The behavioural effects of lesions to the head direction cell circuit on spatial learning in rats

The head direction (HD) cell circuit acts as a neurological compass, providing information on directional heading to the hippocampus. The HD signal originates in reciprocal connections between the lateral mammillary nuclei (LMN) and the dorsal tegmental nuclei (DTN); the signal is then relayed to the anterodorsal thalamic nuclei (ADN) and cortical areas. The functional role of these regions in spatial learning problems is not well understood. Rats underwent neurotoxic lesions of either the LMN or ADN and were tested using three tasks thought to depend on directional heading. Impairments were seen in both lesioned groups in the water T-maze tasks that required discrimination of headings that were 90° (rotation task) or 180° (direction task) apart. Lesioned rats were also impaired on a 12-arm maze if they were required to discriminate between adjacent arms that were 30° apart, but not when arms were separated by at least 90°. Performance on a response reversal task, where the maze was rotated by 90° each time the response-reinforcer contingency changed, was unaffected by lesions to the LMN or ADN, indicating that this task may not be critically dependent on the HD cell signal. Our results suggest that the difficulty of spatial problems relates to the angular specificity of direction discrimination required