Publications

The representation of orientation in primary visual cortex (V1) has been examined extensively at a fine spatial scale corresponding to the columnar architecture. In humans, orientation can be decoded from functional magnetic resonance imaging (fMRI) signals using multivariate classification methods, but it is unknown whether orientation decoding depends on fine-scale, columnar architecture. We used a phase-encoded mapping procedure to test the hypothesis that orientation is represented in human cortex at a coarse spatial scale, and that this organization provides the basis for orientation decoding.

Cortical responses were measured using fMRI (3T Siemens Allegra, 8-ch phased-array surface coil, 2 × 2 × 2 mm, 24 sl perpendicular to calcarine sulcus), while subjects viewed an oriented sinusoidal grating (0.5 cycles/deg) that filled a 4 deg peripheral annulus with a smooth edge. The orientation of the stimulus changed every 1.5 s, cycling through sixteen evenly spaced angles (0-180 deg). The response of each voxel was fit to a sinusoid with period of the stimulus. The phase of the best-fitting sinusoid indicated the preferred orientation of the voxel.

We observed a topographic map of orientation preference in human V1, confirming and extending previous reports of a quadrant bias for radial orientations (Sasaki et al, Neuron, 2006). The map was tightly co-localized with the retinotopic map: at each location within V1, responses exhibited a preference for radial orientations. Circular correlation was used to quantify the similarity between the orientation and polar angle maps (r = 0.75; p < 0.0001). Control experiments confirmed that the orientation map was robust to a variety of stimulus parameters, and was not due to either attention or eye movements. Multivariate classification analyses were applied to decode stimulus orientation. Averaging the data in a manner consistent with the structure of the topographic map did not affect decoding accuracy, demonstrating that the map was sufficient to classify orientation. Our results strongly suggest that orientation decoding does not reflect the irregular spatial arrangements of orientation columns.

There is a coarse-scale topographic map of orientation in V1. The orientation map provides a parsimonious explanation for how multivariate classification methods decode stimulus orientation from fMRI measurements, and challenges the conjecture that decoding reflects random irregularities in the fine-scale columnar architecture.