Role of neural integrators in oculomotor systems: a systematic narrative literature review

Purpose: To evaluate the role of neural integrators (NI) in the oculomotor system.

Methods: A literature search was carried out using several electronic databases during the months of June 2014 to March 2015. The following keywords were used to generate focused results: 'neural integrators', 'gaze-holding', 'oculomotor integration', 'impaired gaze-holding', 'gaze evoked nystagmus' and 'gaze dysfunction'. Further materials were found through searching relevant articles within reference lists. Seventy-one articles were sourced for this review which analysed animal and human subjects and network models; 45 were studies of humans, 16 studies of primates, three studies of felines and one study from rats and network models. The remaining articles were literature reviews.

Results: The horizontal and vertical, including torsional, NI are located logically in the brainstem, nearby their appropriate target extraocular motoneuron nuclei for stable eye position in eccentric position. The nucleus prepositus hypoglossi (NPH) and medial vestibular nuclei (MVN) are closely linked at the caudal pons and dorsal rostral medulla, integrating horizontal conjugate eye movement. The interstitial nucleus of Cajal (INC) integrates vertical and torsional eye movement at the upper midbrain. The integrator time constant is averaged to 25 seconds in human horizontal and animal vertical NI to perform its function. Case reports revealed that dysfunction of horizontal NI also resulted in vertical ocular deviations, indicating some overlap of horizontal and vertical gaze control. Furthermore, pharmacological inactivation of NI exposed a population of inhibitory neurotransmitters that permits its mechanism of action; allowing for smooth conjugate movement.

Conclusions: Neural integrators operate to integrate eye velocity and eye position information to provide signals to extraocular motoneurons to attain and maintain a new position. Therefore, NI allow image stabilization during horizontal and vertical eye movements at eccentric positions for comfortable single vision.