher number of working memory errors (returns to previously visited arms) and were more likely to use non-spatial strategies during training. Together, the findings of the present study support the conclusion that mPAE produces specific deficits in the online processing of spatial information and executing spatial navigation strategies, but spares the ability to discriminate between perceptually similar stimuli.

The Anatomy of Context

For most of my career, I focused on understanding how and where spatial context, the place where things happen, is represented in the brain. My interest in this began in the early 1990's, during my postdoctoral training with David Amaral, when we defined the rodent homolog of the primate parahippocampal cortex, a region implicated in processing spatial and contextual information. We parceled out the caudal portion of the rat perirhinal cortex (PER) and called it the postrhinal cortex (POR). In my own lab at Brown University, I continued to study the anatomy of the PER, POR, and entorhinal cortices. I also began to characterize and differentiate the functions of these regions, particularly the newly defined POR and the redefined PER. Our electrophysiological and behavioral evidence supports a view of POR function that aligns with our anatomical evidence. Briefly, the POR integrates object and feature information from the PER with spatial information from the retrosplenial, posterior parietal, and secondary visual cortices and the pulvinar and uses this information to represent specific environmental contexts, including the spatial arrangement of objects and features within each context. In addition to maintaining a representation of the current context, the POR plays an attentional role by continually monitoring the context for changes and updating the context representation when changes occur. This context representation is accessible to other regions for cognitive processes, including binding life events with context to form episodic memories, guiding context-relevant behavior, and recognizing objects within scenes and contexts.

Ventromedial Prefrontal Cortex Does Not Play a Selective Role in Pattern Separation

Humans have the capacity to form new memories of events that are, at times, highly similar to events experienced in the past, as well as the capacity to integrate and associate new information within existing knowledge structures. The former process relies on mnemonic discrimination and is believed to depend on hippocampal pattern separation, whereas the latter is believed to depend on generalization signals and conceptual categorization supported by the neocortex. Here, we examine whether and how the ventromedial prefrontal cortex (vMPFC) supports discrimination and generalization on a widely used task that was primarily designed to tax hippocampal processes. Ten individuals with lesions to the vMPFC and 46 neurotypical control participants were administered an adapted version of the mnemonic similarity task [Stark, S. M., Yassa, M. A., Lacy, J. W., & Stark, C. E. L. A task to assess behavioral pattern separation (BPS) in humans: Data from healthy aging and mild cognitive impairment. Neuropsychologia, 51, 2442-2449, 2013], which assesses the ability to distinguish previously learned images of everyday objects (targets) from unstudied, highly similar images (lures) and dissimilar images (foils). Relative to controls, vMPFC-lesioned individuals showed intact discrimination of lures from targets but a propensity to mistake studied targets and similar lures for dissimilar foils. This pattern was accompanied by inflated confidence despite low accuracy when responding to similar lures. These findings demonstrate a more general role of the vMPFC in memory retrieval, rather than a specific role in supporting pattern separation.

Demands on perceptual and mnemonic fidelity are a key determinant of age-related cognitive decline throughout the lifespan

Aging results in less detailed memories, reflecting reduced fidelity of remembered compared to real-world representations. We tested whether poorer representational fidelity across perception, short-term memory (STM), and long-term memory (LTM) are among the earliest signs of cognitive aging. Our paradigm probed target-lure object mnemonic discrimination and precision of object-location binding. Across the lifespan, cognitive deficits were observed in midlife when detailed stimulus representations were required for perceptual and short/long-term forced choice mnemonic discrimination. A continuous metric of object-location source memory combined with computational modeling demonstrated that errors in STM and LTM in middle-aged adults were largely driven by a loss of precision for retrieved memories, not necessarily by forgetting. On a trial-by-trial basis, fidelity of item and spatial information was more tightly bound in LTM compared to STM with this association being unaffected by age. Standard neuropsychological tests without demands on memory quality (digit span, verbal learning) were less sensitive to age effects than STM and LTM precision. Perceptual discrimination predicted mnemonic discrimination. Neuropsychological proxies for prefrontal executive functions correlated with STM, but not LTM fidelity. Conversely, neuropsychological indicators of hippocampal integrity correlated with mnemonic discrimination and precision of both STM and LTM, suggesting partially dissociable mechanisms of interindividual variability in STM and LTM fidelity. These findings suggest that reduced representational fidelity is a hallmark of cognitive aging across perception, STM, and LTM and can be observed from midlife onward. Continuous memory precision tasks may be promising for the early detection of subtle age-related cognitive decline. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Muscimol inactivation of dorsal striatum in young and aged male rats does not affect paired associates learning performance

Improving cognitive health for older adults requires understanding the neurobiology of age-related cognitive decline and the mechanisms underlying preserved cognition in old age. During spatial learning tasks, aged humans and rodents shift navigation preferences in favor of a stimulus-response learning strategy. This has been hypothesized to result from competitive interactions of the caudate nucleus/dorsal striatum (DS) memory system with the hippocampus (HPC)-dependent spatial/allocentric memory system. In support of this hypothesis, a recent study reported that inactivation of the DS in aged rodents rescued HPC-dependent spatial learning on a T-maze (Gardner, Gold, & Korol, 2020). Currently, it is unclear whether a shift from HPC-dependent to DS-dependent behavior also contributes to age-related cognitive decline outside of spatial learning and memory. To test the hypothesis that inactivation of the DS can restore age-related cognitive function outside of spatial behavior, the present study bilaterally inactivated the DS of young (n = 8) and aged (n = 7) rats during visuospatial paired associates learning (PAL). This study found that inactivation of the DS did not alter PAL performance in young or aged rats, but did alter a positive control, DS-dependent spatial navigation task. This observation suggests that elevated DS activity does not play a role in the decline of HPC-dependent PAL performance in aged male rats. Given the persistent tendencies of aged rodents toward DS-dependent learning, it will be worthwhile to explore further the coordination dynamics between the HPC and DS that may contribute to age-related cognitive decline. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Functional Dissection of Ipsilateral and Contralateral Neural Activity Propagation Using Voltage-Sensitive Dye Imaging in Mouse Prefrontal Cortex

Prefrontal cortex (PFC) intrahemispheric activity and the interhemispheric connection have a significant impact on neuropsychiatric disorder pathology. This study aimed to generate a functional map of FC intrahemispheric and interhemispheric connections. Functional dissection of mouse PFCs was performed using the voltage-sensitive dye (VSD) imaging method with high speed (1 ms/frame), high resolution (256 × 256 pixels), and a large field of view (∼10 mm). Acute serial 350 μm slices were prepared from the bregma covering the PFC and numbered 1-5 based on their distance from the bregma (i.e., 1.70, 1.34, 0.98, 0.62, and 0.26 mm) with reference to the Mouse Brain Atlas (Paxinos and Franklin, 2008). The neural response to electrical stimulation was measured at nine sites and then averaged, and a functional map of the propagation patterns was created. Intracortical propagation was observed in slices 3-5, encompassing the anterior cingulate cortex (ACC) and corpus callosum (CC). The activity reached area 33 of the ACC. Direct white matter stimulation activated area 33 in both hemispheres. Similar findings were obtained via DiI staining of the CC. Imaging analysis revealed directional biases in neural signals traveling within the ACC, whereby the signal transmission speed and probability varied based on the signal direction. Specifically, the spread of neural signals from cg2 to cg1 was stronger than that from cingulate cortex area 1(cg1) to cingulate cortex area 2(cg2), which has implications for interhemispheric functional connections. These findings highlight the importance of understanding the PFC functional anatomy in evaluating neuromodulators like serotonin and dopamine, as well as other factors related to neuropsychiatric diseases.

Parsing memory and nonmemory contributions to age-related declines in mnemonic discrimination performance: a hierarchical Bayesian diffusion decision modeling approach

The mnemonic discrimination task (MDT) is a widely used cognitive assessment tool. Performance in this task is believed to indicate an age-related deficit in episodic memory stemming from a decreased ability to pattern-separate among similar experiences. However, cognitive processes other than memory ability might impact task performance. In this study, we investigated whether nonmnemonic decision-making processes contribute to the age-related deficit in the MDT. We applied a hierarchical Bayesian version of the Ratcliff diffusion model to the MDT performance of 26 younger and 31 cognitively normal older adults. It allowed us to decompose decision behavior in the MDT into different underlying cognitive processes, represented by specific model parameters. Model parameters were compared between groups, and differences were evaluated using the Bayes factor. Our results suggest that the age-related decline in MDT performance indicates a predominantly mnemonic deficit rather than differences in nonmnemonic decision-making processes. In addition, this mnemonic deficit might also involve a slowing in processes related to encoding and retrieval strategies, which are relevant for successful memory as well. These findings help to better understand what cognitive processes contribute to the age-related decline in MDT performance and may help to improve the diagnostic value of this popular task.

Executive Function Predicts Older Adults' Lure Discrimination Difficulties on the Mnemonic Similarity Task

OBJECTIVES

Older adults often have difficulty remembering the details of recently encountered objects. We previously found this with the Mnemonic Similarity Task (MST). Surprisingly, the older adults' MST Lure Discrimination Index (LDI) was significantly correlated with visual acuity but not with memory or executive function. Here we ran a replication with new, larger samples of young (N = 45) and older adults (N = 70). We then combined the original and replication older adult samples (N = 108) to critically examine the relative contributions of visual acuity, memory, and executive function composite scores to LDI performance using dominance analysis. This provided, to our knowledge, the first direct statistical comparison of all 3 of these factors and their interactions on LDI.

METHODS

Participants completed the MST and a battery assessing visual acuity, memory, and executive function. We examined age group differences on MST performance in the new (i.e., replication) young and older adult samples and performed multiple regression and dominance analysis on the combined older adult sample.

RESULTS

Consistent with previous findings, the older adults showed significantly poorer LDI but preserved item recognition. LDI was significantly correlated with both memory and executive function but not with visual acuity. In the combined older adult sample, all 3 composites predicted LDI, but dominance analysis indicated that executive function was the most important predictor.

DISCUSSION

Older adults' MST LDI difficulty may be predicted by their executive function and visual acuity. These factors should be considered when interpreting older adults' MST performance.

Extra-hippocampal contributions to pattern separation

Pattern separation, or the process by which highly similar stimuli or experiences in memory are represented by non-overlapping neural ensembles, has typically been ascribed to processes supported by the hippocampus. Converging evidence from a wide range of studies, however, suggests that pattern separation is a multistage process supported by a network of brain regions. Based on this evidence, considered together with related findings from the interference resolution literature, we propose the 'cortico-hippocampal pattern separation' (CHiPS) framework, which asserts that brain regions involved in cognitive control play a significant role in pattern separation. Particularly, these regions may contribute to pattern separation by (1) resolving interference in sensory regions that project to the hippocampus, thus regulating its cortical input, or (2) directly modulating hippocampal processes in accordance with task demands. Considering recent interest in how hippocampal operations are modulated by goal states likely represented and regulated by extra-hippocampal regions, we argue that pattern separation is similarly supported by neocortical-hippocampal interactions.

Intrinsic functional connectivity in the default mode network predicts mnemonic discrimination: A connectome-based modeling approach

The ability to distinguish existing memories from similar perceptual experiences is a core feature of episodic memory. This ability is often examined using the mnemonic similarity task in which people discriminate memories of studied objects from perceptually similar lures. Studies of the neural basis of such mnemonic discrimination have mostly focused on hippocampal function and connectivity. However, default mode network (DMN) connectivity may also support such discrimination, given that the DMN includes the hippocampus, and its connectivity supports many aspects of episodic memory. Here, we used connectome-based predictive modeling to identify associations between intrinsic DMN connectivity and mnemonic discrimination. We leveraged a wide range of abilities across healthy younger and older adults to facilitate this predictive approach. Resting-state functional connectivity in the DMN predicted mnemonic discrimination outside the MRI scanner, especially among prefrontal and temporal regions and including several hippocampal regions. This predictive relationship was stronger for younger than older adults, primarily for temporal-prefrontal connectivity. The novel associations established here are consistent with mounting evidence that broader cortical networks including the hippocampus support mnemonic discrimination. They also suggest that age-related network disruptions undermine the extent that the DMN supports this ability. This study provides the first indication of how intrinsic functional properties of the DMN support mnemonic discrimination.

Entorhinal cortical Island cells regulate temporal association learning with long trace period

Temporal association learning (TAL) allows for the linkage of distinct, nonsynchronous events across a period of time. This function is driven by neural interactions in the entorhinal cortical-hippocampal network, especially the neural input from the pyramidal cells in layer III of medial entorhinal cortex (MECIII) to hippocampal CA1 is crucial for TAL. Successful TAL depends on the strength of event stimuli and the duration of the temporal gap between events. Whereas it has been demonstrated that the neural input from pyramidal cells in layer II of MEC, referred to as Island cells, to inhibitory neurons in dorsal hippocampal CA1 controls TAL when the strength of event stimuli is weak, it remains unknown whether Island cells regulate TAL with long trace periods as well. To understand the role of Island cells in regulating the duration of the learnable trace period in TAL, we used Pavlovian trace fear conditioning (TFC) with a 60-sec long trace period (long trace fear conditioning [L-TFC]) coupled with optogenetic and chemogenetic neural activity manipulations as well as cell type-specific neural ablation. We found that ablation of Island cells in MECII partially increases L-TFC performance. Chemogenetic manipulation of Island cells causes differential effectiveness in Island cell activity and leads to a circuit imbalance that disrupts L-TFC. However, optogenetic terminal inhibition of Island cell input to dorsal hippocampal CA1 during the temporal association period allows for long trace intervals to be learned in TFC. These results demonstrate that Island cells have a critical role in regulating the duration of time bridgeable between associated events in TAL.