|Memory > Activation of Silent Neurons by Reminding Cues|
Suppose you had visited an attraction (e.g., Disneyland) many years ago. You may forget most of the events that happened there. However, if you visit the attraction again, the scene may trigger your old memory. This is an example of memory extinction, wherein the memory about the attraction has faded away due to the silence of memory engram cells. The "natural reminding cues" (in this example, the scene of the attraction) may activate the silent neurons, bringing about old memories.
Figure 11-1 shows the firing characteristics of silent neurons. These data were obtained from patients who were implanted with depth electrodes for clinical reasons. The implanted microelectrodes allow for single-cell recordings. In the experiments, patients were presented with pictures according to their preferences and background. The microelectrodes were implanted into the patient's medial temporal lobe which includes the hippocampus, amygdala, and entorhinal, parahippocampal and perirhinal cortices. In each trial, the recording lasts for about 1500 milliseconds. The recorded action potentials (spikes) are represented as dots in this figure. We see that the neurons in Cluster 3 respond selectively to the picture of Russian President Putin while Cluster 5 reacts preferentially to the picture of Taj Mahal in India.
The above result suggests that the memory of an object or event is encoded in a particular set of neurons. These neurons typically remain silent until they are activated by natural reminding cues (in this example, the image of Putin or Taj Mahal). However, modern technology has been able to retrieve memories directly by using optogenetic stimulation, (Liu et al., 2012), even for amnesic memory that cannot be retrieved by natural reminding cues (Ryan et al., 2015; Roy et al., 2017). Remarkably, stimulation of the dentate gyrus is sufficient to retrieve particular fear memories (Liu et al., 2012), despite the fact that memories are stored in distributed brain areas (Herry and Johansen, 2014).
Author: Frank Lee