One picture is worth at least a million neurons.

How many neurons participate in the representation of a single visual image? Answering this question is critical for constraining biologically inspired models of object recognition, which vary greatly in their assumptions from few "grandmother cells" to numerous neurons in widely distributed networks. Functional imaging techniques, such as fMRI, provide an opportunity to explore this issue, since they allow the simultaneous detection of the entire neuronal population responding to each stimulus. Several studies have shown that fMRI BOLD signal is approximately proportional to neuronal activity. However, since it provides an indirect measure of this activity, obtaining a realistic estimate of the number of activated neurons requires several intervening steps. Here, we used the extensive knowledge of primate V1 to yield a conservative estimate of the ratio between hemodynamic response and neuronal firing. This ratio was then used, in addition to several cautious assumptions, to assess the number of neurons responding to a single-object image in the entire visual cortex and particularly in object-related areas. Our results show that at least a million neurons in object-related cortex and about two hundred million neurons in the entire visual cortex are involved in the representation of a single-object image. Copyright 2004 Elsevier Ltd.