Do single neurons in the primary visual cortex (V1) represent a constant speed? Priebe et al. (2006) reported that a fraction of complex cells in macaque V1 are tuned to a constant speed for high-contrast stimuli because they showed tilted response profiles in the 2D frequency domain (1D of space and 1D of time). We examine this issue by measuring the responses of cat V1 neurons at various contrast levels in the 3D frequency domain (2D of space and 1D of time).

Single units were recorded in the V1 of paralyzed and anesthetized adult cats while dynamic 2D white noise stimuli were presented in the dominant eye. These noise stimuli contain various contrasts, or amplitudes, for all spectral components, allowing us to examine the contrast responses of neurons at different spatiotemporal frequencies simultaneously.

Recorded spikes were analyzed by using a variant of reverse correlation methods. First, each stimulus ensemble triggered by spikes was picked up for eight consecutive stimulus frames, and this spike-triggered stimulus was transformed into the 3D frequency domain via Fourier transform. Second, the same Fourier transform was performed for all stimuli to investigate the distribution of the amplitude spectra of raw stimuli. To obtain contrast response functions at various spatiotemporal frequencies, the distribution of the amplitude spectra of spike-triggered stimuli was compared with that of raw stimuli for each spatiotemporal frequency.

Various shapes of contrast responses were observed across neurons; some showed a high threshold (i.e. they responded only to high-contrast stimuli) while others showed a low threshold (i.e. their responses increased gradually as contrast became higher). However, contrast responses were virtually identical across different spatiotemporal frequencies within single neurons. Moreover, no cells in our population showed tilted response profiles in the 3D frequency domain at any contrast level. Therefore, our results rule out the possibility that complex cells in cat V1 can be tuned to a constant speed because of different contrast responses for different spatiotemporal frequencies.