Perceiving Object Length by Dynamic Touch

1
Perceiving Object Length by Dynamic Touch After
a Stroke A Case Study
Paula Silva1, Alen Hajnal1, Steve Harrison1,
Jeffrey Kinsella-Shaw2, Deborah Bubela2, and
Claudia Carello1 1Center for the Ecological
Study of Perception and Action, University of
Connecticut, USA. 2 Collaboratory in
Rehabilitation Research, Department of Physical
Therapy University of Connecticut
Present case study Is dynamic touch preserved in
an individual with atypical kinematics due to
stroke?
What is the basis for the perception of objects
by dynamic touch?
Research has shown that time-invariant moments of
the mass distribution of hand-held objects,
extracted by the nervous system during movement,
constrain perception of object properties by
dynamic touch.
Participant
LW, a 64-year old male, was three years post CVA.
At the time of the experiment, he presented
minimal active movements of the wrist, but was
able to move his arm using the shoulder joint,
albeit with restrictions. He had no neuropathic
impairmenthis two point discrimination was
intact. LW could not grasp the rods in his
affected hand they were secured to his hand with
an elastic band.
Materials, Design and Procedure

• Two sets of three wooden rods differing in length
  (L 45, 60, and 75 cm) were used.
• In order to manipulate the moments of mass
  distribution, A disk of 50g was attached at
  1/4L (Set I) and at 3/4L (Set II).

(A)The angular motions of a wielded object, as
well as the torques used to produce them, vary
over time (B) The wielding dynamics reveal the
invariant moments of the mass distribution
relating the variable torques and angular
motions, which specify the objects properties.
What have we learned about non-visual length
perception by dynamic touch?
Stimulus Sets
Individuals, in the absence of vision and without
practice, are able to accurately judge object
length by wielding or holding. Generally, length
judgments are related to the objects 1st or 2nd
moments.
Set I
Set II

• Trials were blocked by limb each rod was
  presented three times.
• Wielding was primarily via shoulder movements.

Lesson 1 Length judgments are independent of
specific sensations arising from tissue contact
(Carello, 1992)
Results
Mean LP with the affected hand was larger than
with unaffected hand, p lt .03. Apart from this
difference, performance by the two limbs was
comparable

• I. LP was affected by rod length, p lt .001, and
  disk position, p lt .001. The effects were similar
  for both limbs (no interaction).

1 hand
2 hands
handknee handaxle stickaxle
Varying tissue contact with a target rod did not
affect length perception Length judgments were
constrained by the static moment of the rods
whether they were grasped in one hand or contact
was distributed over other body parts.
Lesson 2 Length judgments are independent of
specific kinematics of wielding (Pagano, 1993)
II. Relationship between LP(affected) and
LP(unaffected)
III. Equivalent regressions of LP on the largest
principal moment of inertia
Varying the kinematics of wielding did not affect
length perception Length judgments were
constrained by the largest principal moment of
inertia of the rods (I1), defined at the wrist,
regardless of the joint used for wielding.
Individuals presenting with specific
sensory-motor impairments provide strong tests
for generalizing Lessons 1 and 2
A previous case study Dynamic touch and
peripheral neuropathy
IV. Measures of accuracy (MRS) and consistency
(AD) of length judgments were comparable for the
affected and unaffected limbs.
An individual affected by peripheral neuropathy
presenting with loss of discriminative touch in
the left armwas able to accurately detect the
length of unseen wielded objects (Carello et al.,
2006).
Even though she could not feel objects grasped in
her hand, dynamic touch remained relatively
unimpaired.

• Perceived length LP was reliable and accurate

No sensitivity
Partial sensitivity

• LP correlated with the largest principal moment
  of inertia, I1

POSSIBLE EXPLANATIONS Given the interconnected
nature of the musculoskeletal system, it is
likely that wielding deforms the tissues of the
whole body (not just of the hand), thereby
creating a global flow of mechanical energy that
can be exploited by the haptic subsystem of
dynamic touch.
Conclusions
Conductive tissues that support global detection
of objects inertial properties were preserved
despite significant impairment of the neural
basis for discriminative touch.
The present case study indicates that extracting
invariants of tissue deformation specific to a
to-be-perceived property does not depend on a
particular kinematics of wielding or on its
associated neuromuscular patterning.
Perception is neither sensation-based nor
anatomically specific. Rather, it is constrained
by an invariant flux of stimulation in the
muscles and tendon (Gibson, 1979).
References Carello, C., Fitzpatrick, P.,
Domaniewicz, I., Chan, T-C., Turvey, M. T.
(1992). Effortful touch with minimal movement.
Journal of Experimental Psychology Human
Perception and Performance, 18, 290-302. Carello,
C., Kinsella-Shaw, J. M., Amazeen, E. L.,
Turvey, M. T. (2006). Peripheral neuropathy and
object length perception by effortful (dynamic)
touch A case study. Neuroscience Letters, 405,
159-163. Gibson, J. J. (1966). The senses
considered as perceptual systems. Boston, MA
Houghton Mifflin. Pagano, C. C., Fitzpatrick, P.,
Turvey, M. T. (1993). Tensorial basis to the
constancy of perceived object extent over
variations of dynamic touch. Perception and
Psychophysics, 54, 43-54.