Perceiving Object Length by Dynamic Touch

After a Stroke: A Case StudyPaula 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

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.

(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 object’s properties.

What have we learned about non-visual length perception by dynamic touch?

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 object’s 1st or 2nd moments.

Individuals presenting with specific sensory-motor impairments provide strong tests for generalizing Lessons 1 and 2

A BTorques

Angular Motions

Moments of Mass Distribution

(Invariant)

Object’s properties

Support for Lesson 1: Dynamic Touch is constrained by the objects’ mass moments and independent of specific sensations arising from tissue contact.

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.

Present case study:Is dynamic touch preserved in an individual with atypical kinematics due to stroke?

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 impairment—his 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:

Participant:

II. Relationship between LP(affected) and LP(unaffected)

III. Equivalent regressions of LP on the largest principal moment of inertia:

IV. Measures of accuracy (%MRS) and consistency (%AD) of length judgments were comparable for the affected and unaffected limbs.

Support for Lesson 2: Dynamic Touch is constrained by the objects’ mass moments independent of specific kinematics of wielding.

80

60

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045 60 75

Rod Length

Lp

Unaffected Limb (r2 = .98)

Affected Limb (r2 = .88)

4.8 5.0 5.2 5.4 5.6

2.0

1.8

1.6

1.4

1.2

Log I1

Log LP

ReferencesCarello, 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.

ConclusionsThe 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).

Lesson 1: Length judgments are independent of specific sensations arising from tissue contact (Carello, 1992)

1 hand 2 hands hand+knee hand+axle stick+axle

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)

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.

Wrist Elbow Shoulder

An individual affected by peripheral neuropathy— presenting with loss of discriminative touch in the left arm—was able to accurately detect the length of unseen wielded objects (Carello et al., 2006).

No sensitivity

Partial sensitivity

Even though she could not feel objects grasped in her hand, dynamic touch remained relatively unimpaired.

Perceived length LP was reliable and accurate

LP correlated with the largest principal moment of inertia, I1

A previous case study:Dynamic touch and peripheral neuropathy

•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).

•Trials were blocked by limb; each rod was presented three times.

•Wielding was primarily via shoulder movements.

A little farther

Experimental Set-up

LW wielded the object under an occlusion screen and directed the experimenter to move a visible marker to indicate LP.

ResultsMean LP with the affected hand was larger than with unaffected hand, p < .03. Apart from this difference, performance by the two limbs was comparable:

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

0 20 40 60 80

80

60

40

20

0

Affected limb

Un

affe

cted

lim

b

r2 = .70

Disk Position

0

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60

80

1/4L 3/4L

Lp

Unaffected Limb

Affected Limb

AD (%)

Affected UnaffectedLimb

25

20

15

10

5

0

MRS (%)

Affected UnaffectedLimb

10

8

6

4

2

0

Conductive tissues that support global detection of objects inertial properties were preserved despite significant impairment of the neural basis for discriminative touch.

Stimulus Sets

Set I Set II