Until now, only indirect observations have suggested that the accommodative control system may recalibrate in response to changes in neuromuscular demand.
10 13 18 19 By employing a double-step adaptation paradigm, we have directly demonstrated that the peak velocity and peak acceleration of single-step accommodative responses can change on a short-term basis, at least in some subjects ranging from 18 to 34 years, in response to optically stimulated changes in neuromuscular demand
(Figs. 3 4 6 7) . Unlike response latency
(Table 1) , adaptive changes in peak velocity and peak acceleration were dependent on the direction of training
(Figs. 3 6) , suggesting that stimulus anticipation cannot explain the results completely. In many subjects who showed adaptive changes in the anticipated direction, both peak velocity and peak acceleration changed after training, suggesting that adaptive changes in peak velocity were brought about by integrated changes in the peak acceleration
(Figs. 3 4 6 7) .
17 Of note, the peak velocity increased after increasing-step adaptation even in those subjects where the preadaptation peak velocity saturated at larger response amplitudes
(Figs. 3a 3d 3f) . This suggests that the saturation of peak velocity observed here and in previous experiments
17 33 35 does not reflect an upper limit of velocity that the accommodative system is capable of generating for a given response amplitude.
36 There was no obvious correlation between the magnitude of adaptation and the subject’s age (18–34 years) in the small number of individuals we tested (
n = 15). This result must be confirmed on a larger sample size.
Adaptive changes were larger in the increasing-step paradigm than in the decreasing-step paradigm in several subjects, including those for whom data were available from both training paradigms. This directional bias is unlikely to be due to accommodative fatigue from prolonged periods of stimulation,
37 38 39 for fatigue would only bias the magnitude of adaptation toward lower values in both paradigms. When compared to the increasing-step paradigm, a slightly smaller adaptation stimulus was used in the decreasing-step paradigm (1.75 D vs. 2 D in the increasing-step paradigm) to ensure that the adaptation stimulus was not perceived as a transient change in blur. Although a smaller adaptation stimulus may be expected to produce a smaller change in dynamics, the difference of 0.25 D between the two paradigms appears too small to account for the observed directional bias in adaptation. Further, the results of the decreasing-step paradigm using a 2-D adaptation stimulus (first step size of 4 D) were similar to those seen with a 1.75-D adaptation stimulus in four subjects (results not shown separately). This suggests that the smaller stimulus size did not account for the reduced adaptation in the decreasing-step paradigm. A second possibility for the smaller adaptation response in the decreasing-step paradigm may be that, with the lag of accommodation, the second step decrement in blur brought the visual target into focus while the system was responding to the first stimulus, thereby negating the need for accommodation to respond to the second step. The decreasing-step paradigm may therefore not have stimulated any change in neuromuscular demand. The directional bias in adaptation may also reflect strategies used to minimize errors encountered during the normal growth and ageing processes. For instance, robust adaptation of horizontal vergence
40 and the accommodation–vergence coupling gains
41 to sustained convergence stimuli (than to sustained divergence stimuli) may be a strategy to correct the physiological exophoria and minimize any undue demands on the fusional vergence system during the growth of the cranium.
42 43 Perhaps, the bias in adapting more effectively to increases in neuromuscular effort is also an age-related strategy to compensate for the biomechanical changes in the accommodative plant (e.g., increased lenticular viscoelasticity,
4 increased posterior restriction of ciliary muscle
6 ). Accommodation could therefore be predisposed to increase the neural gain to the increasing step paradigm used in our experiment. Further experiments are warranted to explore these different possibilities.
The results of this study and the indirect observations made by earlier studies
10 13 15 16 17 19 20 qualitatively support the presence of adaptive capability in accommodation. However, our results cannot be directly compared to these earlier studies because the time scale and the magnitude over which neuromuscular effort was modified were very different in these studies. For instance, adaptation to age-related changes would involve a gradual change in neuromuscular effort occurring over several years.
10 12 13 15 16 Similarly, the change in the neural control of disaccommodation observed by Bharadwaj and Schor
18 and the speeding of abnormally sluggish accommodation after orthoptic training
19 20 occurred over a span of several weeks. In contrast, in our experiment, adaptive changes occurred in response to dramatic changes in neuromuscular effort that occurred over a very short period (e.g., 100% change in effort within a few hours in the increasing-step paradigm). It is therefore possible that the neural mechanisms underlying adaptation in our experiment could be different from the mechanisms that triggered adaptation in the earlier studies. Indeed, different neural mechanisms have been proposed for adaptive regulation of saccadic accuracy to errors after muscle paresis (long-term changes) and to errors induced experimentally using a double-step paradigm (short-term changes).
44 Whatever the underlying adaptive mechanisms may be, our results indicate that the accommodative system possesses the capacity to change its dynamic neural control pattern for a given response amplitude and that this capacity may be used to compensate for any age- or environment-related biomechanical changes in the accommodative plant.
Similar to accommodation, the peak velocity and peak acceleration of vergence step responses also increase after double-step increase in disparity and the peak velocity decreases after double-step decrease in disparity.
25 26 The dynamics of saccades also increase after double-step increase in target eccentricity and they reduce after double-step decrease in target eccentricity.
27 28 Qualitative similarities in the adaptive characteristics of accommodative and vergence step responses are somewhat expected given their neural coupling
45 and the similarity in their neural control strategies.
17 46 Adaptive changes in accommodation
(Figs. 2 4)and vergence
25 26 are different from those of saccades in that, unlike accommodation and vergence, adaptive changes in saccade amplitude and dynamics occur in conjunction with each other.
27 28 This difference perhaps stems from the ballistic neural control of saccades.
47 The dynamics of saccades is determined by a preprogramed pulse innervation and its response amplitude is determined by integrating the pulse innervation to produce a step innervation.
47 Any change in the pulse innervation would therefore influence both the dynamics and the amplitude of saccades.
47 However, accommodation
17 48 and vergence
46 step responses are only partially ballistic, with an initial open-loop pulse innervation controlling the response dynamics and an independent closed-loop step innervation that determines the response amplitude. Any change in response amplitude induced by changes in the characteristics of the pulse innervation would therefore be corrected by the adjusting the size of the step innervation in response to blur and disparity feedback.
17 46