Short-term adaptive modification of dynamic ocular accommodation.

Recalibration of neural signals allows a biological control system to restore and maintain optimal motor performance in the event of any age-, pathology-, or environment-related changes in the biomechanical plant. Ocular accommodation of humans and nonhuman primates is a good example of a biological control system whose biomechanical plant (comprising the crystalline lens, ciliary muscle, choroid, and zonules) becomes increasingly noncompliant with age, ultimately leading to a complete loss of focusing ability (absolute presbyopia).1 The loss of focusing ability can be primarily attributed to a decrease in lenticular compliance (compliance = 1/elasticity),2–4 increase in lenticular viscoelasticity,4 mechanical restriction in ciliary muscle movement,5,6 and morphologic changes in the accommodative plant.7–9 During the incipient stages of presbyopia, the increasingly noncompliant accommodative plant increases the neuromuscular demands required to generate the desired amount of accommodation.10 Several indirect observations suggest that the neural control of accommodation may undergo adaptive recalibration to suit the biomechanical changes in the accommodative plant. For instance, neural recalibration to age-related loss of plant compliance is suggested by an increase in the myodiopter (the neuromuscular effort required to generate a diopter of accommodation11) during incipient presbyopia.10,12 An increase in the myodiopter is indicated by an age-related increase in the response AC/A ratio (the amount of vergence generated by a diopter of accommodation) and an age-related decline in the response CA/C ratio (the amount of accommodation generated by a meter angle of vergence), within the linear range of the accommodative stimulus–response function.10,13,14 Neural recalibration to an age-related increase in lenticular viscoelasticity is suggested by the lack of a dramatic reduction in the peak velocity of small accommodative step responses (1–3 D).15–17 On a more general level, neural recalibration of disaccommodation (near-to-far accommodation) is suggested by the serendipitous observation of Bharadwaj and Schor18 that repeated pilot experiments over 1 year reduced the speed and acceleration of small disaccommodative step responses of the first author, minimizing the instabilities that are usually associated with such responses. Neural recalibration may have occurred to optimize stability by slowing down the responses.18 Finally, adaptable dynamics are also suggested by the success of orthoptic training exercises (e.g., accommodative flippers and pencil push-ups) that decrease the time constant, speed up abnormally sluggish accommodative responses, and increase the rate at which accommodation responds to repetitive positive and negative focusing errors (accommodative flipper rates).19,20 Apart from these indirect observations, to our knowledge, there has been no direct demonstration of the adaptation capability of accommodative step response dynamics. The main purpose of this study was to provide direct evidence for such an adaptive capacity by employing a double-step adaptation paradigm that optically stimulates increases (increasing-step paradigm) or decreases (decreasing-step paradigm) in neuromuscular efforts. Our results illustrate that the dynamics of accommodative step responses can be modified on a short-term basis, at least in some subjects ranging in age from 18 to 34 years, with increases in neuromuscular effort adapting more readily than decreases in neuromuscular effort.