As illustrated with the example of Fig. 1, concept cells have a sparse, abstract and explicit representation of the meaning of the stimulus. By “meaning” we refer to the subjective, internal representation created by the subject for memory functions (i.e. how something will be thought of, and potentially remembered). This conclusion is based on the finding on concept cells and the well-established role of the medial temporal lobe in declarative memory [3, 4]. It is therefore not surprising that concept cells tend to encode personally-relevant items [11] – i.e. whatever the subject may mind to store in memory. The representation by concept cells is sparse because they get activated by relatively very few of the presented stimuli [12]; it is abstract because a concept cell fires to the person or object presented and not to particular details of the picture (or other type of stimulus) shown [7, 9] and it is explicit because from the firing of a concept cell we can tell whether the particular concept is being shown [12] or even thought [13]. The finding of concept cells firing to related concepts [5, 9, 14] is in line with the key role of associations in the formation of new declarative memories. Concept cells may then be the key neural substrate for the formation of new declarative (and specifically episodic) memories.