A stable home-base promotes allocentric memory representations of episodic-like everyday spatial memory

Abstract

A key issue in neurobiological studies of episodic-like memory is the geometric frame of reference in which memory traces of experience are stored. Assumptions are sometimes made that specific protocols favour either allocentric (map-like) or egocentric (body-centred) representations. There are, however, grounds for suspecting substantial ambiguity about coding strategy, including the necessity to use both frames of reference occasionally, but tests of memory representation are not routinely conducted. Using rats trained to find and dig up food in sandwells at a particular place in an event arena (episodic-like 'action-where' encoding), we show that a protocol previously thought to foster allocentric encoding is ambiguous but more predisposed towards egocentric encoding. Two changes in training protocol were examined with a view to promoting preferential allocentric encoding-one in which multiple start locations were used within a session as well as between sessions; and another that deployed a stable home-base to which the animals had to carry food reward. Only the stable home-base protocol led to excellent choice performance which rigorous analyses revealed to be blocked by occluding extra-arena cues when this was done after encoding but before recall. The implications of these findings for studies of episodic-like memory are that the representational framework of memory at the start of a recall trial will likely include a path direction in the egocentric case but path destination in the allocentric protocol. This difference should be observable in single-unit recording or calcium-imaging studies of spatially-tuned cells.

Keywords: event arena; frames of reference; hippocampus; path integration; rats.

Figure 1. Three distinct protocols.

(A) Protocol 1: In our standard protocol (as used in Nonaka et al, 2017), there was a single encoding trial consisting of three runs (black path) out to either of two sandwells (one of which was rewarded) followed by returns to the same startbox (green paths). After a memory delay, a choice trial included a run out (red path) to choose amongst 6 sandwells (only one correct) and then a return once again to same startbox. (B) Protocol 2: The primary modification was the use of four different startboxes within a session, thereby changing from 3 rewards at encoding within a single trial (Protocol 1) to three encoding trials each with a single reward pellet. The green return paths are representative in displaying the confusion of the animals about the location to which to return. The choice trial was from the fourth location of the day. (C) Protocol 3: The primary further modification was the use of a fixed 'home-base' (blue) to which the animals had to carry the food that they had dug up (green paths). Encoding was now divided into only 2 trials, but included the opportunity to run out of the home-base back to the sandwell on each trial (i.e. 4 rewards during encoding). The recall choice trial (and any probe trial) was started from a novel location on that session, as in Protocol 2.

Figure 2. Protocol 1 - Experiment 1A (San Diego) - Time-dependent decay of every-day memory and the effect of 180 Startbox rotations on recall trial performance.

(A,B) Sandwell arrangement for the arena used in San Diego for the study of memory retention across delays (30 min to 72 h). Memory for the correct location was significant at short retention intervals of 30 min and 3 h, consistent with Nonaka et al (2017), but memory returned to chance within 24 h. (C, D) Impact of a 180° startbox rotation in well-trained rats. The startbox location varied across successive sessions (not shown). In the recall trial, rats were either started from the same daily location, or the startbox was rotated by 180°. Rotation resulted in a significant decline of the performance index (PI). Note that individual animals vary, with one representative animal (no. 7) displaying only temporary disruption of performance, while another (no. 3) was consistently affected over 10+ sessions. (E, F) Occlusion of extramaze cues and removal of intramaze cues had no effect on performance. Means +/- 1 SEM.

Figure 3. Protocol 1 - Experiment 1B (Edinburgh) - Startbox rotations impair recall trial performance.

Using the same animals as in Nonaka et al (2017), we examined the impact of 180° (A, B) or 90° (C, D) rotations of the startbox used between the encoding and memory recall choice trial. The design allowed counterbalancing for “near” vs “far” (A), and “left” vs “right”(C) in the separate tests. Repeated measures data allowed comparison of an individual animals' scores on rotated and non-rotated trials. Both rotations resulted in a significant decline of the performance index (PI) on recall to chance level. Means +/- 1 SEM and individual animal data plots.

Figure 4. Protocol 2 - Experiment 2 (Edinburgh) - Different startbox locations across successive encoding trials.

Pretraining consisted of a 5-alternative discriminative choice procedure in which all 4 trials of a session started from different startboxes (yellow shading). From session 11 onwards, a single rewarded sandwell was used on each of 3 encoding trials from three different startbox locations (green shading). On the recall trial, there were 5 sandwells, with the obligation to choose the correct sandwell and then return with the food reward to the startbox location of that trial. From sessions 16-18, there were 9 encoding trials (blue shading, 1 pellet each). Performance was initially good upon transfer to the main encoding-choice protocol (session 11), but declined across further training sessions. Means +/- 1 SEM.

Figure 5. Protocol 3 - Experiment 3A - impact of a stable home-base to which food-reward should be carried.

(A) Rapid acquisition of effective performance, with stable above chance performance from session 16, this maintained through to session 70. Two non-encoding control sessions were conducted at the start and end of training (s18, s68) both show performance dropping to chance. Extended regular training provided a stable >80% PI baseline permitting various memory probes tests including a test of retention over 24 h and the impact of withdrawing spatial cues (Figure 6). (B) Memory retention with 3 pellet reward declines from well above chance at 24 min to a lower but still above chance level at 24 h. (C) latency data for the time taken to dig at correct sandwell. Note massive increase in this. time on non-encoding choice-trial sessions as predicted. (D) Inter-experimenter correlation of blind probe test scoring of two experimenters (AA and TT). Means +/- 1 SEM and individual animal data plots (B).

Figure 6. Protocol 3 - Experiment 3A - impact of a stable home-base on performance and its decline upon removal of spatial cues.

(A) Curtains occluding spatial cues were either drawn around the arena, and intra-maze cues removed, or these cues were fully available. (B) In a recall probe test at 24 min, performance was very good with cues available (>70%) or fell to chance (without cues). Means +/- 1 SEM and individual animal data plots.

Figure 7. Protocol 3: Experiment 3A - detailed analysis of paths taken in the arena.

(A) The criterion for identifying whether the rats approached the home-base or not before they reached the correct reward sandwell on successive daily trials. An approach was at an angle > 45°, whereas a by-pass was at < 45°. (B) A possible cryptic 'egocentric' strategy with the 'home-base' might be to run to it and then use it as an anchor-point for the start of a path-integration-associated accumulation of information. This view predicts that approaches to the home-base location would increase within each session, and be high on recall trials (red symbols and shading). In fact, the actual data (black-symbols) shows the opposite trend. Some animals visited the home-base location on encoding trial 1, but this declined as the animals learned the allocentric location of that session 's rewarded sandwell. (C) There was no difference in PI score between a subset of animals that approached the home-base first (grey) and those first visiting the correct sandwell (green). (D) The frequency of different combinations of preferential approach to the home-base before visiting the correct rewarded sandwell. The left is more egocentric, while right is more allocentric. The most egocentric category implies the rats would always visit the home-based before digging at the correct sandwell, while the most allocentric category implies they should visit the correct sandwell directly.

Figure 8. Experiment 3B - Electrophysiology, Histology and. Behaviour - Impact of bilateral inactivation of the dorsal hippocampus.

(A) Experimental design for electrophysiology. Dentate field potentials (fEPSP and PS) were measured over a period from -30 min to +180 min after 2 ul drug infusion of aCSF or CNQX. In the later behavioural study, the blue shading marks time-point after infusion when encoding trials were given (Data in B) and the grey shading marks the time point when recall trials were given (C). Note that both the fEPSP and the PS decline to near zero from a point about 15 min after infusion until 90 min, whereupon both measures return to baseline. (B) The hippocampus was inactivated by bilateral microinfusion of CNQX 15 min before the first of two encoding trials. The recall probe test was conducted 2.5 h later after the effects of CNQX would have dissipated (and thus hippocampal activity should be back to normal). Memory was poor after CNQX but normal after vehicle infusions. (C) The hippocampus was inactivated by bilateral microinfusion of CNQX 15 min before the recall probe test, encoding having been conducted in the absence of the drug. Memory was again poor after CNQX. Means +/- 1 SEM and individual animal data plots. (D) Histological verification of locations of the tips of the bilateral guide cannulae used for drug infusions.