Purpose. We investigated the spatial extent of the summation area in the direction of motion and the direction perpendicular to the motion, using shearing motion stimuli. Based on velocity threshold measurements, we estimated the shape and size of the receptive field of the motion detector responsible for the detection of shearing motion. Method. The thresholds were measured with a random dot kinemotogram of which the direction of the dot motion was vertical and the speed of the dot speed was modulated by a Gabor function. The spatial extent of the motion Gabor path, in terms of the space constant, varied from 0.18 to 3.5 degrees. Spatial frequency of the motion Gabor path was 2 cpd or 0 cpd (a Gaussian). We determined velocity threshold by a two-interval 2AFC staircase method. Results. For both the Gabor and Gauss stimuli, the thresholds decreased rapidly up to a certain value of space constant, beyond which the thresholds were relatively constant. To estimate the receptive field characteristics of motion detector that determined the threshold for each stimulus, a nonlinear least square method fitted a Difference-of-Gaussian (DoG) function to each set of data. The estimation showed that isotropic DoG functions could predict the threshold data for stimuli elongated in the direction of motion and those for stimuli elongated in the direction perpendicular to the motion. The space constant of the DoG function differed for the Gabor and Gauss stimuli (the value for the Gauss stimuli was larger). Conclusions. The results suggest that the shearing motion detector has an isotropic receptive field and that there may be shearing motion detectors with different scales. CR: E.
ASJC Scopus subject areas
- Sensory Systems