The Radial Bias: A Different Slant on Visual Orientation Sensitivity in Human and Nonhuman Primates

Abstract

It is generally assumed that sensitivity to different stimulus orientations is mapped in a globally equivalent fashion across primate visual cortex, at a spatial scale larger than that of orientation columns. However, some evidence predicts instead that radial orientations should produce higher activity than other orientations, throughout visual cortex. Here, this radial orientation bias was robustly confirmed using (1) human psychophysics, plus fMRI in (2) humans and (3) behaving monkeys. In visual cortex, fMRI activity was at least 20% higher in the retinotopic representations of polar angle which corresponded to the radial stimulus orientations (relative to tangential). In a global demonstration of this, we activated complementary retinotopic quadrants of visual cortex by simply changing stimulus orientation, without changing stimulus location in the visual field. This evidence reveals a neural link between orientation sensitivity and the cortical retinotopy, which have previously been considered independent.

Prospective motion correction improves the sensitivity of fMRI pattern decoding

“…Such fMRI effects may originate in the topography of underlying neuronal population codes. Specifically, neurons responding to radial orientations with respect to the fixation point appear to be more frequent, creating a global areal map of radial orientation frequencies (Freeman, Heeger, & Merriam, 2013; Sasaki et al, 2006). Neurons responding independently of radial bias are organized in a much finer‐grained columnar map of orientation preference (Alink et al, 2013; Swindale, Grinvald, & Shmuel, 2003; Tong et al, 2010).…”

Differential human brain activation by vertical and horizontal global visual textures

“…One possible source of the horizontal-vertical diVerence could be the suggested diVerence between radially and tangentially oriented contours (relative to the fovea) (Sasaki et al 2006). To examine the inXuence of such an eVect on our data, we compared the activation to the diVerent stimuli in the ROIs corresponding to the horizontal meridian (where vertical lines are tangential and horizontal lines radial) and in the ROIs corresponding to the vertical meridian (where the geometrical relation is reversed).…”

“…An alternative explanation for the diVerence in response to vertical and horizontal patterns is that it arises from the 'radial orientation bias' proposed by Tootell et al (Sasaki et al 2006). This predicts (and their fMRI study supported) that vertical orientations that are collinear with the centre of gaze will better activate the representation of the vertical meridian, and horizontal orientations collinear with the centre of gaze will better activate the representation of the horizontal meridian.…”

“…We did not Wnd any interaction between the degree of activation to each stimulus type and the meridian representation (horizontal or vertical) ROIs, and so our data do not provide supporting evidence for the radial bias hypothesis. However, our stimuli were considerably diVerent from the contrast reversing, phase-shifting gratings used by Sasaki et al (2006), which were compared with baseline responses to a uniform grey display. Our comparison of oriented texture with random elements was designed to emphasise global processing.…”

“…This latter diVerence could have had a signiWcant impact on the level of brain activity measured over a block. As discussed above with respect to Sasaki et al (2006), the present study also diVers in the 'baseline stimulus' used: in the present study, we used randomly oriented elements rather than the blank screen of mean luminance used by Wilkinson et al (2000) which we argue will reXect responses to local as well as global structure.…”