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ANC/DTC Seminar: Michael Hoffmann, Otto von Guericke University (Host: Jim Bednar)

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Consequences of optic chiasm malformations on the organization of the visual system

What
  • ANC/DTC Seminar
When Apr 01, 2014
from 11:00 AM to 12:00 PM
Where IF 4.31/4.33
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Department of Ophthalmology, Visual Processing Laboratory, Otto-von-Guericke-University Magdeburg, Leipziger Str. 44, 39120 Magdeburg, Germany

In albinism part of the temporal retina projects erroneously to the contralateral hemisphere, in achiasma the nasal retina projects erroneously to the ipsilateral hemisphere. Due to these congenital malformations of the optic chiasm, processing in the early visual cortex is in albinism and in achiasma not only dedicated to the representation of the contralateral, but also of a sizable proportion of the ipsilateral visual field. These conditions therefore provide powerful models to study principles governing the self-organisation of the human visual system. Here, a series of studies will be presented, detailing the projection abnormality, its consequence on cortical organisation, and its relevance for visual perception and visuo-motor processing. Results: In albinism the representation abnormality (i) extends on average eight degrees into the temporal retina, (ii) varies in its extent between subjects in correlation with the pigmentation defect, (iii) is organised as a retinotopic map in the primary visual cortex, (iv) is relevant for visual perception, (v) does not interfere with perception in the contralateral hemifield, and (vi) leaves major lateralisation patterns in the primary motor and the somato-sensory cortex unaffected. (vii) In albinism and achiasma the abnormal cortical representation of the ipsilateral visual field is superimposed as a retinotopic mirror-symmetric overlay onto the normal cortical retinotopic representation of the contralateral visual field. In conclusion, in both conditions, i.e., enhanced and reduced crossing at the optic chiasm, the abnormal input to the visual cortex does not undergo a topographic reorganisation of the geniculo-striate projection. Such a conservative projection scheme requires neural plasticity to make the represented visual information available for perception. We therefore conclude that instead of large-scale reorganisations, comparatively subtle intra-cortical mechanisms mediate the ability of the achiasmic and albinotic subjects to cope with the abnormal visual input.

Supported by German Research Foundation (HO 2002/3-1, HO 2002/4-1, HO 2002/10-1/2)