Subiculum neurons map the current axis of travel

B R I E F COM M U N I C AT I O N S

Subiculum neurons map the current axis of travel

Jacob M Olson, Kanyanat Tongprasearth & Douglas A Nitz

Flexible navigation demands knowledge of boundaries, routes and their relationships. Within a multi-path environment, a subpopulation of subiculum neurons robustly encoded theaxis of travel. The firing of axis-tuned neurons peaked bimodally,at head orientations 180 apart. Environmental manipulations showed these neurons to be anchored to environmental boundaries but to lack axis tuning in an open arena. Axis-tuned neurons thus provide a powerful mechanism for mapping relationships between routes and the larger environmental context.

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Hippocampal CA1 neurons are known for the location specificity of their action potential firing during free foraging within an arena1. Such location-specific firing is modulated by constraints governing running behavior and available trajectories25. In this way, CA1 can and does encode multiple navigationally relevant spatial relationships.

As an efferent target of CA1, the dorsal subiculum may be primed to encode more complex spatial relationships6,7. Subiculum neurons sometimes exhibit place-specific firing akin to that of CA1 (refs. 8,9), yet reported differences include increased generalization of place fields across environments, scaling of firing fields to match arena size, and increased numbers and sizes of fields911. Some subiculum neurons, termed boundary vector cells (BVCs), exhibit spatial tuning reflecting proximity and orientation to arena borders12,13.

Multiple-pathway environments greatly increase the prevalence of task-relevant spatial relationships, but whether subiculum encodes such spatial features is unknown. We therefore obtained single- neuron recordings in rats performing a navigational task (Fig. 1a and Supplementary Figs. 1 and 2) wherein the layout of six interconnected routes ensured that each track section had a characteristic direction and axis of travel (Fig. 1b).

Immediately apparent from firing rate maps and directional tuning plots is a distinctive neuron subpopulation firing strongly whenever the animal ran in either of two opposing directions (Fig. 1c). Such firing was largely independent of room location. Put another way, such neurons fire when the animal travels in either direction along a single axis.

Since neurons with axis-tuned activity were not reported in work using open-field foraging tasks8,13,14, we considered the possibility that axis-specific firing emerges during...