Biomedical subjects

A A Fenton

Publications and source records attributed to A A Fenton.

26 records · Page 2Linked to original sources

PubMed1997

Place cells and place navigation.

The assumption that hippocampal place cells (PCs) form the neural substrate of cognitive maps can be experimentally tested by comparing the effect of experimental interventions on PC activity and place navigation. Conditions that interfere with place navigation (darkness, cholinergic blockade) but leave PC activity unaffected obviously disrupt spatial memory at a post-PC level. Situations creating a conflict between egocentric and allocentric orientation (place navigation in the Morris water maze filled with slowly rotating water) slow down spatial learning. PC recording in rats searching food pellets in a rotating arena makes it possible to determine which firing fields are stable relative to the room (allocentrically dependent on sighted extramaze landmarks), to the surface of the arena (dependent on egocentric path integration mechanisms and intra-arena cues), or disappear during rotation. Such comparison is made possible by the computerized tracking system simultaneously displaying a rat's locomotion and the respective firing rate maps both in the room reference and arena reference frames. More severe conflict between allocentric and egocentric inputs is produced in the field clamp situation when the rat searching food in a ring-shaped arena is always returned by rotation of the arena to the same allocentric position. Ten-minute exposure to this condition caused subsequent disintegration or remapping of 70% PCs (n = 100). Simultaneous examination of PC activity and navigation is possible in the place avoidance task. A rat searching food in a stationary or rotating arena learns to avoid an allocentrically or egocentrically defined location where it receives mild electric footshock. In the place preference task the rat releases pellet delivery by entering an unmarked goal area and staying in it for a criterion time. Both tasks allow direct comparison of the spatial reference frames used by the PCs and by the behaving animal.

Animals↗

PubMed1996

Using digital video techniques to identify correlations between behavior and the activity of single neurons.

Digital movies were made of freely behaving rats in a cylinder. At the same time, action potentials were recorded from single hippocampal neurons. We describe a data processing scheme that allows the spike data and the movie to be merged. A display of the ongoing unit activity was added to each frame in the movie. This provided a powerful new way to examine the relationships between behavior and spike discharge. A second type of data processing, activity-edited video identified and used interesting episodes of neuronal activity (e.g. fast firing) to select the corresponding frames from the original movie. The activity-edited movie can then be viewed to look for behavioral invariances during the episodes that were selected according to the cell's firing rate. The feasibility of these methods was tested by applying them to post-subicular head direction cells and hippocampal place cells because these cell classes have well characterized behavioral correlates. The digital video techniques captured the head direction and place cell phenomena and provided a novel and robust description of the relationship between the firing of these cells and the overall behavior of the rat.

Action Potentials↗

PubMed1995

Interhippocampal synthesis of lateralized place navigation engrams.

Rats were trained in a water maze in a dark room with the extramaze cues restricted to only dimly back-lit shapes. We used lidocaine to reversibly lesion the dorsal hippocampus and this controlled-cue room in order to examine interhippocampal synthesis of lateralized place engrams. Experiment 1 showed that lidocaine injected into both hippocampi effectively abolished place navigation for up to 25 min but not at 45 min. In experiment 2, each day under lidocaine blockade of one hippocampus, pretrained rats were trained in the water maze to locate the target according to two cues (e.g., AB). Two hours later, the contralateral hippocampus was inactivated and the rats were trained to the same location with two other cues (CD). On day 5, intact brain retrieval was tested in one of three conditions: ACQ (e.g., AB), one of the pairs of cues used in acquisition training; SYNTH (e.g., AC), one cue from each of the pairs used in acquisition; CONT (e.g., AE), one cue that was used in acquisition training and a novel cue. The results show that the hippocampi learned the two tasks independently and similarly [latency (L) at the asymptote = 7 s]. Retrieval performance was at the asymptote for ACQ (AB) and SYNTH (AC) (L = 6 and 7, respectively) but was disrupted for CONT (L = 12). In experiment 3 as in experiment 2, the rats were trained, under unilateral blockade, to a new place for 4 days. On day 5, retrieval with the trained hippocampus blocked was worse (L = 11) than with the untrained side blocked (L = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Interhippocampal transfer of place navigation monocularly acquired by rats during unilateral functional ablation of the dorsal hippocampus and visual cortex with lidocaine.

To study the neural mechanisms of interhippocampal transfer of lateralized place navigation engrams in rats, lidocaine was injected via chronically implanted cannulae to reversibly inactivate the hippocampal formation and the visual cortex on one side. The eye opposite the blocked side was occluded. Under these conditions, rats learned the location of an invisible platform in a water maze [mean escape latencies per four-trial block (t) = 5-6 s at the performance asymptote]. Monocular intact brain retrieval with the trained eye (t = 7) was better than with the untrained eye (t = 13). However, analysis of each retrieval trial indicated untrained eye performance was only poor on the first trial (t = 30). To test whether trans-commissural read-out alone or write-in (i.e. interhippocampal transfer) of the lateralized engram explains the above results, rats acquired a new platform location (t = 5). Two groups were then given a 30-s "free swim" (the platform was removed) with intact brain and either the trained or untrained eye occluded. A third group did not have this "transfer" trial. Retrieval was tested with the trained hippocampus and visual cortex blocked. With the trained eye occluded, retrieval in the rats that had the transfer trial (t = 11) was better than in those that did not (t = 25), but slightly worse than in rats tested with the untrained eye, hippocampus and visual cortex blocked (t = 7). Additionally, retrieval was similar, independent of whether the trained (t = 12) or untrained (t = 11) eye was open on the transfer swim. The 30-s swim alone did not induce comparable savings. We conclude that interhippocampal transfer of lateralized place learning is easily induced, is equal if the transfer is facultative or imperative, and involves both trans-commissural read-out and write-in processes.

Animals↗

PubMed1994

Place navigation in the Morris water maze under minimum and redundant extra-maze cue conditions.

Complex relational processes underlying place navigation learning were analyzed by minimizing the relational elements available to rats. The animals navigated in a standard water maze in darkness using controlled remote visual cues (back-lit shapes in opaque buckets aimed at the pool to keep the background dark) while being tracked by an infrared camera and computer. Learning was similar with 2 (AB) or 4 (ABCD) cues and as good as in a fully lit room with many cues (asymptotic escape time t = 5-7 s). The ABCD-trained rats were not impaired by removal of any 2 cues (t = 7). For AB-trained rats, adding 2 new cues (ABEF) or replacing AB with EF (EF) caused small (t = 11) or big disruptions (t = 20), respectively. By block 2, both groups (ABEF, EF) returned to asymptotic performance. But testing the ABEF rats on block 2 with only EF indicated that EF was learned (t = 12) but not as well as when only EF was present (t = 5). Thus transfer from a redundant to a minimal cue condition is immediate and easier than vice versa. Theoretical implications are discussed.

Association Learning↗

PubMed1993

Place navigation in rats with unilateral tetrodotoxin inactivation of the dorsal hippocampus: place but not procedural learning can be lateralized to one hippocampus.

Lateralization of the Morris water maze task was examined by injecting 5 ng of tetrodotoxin (TTX) into one dorsal hippocampus (HPC) of chronically cannulated rats (n = 20). The task, acquired (A) during unilateral HPC block, was retrieved (R) during TTX blockade of the ipsi- (I) or contralateral (C) HPC. Escape latencies (in seconds) at 3 levels of acquisition and in the first 4-trial block of I or C retrieval suggest lateralization was absent after the first block (A = 49, I = 40, C = 44), partial after training to an efficient search strategy (A = 6, I = 9, C = 14), and complete after overtraining to goal-directed escape (A = 4, I = 4, C = 12), as further indicated by comparisons of A vs. R behavior and probe trials. The analyses also indicate that (a) HPC contributes to the acquisition but is unnecessary for retrieval of the procedural engram, (b) overtrained learning with one HPC or intact brain is similar, and (c) there is no left-right hemispheric specialization for place learning.

Animals↗

PubMedDate not supplied

Rodent navigation after dissociation of the allocentric and idiothetic representations of space.

Analysis of the neural mechanisms of place navigation requires isolation of the landmark dependent allocentric and self-motion related idiothetic orientation modes. To assess their importance, rats were trained on a rotating (360 degrees/min) arena to avoid foot shocks applied in either a room frame defined sector of the arena or an idiothetically defined region of the floor. Independence of the respective allocentric and idiothetic engrams was revealed by simultaneous avoidance of both locations. The possibility that idiothetic orientation was confounded by allocentric intramaze cues was examined in an apparatus consisting of an inner rotating disc surrounded by a stationary belt. As long as the rat was on the moving disc, position of the 60 degrees shock sector was stable on the disk but projected from it to different parts of the belt. When the rat moved to the belt the shock sector was now stable on the belt, but its projection to the disk travelled over its moving surface. The rat always found the shock sector in an idiothetically correct position but the mutual shifts of the disk and belt eliminated the utility of local cues like scent marks for the idiothetic solution of the task. Purely allocentric orientation was required in a place recognition task in which pressing a lever mounted on a rotating arena was rewarded only when the operandum moved through an allocentrically defined 60 degrees segment of its trajectory. Place recognition was manifest by increased bar pressing rates on approach to and inside the reward zone. These methods may reveal how hippocampal place cell activity correlates with both allocentric and idiothetic aspects of spatial orientation.

Animals↗

PubMedDate not supplied

On the location-specific positional and extra-positional information in the discharge of rat hippocampal cells.

The informational content in the location-specific discharge of rat hippocampal cells is usually quantified by an average for the entire experimental space. In contrast, in the present work the information that can be obtained from spike counts at each position is considered. Along with the local positional information, measures for the total and extra-positional information are introduced. Their properties are studied and illustrated on simulated and experimentally obtained data. It is demonstrated that these information measures provide a new valuable tool for studying the location-specific activity of rat hippocampal cells even on time scales as short as 100 ms. The measures can be used to investigate place cell responses to arbitrary signals in addition to current position.

Action Potentials↗