Cerebral Cortex, Vol. 13, No. 3, 225-238,
March 2003
© 2003 Oxford University Press
The Spatial Pattern of Response Magnitude and Selectivity for Orientation and Direction in Cat Visual Cortex
Department of Ophthalmology and Visual Sciences, University of British Columbia, Vancouver, Canada V5Z 3N9, , 1 Department of Neurobiology, The Weizmann Institute of Science, Rehovot, 76100, Israel and , 2 Center for Magnetic Resonance Research, University of Minnesota, 2021 6th St. SE, Minneapolis, MN 55455, USA
Address correspondence to Dr Nicholas V. Swindale, Department of Ophthalmology, University of British Columbia, 2550 Willow St., Vancouver, BC, Canada, V5Z 3N9. Email: swindale{at}interchange.ubc.ca.
Optical imaging studies of orientation and direction preference in visual cortex have typically used vector averaging to obtain angle and magnitude maps. This method has shown half-rotation orientation singularities (pinwheels) located within regions of low orientation vector magnitude. Direction preference is generally orthogonal to orientation preference, but often deviates from this, particularly in regions of low direction vector magnitude. Linear regions of rapid change in direction preference terminate in or near orientation singularities. The vector-averaging method is problematic however because it does not clearly disambiguate spatial variation in orientation tuning width from variation in height. It may also wrongly estimate preferred direction in regions where preference is weak. In this paper we analyze optical maps of cat visual cortex by fitting model tuning functions to the responses. This new method reveals features not previously evident. Orientation tuning height and width vary independently across the map: tuning height is always low near singularities, however regions of broad and narrow orientation tuning width can be found in regions of low tuning height, often alternating in a spoke-like fashion around singularities. Orientation and direction preference angles are always closely orthogonal. Reversals in direction preference form lines that originate precisely in orientation singularities.
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