Neuroscience Poster Session
Rodents moving through their environment use various information sources to determine their position with respect to a hidden goal. On the basis of their origin, these sources can be described in terms of two main reference frames. The allothetic frame determines the animal's position relative to perceived landmarks (mainly the visual ones), whereas the idiothetic frame uses the information obtained by integrating the elements of the previously traversed path (i.e. path-integration) with respect to a starting point. The latter system uses internal self-motion information (proprioceptive, vestibular etc.) and allows the animal to navigate in absence of any exteroceptive cues. We decided to test the hypothesis that substrate-related exteroceptive cues (e.g., fine irregularities of the arena floor, scent marks) are necessary for frequent corrections of the error-prone path-integration. We designed an open field task in which both extra-arena and intra-arena exteroceptive cues are made irrelevant for spatial navigation, thus forcing the rats to rely on path integration only. In this way pure idiothesis can be studied over extended period of time (~30 min), which is not possible in typical homing experiments.
Materials and Methods
General methods: All experiments proceeded in complete darkness. Adult male Long-Evans rats were trained to search food pellets randomly thrown (in 8 sec intervals) on the arena floor. At the same time they avoided a 60o sector entering of which was punished by a mild footshock (place avoidance task, PA). The arena was formed by two concentric segments: a rotary inner disk and a surrounding fixed belt. The computer-based interactive system tracked the rat marked by an infrared LED above the arena and kept the shock sector in a fixed position relative to a defined reference frame (see the mechanical model). by mutual displacement of arena segments. For definition of the shock sector we set 4 following conditions.
1. Stable arena: Both segments of the arena were fixed relative to each other and to the room. In this configuration, shock sector was defined in both the arena and room frame and in the dark rats could use surface-bound cues for navigation.
2. Both segments shuffled: This was done as follows: As long as the rat was on the inner disk, the disk was slowly rotating (180o/min) together with the shock sector, which projected over the belt. When the rat left for the outer belt, the inner disk continued to rotate physically, but the shock sector remained stable with respect to the belt. After the rat entered the inner disk again, shape of the shock sector was projected idiothetically correctly back to it. The shock sector was thus defined in a “purely idiothetic frame”, i.e., maintained in an idiotheticaly predictable position, while putative intramaze cues in both segmets were “shuffled”, i.e., their position was irrelevant for navigation.
3. Inner disk shuffled-When the rat was on the inner disk, it was stable, but it started rotation when the rat left for the outer belt. The shock sector was always stable with respect to the belt and to the room. Intramaze cues were thus useless on the inner disk, but on the belt they were useful for navigation.
4. Outer belt shuffled- This condition was analogous as condition 2, except that when the animal was on the outer belt, the inner disk was not rotating. Possible cues on the central disk were reliable for orientation, while the ones on the belt were uninformative.
Another group of rats (n=7) was firstly trained to avoid a sector defined in the arena and room frame (condition 1) for 6 consecutive sessions. Afterwards, rats learned the task with the central disk shuffled (condition 3) for 6 sessions and with the peripheral belt shuffled (condition 4) for 6 sessions. Subsequently these 3 conditions were alternated daily for 3x4 sessions for comparison of performance of the rats in the final stage of training.
On the stable arena, rats reached the asymptotic level of PA performance in four days (5.6±1.03 shocks per 30 min). When navigating only by idiothesis, their performance did not improve in the course of sessions (~9 sessions), but varied around 20.8±4.08 shocks. In subsequent sessions when both conditions were alternated, rats performed with 4.8±0.38 shocks and 15.0±0.92 shocks per session in the arena frame and idiothetic frame, respectively. Extinction of place avoidance:
Rate of PA extinction was analyzed in 2-minute epochs during 30 min period, when shock was turned off. An arbitrary extinction criterion was reached when a rat spent more than 10% of total epoch time in the previously punished sector for 2 consecutive epochs.
On the stable arena, memory of the shock sector location was highly stable and long-lasting. At the asymptotic level of performance, rats did not reach this criterion during 30 min of extinction testing at all.
On the contrary, purely idiothetic memory, tested on shuffled arena, was strikingly unstable and short-lasting. The criterion was reached on average in 5.13±0.9 min after turning the shock off.
During six sessions of initial training, rats reached the asymptotic level of 5.714±0.68 shocks per 30 min on the stable arena (condition 1), 11.25±1.43 shocks on arena with the belt shuffled (condition 3), and 16.19±1,22 shocks on arena with disk shuffled (condition 4).
In subsequent sessions where these 3 conditions were interchanged daily, rats exhibited the same level of performance (5.39±0.50 shocks ) on the stable arena. On arena with the disk shuffled, rats even improved (to the level of 5.82±0.57 shocks) in comparison to the initial training as well as on arena with the belt shuffled (8.00±0.55 shocks).
This suggests that rats learned to use partly intact idiothetic information (in some parts of arena) for efficient navigation in the whole arena. It is also possible that they used some residual room frame information in condition 3, where the arena reference frame corresponds to the reference frame of darkened room. Slightly impaired navigation in condition 4 may be partly caused by inertial stimulation of rats on slowly rotating central disk. This will be possible to exclude in a new experimental apparatus presently developed in our laboratory.
Discussion and Conclusion
1. It is possible to experimentally dissociate exteroceptive spatial information from the idiothetic navigation and thus to examine the properties of pure path-integration, when both intra-maze and extra-maze cues are made irrelevant for succesful solution of the spatial task.
2. Navigation by pure path-integration is accurate only for a short period of time. At longer time intervals, it must be fixed by exteroceptive information in order to provide reliable and effective orientation in the space.
3. Studying path-integration in situations, where exteroceptive information is irrelevant for navigation, offers possibility to investigate neural mechanisms of idiothesis, e.g. by means of lesion and electrophysio-logical studies. Both these metodological approaches are applicable in presented open-field spatial task.
|Stuchlik, A.; Bures, J.; (1998). Solving a Spatial Problem in the Dark: Importance of Substratal Cues for the Rat. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Available at URL http://www.mcmaster.ca/inabis98/neuroscience/stuchlik0597/index.html|
|© 1998 Author(s) Hold Copyright|