Abstract
Random-Dot Kinematograms (RDK) have been extensively used to study motion-detecting mechanisms in human vision. While the measurements of the limits of the direction discrimination performance (Dmax) have primarily been the subject of such studies, surprisingly little research (e.g., Ledgeway, 1996) has asked the question as to how similar the spatial frequency (SF) content of individual frames of a RDK must be to support motion detection. Here we used a 2-frame 1D vertical pink noise kinematogram, in which both frames were bandpass filtered, with the central SF of the filter manipulated independently for each frame. To avoid spatial aliasing, there was no actual leftward-rightward shift of the image: instead, the phases of all Fourier components of the 2nd image were shifted by ±¼ wavelength with respect to those of the 1st (Quaia et al. 2017). We recorded the OFRs and perceptual direction discrimination in 3 human subjects. The OFRs showed a smooth decline in amplitude, well fit by Gaussian functions, as the difference between the central SFs of the 1st and 2nd images increased. Instead, 100% correct perceptual direction-discrimination performance was observed when the central SF difference was small, deteriorating sharply to chance level when the difference was increased further. Allowing the subjects to grade the saliency of perceived motion moved perceptual dependencies closer to the OFR ones. Observed response asymmetries suggest a possibility that the perceptual judgements rely on the same visual processing mechanisms as do the early OFRs. The OFR data were quantitatively well described by a model which combined two factors: (1) an excitatory drive determined by a weighted sum of stimulus Fourier components, scaled by (2) a contrast normalization mechanism. Thus, in addition to traditional studies relying on perceptual reports, the OFRs represent a valuable behavioral tool for studying early motion processing on a fine scale.