Hippocampal place cells in rats display spatially selective firing in relation to both external and internal cues. In the present study, we assessed the effects of removing visual and/or olfactory cues on place field stability. Place cell activity was recorded as rats searched for randomly scattered food in a cylinder. During an initial recording session, the lights were on and the only available cue was a single white cue card. Following this session, three sessions were run in a row with the cue card removed. In addition, the lights were either turned off or left on and the floor was either cleaned or left unchanged, thus creating four conditions: dark/cleaning, dark/no cleaning, light/cleaning, and light/no cleaning. A fifth session was run with the cue card back on the cylinder wall and the lights turned on. The rat remained in the cylinder during all sessions without being removed at any time. In the dark/cleaning and light/cleaning conditions, most place fields were not stable (i.e., abruptly shifted position). In addition, half of the cells stopped firing in the dark/cleaning condition. In contrast, in the dark/no cleaning and light/no cleaning conditions, most place fields remained stable across sessions. These results suggest that 1) rats are not able to rely on only movement-related information to maintain a stable place representation, 2) visual input is necessary for the firing of a large number of cells, and 3) olfactory information can be used to compensate for the lack of visuospatial information.

Hippocampal place cells in rats display spatially selective firing in relation to both external and internal cues. In the present study, we assessed the effects of removing visual and/or olfactory cues on place field stability. Place cell activity was recorded as rats searched for randomly scattered food in a cylinder. During an initial recording session, the lights were on and the only available cue was a single white cue card. Following this session, three sessions were run in a row with the cue card removed. In addition, the lights were either turned off or left on and the floor was either cleaned or left unchanged, thus creating four conditions: dark/cleaning, dark/no cleaning, light/cleaning, and light/no cleaning. A fifth session was run with the cue card back on the cylinder wall and the lights turned on. The rat remained in the cylinder during all sessions without being removed at any time. In the dark/cleaning and light/cleaning conditions, most place fields were not stable (i.e., abruptly shifted position). In addition, half of the cells stopped firing in the dark/cleaning condition. In contrast, in the dark/no cleaning and light/no cleaning conditions, most place fields remained stable across sessions. These results suggest that 1) rats are not able to rely on only movement‐related information to maintain a stable place representation, 2) visual input is necessary for the firing of a large number of cells, and 3) olfactory information can be used to compensate for the lack of visuospatial information. Hippocampus 2000;10:64–76. © 2000 Wiley‐Liss, Inc.

Objective: Survivors of traumatic brain injury (TBI) often have spatial navigation deficits. This study examined such deficits and conducted a detailed analysis of navigational behaviour in a virtual environment.Design: TBI survivors were tested in a computer simulation of the Morris water maze task that required them to find and remember the location of an invisible platform that was always in the same location. A follow-up questionnaire assessed everyday spatial ability. Method: Fourteen survivors of moderate-to-severe TBI were compared to 12 non-injured participants. Results: TBI survivors navigated to a visible platform but could not learn the location of the invisible platform. The difference between TBI survivors and uninjured participants was best indicated by two new dependent variables, path efficacy and spatial scores. Conclusion: This study confirms the capacity of virtual environments to reveal spatial navigation deficits after TBI and establishes the best way to identify such deficits.

A characteristic feature of the electroencephalogram (EEG) of the hippocampus and rhinal (entorhinal and perirhinal) cortex of the freely moving rat is theta rhythm, a prominent oscillation of approximately 8 Hz. Here we demonstrate that a novel rhythm that occurs at the border between the theta and alpha range of frequencies (10-12 Hz) can also be recorded from these structures. This rhythm (referred to here as "flutter") appears to be of non-theta origin as it can occur simultaneously with theta and it does not display the phase inversion across the hippocampus that characterizes theta activity. Flutter is observed in locomoting rats that are foraging for food reward in a familiar environment. Flutter disappears when rats are placed into a novel (although visually identical) environment, even though their foraging behavior does not appear to be altered. It is, at the present time, unclear what function flutter subserves. The presence of flutter may relate to a particular motivational state of the animal or to a particular type of information processing.

Individual neurons in the medial septum and diagonal band fire in phase with, and appear to act as a 'pacemaker' of, the hippocampal theta rhythm. We investigated the relationships of periodic EEG both among various parts of the septum and with dorsal hippocampal theta recorded concurrently in freely moving rats. Most septal sites showed theta rhythm concurrent with hippocampal theta during locomotion. However, periods with theta at hippocampal but not septal sites were more frequent than the reverse. Theta waves in different parts of the septum were synchronized with each other but medial septal sites showed less frequent theta than other sites. The phase delays between medial and lateral septal sites were < 10 ms, suggesting that the hippocampus does not act as a simple relay between the two. Spectral analysis revealed periods (> 5 s) of theta at hippocampal sites co-occurring with rhythms at multiple septal sites that were slower than theta. Even slower were the 'slow septal waves' (mean 2.7 Hz), which were present in the absence of locomotion and did not 'drive' the hippocampus. Our data suggest that the pacemaker of hippocampal theta may best be thought of as a set of functionally differentiated components rather than as a single homogenous unit.

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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)

Contribution of visual and nonvisual mechanisms to spatial behavior of rats in the Morris water maze was studied with a computerized infrared tracking system, which switched off the room lights when the subject entered the inner circular area of the pool with an escape platform. Naive rats trained under light-dark conditions (L-D) found the escape platform more slowly than rats trained in permanent light (L). After group members were swapped, the L-pretrained rats found under L-D conditions the same target faster and eventually approached latencies attained during L navigation. Performance of L-D-trained rats deteriorated in permanent darkness (D) but improved with continued D training. Thus L-D navigation improves gradually by procedural learning (extrapolation of the start-target azimuth into the zero-visibility zone) but remains impaired by lack of immediate visual feedback rather than by absence of the snapshot memory of the target view.