Episodic memories are tagged with information about time and place. If we remember an event then it is almost certain we will remember where it happened and where it lies in the temporal sequence of events. Research has shown that an activity pattern in a part of the brain involved in memory, the entorhinal cortex, feeds where and when information to the hippocampus which forms the new memory.
The research is reported in a recent paper: Takashi Kitamura, Chen Sun, Jared Martin, Lacey J. Kitch, Mark J. Schnitzer, Susumu Tonegawa. Entorhinal Cortical Ocean Cells Encode Specific Contexts and Drive Context-Specific Fear Memory. Neuron, 2015; DOI: 10.1016/j.neuron.2015.08.036.
The entorhinal area involved has been likened to an ocean of context specific ‘where’ cells with islands of ‘when’ cells. The ocean cells signal the CA3 cells of the hippocampus and the island cells signal the CA1 cells. If ocean cells are blocked, animals cannot learn to connect fear with a particular environment. Island cells seem to react to the speed an animal is moving at and manipulating their signals changed the gap between events being linked in an animals memory. This is probably one of many ingredients in the processing of time-and-space.
Absract: “Forming distinct representations and memories of multiple contexts and episodes is thought to be a crucial function of the hippocampal-entorhinal cortical network. The hippocampal dentate gyrus (DG) and CA3 are known to contribute to these functions, but the role of the entorhinal cortex (EC) is poorly understood. Here, we show that Ocean cells, excitatory stellate neurons in the medial EC layer II projecting into DG and CA3, rapidly form a distinct representation of a novel context and drive context-specific activation of downstream CA3 cells as well as context-specific fear memory. In contrast, Island cells, excitatory pyramidal neurons in the medial EC layer II projecting into CA1, are indifferent to context-specific encoding or memory. On the other hand, Ocean cells are dispensable for temporal association learning, for which Island cells are crucial. Together, the two excitatory medial EC layer II inputs to the hippocampus have complementary roles in episodic memory.”