Grandmother Cells and Distributed Representations
Résumé
It is generally accepted that a typical visual stimulus will be represented by the activity of many millions of neurons distributed across many regions of the visual cortex. However, there is still a long-running debate about the extent to which information about individual objects and events can be read out from the responses of individual neurons. Is it conceivable that neurons could respond selectively and in an invariant way to specific stimuli--the idea of "grandmother cells"? Recent single-unit recording studies in the human medial lobe seem to suggest that such neurons do indeed exist, but there is a problem, because the hit rate for finding such cells seems too high. In this chapter, I will look at some of the implications of this work and raise the possibility that the cortical structures that provide the input to these hippocampal neurons could well contain both highly distributed and highly localizt coding. I will discuss how a combination of STDP and temporal coding can allow highly selective responses to develop to frequently encountered stimuli. Finally, I will argue that "grandmother cell" coding has some specific advantages not shared by conventional distributed codes. Specifically, I will suggest that when a neuron becomes very selective, its spontaneous firing rate may drop to virtually zero, thus allowing visual memories to be maintained for decades without the need for reactivation.