The contents of episodic memory (but not other types of memory) are formed in the medial temporal lobe (the hippocampus and neocortical areas adjacent to it, including the entrorhinal area). This part of the brain seems to pick out moments of our conscious experience and commit them to memory. A recent paper looks at individual neurons involved in this memory formation. (Matias J. Ison, Rodrigo Quian Quiroga, Itzhak Fried. Rapid Encoding of New Memories by Individual Neurons in the Human Brain. Neuron, 2015; 87 (1): 220 DOI: 10.1016/j.neuron.2015.06.016)
The researchers used the method of asking cooperation from patients who are awaiting surgery for epilepsy and have been fitted with electrodes in a particular area of the brain to locate the exact focus of the epilepsy. This is probably the only ethical way in which studies can be done involving the recording from individual neurons in humans.
It is thought that memories are established by associations. But it is a puzzle how associations can be made by single exposures to unique natural events. The researchers showed patients many images of people, animals and places. Images that activated a single neuron from those that were being recorded were set aside for use. The images were made into composites where one person or animal image was put into one place image to appear to be the person/animal in that setting. The neurons that had responded to one of the images but not the other began responding to both original images after being exposed to the composite. A neuron was forming an association of the two parts of the composite. Associations formed rapidly in an all-or-nothing manner.
After rapid capture, these associations are probably manipulated and consolidated (or destroyed) in the production of more permanent and complex memories.
Abstract: “The creation of memories about real-life episodes requires rapid neuronal changes that may appear after a single occurrence of an event. How is such demand met by neurons in the medial temporal lobe (MTL), which plays a fundamental role in episodic memory formation? We recorded the activity of MTL neurons in neurosurgical patients while they learned new associations. Pairs of unrelated pictures, one of a person and another of a place, were used to construct a meaningful association modeling the episodic memory of meeting a person in a particular place. We found that a large proportion of responsive MTL neurons expanded their selectivity to encode these specific associations within a few trials: cells initially responsive to one picture started firing to the associated one but not to others. Our results provide a plausible neural substrate for the inception of associations, which are crucial for the formation of episodic memories.”