Combining speed information across space
In this thesis, we investigate the integration of local motion signals across space. We
examined how both perceived speed and speed discrimination is affected by an increase in
the number of local motion elements, their spatial separation, and their contrast.
We found that the rate of integration was affected by contrast and the number of elements;
the lower the contrast the greater the rate of integration. At a contrast close to threshold
(7%) the rate of integration could be accounted for by probability summation of local
speed signals across space. The effect of distance between speed cues was negligible. The
improved speed discrimination sensitivity with an increase in element number could not be
accounted for by increased detectability; for equally detectable stimuli, speed
discrimination thresholds are decreasing with element number. Whereas element number
strongly increases speed discrimination sensitivity, the effect on perceived speed is small
and not always consistent across observers. Finally, the integrative process was found to be
related to the direction of the local speed cue at high contrast (20%) whereas at low
contrast (7%) the integrative process is direction blind. The implications of the results
obtained within the context of this research are discussed.