Spatial Frequency Maps in Cat Visual Cortex

1. Spatial Frequency Maps in Cat Visual Cortex Naoum P. Issa, Christopher Trepel, and Michael P. Stryker Jasmine Kwong

2. The Question • What is the fundamental functional organizational unit of the brain? ---> The organization of orientation selectivity has been thoroughly described, but what is the arrangement of spatial frequency (SF) preference in V1?

3. Suggested layouts • Laminar - layers • Columnar - columns • Clustered - random • Pinwheel - ordered • Binary - high and low SFs

4. Which one is correct? • Present stimulus and obtain optical imaging of intrinsic signals • Look at geometry of patterns of images to examine layout of SF preference

5. Methods • Optical imaging of intrinsic signals • 7 cats, 7 - 16 weeks old (contrast sensitivity has reached adult levels), anesthetized • Experiment begins with mapping of orientation and ocular dominance by showing 20 stimulus conditions • Spatial frequency mapping required much more stimulus conditions, so were distributed among either 2 or 4 sets that were run sequentially

6. Methods • Each stimulus set included a wide range of SFs and orientations and was designed to provide a coarse map before combination with data from other sets • A map from 8 orientations and 8 SFs was constructed from 4 stimulus sets, each consisting of 22 conditions

7. Methods • Additional sporadic multi-unit extracellular recordings (electrophysiology) to confirm that optically imaged SF maps correspond to neuronal SF activity • After the experiments, the animal was sacrificed --> Histology of visual cortex (electrode tracks and recording positions reconstructed)

8. Results • Area 18 was well stimulated by the 2 lowest SFs presented - 0.1, 0.16 c/deg • Area 17 was activated by gratings of higher SF

9. Results

10. Results - Regions of cortex respond to narrow ranges of SFs - there really are domains that prefer intermediate SFs

11. Results • SF preference in area 17 is continuous over most of the cortex but has distinct linear discontinuities

12. Relationship between SF and orientation • Colocalization of pinwheel centers with domains of extreme SF, low or high • Preferred SF not independent of stimulus orientation for many single units, BUT… • Orientation preference changes little with stimulus SF --> orientation preference is primarily independent of stimulus SF

13. Conclusions Four characteristics of organization of SF preference in V1 • Cells with SF preferences between 0.2 and 1.8 c/ are clustered into domains with other cells of similar SF preference • These SF domains are organized into a map that is locally continuous across V1, though the spatial gradient of SF preference is not constant, and there are often clear fractures in the SF map before the entire range of SF preference is represented

14. Conclusions 3) Distance between domains of very different SF preference conforms to the hypercolumn description of cortical organization 4) Cortical domains containing cells that prefer extremes of SF continuum tend to colocalize with pinwheel centers in cortical map of orientation preference

15. Problems • Anesthetized cats (changes cortical properties) • Low Signal to noise ratio of method --> Need to average pictures. Controversial. More than one way to do it. Arbitrary? Can create artifacts. • Coarse sampling of spatial frequency - only 8. • Though SF preference sometimes varied with deeper penetrations into cortex, no systematic investigation of possibility of laminar organization of SF