Dissociable roles of the medial and lateral divisions of the entorhinal cortex for scene memory-dependent behavior

Abstract

The medial temporal lobe, which includes the hippocampal system, has been known to play an important role in forming episodic memory. A number of studies have described visual scene as one of the most important sensory stimuli for the hippocampal system in humans as well in rodents and nonhuman primates. How scene-dependent memory, however, is processed by upstream structures of the hippocampus is largely unknown. In the present thesis, differential roles of the lateral and medial subdivisions of the entorhinal cortex (LEC and MEC, respectively) were examined in scene-dependent memory tasks. When the rat made spatial choices, namely turning left or right in a T-maze using scene stimuli, inactivation of the MEC, but not the LEC, by muscimol (MUS) severely impaired behavioral performance. However, when the task required the rat to make non-spatial choices between digging in the sand in the jar and pushing that same jar, based on scene stimuli, the LEC became more important than the MEC. This double dissociation demonstrates that (i) the subdivisions of the entorhinal cortex are involved in making distinct behavioral responses using scenes and (ii) motor responses seem to be important in determining the involvement of the LEC and MEC in scene-dependent memory tasks. While previous studies support a simple what versus where dichotomy for the roles of the LEC and MEC, respectively, the present thesis suggests the role of the LEC for What should I do to this object in this context? and the MEC for Where should I go from here in this context?.