The involvement of entorhinal cortex in cross-modal associative memory

Abstract

As the gateway of the hippocampal system to the neocortex, the entorhinal cortex (EC) is hypothesized as a hub in the transformation process of recent memory to remote memory. The purpose of this research is to explore the EC's role in the retrieval of recent and remote cross-modal associative memory and identify another behavioral indicator of this kind of association besides freezing percentage. In the first part of the thesis, a within-subject approach was adopted to examine the EC's role in cross-modal associative memory. An association between a visual stimulus and an auditory stimulus was induced by classical fear conditioning. The rats in the EC-intact condition exhibited 96%±2% freezing to the light stimulus in Week 1, while the rats in the EC-inactivated condition showed 2%±1% freezing. This difference lasted for the first three weeks. The result indicated that the EC participated in the retrieval of cross-modal associative memory. The freezing percentages in both conditions approached each other in the last two weeks. The mean freezing percentage in the EC-intact condition decreased to 54%±17% in Week 4, while in the EC-inactivated condition it increased to 34%±5%. This trend indicated that the dependence of the retrieval on EC decreased over time. In the second part of the thesis, a cross-modal association between light and sound was demonstrated by a delay fear conditioning and a cued-reward task. Firstly, the light and sound stimuli were combined as the conditioned stimulus and associated with a foot shock in the experiment group; only the light stimulus and the foot shock were presented to the control group. Secondly, the sound stimulus was used as the cue for water reward. Thirdly, the sound was replaced by the light to examine whether the cued-reward task was an indicator for the cross-modal associative memory. Six out of 7 rats in the experiment group, compared with 1 out of 8 rats in the control group, succeeded in getting the water reward in the first trials of the replacement. This difference between the two groups lasted for the next 20 trials. It indicated that visuoauditory association was formed in the conditioning phase and could be retrieved in the reward phase with the cued-reward task. In summary, the retrieval of cross-modal associative memory depends on the EC and the dependence decreases over time. The cued-reward task can be used as an indicator to represent the cross-modal association between visual and auditory stimuli.