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1 Smith-Kettlewell Eye Research Institute, San Francisco, CA 94115, USA
* To whom correspondence should be addressed. E-mail: valerio{at}ski.org.
A recent optical imaging study of primary visual cortex (V1) by Basole, White and Fitzpatrick (Nature, 2003) demonstrated that maps of preferred orientation depend on the choice of stimuli used to measure them. These authors measured population responses expressed as a function of the optimal orientation of long drifting bars. They then varied bar length, direction and speed, and found that (1) stimuli of a same orientation can elicit different population responses, and (2) stimuli with different orientation can elicit similar population responses. We asked whether these results can be explained from known properties of V1 receptive fields. We implemented an "energy model" where a receptive field integrates stimulus energy over a region of three-dimensional frequency space. The population of receptive fields defines a volume of visibility, which covers all orientations and a plausible range of spatial and temporal frequencies. This energy model correctly predicts the population response to bars of different length, direction, and speed, and explains the observations made with optical imaging. The model also readily explains a related phenomenon, the appearance of motion streaks for fast moving dots (Geisler, Nature, 1999). We conclude that the energy model can be applied to activation maps of V1 and predicts phenomena that may otherwise appear to be surprising. These results indicate that maps obtained with optical imaging reflect the layout of neurons whose receptive field responds to the stimulus energy, not the layout of neurons that are selective to isolated stimulus features such as orientation, direction, and speed.
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