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Asymmetric connections, duplicate layers, and a vertically inverted map in the primary visual system
D Hogan1, P E Garraghty, R W Williams
1Department of Anatomy and Neurobiology, University of Tennessee, Memphis, Tennessee 38163, USA. dhogan@utmb.edu
Summary
A rare achiasmatic mutation in dogs causes unique visual system wiring. Despite significant retinal and brain abnormalities, these dogs exhibit normal behavior and vision, with an unexplained vertical inversion in cortical maps.
Area of Science:
- Neuroscience
- Genetics
- Ophthalmology
Background:
- The achiasmatic mutation is a rare genetic condition affecting the visual system, characterized by the absence of the optic chiasm.
- In affected animals, retinal axons project ipsilaterally, leading to mirror-reversed visual maps in the lateral geniculate nucleus (LGN) and congenital nystagmus.
Purpose of the Study:
- To investigate a novel variant of the achiasmatic mutation in dogs.
- To analyze the structural and functional consequences of atypical retinal axon projections and LGN hyperinnervation.
Main Methods:
- Analysis of a novel achiasmatic mutation in black sheepdogs.
- Multiunit mapping of the visual cortex (area 17) to assess receptive field topography and orientation selectivity.
- Examination of LGN lamination patterns and retinal axon projections.
Main Results:
- A variant achiasmatic mutation resulted in one eye's axons crossing the midline and most fibers terminating in a single LGN.
- The hyperinnervated LGN displayed duplicate and interwoven layers.
- Visual cortex mapping revealed normal receptive fields and orientation selectivity, with near-normal binocular vision.
- An unexplained vertical inversion of visual fields was observed in the cortical representation.
Conclusions:
- This novel achiasmatic variant presents a unique pattern of visual system organization with significant axonal misrouting.
- Despite severe structural abnormalities, the affected dogs showed normal behavior and oculomotor function.
- The study highlights the brain's plasticity and compensatory mechanisms in response to major visual pathway disruptions.