Dynamics of Hippocampal and Cortical Activation during Consolidation of a Nonspatial Memory

Observations of temporally graded retrograde amnesia after hippocampal damage suggest that the hippocampal region plays a critical, time-limited role in memory consolidation. However, these observations do not indicate where permanentmemory is stored, nor do they clarify whether the hippocampus normally remains involved in a nonessential way. Evidence from multiple neural imaging studies indicate the time-limited role of the hippocampus and suggest that the anterior cingulate cortex is a critical storage site of different types of long-term memory. However, each of the previous studies examined spatial memory, leaving open the question of whether different cortical areas support long-term memory for other types ofmaterial. We characterized the course of involvement of cortical and hippocampal areas in animals trained in an explicitlynonspatial task. First, we confirmed previous findings that hippocampal damage produces temporally graded retrogradeamnesia for the social transmission of a food preference (STFP) within our experimental protocol. Damage to thehippocampal region 1 d, but not 21 d, after training impaired subsequent recall of STFP. Then, we characterized theanatomical patterns of activation of the immediate early gene c-fos during retrieval of STFP immediately and 1, 2, and 21 dafter training. The ventral subiculum was activated during retrieval shortly after learning, but the level of activation declined at successive times. In contrast, olfactory recipient regions including piriform, entorhinal, and orbitofrontal cortex showed the opposite pattern, increasingly greater activation in successively later retrieval tests. These findings support the view that different cortical networks support long-term memory for different types of information

Hippocampus: Mapping or memory?

AbstractThe identification of ‘place cells’ in the hippocampus has suggested a special role for this brain structure in spatial mapping, but other studies have indicated a more general role in memory. New findings on place cells point the way towards a reconciliation of the mapping and memory views

Neurotoxic Hippocampal Lesions Have No Effect on Odor Span and Little Effect on Odor Recognition Memory But Produce Significant Impairments on Spatial Span, Recognition, and Alternation

Recent work has shown that lesions of the hippocampus in monkeys cause deficits in the capacity to remember increasing numbers of objects, colors, and spatial locations (Beason-Heldet al., 1999). However, others have observed that hippocampectomized monkeys can show intact memory for a list of objects or locations (Murray and Mishkin, 1998). We wished to explore the effects of hippocampal damage on the capacity of memory in the rodent and, to do so, developed novel "span" tasks in which a variable number of odors or locations had to be remembered. In the odor span task (experiment 1), rats were trained on a nonmatching to sample task in which increasing numbers of odors had to be remembered. Half of the trained rats received ibotenic acid lesions of the hippocampus. Postoperatively, hippocampectomized animals did not differ from control animals even when required to remember up to 24 odors. However, when tested on delayed retention of a list of 12 odors, rats with hippocampal lesions were impaired at a long delay. Also, these rats were impaired on a subsequent test of delayed spatial alternation. In a spatial span task (experiment 2), naive rats were trained on a nonmatching to sample task in which a variable number of locations had to be remembered. After this, half of the animals received ibotenic acid lesions. Postoperatively, hippocampectomized animals performed above chance levels when required to remember a single cup location, but were unable to remember more. Subsequent testing on another spatial delayed alternation task suggested that hippocampectomized rats could recognize, but could not inhibit their approach to previously visited locations

A mechanism for the formation of hippocampal neuronal firing patterns that represent what happens where

The association of specific events with the context in which they occur is a fundamental feature of episodic memory. However, the underlying network mechanisms generating what–where associations are poorly understood. Recently we reported that some hippocampal principal neurons develop representations of specific events occurring in particular locations (item-position cells). Here, we investigate the emergence of item-position selectivity as rats learn new associations for reward and find that before the animal’s performance rises above chance in the task, neurons that will later become itemposition cells have a strong selective bias toward one of two behavioral responses, which the animal will subsequently make to that stimulus. This response bias results in an asymmetry of neural activity on correct and error trials that could drive the emergence of particular item specificities based on a simple reward-driven synaptic plasticity mechanism

Gradual Translocation of Spatial Correlates of Neuronal Firing in the Hippocampus toward Prospective Reward Locations

SummaryIn a continuous T-maze alternation task, CA1 complex-spike neurons in the hippocampus differentially fire as the rat traverses overlapping segments of the maze (i.e., the stem) repeatedly via alternate routes. The temporal dynamics of this phenomenon were further investigated in the current study. Rats learned the alternation task from the first day of acquisition and the differential firing pattern in the stem was observed accordingly. More importantly, we report a phenomenon in which spatial correlates of CA1 neuronal ensembles gradually changed from their original firing locations, shifting toward prospective goal locations in the continuous T-maze alternation task. The relative locations of simultaneously recorded firing fields, however, were preserved within the ensemble spatial representation during this shifting. The within-session shifts in preferred firing locations in the absence of any changes in the environment suggest that certain cognitive factors can significantly alter the location-bound coding scheme of hippocampal neurons

Rates of distress vocalizations in naive domestic chicks as an index of approach tendency to an imprinting stimulus

Naive chicks were exposed to an imprinting target, consisting of a bank of miniature flashing lights, under normal (100[deg]F) and reduced temperatures (75 and 57[deg]F). It was found that cold reliably evoked distress calls and that the cyclic presentations of the target substantially reduced distress calling when the target was present, relative to when it was absent. When later tested for approach tendencies to the target, clear differences between temperature groups did not emerge. Evidence was obtained, however, that chicks that showed the greatest reduction in distress calling in response to the presentation of the target under the cold conditions showed significantly greater approach tendencies in the subsequent tests.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33786/1/0000041.pd

The role of frontal cortex-reticular interactions in performance and extinction of the classically conditioned nictitating membrane response in the rabbit

In order to investigate the behavioral role of interactions between frontal cortex and reticular nuclei, we examined the effects of single and combined lesions of these structures on the classically conditioned nictitating membrane response (NMR) of rabbits. Lesions of frontal cortex decreased latencies of the conditioned NMRs in reacquisition and retarded extinction of the conditioned response. Lesions of nucleus reticularis pontis oralis (NRPO) produced similar effects. In contrast, lesions of nucleus reticularis tegmenti (NRT) increased NMR latencies during reacquisition. The opposite effects of frontal cortex and NRT lesions were abolished when the two lesions were combined, indicating that the two lesion effects summed. In contrast, the deficits due to frontal and NRPO lesions did not sum; combined frontal--NRPO lesions produced deficits very similar in magnitude and time course to those of the NRPO lesions alone. These findings suggest that frontal cortex may exert its inhibitory effects on behavior not by directly interacting with NRT, but by facilitating NRPO, which in turn may inhibit the nucleus of the VIth nerve, the final common pathway to the NMR. NRT may facilitate the motor pathway by modulating the inhibitory effect of NRPO on the VIth nerve nucleus.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23959/1/0000208.pd

Correlates of hippocampal complex-spike cell activity in rats performing a nonspatial radial maze task

The observation of hippocampal place cells forms a major line of evidence supporting the view that the hippocampus is dedicated to spatial processing. However, most studies demonstrating the spatial properties of hippocampal unit activity have employed tasks that emphasize spatial cues but minimize nonspatial cues. In the present experiment we recorded the activity of hippocampal complex-spike cells from rats performing a nonspatial radial maze task. Performance in this task was guided by local visual-tactile cues on the maze arms, while distal spatial cues were minimized and made irrelevant. The influence of three variables on unit activity was examined:type of cue on an arm, spatial location of an arm, and the relative position of the animal on an arm. Of the units recorded, almost one-fifth were classified as "cue cells" in that their activity was associated with cue type but not spatial location. Conversely, a similar proportion of the units were classified as "place cells" in that their activity was associated with location, but not cue type. In an additional similar proportion of units, firing was influenced only by relative position and not by local cues or spatial locations. For the majority of units, however, firing was related to combinations of these three variables, indicating that most hippocampal neurons encoded conjunctions or relations between spatial and local cue information. This pattern of results indicates that when local rather than distal spatial cues are emphasized, hippocampal neural activity is strongly influenced by salient nonspatial cues and shows no overwhelming predominance of place coding. These findings are at odds with the hypothesis that the hippocampus is selectively involved in spatial processing and, conversely, support the broader view that the hippocampus encodes both spatial and nonspatial relations among important experimental variables

Radioautographic localization of inhibition of protein synthesis in specific regions of monkey brain

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33800/1/0000055.pd