Spatial context and the functional role of the postrhinal cortex

Abstract

The postrhinal cortex (POR) serves as a key input area to the hippocampal system. It receives highly processed information from the ventral visual stream and other limbic areas including the retrosplenial cortex, parahippocampal areas, and portions of the limbic thalamus. The POR was studied early on by David Bucci and colleagues who first postulated that the POR plays a major role in contextual learning. Here we review a number of approaches and experimental studies that have explored POR's role in contextual processing. We discuss POR lesion studies that monitored deficits in fear conditioning tasks and the effects that these lesions had on processing visual landmark information. We then review the types of spatial correlates encoded by POR cells. A large number of head direction (HD) cells are present, although recent findings suggest that many of them are more accurately characterized as landmark modulated-HD cells as opposed to classic HD cells. A significant number of POR cells are also tuned to egocentric properties of the environment, such as the spatial relationship of the animal to the center of its environment, or the distance between the animal and either the environment's center or its boundaries. We suggest potential frameworks through which these functional cell types might support contextual processing. We then discuss deficits seen in humans who have damage to the homologous parahippocampal cortex, and we finish by reviewing functional imaging studies that found activation of this area while human subjects performed various tasks. A preponderance of evidence suggests that the POR, along with its interactions with retrosplenial cortex, plays a key role in contextual information processing.

Keywords: Context; Egocentric; Postrhinal cortex; Spatial orientation; Visual landmarks.

Figure 1 –

Egocentric and allocentric spatial correlates in POR. A) Schematic top-down view of the recording arena defining egocentric and allocentric spatial variables encoded by POR cells. Star represents the geometric centroid of the environment. B) Directional spike plot and center-bearing tuning curve for an example POR center-bearing cell. The gray trace represents the path of the animal during the recording session, while the dots represent locations where the cell fired a spike. Dots are colored according to the animal’s allocentric head direction when the spike was fired (color wheel on left). C) Directional spike plot and center-distance tuning curve for an example POR center-distance cell. Note the approximately linear increase in firing rate with distance from the centroid. D) Directional spike plot and head direction tuning curve for an example POR HD cell. E) Directional spike plot and tuning curves for a POR cell that showed conjunctive tuning to multiple variables. Figure modified from LaChance et al., 2019.

Figure 2 –

Modulation of POR LM-HD cell firing by visual landmarks. A) Schematic top-down view of the recording arena across the three sessions of the AB experiment, including the reference frame for measuring HD. B) Tuning curves for two example POR LM-HD cells (left peak-locked, right trough-locked) across the three sessions of the AB experiment. Cue positions in an allocentric reference frame are indicated by red lines. Note that both cells became bidirectionally tuned during the AB session. C) Same as (A) but for the no cue experiment. D) Tuning curves for two example POR LM-HD cells (left peak-locked, right trough-locked) across the three sessions of the no cue experiment. Note that both cells became less strongly tuned when cue A was removed. E) Same as (A) but for the B experiment. F) Tuning curves for two example POR LM-HD cells (left peak-locked, right trough-locked) across the three sessions of the B experiment. Note that both cells became bidirectionally tuned when cue B was introduced and cue A was removed. G) Same as (A) but for the AC experiment. H) Tuning curves for two example co-recorded peak-locked POR LM-HD cells across the three sessions of the AC experiment. Note that, while the cell on the left did not become bidirectional in response to the addition of cue C (the most typical response), the cell on the right did, suggesting that both types responses are simultaneously possible among POR cells. Figure modified from LaChance et al., 2022.

Figure 3 –

Centroid-boundary distance encoding of environmental geometry. A) left, top-down schematic view of the recording arena showing measurement of egocentric center-bearing and HD. right, egocentric center-bearing and HD are summed to compute allocentric center-bearing (α). This calculation can be reversed to compute the allocentric bearing of the animal’s location with respect to the centroid (1- α). B) left, top-down schematic view of a 120 × 120 cm square recording arena. Arrows pointing from the arena centroid to the boundary at specific allocentric bearings indicate measurement of centroid-boundary distance (CBD) along that angle. right, CBDs plotted as a function of allocentric bearing relative to the centroid. The result is a CBD signature that is unique to the shape and size of the arena. C-E) Same as B but for a 60 × 60 cm square, 120 × 60 cm rectangle, and 120 cm diameter circle, respectively. Note that while the arenas in B and C are both squares, they have unique CBD signatures because of their different sizes.

Figure 4 –

Potential interactions between egocentric and HD signals in representing spatial context. A) Top-down schematic views of a recording arena with identical geometries but different visual landmarks, along with the locations of the unweighted centroid c based on all physical cues of the environment and the salience centroid s which is weighted by the salience of each physical cue. Note that s is displaced preferentially toward the location of a white cue card in each context, and to a lesser extent toward the black cue card in context 2. The salience vector $\overrightarrow{CS}$ connects the unweighted centroid to the salience centroid and is unique for each context. B) Left, vector field showing the firing preferences of a center-bearing by center-distance cell; middle, vector field showing the firing preferences of an HD cell with PFD pointing northeast; right, resultant vector field after summing the center-bearing/distance and HD fields. Note that the focal point of the center-bearing/distance field (anchor point; indicated by a red X) has shifted toward the northeast after summing with an HD signal.