Pulse modulation detection in human motion vision.
We present data on the human sensitivity to temporal pulse modulations of target velocity. We measured threshold detection modulation amplitudes for pulse-shaped speed modulations, as a function of pulse duration and temporal frequency. At short pulse durations (up to 50 msec) and low modulation frequency (1 Hz), detection amplitudes are ruled by Bloch's law: the product of pulse duration and threshold modulation amplitude is a constant. This constant corresponds to a position modulation with an amplitude of 3 arc min in a coordinate frame that moves at the average speed (3 deg/sec) of the target. At longer pulse durations we find deviations from Bloch's law. Speed modulation thresholds are not critically dependent on target luminance contrast. These results are modeled by a modulation detection process in two stages. A functional description of the first stage is filtering of the true speed modulation signal by a second order low-pass filter with a characteristic time constant of 20-25 msec. The second (decision) stage is variance detection: modulations are detected when the variance of the filtered modulation function exceeds a certain threshold variance. The square-root threshold variance is estimated 8-10%. This two-parameter model accurately predicts the measured dependence of pulse modulation detection thresholds on pulse duration and pulse density.
Snippe
H P
HP
Department of Psychology, New York University, NY 10003.
Werkhoven
P
P
D005374:Filtration; D006801:Humans; D008027:Light; D008960:Models, Psychological; D009039:Motion Perception / Q000502:physiology*; D011601:Psychophysics; D012684:Sensory Thresholds / Q000502:physiology; D013997:Time Factors