Functional Differentiation of Dorsal and Ventral Posterior Parietal Cortex of the Rat: Implications for Controlled and Stimulus-Driven Attention.

The posterior parietal cortex (PPC) is important for visuospatial attention. The primate PPC shows functional differentiation such that dorsal areas are implicated in top-down, controlled attention, and ventral areas are implicated in bottom-up, stimulus-driven attention. Whether the rat PPC also shows such functional differentiation is unknown. Here, we address this open question using functional neuroanatomy and in vivo electrophysiology. Using conventional tract-tracing methods, we examined connectivity with other structures implicated in visuospatial attention including the lateral posterior nucleus of the thalamus (LPn) and the postrhinal cortex (POR). We showed that the LPn projects to the entire PPC, preferentially targeting more ventral areas. All parts of the PPC and POR are reciprocally connected with the strongest connections evident between ventral PPC and caudal POR. Next, we simultaneously recorded neuronal activity in dorsal and ventral PPC as rats performed a visuospatial attention (VSA ) task that engages in both bottom-up and top-down attention. Previously, we provided evidence that the dorsal PPC is engaged in multiple cognitive process including controlled attention (Yang et al. 2017). Here, we further showed that ventral PPC cells respond to stimulus onset more rapidly than dorsal PPC cells, providing evidence for a role in stimulus-driven, bottom-up attention.

Postrhinal cortex contributions to the expression of auditory fear conditioning.

The postrhinal cortex (POR), the rodent homologue of the primate parahippocampal cortex (PHC), has been implicated in contextual and spatial processing. For instance, prior studies have demonstrated that permanent lesions of POR impair contextual fear conditioning. In contrast, permanent lesions of POR, specifically prior to training, do not impact auditory fear conditioning. In the current experiments, we examined the role of POR in the expression of auditory fear conditioning by using chemogenetics to silence neural activity in POR at the time of retrieval testing. Considering that extinction is context-dependent, and POR contributes to contextual memory, we hypothesized that POR would be necessary for expression of auditory fear conditioning following extinction. We found that POR inactivation during retrieval impaired freezing to an auditory cue that was tested in the conditioning context (A) after it had been extinguished in a different context (B). However, the involvement of POR was not specific to extinction. POR inactivation also impaired freezing to an auditory fear cue that had not undergone extinction. Thus, while prior studies have identified a role for POR in contextual fear conditioning, the current findings extend the functional role of POR to include the expression of auditory fear conditioning.

Beyond the hippocampus: The role of parahippocampal-prefrontal communication in context-modulated behavior.

Multiple paradigms indicate that the physical environment can influence spontaneous and learned behavior. In rodents, context-dependent behavior is putatively supported by the prefrontal cortex and the medial temporal lobe. A preponderance of the literature has targeted the role of the hippocampus. In addition to the hippocampus proper, the medial temporal lobe also comprises parahippocampal areas, including the perirhinal and postrhinal cortices. These parahippocampal areas directly connect with multiple regions in the prefrontal cortex. The function of these connections, however, is not well understood. This article first reviews the involvement of the perirhinal, postrhinal, and prefrontal cortices in context-dependent behavior in rodents. Then, based on functional and anatomical evidence, we suggest that perirhinal and postrhinal contributions to context-dependent behavior go beyond supporting context representation in the hippocampus. Specifically, we propose that the perirhinal and postrhinal cortices act as a contextual-support network that directly provides contextual and spatial information to the prefrontal cortex. In turn, the perirhinal and postrhinal cortices modulate prefrontal input to the hippocampus in the service of context-guided behavior.

Disconnection of the Perirhinal and Postrhinal Cortices Impairs Recognition of Objects in Context But Not Contextual Fear Conditioning.

The perirhinal cortex (PER) is known to process object information, whereas the rodent postrhinal cortex (POR), homolog to the parahippocampal cortex in primates, is thought to process spatial information. A number of studies, however, provide evidence that both areas are involved in processing contextual information. In this study, we tested the hypothesis that the rat POR relies on object information received from the PER to form complex representations of context. Using three fear-conditioning (FC) paradigms (signaled, unsignaled, and renewal) and two context-guided object recognition tasks (with 3D and 2D objects), we examined the effects of crossed excitotoxic lesions to the POR and the contralateral PER. Performance of rats with crossed lesions was compared with that of rats with ipsilateral POR plus PER lesions and sham-operated rats. We found that rats with contralateral PER-POR lesions were impaired in object-context recognition but not in contextual FC. Therefore, interaction between the POR and PER is necessary for context-guided exploratory behavior but not for associating fear with context. Our results provide evidence for the hypothesis that the POR relies on object and pattern information from the PER to encode representations of context. The association of fear with a context, however, may be supported by alternate cortical and/or subcortical pathways when PER-POR interaction is not available. Our results suggest that contextual FC may represent a special case of context-guided behavior.SIGNIFICANCE STATEMENT Representations of context are important for perception, memory, decision making, and other cognitive processes. Moreover, there is extensive evidence that the use of contextual representations to guide appropriate behavior is disrupted in neuropsychiatric and neurological disorders including developmental disorders, schizophrenia, affective disorders, and Alzheimer's disease. Many of these disorders are accompanied by changes in parahippocampal and hippocampal structures. Understanding how context is represented in the brain and how parahippocampal structures are involved will enhance our understanding and treatment of the cognitive and behavioral symptoms associated with neurological disorders and neuropsychiatric disease.

Single neuron activity and theta modulation in the posterior parietal cortex in a visuospatial attention task.

The posterior parietal cortex (PPC) is implicated in directing and maintaining visual attention to locations in space. We hypothesized that the PPC also engages other cognitive processes in the transformation of behaviorally relevant visual inputs into appropriate actions, for example, monitoring of multiple locations, selection of responses to locations in space, and monitoring the outcome of response selections. We recorded single cells and local field potentials in the rat PPC during performance on a novel visuospatial attention (VSA) task that requires visually monitoring locations in space in order to make appropriate stimulus-guided locomotor responses. In each trial, rats attended to four locations on the floor of a maze. A randomly chosen location was briefly illuminated. Approach to the correct target location was followed by food reward. We observed that PPC activity correlated with multiple phases of the VSA task, including monitoring for stimulus onset, detection of a target, spatial location of the target, and target choice. A substantial proportion of cells with behavioral correlates were also modulated by outcome of the trial. Our analyses of local field potentials revealed strong oscillatory rhythms in the theta frequency band, and more than a third of PPC neurons were phase locked to theta oscillations. As in other brain regions, theta power correlated with running speed. Peak theta power was higher in superficial layers than deep layers providing evidence against volume conduction from the hippocampus. In addition, theta power was sensitive to the outcome of a choice. Theta power was significantly higher following incorrect choices compared with correct choices, possibly providing a prediction error signal. Our study provides evidence that the rat PPC has multiple roles in the translation of visual information into appropriate behavioral actions. © 2016 Wiley Periodicals, Inc.

Bidirectional Modulation of Recognition Memory.

Perirhinal cortex (PER) has a well established role in the familiarity-based recognition of individual items and objects. For example, animals and humans with perirhinal damage are unable to distinguish familiar from novel objects in recognition memory tasks. In the normal brain, perirhinal neurons respond to novelty and familiarity by increasing or decreasing firing rates. Recent work also implicates oscillatory activity in the low-beta and low-gamma frequency bands in sensory detection, perception, and recognition. Using optogenetic methods in a spontaneous object exploration (SOR) task, we altered recognition memory performance in rats. In the SOR task, normal rats preferentially explore novel images over familiar ones. We modulated exploratory behavior in this task by optically stimulating channelrhodopsin-expressing perirhinal neurons at various frequencies while rats looked at novel or familiar 2D images. Stimulation at 30-40 Hz during looking caused rats to treat a familiar image as if it were novel by increasing time looking at the image. Stimulation at 30-40 Hz was not effective in increasing exploration of novel images. Stimulation at 10-15 Hz caused animals to treat a novel image as familiar by decreasing time looking at the image, but did not affect looking times for images that were already familiar. We conclude that optical stimulation of PER at different frequencies can alter visual recognition memory bidirectionally. Significance statement: Recognition of novelty and familiarity are important for learning, memory, and decision making. Perirhinal cortex (PER) has a well established role in the familiarity-based recognition of individual items and objects, but how novelty and familiarity are encoded and transmitted in the brain is not known. Perirhinal neurons respond to novelty and familiarity by changing firing rates, but recent work suggests that brain oscillations may also be important for recognition. In this study, we showed that stimulation of the PER could increase or decrease exploration of novel and familiar images depending on the frequency of stimulation. Our findings suggest that optical stimulation of PER at specific frequencies can predictably alter recognition memory.

Subcortical connections of the perirhinal, postrhinal, and entorhinal cortices of the rat. I. afferents.

In this study the subcortical afferents for the rat PER areas 35 and 36, POR, and the lateral and medial entorhinal areas (LEA and MEA) were characterized. We analyzed 33 retrograde tract-tracing experiments distributed across the five regions. For each experiment, we estimated the total numbers, percentages, and densities of labeled cells in 36 subcortical structures and nuclei distributed across septum, basal ganglia, claustrum, amygdala, olfactory structures, thalamus, and hypothalamus. We found that the complement of subcortical inputs differs across the five regions, especially the PER and POR. The PER receives input from the reuniens, suprageniculate, and medial geniculate thalamic nuclei as well as the amygdala. Overall, the subcortical inputs to the PER were consistent with a role in perception, multimodal processing, and the formation of associations that include the motivational significance of individual items and objects. Subcortical inputs to the POR were dominated by the dorsal thalamus, particularly the lateral posterior nucleus, a region implicated in visuospatial attention. The complement of subcortical inputs to the POR is consistent with a role in representing and monitoring the local spatial context. We also report that, in addition to the PER, the LEA and the medial band of the MEA also receive strong amygdala input. In contrast, subcortical input to the POR and the MEA lateral band includes much less amygdala input and is dominated by dorsal thalamic nuclei, particularly nuclei involved in spatial information processing. Thus, some subcortical inputs are consistent with the view that there is functional differentiation along the septotemporal axis of the hippocampus, but others provide considerable integration. Overall, we conclude that the patterns of subcortical inputs to the PER, POR, and the entorhinal LEA and MEA provide further evidence for functional differentiation in the medial temporal lobe. © 2016 Wiley Periodicals, Inc.

Subcortical connections of the perirhinal, postrhinal, and entorhinal cortices of the rat. II. efferents.

This is the second of two studies detailing the subcortical connections of the perirhinal (PER), the postrhinal (POR) and entorhinal (EC) cortices of the rat. In the present study, we analyzed the subcortical efferents of the rat PER areas 35 and 36, POR, and the lateral and medial entorhinal areas (LEA and MEA). Anterograde tracers were injected into these five regions, and the resulting density of fiber labeling was quantified in an extensive set of subcortical structures. Density and topography of fiber labeling were quantitatively assessed in 36 subcortical areas, including olfactory structures, claustrum, amygdala nuclei, septal nuclei, basal ganglia, thalamic nuclei, and hypothalamic structures. In addition to reporting the density of labeled fibers, we incorporated a new method for quantifying the size of anterograde projections that takes into account the volume of the target subcortical structure as well as the density of fiber labeling. The PER, POR, and EC displayed unique patterns of projections to subcortical areas. Interestingly, all regions examined provided strong input to the basal ganglia, although the projections arising in the PER and LEA were stronger and more widespread. PER areas 35 and 36 exhibited similar pattern of projections with some differences. PER area 36 projects more heavily to the lateral amygdala and much more heavily to thalamic nuclei including the lateral posterior nucleus, the posterior complex, and the nucleus reuniens. Area 35 projects more heavily to olfactory structures. The LEA provides the strongest and most widespread projections to subcortical structures including all those targeted by the PER as well as the medial and posterior septal nuclei. POR shows fewer subcortical projections overall, but contributes substantial input to the lateral posterior nucleus of the thalamus. The MEA projections are even weaker. Our results suggest that the PER and LEA have greater influence over olfactory, amygdala, and septal nuclei, whereas PER area 36 and the POR have greater influence over thalamic nuclei. © 2016 Wiley Periodicals, Inc.

Objects and landmarks: hippocampal place cells respond differently to manipulations of visual cues depending on size, perspective, and experience.

Human navigation studies show that landmarks are used for orientation, whereas objects contribute to the contextual representation of an environment. What constitutes a landmark? Classic rodent studies show that hippocampal place fields are controlled by distal, polarizing cues. Place fields, however, are also influenced by local cues. One difficulty in examining mechanisms by which distal and local cues influence the activity of hippocampal cells is that distant cues are necessarily processed visually, whereas local cues are generally multimodal. Here, we compared the effects of 90° rotations under different cue conditions in which cues were restricted to the visual modality. Three two-dimensional visual cue conditions were presented in a square open field: a large vertical cue on one wall, a large floor cue in a corner abutting two walls, and a smaller complex floor cue in a corner adjacent to two walls. We show that rotations of large distal cues, whether on the wall or floor, were equally effective in controlling place fields. Rotations of the smaller floor cues were significantly more likely to result in remapping, whether or not animals were also exposed to the distal polarizing cues. Responses of distal and local cues were affected differently by extended experience. Our data provide evidence that hippocampal place cell responses to visual cues are influenced by perspective, salience of the cue, and prior experience. The hippocampus processes visual cues either as stable landmarks useful for orientation and navigation or as nonstationary objects or features of the local environment available for associative learning or binding items in context.

Borders and comparative cytoarchitecture of the perirhinal and postrhinal cortices in an F1 hybrid mouse.

We examined the cytoarchitectonic and chemoarchitectonic organization of the cortical regions associated with the posterior rhinal fissure in the mouse brain, within the framework of what is known about these regions in the rat. Primary observations were in a first-generation hybrid mouse line, B6129PF/J1. The F1 hybrid was chosen because of the many advantages afforded in the study of the molecular and cellular bases of learning and memory. Comparisons with the parent strains, the C57BL6/J and 129P3/J are also reported. Mouse brain tissue was processed for visualization of Nissl material, myelin, acetyl cholinesterase, parvalbumin, and heavy metals. Tissue stained for heavy metals by the Timm's method was particularly useful in the assignment of borders and in the comparative analyses because the patterns of staining were similar across species and strains. As in the rat, the areas examined were parcellated into 2 regions, the perirhinal and the postrhinal cortices. The perirhinal cortex was divided into areas 35 and 36, and the postrhinal cortex was divided into dorsal (PORd) and ventral (PORv) subregions. In addition to identifying the borders of the perirhinal cortex, we were able to identify a region in the mouse brain that shares signature features with the rat postrhinal cortex.

Transcranial direct current stimulation impairs updating of avoidance-based associative learning.

Introduction: Exposure-based psychotherapies for the treatment of anxiety- and fear-based disorders rely on "corrective" associative learning. Namely the repeated confrontation with feared stimuli in the absence of negative outcomes allows the formation of new, corrected associations of safety, indicating that such stimuli no longer need to be avoided. Unfortunately, exposure-facilitated corrective learning tends to be bound by context and often poorly generalizes. One brain structure, the prefrontal cortex, is implicated in context-guided behavior and may be a relevant target for improving generalization of safety learning. Here, we tested whether inhibition of the left prefrontal cortex causally impaired updating of context-bound associations specifically or, alternatively, impaired updating of learned associations irrespective of contextual changes. Additionally, we tested whether prefrontal inhibition during corrective learning influenced subsequent generalization of associations to a novel context. Methods: In two separate experiments, participants received either 10 min of 2 mA cathodal transcranial direct current stimulation (tDCS) over EEG coordinate F3 (Experiment 1 n = 9, Experiment 2 n = 22) or sham stimulation (Experiment 1 n = 10, Experiment 2 n = 22) while previously learned associations were reversed in the same or a different context from initial learning. Next, to assess generalization of learning, participants were asked to indicate which of the previously seen images they preferred in a novel, never seen before context. Results: Results indicate that tDCS significantly impaired reversal irrespective of context in Experiment 2 only. When taking learning rate across trials into account, both experiments suggest that participants who received sham had the greatest learning rate when reversal occurred in a different context, as expected, whereas participants who received active tDCS in this condition had the lowest learning rate. However, active tDCS was associated with preferring the originally disadvantageous, but then neural stimulus after stimulus after reversal occurred in a different context in Experiment 1 only. Discussion: These results support a causal role for the left prefrontal cortex in the updating of avoidance-based associations and encourage further inquiry investigating the use of non-invasive brain stimulation on flexible updating of learned associations.

The effects of combined perirhinal and postrhinal damage on complex discrimination tasks.

Rats with combined lesions of the perirhinal (PER) and postrhinal (POR) cortices were trained on a complex discrimination in the simultaneous feature-positive and feature-negative discrimination task. In this task, a panel light (L) paired with an auditory stimulus determined whether a tone (T) or white noise (N) would be rewarded (+) or not rewarded (-). Thus, the light feature determined whether the target auditory stimuli were rewarded or not. In each session, trial types were LT+, T-, N+, and LN-. We had hypothesized that damage to the target regions would impair performance on this task. Acquisition was altered in the lesioned rats, but not in the predicted direction. Instead, lesioned rats exhibited significantly enhanced acquisition of the discrimination. Manipulation of intertrial intervals indicated that reduction of proactive interference did not explain the enhancement. Lesioned rats were not different from controls on a multiple-cued interval timing task, providing evidence that the enhancement does not extend to all types of discriminations and is not due to a deficit in timing. Other research shows that rats with PER lesions are impaired on similar tasks, thus the enhancement is likely due to the effects of POR damage. Normally in this task, context is thought to accrue inhibitory control over other cues. Without this inhibitory control, animals might be expected to learn the task more efficiently. Our conclusion is that deficits in processing contextual information underlie the enhanced acquisition observed in rats with combined PER and POR damage on this complex discrimination task.

Electrophysiological and morphological properties of neurons in layer 5 of the rat postrhinal cortex.

The postrhinal (POR) cortex of the rat is homologous to the parahippocampal cortex of the primate based on connections and other criteria. POR provides the major visual and visuospatial input to the hippocampal formation, both directly to CA1 and indirectly through connections with the medial entorhinal cortex. Although the cortical and hippocampal connections of the POR cortex are well described, the physiology of POR neurons has not been studied. Here, we examined the electrical and morphological characteristics of layer 5 neurons from POR cortex of 14- to 16-day-old rats using an in vitro slice preparation. Neurons were subjectively classified as regular-spiking (RS), fast-spiking (FS), or low-threshold spiking (LTS) based on their electrophysiological properties and similarities with neurons in other regions of neocortex. Cells stained with biocytin included pyramidal cells and interneurons with bitufted or multipolar dendritic patterns. Similarity analysis using only physiological data yielded three clusters that corresponded to FS, LTS, and RS classes. The cluster corresponding to the FS class was composed entirely of multipolar nonpyramidal cells, and the cluster corresponding to the RS class was composed entirely of pyramidal cells. The third cluster, corresponding to the LTS class, was heterogeneous and included both multipolar and bitufted dendritic arbors as well as one pyramidal cell. We did not observe any intrinsically bursting pyramidal cells, which is similar to entorhinal cortex but unlike perirhinal cortex. We conclude that POR includes at least two major classes of neocortical inhibitory interneurons, but has a functionally restricted cohort of pyramidal cells.

The anatomy and function of the postrhinal cortex.

The parahippocampal cortex (PHC) in the primate brain is implicated in the medial temporal lobe (MTL) memory network for spatial and episodic memory, but the precise function of this region remains unclear. Importantly, the rodent postrhinal cortex (POR) provides a structural and connectional homolog to the primate PHC. This homology permits the use of the powerful tools available in rodent models to better understand the function of the PHC in the human and nonhuman primate brains. Although many articles have compared and dissociated the function of the rodent POR from other areas in the MTL implicated in learning, memory, and memory-guided behavior, there are no in-depth reviews, particularly covering the last two decades of research. Nor has there been a review of the literature on the potential role of the POR in attention. Here, we review the anatomical and functional connectivity of the POR in rats, examine the evidence for proposed behavioral functions of this region, and suggest a model that accounts for the array of observations. We propose that the rodent POR binds nonspatial information and spatial information to represent the current local physical environment or context, including the geometry of the space and the spatial layout of objects and features in the environment. The POR also automatically monitors the environment for changes and updates representations when changes occur. These representations of context are available to be used by multiple brain regions, including prefrontal, posterior cortical, and hippocampal areas, for context-guided behavior, associative learning, and episodic memory. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Remembering David Bucci.

In the year of 2021, we are honoring David J. Bucci, our beloved and respected colleague, who died at the age of 50 in 2019. In the Special Section entitled Remembering David Bucci, we reprint 6 of the 29 articles that Dave published in Behavioral Neuroscience beginning with the first one in 1995. Of the 29 articles, these 6 were chosen in an attempt to span Dave's research interests and to highlight a few of his many collaborators and students. These articles are meant to represent his interests in the functions of the hippocampus, surrounding brain regions, and beyond, particularly with regard to memory, attention, and other cognitive functions. Reprinting these articles is a small tribute to the many contributions David Bucci made to behavioral neuroscience through his research, the many students he has trained and mentored, and his leadership in the field. In this way, we honor the life and contributions of David J. Bucci (1968-2019). (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Contributions of postrhinal and perirhinal cortex to contextual information processing.

The role of the postrhinal cortex (POR) and the perirhinal cortex (PER) in processing relational or contextual information was examined with Pavlovian fear conditioning. Rats with electrolytic or neurotoxic lesions of the POR or PER were tested in 2 contextual fear conditioning paradigms. In Experiment 1, electrolytic lesions of the POR or PER produced impairments in contextual fear conditioning but not in conditioning to a phasic auditory conditioned stimulus. Neurotoxic lesions of the POR or PER likewise resulted in anterograde (Experiment 2) and retrograde (Experiment 3) deficits in fear conditioning to the training context in an unsignaled shock paradigm. The results suggest that operations performed on sensory information by the POR and PER are necessary to support contextual learning. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Perirhinal and Postrhinal Damage Have Different Consequences on Attention as Assessed in the Five-Choice Serial Reaction Time Task.

The perirhinal (PER) and postrhinal (POR) cortices, structures in the medial temporal lobe, are implicated in learning and memory. The PER is understood to process object information and the POR to process spatial or contextual information. Whether the medial temporal lobe is dedicated to memory, however, is under debate. In this study, we addressed the hypothesis that the PER and POR are also involved in non-mnemonic cognitive functions. Rats with PER or POR damage and SHAM surgical controls were shaped, trained, and tested on the five-choice serial reaction time (5CSRT) task, which assesses attention and executive function. Rats with PER damage were impaired in acquiring the task and at asymptote, although processing information about objects was not relevant to the task. When confronted with attentional challenges, rats with PER damage showed a pattern consistent with decreased attentional capacity, increased response errors, and increased impulsive behavior. Rats with POR damage showed intact acquisition and normal asymptotic performance. They also exhibited faster latencies in the absence of speed accuracy trade-off suggesting enhanced response readiness. We suggest this increased response readiness results from decreased automatic monitoring of the local environment, which might normally compete with response readiness. Our findings are consistent with a role for PER in controlled attention and a role for POR in stimulus-driven attention providing evidence that the PER and POR cortices have functions that go beyond memory for objects and memory for scenes and contexts, respectively. These findings provide new evidence for functional specialization in the medial temporal lobe.

Neuronal Activity in the Rat Pulvinar Correlates with Multiple Higher-Order Cognitive Functions.

The pulvinar, also called the lateral posterior nucleus of the thalamus in rodents, is one of the higher-order thalamic relays and the main visual extrageniculate thalamic nucleus in rodents and primates. Although primate studies report the pulvinar is engaged under attentional demands, there are open questions about the detailed role of the pulvinar in visuospatial attention. The pulvinar provides the primary thalamic input to the posterior parietal cortex (PPC). Both the pulvinar and the PPC are known to be important for visuospatial attention. Our previous work showed that neuronal activity in the PPC correlated with multiple phases of a visuospatial attention (VSA) task, including onset of the visual stimuli, decision-making, task-relevant locations, and behavioral outcomes. Here, we hypothesized that the pulvinar, as the major thalamic input to the PPC, is involved in visuospatial attention as well as in other cognitive functions related to the processing of visual information. We recorded the neuronal activity of the pulvinar in rats during their performance on the VSA task. The task was designed to engage goal-directed, top-down attention as well as stimulus-driven, bottom-up attention. Rats monitored three possible locations for the brief appearance of a target stimulus. An approach to the correct target location was followed by a liquid reward. For analysis, each trial was divided into behavioral epochs demarcated by stimulus onset, selection behavior, and approach to reward. We found that neurons in the pulvinar signaled stimulus onset and selection behavior consistent with the interpretation that the pulvinar is engaged in both bottom-up and top-down visuospatial attention. Our results also suggested that pulvinar cells responded to allocentric and egocentric task-relevant locations.

Instantaneous amplitude and shape of postrhinal theta oscillations differentially encode running speed.

Hippocampal theta oscillations have a temporally asymmetric waveform shape, but it is not known if this theta asymmetry extends to all other cortical regions involved in spatial navigation and memory. Here, using both established and improved cycle-by-cycle analysis methods, we show that theta waveforms in the postrhinal cortex are also temporally asymmetric. On average, the falling phase of postrhinal theta cycles lasts longer than the subsequent rising phase. There are, however, rapid changes in both the instantaneous amplitude and instantaneous temporal asymmetry of postrhinal theta cycles. These rapid changes in amplitude and asymmetry are very poorly correlated, indicative of a mechanistic disconnect between these theta cycle features. We show that the instantaneous amplitude and asymmetry of postrhinal theta cycles differentially encode running speed. Although theta amplitude continues to increase at the fastest running speeds, temporal asymmetry of the theta waveform shape plateaus after medium speeds. Our results suggest that the amplitude and waveform shape of individual postrhinal theta cycles may be governed by partially independent mechanisms and emphasize the importance of employing a single cycle approach to understanding the genesis and behavioral correlates of cortical theta rhythms. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat.

We investigated the cortical efferents of the parahippocampal region by placing injections of the anterograde tracers, Phaseolus vulgaris-leuccoagglutinin, and biotinylated dextran amine, throughout the perirhinal (PER), postrhinal (POR), and entorhinal cortices of the rat brain. The resulting density of labeled fibers was evaluated in 25 subregions of the piriform, frontal, insular, temporal, cingulate, parietal, and occipital areas. The locations of labeled terminal fibers differed substantially depending on whether the location of the injection site was in PER area 35, PER area 36, POR, or the lateral or the medial entorhinal (LEA and MEA). The differences were greater for sensory regions. For example, the POR efferents preferentially target visual and spatial regions, whereas the PER efferents target all sensory modalities. The cortical efferents of each region largely reciprocate the cortical afferents, though the degree of reciprocity varied across originating and target regions. The laminar pattern of terminal fibers was consistent with the notion that the efferents are feedback projections. The density and amount of labeled fibers also differed substantially depending on the regional location of injection sites. PER area 36 and POR give rise to a greater number of heavy projections, followed by PER area 35. LEA also gives rise to widespread cortical efferents, arising mainly from a narrow band of cortex adjacent to the PER. In contrast, the remainder of the LEA and the MEA provides only weak efferents to cortical regions. Prior work has shown that nonspatial and spatial information is transmitted to the hippocampus via the PER-LEA and POR-MEA pathways, respectively. Our findings suggest that the return projections follow the same pathways, though perhaps with less segregration.