Quantitative comparison of the hemodynamic activation elicited by cardinal and oblique gratings with functional near-infrared spectroscopy.

Evidence has been accumulated for over a century indicating that the visual system of humans and many animals is more sensitive to contour stimulation at vertical or horizontal orientations than oblique orientations. However, the neural basis for this orientation anisotropy is still a subject of debate. In the present study, we recorded brain activity over the parietal-occipital and frontal lobes with functional near-infrared spectroscopy (fNIRS) when human participants were presented with gratings in different orientations. The oblique gratings induced a much larger change in the oxygenated hemoglobin concentration than vertical and horizontal gratings in the left occipital lobe. However, we did not find any significant orientation anisotropy in the frontal lobe. Our study showed that different quantitative changes in the hemoglobin concentrations occurred in response to differently oriented stimuli in the visual cortex and that fNIRS could potentially be a valuable tool in the assessment of the hemodynamic responses of the visual system.