MOTION DISCRIMINATION IN 2-FRAME SEQUENCES WITH DIFFERING SPATIAL-FREQUENCY CONTENT

We measured the upper threshold for directional motion discrimination (D(max)) in two-frame random binary luminance patterns (random dot kin ematograms) in which either one or both frames was spatially low-pass filtered by convolution with a Gaussian filter. When both frames were low-pass filtered, D(max) increased as a function of the standard devi ation of the Gaussian blurring function, in agreement with previous fi ndings. However, when only one of the two frames was blurred, D(max) s howed little change with blurring space constants below about 20 min a rc, and at larger space constants motion discrimination became impossi ble. We take this as evidence against the proposal that D(max) is pref erentially determined by motion signals from high spatial frequencies; and as evidence for the alternative that D(max) depends upon the mean spatial interval between features in the pattern after a single stage of spatial frequency pre-filtering. The breakdown in motion discrimin ation for space constants above about 20 min arc can be predicted from the computed effects of blurring upon the correlation between feature s (zero-bounded regions) in the broad-band and spatially filtered patt erns. At values of blur where motion discrimination began to collapse there was a temporal order asymmetry: discrimination was easier when t he low-pass pattern preceded the broadband pattern than when the broad band pattern appeared first. We propose that the temporally sustained high spatial frequency signal in the broadband pattern is delayed rela tive to the more transient low frequency signal; or alternatively, tha t the inhibitory surround of the spatial prefilter is switched in afte r a delay relative to the excitatory centre. The processing-delay inte rpretation was tested and confirmed in a second experiment by manipula ting the frame duration.