GEE was used to assess the relationship between 1 min means of contrast thresholds (contrast threshold1-min averages) and accommodation responses (accommodation response1-min averages) with time (min 1 7) to ascertain if the two sets of response measures were stable.
In the measurement records of all subjects a lag (residual negative response) of accommodation to the 2 D stimulus was observed.
Intravenous infusion of pilocarpine was a rapid and reliable method of producing a nearly maximal accommodative response and maintaining accommodation when desired.
The typical control strategy of disaccommodation results in faster responses than accommodation but at the cost of overshooting the required steady-state level.
In viewing conditions with relatively high accommodative demands, the level of accommodation response explained some part of the trapezius muscle activity, irrespective of whether incongruence between accommodation and convergence was present or not.
In the two viewing conditions with high accommodative demands, the accommodation responses were positively associated with the muscle activity levels, see Table 2.
Since there was a significant effect of time on accommodation response in condition MM (see Sect. " Accommodation responses and contrast thresholds"; Fig. 3), the variable accommodation response1-min averages was used when analyzing condition MM.
To control for the effect of group (neck or control group), trapezius muscle activity during rest, heart rate variability, and accommodation response, the variables group, logEMGrest 3-min average, HRVfix 1-min averages, and accommodation response 7-min average were included as covariates in the GEE model.
The control GEE analyses which related accommodation response and contrast thresholds to time indicated that participants generally maintained their level of accommodation response during the visual tasks but that the contrast threshold decreased.
However, the main effects of time and accommodation response were significant in the minus lens conditions (BM and MM) and these two variables (time and accommodation response), therefore, contribute to an increased trapezius muscle activity in an additive way.
