Voluntary And Edinger-Westphal (EW) Stimulated Accommodative Dynamics In An Awake-behaving Monkey

Abstract

Purpose: : To bridge understanding of the differences between EW-stimulated accommodation in anaesthetized monkeys and voluntary accommodation in conscious humans, accommodative dynamics were studied during voluntary and EW-stimulated accommodation in an awake-behaving monkey.

Methods: : All experiments conformed to the ARVO Statement for the Use of Animals in Vision Research and were in accordance with institutionally approved animal protocols. A 6-year-old monkey was trained with a juice reward to make saccades to visual stimuli at a distance of 0.57 m (1.7 D) and to accommodate between a 'far' (0.57 m) and 'near' LEDs at varied distances. Binocular accommodation was recorded at 30 Hz with a dual-photorefractor in three protocols: 1) The EW nucleus was identified by recording single-unit responses when the monkey was performing accommodative tasks, and the EW nucleus was then stimulated with 1-second, 72 Hz pulses, with currents from 10 µA to 100 µA; 2) Behavioral accommodative demands were set by progressively moving the near LED along the left eye visual axis from 2.7 D to 11.7 D; 3) For 2 D fixed amplitude LEDs, the accommodative starting point was shifted by adding trial lenses of different powers (-1 D to -7 D). From many experimental sessions, a total of 1296 accommodative responses were recorded. Peak velocity and time-to-63% of amplitude (T_0.63Amp) of each response were analyzed as a function of response amplitude and starting point using a two-point difference algorithm.

Results: : Maximal accommodative response of 11.16 D for EW-stimulation and of 11.30 D was achieved for near-LED stimulation. For EW stimulation, peak velocity increased linearly with response amplitude (OD: PV=6.80•Amp+0.90, n=245, r²=0.89, p<0.001; OS: PV=7.78•Amp+0.01, n=257, r²=0.94, p<0.001). Peak velocity of behavioral responses saturated at amplitudes above 4 D for near-LED stimulation. T_0.63Amp showed no relationship for EW-stimulation but increased linearly with amplitude for near-LED stimulation (OD: T_0.63Amp=0.019•Amp+0.043, n=181, r²=0.43, p<0.001; OS: T_0.63Amp=0.033•Amp+0.012, n=406, r²=0.65, p<0.001). For fixed amplitude accommodation, the peak velocity increased with the starting point in one eye (OD: PV=1.11•Amp+19.49, n=105, r²=0.12, p<0.001), but not in the other (OS: n=105, p=0.623).

Conclusions: : Saturation of peak velocity is similar to what has been reported previously with voluntary accommodation in conscious humans. A linear increase in peak velocity with amplitude with EW-stimulated accommodation is similar to what has been reported previously in anesthetized monkeys. Differences between EW-stimulated and behavioral accommodation are likely due to differences in neural control and not anesthesia.

Keywords: accommodation • ocular motor control • innervation: neural regulation