Old topic, I know, but thought I’d add my two cents on the “grandmother cells” idea.
There isn’t going to be “A” grandmother cell (i.e that cell dies, you forget about grandma). The representation for an abstract concept of “grandmother” (the whole concept, not simply the word) will consist of many active cells, with the normal noise and error tolerances that come with SDRs.
Most of the cells in the representation will NOT only be active for the concept of “grandmother”. The majority of the cells will also activate as part of representations for other abstract concepts which share semantics, such as “elders”, “Christmas cookies”, “nursing home”, etc. depending on the specific experiences which built the “grandmother” concept.
There will be a diminishing percentage of cells in the representation which have receptive fields that strongly match the one specific concept but weakly match any other concepts. You could consider only those few that breach some minimum threshold to be “true” grandmother cells (i.e. almost never activate as part in any other representation).