Locomotion-induced hippocampal theta is independent of visual information in rats during movement through a pipe

Behavioural correlates of the hippocampal theta rhythm have been suggested to include voluntary motor behaviours and spatial learning. The involvement of visual information during these processes is still undetermined. Therefore, our aim was to clarify the contribution of locomotion and visual information to the generation of hippocampal theta during locomotion. Forty-one Wistar-Kyoto male rats (8-9 weeks old) were separated into active or passive movement groups that travelled through a pipe, which was either lit or unlit. Animals were implanted with a bipolar electrode in the hippocampus for local field potential recording. Head and leg movements were recorded by accelerometer and leg electromyogram, respectively, and stress levels were assessed by heart rate measurement. Theta power (4-12 Hz) was divided into medium theta (MT, 6-10 Hz) and low theta (LT, 4-6 Hz) power. There was a significant effect of locomotion (p<0.001, two-way ANOVA) on theta power, MT power, and theta mean power frequency. Visual information, however, had no significant effect, nor did the interaction between locomotion and visual information. The lack of visual information effect could not be explained by differences in movement patterns or stress levels, because these two measures did not differ between the lit and unlit conditions. Our results indicate that visual information is not essential for locomotion-induced hippocampal theta, implying that theta oscillation during spatial learning does not reflect sensory processing of visual information.