The Accessory Oculomotor Nucleus (AON), also known as the Accessory Optic System (AOS), is a collection of brainstem nuclei that process visual motion information and coordinate reflexive eye movements. This system plays a crucial role in stabilizing images during self-motion.
Neurons
The Accessory Oculomotor Nucleus (AON), also known as the Accessory Optic System (AOS), is a collection of brainstem nuclei that process visual motion information and coordinate reflexive eye movements. This system plays a crucial role in stabilizing images during self-motion.
The Accessory Oculomotor Nucleus is located in the midbrain and pretectal region, comprising several distinct nuclei:
- Medial terminal nucleus (MTN) - Largest component
- Lateral terminal nucleus (LTN)
- Dorsal terminal nucleus (DTN)
- Ventral terminal nucleus (VTN)
The AON processes retinal slip - the movement of visual images across the retina that occurs during head movements. This information is used to:
- Image stabilization - Compensate for head movements during visual fixation
- Optokinetic reflex - Generate smooth eye movements to track moving visual fields
- Vestibulo-ocular reflex (VOR) modulation - Fine-tune gaze stabilization
- Self-motion perception - Process optic flow information
AON neurons are highly sensitive to:
- Direction-selective visual motion
- Optic flow patterns generated by self-motion
- Wide-field moving stimuli (especially whole-field motion)
- Sustained responses to continuous motion
- Retina - Direct input from direction-selective retinal ganglion cells
- Superior colliculus - Multisensory integration
- Visual cortex - Higher-order motion processing
- Vestibular nuclei - Modulate VOR
- Oculomotor nuclei - Coordinate eye movements
- Nucleus of the optic tract - Part of optokinetic system
- Thalamus - Information relay to cortex
The accessory optic system shows dysfunction in PD:
- Impaired optokinetic responses - Reduced eye tracking of moving visual fields
- Abnormal VOR adaptation - Difficulty adjusting to changed visual conditions
- Gait and balance deficits - Motion processing deficits contribute to spatial disorientation
Research shows that PD patients have:
- Reduced optokinetic nystagmus gain
- Impaired visual-vestibular integration
- Difficulty with visually-guided locomotion
- Marked impairment of the accessory optic system
- Contributes to severe visual tracking deficits
- Characteristic downgaze palsy involves AON dysfunction
- Moderate AON involvement
- Contributes to oculomotor dysfunction
- Vestibular deficits compound visual motion processing issues
The AON may play a role in visual hallucinations in neurodegenerative diseases:
- Misdirected visual motion signals may contribute to misinterpreted visual phenomena
- Dysfunction in motion detection may produce false visual perceptions
- Particularly relevant in Dementia with Lewy Bodies
AON neurons exhibit:
- Direction selectivity - Prefer motion in specific directions
- Large receptive fields - Respond to wide-field motion
- Sustained firing - Respond throughout motion presentation
- Motion-sensitive - Respond to both retinal and whole-field motion
- Optokinetic nystagmus testing - Evaluates AON function
- Video oculography - Records pursuit and tracking movements
- Subjective visual vertical - Assesses vestibulo-ocular integration
- Visual motion training may improve balance in PD
- Optokinetic stimulation used in vestibular rehabilitation
- Virtual reality approaches target AON-mediated functions
- Gates, Visual-vestibular integration in movement disorders (2015)
- Bronstein et al., Vestibular and optokinetic dysfunction in Parkinson's disease (2019)
- Lencer et al., The accessory optic system in health and disease (2018)
- Kellman et al., Optokinetic therapy for balance disorders (2016)
- Warmerdam et al., Visual dysfunction in progressive supranuclear palsy (2020)