The neural control of convergence eye movements and accomodation

Thesis

Vergence eye movements and ocular accommodation were measured in three monkeys (Macacca mulatta) trained to track a haploscopically presented target which appeared to move in depth. The dynamic performance of both vergence and accommodation were found to be much better during binocular viewing than during monocular viewing. Responses during binocular viewing in which blur stimuli were removed (AOL viewing) were comparable to those in normal binocular viewing. It was concluded that disparity stimuli were more effective in the dynamic control of accommodation than blur cues.

In two of the three monkeys extracellular recordings were made from cells in a midbrain region dorsal and dorso-lateral to the oculomotor nucleus, which modulated their firing rate when the awake animals tracked target motion in depth, but not during side-to-side target motion. The amplitude of this modulation was similar during monocular viewing, normal binocular viewing, and AOL viewing, demonstrating a relationship to the motor output of accommodation or vergence, rather than any relationship to the blur or disparity stimuli. When presented with conflicting accommodation and vergence stimuli the accommodation and vergence responses, which are normally tightly coupled, were partially dissociated. Comparing unit activity during this task with that during normal binocular viewing, it was possible to show that nineteen cells were related to the vergence response alone, and eight cells were related to the accommodation response alone. For cells related to the vergence response alone, the phase advance of the cells' activity over the vergence response was much larger than that of motoneurones recorded by others, suggesting that these cells may project (directly or indirectly) to motoneurones with time constants longer than the mean.

Extracellular microstimulation in the same midbrain area of one monkey evoked accommodation and vergence responses, confirming the motor nature of the cells recorded in this region. Stimulation over a wide area of the occipito-temporal cortex failed to evoke accommodation or vergence responses, calling into question the assertion of Jampel (1960) that this area of the macaque cortex contains a centre for co-ordinating accommodation, vergence, and pupil constriction.

Authors: Cumming, BG

• Publication date: 1985
• DOI: 10.5287/ora-dxb4bomma
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Eye; Movements; Accommodation and refraction; Vision