The Role of Speed Lines in Subtle Motion Judgments

1. The Role of Speed Lines in Subtle Motion Judgments Jason Allen & Nestor Matthews Department of Psychology, Denison University, Granville OH 43023 USA Purpose:To determine whether our sensitivity to ‘speed lines' sets the limit on how finely we see direction differences. Stimulus Conditions Discussion The ability to reliably judge subtly different stimuli depends on the variability of the nervous system’s response. More variable nervous-system responses result in less precise judgments from a behaving organism (or robot!). The precision with which a a behaving organism (or robot) can judge subtle differences is limited by the variability in its nervous-system’s response. If the precision of our participants’ judgments had been limited by the variability of the visual system’s response to speed lines, performance would have been comparable across conditions. This was not the case, indicating that the limit was not set by orientation-tuned neural responses, i.e, the response to motion trajectory integrated over time (the speed lines). While our visual system may use speed lines to detect moving objects (Geisler, 1999) and render gross direction judgments (Burr & Ross, 2002) our data suggest that… Stimuli: In this schematic, the second stimulus is always oriented 30 deg clockwise to the first. In the actual experiment, we set the angular difference to each participant’s 75% discrimination threshold. Background:To depict a moving object in a stationary picture, cartoonists often draw ‘speed lines’–stationary lines that are parallel to the orientation of an object’s trajectory. Examples of speed lines in cartoons are shown here. Recent experiments have suggested that the human visual system may rely on speed lines to detect moving stimuli (Geisler, 1999), and to make gross direction judgments (Burr & Ross 2002). Sensitivity to speed lines could arise from the fact that the visual system integrates light over time. This temporal integration is somewhat analogous to a camera with a slow shutter speed, “blending” together successive positions of a moving object to record the orientation of the trajectory (see photos above). Here we extend earlier research by investigating the role of speed lines in judging subtle (as opposed to gross) direction differences. Results The Bottom Line Proficiency Orientation Sensitivity Reaction Time Speed lines do not set the limit on subtle direction judgments. References Method:Twenty naive participants made 'clockwise' / 'anti-clockwise' judgments on successively presented random-dot stimuli that differed from each other in the orientation along which the dots were positioned. The angular difference between the two orientations was constant across trials, while the temporal characteristics of the stimuli varied randomly among the five conditions shown in the next panel. Geisler, W.S. (1999). Motion streaks provide a spatial code for motion direction. Nature, 400(6739), 65-69. Burr, D.C., & Ross, J. (2002). Direct evidence that "speed-lines" influence motion mechanisms. Journal of Neuroscience, 22(19), 8661-8664. This poster can be viewed on-line at the following web site: http://www.denison.edu/~matthewsn/speedlines04.html