A Case of Tactile Synchiria Preserved Somatotopy

1
A Case of Tactile Synchiria Preserved
Somatotopy Jared Medina and Brenda
Rapp Department of Cognitive Science, Johns
Hopkins University

• Introduction
• Synchiria
• Disorder in which stimuli presented to one side
  of an individuals body are experienced on both
  sides (Obersteiner, 1882 Drinkwater, 1913),
  creating what could be called a phantom
  sensation
• Historical cases
• Obersteiner (1882), Case I
• Experienced contralesional sensations when
  presented w/stimuli on the ipsilesional side
• Phantom sensations were experienced at the
  homologous location on the contralesional side of
  the body
• Some cases of synchiria co-occur with deficits in
  localization of sensation (see Leyden, 1863)
• Little research has been done to explore the
  mechanisms that give rise to this phenomenon
• Goals
• Examine the link between right and left
  hemisphere somatosensory representations and
  interhemispheric connectivity

Summary
Experiment 2
Discussion
Experiment 1
Results
RESPONSE

• We describe an individual with both poor tactile
  perception of location and phantom synchiric
  sensations on the contralesional hand
• We report a striking dissociation between the
  localization accuracy for phantom versus
  veridical sensations, with greater accuracy for
  the phantom sensation
• This dissociation provides evidence for early
  connectivity between homologous somatosensory
  areas

• Methods/Results
• Both hands of subject placed flat on a table
• Light touch presented to the center of the dorsal
  surface of either the left hand, right hand, both
  hands simultaneously, or neither hand with
  subjects eyes closed
• DLE asked to report which hand(s) was stimulated

• What is the evidence for interhemispheric
  connectivity between somatosensory cortices?
• Calford Tweedale (1990)
• Unilateral anesthetization or amputation of
  flying fox thumbs
• Receptive fields of both contralateral and
  ipsilateral neurons expand
• Krubitzer et al. (1998)
• Callosal projections between primary and
  secondary somatosensory cortices in flying fox
• Some projections are matched to the body region
  representations in the opposite hemisphere
• Disbrow et al. (2001)
• fMRI indicates that areas of somatosensory cortex
  in humans (S2/PV) receive inputs from the
  contralateral and ipsilateral hand
• Theorized that contralateral input is due to
  callosal connections
• The accuracy of localization of the phantom
  percept suggests that there are precise
  connections between representations of the
  homologous body regions in the two hemispheres
• Expression of this phantom percept in DLE may
  be due to removal of intrahemispheric inhibition
  in conjunction with activation of bilateral
  somatosensory representations

• We documented DLEs impairment in the perception
  of CL tactile location
• Methods
• Light tap presented to one of 22 different
  locations on the hand
• Tap presented with subjects eyes closed
• After each trial, the subject opened his eyes and
  immediately pointed to the perceived location of
  the stimulus
• N 6 for each point on the right hand

STIMULUS

• Correct response in bold italics

• 84 of stimuli presented to different locations
  on both hands are perceived at the same location
  on both hands
• Analysis 1
• When DLE reports that CL and IL stimuli are
  perceived at the same location does the
  perceived location correspond to the location of
  the IL or the CL stimulus?
• We measured the displacement from the location
  of contralesional sensation to the location of
  the actual stimulus on each hand (n 51)

• Acknowledgments
• We would like to acknowledge DLE for his time and
  dedication.

• N 101 for each group
• 90 of incorrect responses to stimuli presented
  to the ipsilesional hand consisted of both
  hands responses

References

• Summary
• DLE is proficient at identifying the stimulated
  hand when stimuli were presented to the
  contralesional hand and both hands
• However, DLE consistently reports that stimuli
  presented to the ipsilesional hand are perceived
  on both hands (phantom contralesional hand
  sensation)

Calford, M.B., Tweedale, R. (1990).
Interhemispheric transfer of plasticity in the
cerebral cortex. Science, 249, 805-807. Clarey,
J.C, Tweedale, R., Calford, M.B. (1996).
Interhemispheric modulation of somatosensory
receptive fields Evidence for plasticity in
primary somatosensory cortex. Cerebral Cortex,
6, 196-206. Disbrow, E., Roberts, T., Poeppel,
D., Krubitzer, L. (2001). Evidence for
Interhemispheric Processing of Inputs From the
Hand in Human S2 and PV. Journal of
Neurophysiology, 85, 2236-2244. Drinkwater, H.
(1913). Obligatory bi-manual synergia with
allocheiria in a boy otherwise normal.
Proceedings of the XVIIth International Congress
of Medicine, 117-124. Krubitzer, L., Clarey,
J.C., Tweedale, R., Calford, M.B. (1998).
Interhemispheric connections of somatosensory
cortex in the flying fox. The Journal of
Comparative Neurology, 402, 538-559. Leyden, F.
(1863). Die graue Degeneration der hinteren
Ruckenmarks-Strange, Berlin. Obersteiner, H.
(1882). On Allochiria A Peculiar Sensory
Disorder. Brain, 4, 153-163.

• Stimulation points on dorsal surface of DLEs
  right hand
• Results
• Displacement analysis
• Little somatotopy preserved for contralesional
  hand

• Case Study
• DLE
• 69 year old male
• Left middle cerebral artery CVA two years prior
  to testing
• Lesion in left posterior frontal lobe and
  parietal lobe in the area of the post central
  sulcus
• Aphasia, right hemiparesis, spelling problems
• Synchiria
• Tests given to further evaluate the nature of the
  synchiric sensations

• Localization judgments of contralesional
  sensations are significantly closer to the
  ipsilesional hand than the contralesional hand
  along both the x- and y-axes (p gt .00000001 for
  both)

• Analysis 2
• How accurately are phantom and veridical
  contralesional sensations perceived?

• Experiment 3
• What are the characteristics of the phantom
  sensation on the contralesional hand?
• Is it localizable?
• If so, where is the phantom sensation
  experienced?
• Methods
• Light touch presented to 22 different locations
  on the dorsal surface of
• Ipsilesional (left) hand only
• Contralesional (right) hand only
• Both hands, same location
• Both hands, different location
• After each trial, the subject opened his eyes and
  immediately pointed to the perceived location of
  the stimulus/stimuli

• Absolute displacement from the stimulus position
  to the location of sensation on each hand,
  measured in x-y coordinates (in mm)
• Significantly more displacement of localization
  judgments in CL vs. IL along x-axis (p gt .000001)
  and y-axis (p gt .0007)

Correspondence Jared Medina (medina_at_cogsci.jhu.ed
u)

• Absolute displacement from the location of
  sensation on the CL hand to the stimulus location
  on the CL hand (n51) or IL hand (n49), measured
  in x-y coordinates (in mm)
• Phantom sensations are significantly closer to
  the homologous location on the ipsilesional hand
  than the actual stimulus locationon the
  contralesional hand for the x- and y-axes (p gt
  .0000001)

Summary

• When stimuli are presented in different locations
  on both hands, the synchiric sensation
  overrides the actual stimulus presented to the
  right hand
• The synchiric contralesional phantom sensation
  is more accurately localized than actual stimuli
  presented to the contralesional hand

• Mean localization judgments of responses
  presented to the dorsal surface of DLEs
  contralesional hand

• Summary
• DLE is highly impaired in perception of the
  location of contralesional tactile stimuli