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A A Fenton

Publications and source records attributed to A A Fenton.

At least 19 recordsLinked to original sources
PubMed2005

Evidence for hippocampal role in place avoidance other than merely memory storage.

Spatial navigation is used as a popular animal model of higher cognitive functions in people. The data suggest that the hippocampus is important for both storing spatial memories and for performing spatial computations necessary for navigation. Animals use multiple behavioral strategies to solve spatial tasks often using multiple memory systems. We investigated how inactivation of the rat hippocampus affects performance in a place avoidance task to determine if the role of the hippocampus in this task could be attributed to memory storage/retrieval or to the computations needed for navigation. Injecting tetrodotoxin (TTX) into both hippocampi impaired conditioned place avoidance, but after injecting only one hippocampus, the rats learned the place avoidance as well as without any injections. Retention of the place avoidance learned with one hippocampus was not impaired when the injection was switched to the hippocampus that had not been injected during learning. The result suggests that during learning, the hippocampus did not store the place avoidance memory.

Anesthetics, Local↗
PubMed2003

Quantifying location-specific information in the discharge of rat hippocampal place cells.

The informational content in the location-specific discharge of rat hippocampal cells is usually quantified by an average for the entire behaviorally accessible space. In contrast to such "global" information measures, we consider here information that can be obtained from "local" spike counts at each position. The properties of these local information measures are first illustrated using simulated data with predetermined distributions of location-specific spike counts. Next, place cell recordings from rats foraging in a cylindrical arena with two cue cards on its walls are analyzed; time windows as short as 100 ms were used to accumulate spike counts in locations. We show that information at the centers of firing fields is higher for fields nearer to the cues. Neither firing rates or "global" information measures detected differences between fields near and far from the cues. Thus, analyses of the location-specific information provides a new valuable tool for studying the location-specific activity of rat hippocampal cells. Generalizations of location-specific information can be used to investigate place cell responses to other factors such as running speed or the state of the hippocampal EEG in addition to current position.

Action Potentials↗
PubMed2002

Properties of the extra-positional signal in hippocampal place cell discharge derived from the overdispersion in location-specific firing.

There is a good deal of evidence that in the rodent, an internal model of the external world is encoded by hippocampal pyramidal cells, called 'place cells'. During free exploration, the activity of place cells is higher within a small part of the space, called the firing field, and virtually silent elsewhere. We have previously shown that the spiking activity during passes through the firing field is characterized not only by the high firing rate, but also by its very high variability ('overdispersion'). This overdispersion indicates that place cells carry information in addition to position. Here we demonstrate by simulations of an integrate-and-fire neuronal model that while a rat is foraging in an open space this additional information may arise from a process that alternatingly modulates the inputs to place cells by about 10% with a mean period of about 1 s. We propose that the overdispersion reflects switches of the rats attention between different spatial reference frames of the environment. This predicts that the overdispersion will not be observed in rats that use only room-based cues for navigation. We show that while place cell firing is overdispersed in rats during foraging in an open arena, the firing is less overdispersed during the same behavior in the same environment, when the rats have been trained to use only room-based and not arena-based cues to navigate.

Action Potentials↗
PubMed2002

Hippocampus-dependent retrieval and hippocampus-independent extinction of place avoidance navigation, and stress-induced out-of-context activation of a memory revealed by reversible lesion experiments in rats.

The use of reversible lesion techniques in memory research was pioneered in the laboratory of Jan Bures and Olga Buresova. We use the occasion of Jan's 75th birthday to briefly review the experimental utility of this approach. Two experiments from our current research are reported in which reversible lesioning methods are used to ask otherwise experimentally untenable questions about memory retrieval. The first experiment used intra-hippocampal injections of tetrodotoxin to temporarily inactivate the hippocampus during retrieval of a well-learned place avoidance navigation memory. This revealed that the hippocampus is necessary for place avoidance retrieval but that the extinction of place avoidance can occur independently of retrieving the memory and intact hippocampal function. The second experiment used KCl-induced cortical spreading depression in an interhippocampal transfer paradigm to demonstrate that a Y-maze memory that is learned by only one cortical hemisphere can be made to transfer to the other hemisphere by forcing the rat to swim, a unique stressful experience that occurred in a different apparatus, different behavioral context, and involved different behaviors than the Y-maze training. This demonstrates, we believe for the first time behaviorally, that memories can be activated outside of the behavioral context of their acquisition and expression in rats.

Animals↗
PubMed2001

New spatial cognition tests for mice: passive place avoidance on stable and active place avoidance on rotating arenas.

Dry arenas are a convenient tool for assessing the spatial navigation abilities of rodents. In this paper, mice must avoid a punished sector of a dry arena from which they are expelled by a puff of compressed air. The position of the punished sector is defined relative to the coordinate system of the room. In a stable environment the mice can use both extramaze and intramaze landmarks to orient themselves accurately. However, when the shock area is defined by extramaze landmarks, continuous rotation of the arena at 1 rpm makes it impossible to solve the avoidance task using arena-based cues or idiothesis. The avoidance can only be solved by paying attention to extramaze cues. Our protocol tested spatial abilities on stable and rotating arenas. The acquisition of the task was manifested under both conditions by a significant improvement of performance within the first session (short-term memory component) and at the beginning of the 24-h delayed second session (long-term memory component).

Animals↗
PubMed2001

Inactivating one hippocampus impairs avoidance of a stable room-defined place during dissociation of arena cues from room cues by rotation of the arena.

Unilateral intrahippocampal injections of tetrodotoxin were used to temporarily inactivate one hippocampus during specific phases of training in an active allothetic place avoidance task. The rat was required to use landmarks in the room to avoid a room-defined sector of a slowly rotating circular arena. The continuous rotation dissociated room cues from arena cues and moved the arena surface through a part of the room in which foot-shock was delivered. The rat had to move away from the shock zone to prevent being transported there by the rotation. Unilateral hippocampal inactivations profoundly impaired acquisition and retrieval of the allothetic place avoidance. Posttraining unilateral hippocampal inactivation also impaired performance in subsequent sessions. This allothetic place avoidance task seems more sensitive to hippocampal disruption than the standard water maze task because the same unilateral hippocampal inactivation does not impair performance of the variable-start, fixed hidden goal task after procedural training. The results suggest that the hippocampus not only encodes allothetic relationships amongst landmarks, it also organizes perceived allothetic stimuli into systems of mutually stable coordinates. The latter function apparently requires greater hippocampal integrity.

Animals↗
PubMed2001

Substratal idiothetic navigation of rats is impaired by removal or devaluation of extramaze and intramaze cues.

The spatial orientation of vertebrates is implemented by two complementary mechanisms: allothesis, processing the information about spatial relationships between the animal and perceptible landmarks, and idiothesis, processing the substratal and inertial information produced by the animal's active or passive movement through the environment. Both systems allow the animal to compute its position with respect to perceptible landmarks and to the already traversed portion of the path. In the present study, we examined the properties of substratal idiothesis deprived of relevant exteroceptive information. Rats searching for food pellets in an arena formed by a movable inner disk and a peripheral immobile belt were trained in darkness to avoid a 60 degrees sector; rats that entered this sector received a mild foot shock. The punished sector was defined in the substratal idiothetic frame, and the rats had to determine the location of the shock sector with the use of substratal idiothesis only, because all putative intramaze cues were made irrelevant by angular displacements of the disk relative to the belt. Striking impairment of place avoidance by this "shuffling procedure" indicates that effective substratal idiothesis must be updated by exteroceptive intramaze cues.

Animals↗
PubMed2001

Transient sex differences in the between-sessions but not in the within-session memory underlying an active place avoidance task in weanling rats.

Spatial abilities were tested in male and female rats by training them to avoid an area in which there was a mild footshock while the arena rotated at 1 revolution/minute. The to-be-avoided area was stable in the coordinates of the room, so extramaze landmarks had to be used for accurate navigation, as the rotation made intramaze cues and substrate-based path integration useless for the avoidance. From Postnatal Day (PD) 19, rats were trained for 22 consecutive days. When the shock area was the same across sessions male rats reached optimal performance on PDs 23-24, 10 days before female rats, but when the location of the shock changed daily there were no sex differences. The results indicate that there are separate memory components underlying spatial competence: a within-session component that develops similarly in male and female rats and a between-sessions component that lasts at least 24 hr and appears earlier in male than in female rats.

Animals↗
PubMed2000

Functional inactivation of dorsal hippocampus impairs active place avoidance in rats.

This paper assesses the contribution of hippocampus to the spatial orientation of Long-Evans rats in a new place avoidance task. The animals learn to avoid a mild footshock in a segment of a rotating arena. Since the punished region is defined in the coordinate system of the stationary room the subject is forced to move away from the prohibited segment even if it is immobile. After bilateral injection of tetrodotoxin (5 ng in 1 microl of saline) into the dorsal hippocampus rats were not able to avoid the punished place while a similar injection of saline did not affect performance. The results suggest the task is suitable for assessing the hippocampus-dependent spatial abilities of laboratory rodents.

Animals↗
PubMed2000

Understanding hippocampal activity by using purposeful behavior: place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames.

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.

Animals↗
PubMed2000

Continuous place avoidance task reveals differences in spatial navigation in male and female rats.

A new place navigation test was used to estimate the spatial orientation abilities of male and female rats. Animals had to avoid a room frame defined area on a rotating arena, entering of which was punished by mild footshock, i.e. rats had to avoid the same place in the room but different parts of the floor, which was rotated through the punished zone. Because of the rotation of the arena (one revolution per min), animals could not rely on intramaze cues and only extramaze landmarks could be used for accurate navigation. During 8 consecutive days rats were exposed to daily 40-min sessions, consisting of 20-min acquisition and 20-min extinction (shock discontinued). The position of the punished sector centered around one of the four mutually perpendicular azimuths was daily changed in a predetermined sequence. The results showed no male female differences during acquisition and better performance of males during extinction. The performance of females was not affected by estral cycle-related hormonal changes. The findings are discussed in the light of controversial results of research into sex differences in spatial abilities.

Animals↗
PubMed2000

Conjoint control of hippocampal place cell firing by two visual stimuli. I. The effects of moving the stimuli on firing field positions.

To better understand how hippocampal place cell activity is controlled by sensory stimuli, and to further elucidate the nature of the environmental representation provided by place cells, we have made recordings in the presence of two distinct visual stimuli under standard conditions and after several manipulations of these stimuli. In line with a great deal of earlier work, we find that place cell activity is constant when repeated recordings are made in the standard conditions in which the centers of the two stimuli, a black card and a white card, are separated by 135 degrees on the wall of a cylindrical recording chamber. Rotating the two stimuli by 45 degrees causes equal rotations of place cell firing fields. Removing either card and rotating the other card also causes fields to rotate equally, showing that the two stimuli are individually salient. Increasing or decreasing the card separation (card reconfiguration) causes a topological distortion of the representation of the cylinder floor such that field centers move relative to each other. We also found that either kind of reconfiguration induces a position-independent decrease in the intensity of place cell firing. We argue that these results are not compatible with either of two previously stated views of the place cell representation; namely, a nonspatial theory in which each place cell is tuned to an arbitrarily selected subset of available stimuli or a rigid map theory. We propose that our results imply that the representation is map-like but not rigid; it is capable of undergoing stretches without altering the local arrangement of firing fields.

Animals↗
PubMed2000

Conjoint control of hippocampal place cell firing by two visual stimuli. Ii. A vector-field theory that predicts modifications of the representation of the environment.

Changing the angular separation between two visual stimuli attached to the wall of a recording cylinder causes the firing fields of place cells to move relative to each other, as though the representation of the floor undergoes a topological distortion. The displacement of the firing field center of each cell is a vector whose length is equal to the linear displacement and whose angle indicates the direction that the field center moves in the environment. Based on the observation that neighboring fields move in similar ways, whereas widely separated fields tend to move relative to each other, we develop an empirical vector-field model that accounts for the stated effects of changing the card separation. We then go on to show that the same vector-field equation predicts additional aspects of the experimental results. In one example, we demonstrate that place cell firing fields undergo distortions of shape after the card separation is changed, as though different parts of the same field are affected by the stimulus constellation in the same fashion as fields at different locations. We conclude that the vector-field formalism reflects the organization of the place-cell representation of the environment for the current case, and through suitable modification may be very useful for describing motions of firing patterns induced by a wide variety of stimulus manipulations.

Action Potentials↗
PubMed1999

Is the hippocampus of the rat part of a specialized navigational system?

The spatial mapping theory of hippocampal function proposes that the rat hippocampus is specialized for navigational computations, computations that allow the animal to solve difficult spatial problems. In this paper, we review evidence obtained by recording place cells and other "spatially tuned" cells from freely moving rats. Our main conclusion is that the nature of the signals carried by these cells and the ways in which the signals transform after changing the environment imply that the hippocampus and associated structures are able to represent aspects of the geometry of the environment.

Animals↗
PubMed1998

Both here and there: simultaneous expression of autonomous spatial memories in rats.

Foraging rats learned to avoid footshock that was present in a part of a circular arena that was either stable or rotating slowly in a lighted room. The rotation dissociated spatial information in the separate reference frames of the room and arena. After learning to avoid the shocked region in either condition, in the absence of shock, memory for this place was expressed by simultaneous avoidance of an area defined in the reference frame of the room as well as of an area defined in the reference frame of the rotating arena. Spatial memories in these distinct reference frames were acquired, retrieved, and extinguished autonomously.

Animals↗
PubMed1998

Place cell discharge is extremely variable during individual passes of the rat through the firing field.

The idea that the rat hippocampus stores a map of space is based on the existence of "place cells" that show "location-specific" firing. The discharge of place cells is confined with remarkable precision to a cell-specific part of the environment called the cell's "firing field." We demonstrate here that firing is not nearly as reliable in the time domain as in the positional domain. Discharge during passes through the firing field was compared with a model with Poisson variance of the location-specific firing determined by the time-averaged positional firing rate distribution. Place cells characteristically fire too little or too much compared with expectations from the random model. This fundamental property of place cells is referred to as "excess firing variance" and has three main implications: (i) Place cell discharge is not only driven by the summation of many small, asynchronous excitatory synaptic inputs. (ii) Place cell discharge may encode a signal in addition to the current head location. (iii) The excess firing variance helps explain why the errors in computing the rat's position from the simultaneous activity of many place cells are large.

Action Potentials↗
PubMed1997

Dissociation of exteroceptive and idiothetic orientation cues: effect on hippocampal place cells and place navigation.

Navigation by means of cognitive maps appears to require the hippocampus; hippocampal place cells (PCs) appear to store spatial memories because their discharge is confined to cell-specific places called firing fields (FFs). Experiments with rats manipulated idiothetic and landmark-related information to understand the relationship between PC activity and spatial cognition. Rotating a circular arena in the light caused a discrepancy between these cues. This discrepancy caused most FFs to disappear in both the arena and room reference frames. However, FFs persisted in the rotating arena frame when the discrepancy was reduced by darkness or by a card in the arena. The discrepancy was increased by 'field clamping' the rat in a room-defined FF location by rotations that countered its locomotion. Most FFs dissipated and reappeared an hour or more after the clamp. Place-avoidance experiments showed that navigation uses independent idiothetic and exteroceptive memories. Rats learned to avoid the unmarked footshock region within a circular arena. When acquired on the stable arena in the light, the location of the punishment was learned by using both room and idiothetic cues; extinction in the dark transferred to the following session in the light. If, however, extinction occurred during rotation, only the arena-frame avoidance was extinguished in darkness; the room-defined location was avoided when the lights were turned back on. Idiothetic memory of room-defined avoidance was not formed during rotation in light; regardless of rotation, there was no avoidance when the lights were turned off, but room-frame avoidance reappeared when the lights were turned back on. The place-preference task rewarded visits to an allocentric target location with a randomly dispersed pellet. The resulting behaviour alternated between random pellet searching and target-directed navigation, making it possible to examine PC correlates of these two classes of spatial behaviour. The independence of idiothetic and exteroceptive spatial memories and the disruption of PC firing during rotation suggest that PCs may not be necessary for spatial cognition; this idea can be tested by recordings during the place-avoidance and preference tasks.

Animals↗