Cortical and behavioral sensitivity to eccentric polar form.

Patterns composed of local features aligned relative to polar angle, yielding starbursts, concentric circles, and spirals, can inform the understanding of spatial form perception. Previous studies have shown that starburst and concentric form instantiated in Glass patterns are, relative to spirals, both more readily detected in noise and evoke higher levels of blood-oxygen level-dependent (BOLD) signal, as measured with functional magnetic resonance imaging (fMRI), in the retinotopic cortex. However, such studies have typically presented the polar form at the center of gaze, which confounds the distribution of local orientations relative to fixation with variations in polar form. Here, we measure psychophysical detection thresholds and evoked BOLD signal to Glass patterns of varying polar orientation centered at eccentricity. We find an enhanced behavioral sensitivity to starburst and concentric form, consistent with previous studies. While visual areas V1, V2, V3, V3A/B, and hV4 showed elevated levels of BOLD activity to concentric patterns, V1 and V2 showed little to none of the increased activity to starburst patterns evident in areas V3, V3A/B, and hV4. Such findings demonstrate the anisotropic response of the human visual system to variations in polar form independent of variations in local orientation distributions.