Measuring bias in movement perception

1
A behavioral method for studying mirror neurons 
Repetitive action affects visual perception
Arthur M. Glenberg1,2, Gabriel Lopez-Mobilia1,
Michael McBeath1, Michael Toma1, Marc Sato3, and
Luigi Cattaneo4 1Arizona State University,
2University of Wisconsin - Madison,3University of
Grenoble, 4University of Trento

• Experiment 1
• Procedure
• 1. Bias in motion perception was measured for the
  three stimulus shapes a hand holding a bean, an
  open hand with palm up, and a plain diamond (see
  Figure 3a). The three stimuli were used to
  determine if we are tapping strictly-congruent
  (only the hand stimulus should show evidence of
  change in bias) or broadly-congruent mirror
  neurons (all directional stimuli may show a
  change in bias). The type of stimulus was
  manipulated within-subjects, and the tests of
  bias for each of the three stimuli were
  interleaved so that a participant might see a
  diamond shift, a hand holding a bean shift, and
  then an open hand shift.
• 2. Next, 48 participants moved 600 beans in one
  direction or another. The movement direction was
  manipulated between-subjects.
• 3. Last, bias in motion perception was measured
  again for the three stimuli.
• Results
• Figure 3 (right) shows the change in measured
  bias as a function of bean movement direction on
  the second, post-movement, measure of bias. The
  interaction of bean movement direction and
  pre-movement or post-movement measure of bias was
  significant, F(1, 46) 5.16, p .03.
• Figure 3.
• Stimuli (left)
• Results (right)
• for Exp. 1.

The bi-directional action task Participants move
beans, one at a time, from a wide-mouth container
to a narrow-mouth container an arms-length away
(Figure 1.) Half of the participants move the
beans in a direction Away from the self, and half
move the beans in a direction Toward the self.
Note that very similar muscles are used in the
two conditions. Thus the actions are defined in
terms of target location and specifics of the
movement (e.g., a power grip is used in the Away
movement but not the return toward the body, and
vice versa in the Toward movement). Fi
gure 1. Bean containers illustrating the Away
condition.
Plasticity and language comprehension Glenberg,
Sato, and Cattaneo (2008) used the same bean task
as in Experiment 1. Participants then read and
responded to sentences describing action Toward
the participant (e.g., Art gives the pen to
you) or Away from the participant (e. g., You
give the pen to Art). The response was whether
the sentence was sensible or nonsense (e.g., You
give Art to the pen). Participants indicated
sensible by depressing a key with the right index
finger, and they indicated nonsense by depressing
a key with the left index finger. Neither
response required arm movements. The findings
indicated an interaction such that the time to
respond sensible to Toward sentences was faster
when the beans were moved in the away direction
and the time respond sensible to the Away
sentences was faster when the beans were moved in
the toward direction. Interestingly, the same
effects were found for sentences describing the
transfer of concrete objects and sentences
describing the transfer of abstractions (e.g.,
Anna delegates the responsibilities to you).
Apparently, neural systems used in action control
were also used for comprehending sentences
describing actions of the same general sort.
Plasticity and speech perception Sato,
Brisebois, Grabski, Basirat, Menard, Glenberg,
Cattaneo (2008) had participants purse their
lips 150 times to induce changes in corticomotor
control of the orofacial musculature.
Subsequently, they performed a speeded two-choice
identification task of acoustically presented
/pa/ and /ta/ CV syllables either embedded in
acoustic noise or not. A control task was
identical but without the lip pursing. They
observed a decrease in pa responses in the
control session and overall faster RTs in the
motor session.
Measuring bias in movement perception Our
measure of visual perception (that could be
adapted by movement practice) is the bias to see
ambiguous apparent movement toward the observer
or away from the observer. The staircase
procedure used to measure the bias was developed
by Lewis McBeath (2004). Participants view on
a computer monitor a screen tiled with stimuli
that appear to recede into the distance (Figure
2). Figure 2. The tiling for one
stimulus, the bean hand A second screen frame is
presented one second later with shape tiles
slightly shifted, which induces an illusion of
motion that can be in either depth direction. A
shift of 50 of the inter-tile distance is
completely ambiguous, whereas a shift of 20 in
either direction is perceived as movement in the
smaller direction of shift. We start with a shift
of 20 and then small increments in shift are
added until the observer perceives movement in
the opposite direction (a turn point). The
increment is then reduced and the turn point
measured in the opposite direction. Two sets of
eight turn points are collected, and the averages
of the last seven (in each set) are used as the
measure of the threshold, or bias, to perceive
movement in one direction or another.

• Why not fMRI?
• Because the fMRI voxel is sensitive to activity
  from many neurons, simply showing that an area is
  active both during action and perception does not
  guarantee that the same neurons are responding to
  both action and perception (e.g., Dinstein et al,
  2007).
• 2. In most fMRI experiments, the signal only
  indicates a correlation, not a causal or
  functional relation.
• fMRI is difficult to use with children.
• Its expensive.

Conclusions Our technique appears to have
adapted a neural system tuned to multiple
modalities including motor, visual, and language
processes. Although more research is needed, so
far these results are consistent with claims that
a human mirror neuron system exists and that it
contributes to action perception, speech
perception, and language comprehension.
Furthermore, this simple method provides
researchers with a promising instrument to study
the role of mirror neuron systems in these and
other cognitive processes without the expense and
inconvenience of fMRI.
Experiment 2 We made several changes in the
methodology to rule out alternative explanations.
Most importantly, participants were blindfolded
during the bean movement task. Thus, any affect
of movement on the bias measure can be more
securely attributed to adapting an action system
rather than visual stimulation. Second, to
eliminate any sensitization to toward and away
movement due to the initial bias measure, the 24
participants first moved 300 beans in one
direction, and then we measured bias.
Participants then moved an additional 300 beans
in the same direction, and the bias was measured
again. The data for the first, post-movement
bias measure are in Figure 4. The effect of bean
movement direction was significant, F(1,22)
5.89, p .02. The effects were not significant
when measured after the second set of bean
movements. Figure 4. Experiment 2
Results for the first, post-movement bias
measure. Error bars are one standard error. A
bean movement away from the body T bean
movement toward the body.
Use-induced plasticity Repetitive practice of a
movement can produce a temporary re-organization
of the brain. For example, Classen et al.
(1998) used a TMS pulse over motor cortex
controlling the thumb and elicited a movement in
a particular direction. Then participants
practiced moving the thumb in a different
direction for about 20 minutes. Finally, TMS
over the same area of motor cortex now tended to
elicit movement in the practiced direction.
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131-41. Sato, M., Brisebois, A., Grabski, K.,
Basirat, A, Ménard, L, Glenberg, A. M.,
Cattaneo, L. (October, 2008). Paper presented at
the Speech and Face to Face Communication
Workshop, Grenoble, France. Author Note This
research was support in part by NSF grant BCS
0744105. Any opinions, findings, and conclusions
or recommendations expressed in this material are
those of the author and do not necessarily
reflect the views of the National Science
Foundation. Direct correspondence to Arthur
Glenberg, glenberg_at_asu.edu
Use-induced plasticity and mirror neurons The
logic Mirror neurons play a role in both
producing action and perceiving action. Thus,
if practice adapts part of a mirror neuron
system, then the effects of that practice should
be revealed in a perceptual task that uses the
same mirror neuron system. In our experiments,
we rule out non-specific effects of practice by
adapting the motor system using two opposing
practice directions. Showing different effects
of the direction of practice on visual perception
rules out non-specific (e.g., fatigue) effects.
Two-frame apparent motion tasks show promise as a
hit with some observers!