Abstract
The topographical structure of the visual system in individual subjects can be visualized using fMRI. Recently, a radial bias for the long axis of population receptive fields (pRF) has been shown using fMRI. It has been theorized that the elongation of receptive fields pointing toward the fovea results from horizontal local connections bundling orientation selective units mostly parallel to their polar position within the visual field. In order to investigate whether there is a causal relationship between orientation selectivity and pRF elongation the current study employed a global orientation adapter to modulate the orientation bias for the visual system while measuring spatial pRF characteristics. The hypothesis was that the orientation tuning change of neural populations would alter pRF elongations toward the fovea particularly at axial positions parallel and orthogonal to the affected orientation. The results indeed show a different amount of elongation of pRF units and their orientation at parallel and orthogonal axial positions relative to the adapter orientation. Within the lower left hemifield, pRF radial bias and elongation showed an increase during adaptation to a 135° grating while both parameters decreased during the presentation of a 45° adapter stimulus. The lower right visual field showed the reverse pattern. No modulation of the pRF topographies were observed in the upper visual field probably due to a vertical visual field asymmetry of sensitivity toward the low contrast spatial frequency pattern of the adapter stimulus. These data suggest a direct relationship between orientation selectivity and elongation of population units within the visual cortex.
Highlights
The region of visual space eliciting the response of a particular neuron in the visual cortex determinates the receptive field for that neuron (Felleman & Van Essen, 1987; Hubel & Wiesel, 1962)
The current results suggest that adaptation to an orientation modulates the spatial extent of population receptive fields (pRF) in the visual cortex
PRFs exhibit larger elongations with increased radial bias at axial locations orthogonal to an orientation of a simultaneously presented adapter stimulus compared to parallel locations, at which receptive fields spatial elongations and orientation biases decrease
Summary
The region of visual space eliciting the response of a particular neuron in the visual cortex determinates the receptive field for that neuron (Felleman & Van Essen, 1987; Hubel & Wiesel, 1962). The current study aims to link orientation selectivity directly to the elongation of the receptive field at the population level by globally manipulating the orientation bias while measuring spatial pRF characteristics. Since scanning pRFs with fMRI is time-consuming, the modulation of orientation selectivity must be characterized by a fairly time constant process For this purpose, adaptation was employed as a low-level manipulation to globally decrease the responsiveness toward one particular orientation. Since the response saturation of the visual system toward the adapter orientation follows approximately a power function (Dong, Engel, & Bao, 2014; Greenlee, Georgeson, Magnussen, & Harris, 1991), a stable modulation of orientation selectivity can be achieved in a fraction of the duration needed for the spatial sampling of the receptive fields. The opposite effect should be observed at orthogonal axial positions, that is, an increase in spatial elongation combined with a higher radial bias (Figure 1c)
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