STIMULUS UNCERTAINTY AFFECTS VELOCITY DISCRIMINATION

Velocity discrimination thresholds were determined for 1 c/deg driftin g gratings when uncertainty about the reference velocity was introduce d by interleaving stimuli with different reference velocities from tri al to trial. When drifting gratings with reference velocities spanning 4 octaves (1-16 deg/sec) were mixed randomly within a series of trial s, the velocity discrimination threshold for a 4 deg/sec stimulus incr eased by more than a factor of 3, The threshold elevation decreased as the range of interleaved velocities was reduced from 4 to approx. 0.7 5 octaves, below which velocity interleaving had Little effect, In con trast, when gratings that spanned a 4-octave range in spatial frequenc y were interleaved on successive trials, velocity discrimination for 4 deg/sec was essentially unaffected. Our results indicate that the psy chophysical mechanisms underlying velocity discrimination are not spat ial-frequency specific, but are tuned to the velocity or speed of the stimulus. (C) 1998 Elsevier Science Ltd. All rights reserved.