COMBINING SPEED INFORMATION ACROSS SPACE

We used speed discrimination tasks to measure the ability of observers to combine speed information from multiple stimuli distributed across space. We compared speed discrimination thresholds in a classical dis crimination paradigm to those in an uncertainty/search paradigm. Thres holds were measured using a temporal two-interval forced-choice design . In the discrimination paradigm, the n gratings in each interval all moved at the same speed and observers were asked to choose the interva l with the faster gratings. Discrimination thresholds for this paradig m decreased as the number of gratings increased. This decrease was not due to increasing the effective stimulus area as a control experiment that increased the area of a single grating did not show a similar im provement in thresholds. Adding independent speed noise to each of the n gratings caused thresholds to decrease at a rate similar to the ori ginal no-noise case, consistent with observers combining an independen t sample of speed from each grating in both the added- and no-noise ca ses. In the search paradigm, observers were asked to choose the interv al in which one of the n gratings moved faster. Thresholds in this cas e increased with the number of gratings, behavior traditionally attrib uted to an input bottleneck. However, results from the discrimination paradigm showed that the increase was not due to observers' inability to process these gratings. We have also shown that the opposite trends of the data in the two paradigms can be predicted by a decision theor y model that combines independent samples of speed information across space. This demonstrates that models typically used in classical detec tion and discrimination paradigms are also applicable to search paradi gms. As our model does not distinguish between samples in space and ti me, it predicts that discrimination performance should be the same reg ardless of whether the gratings are presented in two spatial intervals or two temporal intervals. Our last experiment largely confirmed this prediction.