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.
Discussion
Stimulus Conditions

• The ability to reliably judge subtly different
  stimuli depends on the variability of the nervous
  systems 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-systems response. If the precision of
  our participants judgments had been limited by
  the variability of the visual systems 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 participants 75
discrimination threshold.
Background To depict a moving object in a
stationary picture, cartoonists often draw speed
linesstationary lines that are parallel to the
orientation of an objects 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.deni
son.edu/matthewsn/speedlines04.html