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MS: 2002-12021B
Neural Correlates of Implied Motion
Bart Krekelberg, Sabine Dannenberg, Klaus-Peter Hoffmann, Frank Bremmer, John Ross
MS: 2002-12021B
Supplementary Information
A. Local Motion Signals.
Introduction. Glass patterns do not contain globally coherent motion vectors1. It
is possible, however, that accidental local correlations biased the subjects percept of
motion enough to induce a global percept of motion. We investigated whether this was
the case by presenting the same sequence of rotational Glass patterns twice, first in
forward order, and then in reverse order. If the percept of coherent global rotation were
based on accidental local correlations in the randomly chosen sequences, the reversal of
the order of presentation should change the percept from clockwise to counter clockwise
and vice-versa.
Method. A set of 20 rotational Glass patterns, each containing 200 pairs of points,
was newly constructed for each experimental session. Paired dots (white on a gray
background) were separated by 5 degrees of rotation. Luminance and dot size was the
same as in the experiments reported in the paper. On each trial 5 patterns were selected at
random from the available set of 20 and presented each once only in a random order at a
rate of 24 Hz. In the Reverse condition (100 trials) the same 5 patterns were shown again
in reverse order. On each presentation (initial and reverse) subjects judged whether the
direction of rotational motion they had seen was clockwise (CW) or counter clockwise
MS: 2002-12021B
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(CCW). The whole procedure was repeated with non-Glass patterns made up of single,
unpaired points.
Results. Averaged over all trials, the two subjects reported 52% and 46%
clockwise decisions. This confirms that on average the direction of motion was equally
likely to be clockwise or counter clockwise, hence the direction of motion is truly
ambiguous on average. If the subjects based their decisions on accidental local motion
cues, a CW decision should be followed by a CCW decision in the following trial (in
which the order of presentation and hence the local motion cues were reversed). Hence,
the percentage of successive same-direction decisions (CCW/CCW or CW/CW) should
be near zero. This was not the case. Two naive subjects reported 76% and 72% samedirection decisions.
Discussion. This experiment demonstrates that observers did not base their
percept of motion direction on accidental local non-randomness. Taken together with the
known global randomness1, this convincingly shows that the motion implied by the form
of the Glass patterns drives subjects’ motion percepts. For completeness, these results
also indicate, however, that subjects’ successive responses were not entirely independent.
If they were, the same-direction percentages would have been ~50%. That this is not the
case reflects the fact that these subjects typically clustered their decisions: a few trials
CW, followed by some CCW. That these decisions were based on truly perceived motion
can be inferred from the perceptual rating experiments discussed by Ross et al1.
Moreover, the clustering itself is not peculiar to sequences of Glass patterns. When
forced to report a CW or CCW motion direction for random noise, the same clustering
was observed.
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B. Ambiguous motion directions.
Because the direction of motion in Glass patterns is inherently ambiguous, one might
expect that the cells’ responses alternated between the response to the preferred direction
and the anti-preferred direction. Such an alternation may take place on a trial-by-trial
basis, but also in a single trial. We investigated this for rotational Glass patterns by
testing whether a cell’s Glass response was statistically indistinguishable from the mean
of the counter- and clockwise rotations (the ‘motion-mean’). To exclude cells whose
responses were simply noisy and therefore indistinguishable from the motion-mean, we
excluded cells whose response to the noise stimulus was indistinguishable from the
motion-mean. Of the remaining 33 cells, 25 cells responded to a Glass pattern with a rate
that was indistinguishable from the motion-mean. On average, 3.6 Glass patterns per cell
showed this behaviour. The distribution of these ‘switching’ Glass shifts was
significantly peaked in central range of Glass shifts. This suggests that these cells
responded to the two ambiguous implied directions of motion. Note that such an
alternation of responses increases the variance of trial and time averages. This increased
variance made it less likely that the null-hypothesis of ‘no-tuning’ could be rejected. Our
estimate of the fraction of cells that were significantly tuned to the Glass-shift may
therefore in fact be an underestimate.
C. Spiral Space.
Studies of judged motion direction were also conducted with spiral glass patterns. The
apparent pitch of spiral motion was again a compromise between the actual pitch of
motion and Glass pattern pitch. The influence of implied motion was greater than for
linear Glass patterns in linear motion. Real and implied motion had equal influence when
pattern signal strength was 25% and motion signal strength was 50%, the reverse of the
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values for linear patterns. Once again, implied motion exerted a strong attractive
influence only when conflict between the implied motion in the pair orientation and
motion direction was small. Further evidence of the influence of Glass patterns on
motion, both linear and spiral, is provided by Burr and Ross.2 They show that varying the
orientation of Glass pattern dipoles randomly from trial to trial has the same effect as
motion noise: it raises thresholds for detecting deviations in the direction of global
motion, provided differences between local motion direction and pair orientation are
small.
References
1.
2.
Ross, J., Badcock, D. R. & Hayes, A. Coherent global motion in the absence of
coherent velocity signals. Curr Biol 10, 679-82. (2000).
Burr, D. C. & Ross, J. Direct evidence that "Speedlines" influence motion
mechanisms. J Neurosci 22 (2002).