Navigation and the underlying brain signals are influenced by various allothetic and idiothetic cues, depending on environmental conditions and task demands. Visual landmarks typically control navigation in familiar environments but, in the absence of landmarks, self-movement cues are able to guide navigation relatively accurately. These self-movement cues include signals from the vestibular system, and may originate in the semicircular canals or otolith organs. Here, we tested the otolithic contribution to navigation on a food-hoarding task in darkness and in light. The dark test prevented the use of visual cues and thus favored the use of self-movement information, whereas the light test allowed the use of both visual and non-visual cues. In darkness, tilted mice made shorter-duration stops during the outward journey, and made more circuitous homeward journeys than control mice; heading error, trip duration, and peak error were greater for tilted mice than for controls. In light, tilted mice also showed more circuitous homeward trips, but appeared to correct for errors during the journey; heading error, trip duration, and peak error were similar between groups. These results suggest that signals from the otolith organs are necessary for accurate homing performance in mice, with the greatest contribution in non-visual environments.

The role of limbic system structures in spatial orientation continues to be debated. The hippocampus (HPC) has been implicated in encoding symbolic representations of environments (i.e., cognitive map), whereas entorhinal cortex (EC) function has been implicated in self-movement cue processing (i.e., dead reckoning). These distinctions largely depend on the electrophysiological characteristics of cells within these regions and behavioral tasks that typically fail to dissociate environmental and self-movement cue processing. Topographic and kinematic characteristics of exploratory trip organization have been shown to differentially depend on environmental and self-movement cue processing. The present study examines the effects of either HPC or EC lesions on exploratory trip organization under varying lighting conditions. HPC lesions selectively impaired all measures of performance under dark conditions, but spared all measures of performance under light conditions. EC lesions impaired kinematic measures related to distance estimation under all conditions and impaired all measures of performance under light conditions. These results provide evidence that the HPC is involved in processing self-movement cues but not environmental cues, and EC is involved in processing distance estimates generated from either self-movement or environmental cues. These observations provide further support for serial processing of self-movement cues through limbic system structures that converge on the HPC.

The current set of studies examines the contribution of movement segmentation to self-movement cue processing for estimating direction and distance to a start location in humans and rats. Experiments 1 and 2 examined the extent that ambulatory dead reckoning tasks can be adapted to the manipulatory scale in humans. Experiments 3 and 4 investigated the performance of rats in similar tasks at their ambulatory scale. Movement segmentation had differential effects on absolute heading error for humans and rats when only comparing performance on specific tasks; however, movement segmentation had similar effects for both species when performance was examined across all tasks. In general, magnitude of movement segmentation was associated with absolute heading error in both humans and rats. In contrast, both species modified homeward segment kinematics based on the distance to the start location in all tasks, consistent with the use of self-movement cues to estimate distance. The current study provides evidence for a role of movement segmentation in processing self-movement cues selective to direction estimation and develops a foundation for future studies investigating the neurobiology of spatial orientation.

Degeneration of the septohippocampal system is associated with the progression of Dementia of the Alzheimer's type (DAT). Impairments in mnemonic function and spatial orientation become more severe as DAT progresses. Although evidence supports a role for cholinergic function in these impairments, relatively few studies have examined the contribution of the septohippocampal GABAergic component to mnemonic function or spatial orientation. The current study uses the rat food-hoarding paradigm and water maze tasks to characterize the mnemonic and spatial impairments associated with infusing GAT1-Saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). Although infusion of GAT1-Saporin significantly reduced parvalbumin-positive cells in the MS/VDB, no reductions in markers of cholinergic function were observed in the hippocampus. In general, performance was spared during spatial tasks that provided access to environmental cues. In contrast, GAT1-Saporin rats did not accurately carry the food pellet to the refuge during the dark probe. These observations are consistent with infusion of GAT1-Saporin into the MS/VDB resulting in spared mnemonic function and use of environmental cues; however, self-movement cue processing was compromised. This interpretation is consistent with a growing literature demonstrating a role for the septohippocampal system in self-movement cue processing.

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