Accommodation is achieved by changing the degree of curvature of the internal crystalline lenses of the eyes.
In youth, these are naturally elastic and become more curved when the pull on the ligament by which they are suspended is reduced by contraction of the surrounding muscle ring (CILIARY MUSCLE). The elasticity of the lenses drops progressively with age so the power of accommodation lessens.
In some animals this adjustment occurs either as a result of an anteroposterior movement of the crystalline lens, or of an alteration in the curvature of the cornea.
Along with these changes are an increase in the thickness of the lens, a decrease in its equatorial diameter and a reduction in pupil size.In the learning theory of Jean Piaget, the process through which a person's schema of understanding incorporates new experiences that do not fit existing ways of understanding the world.See: adaptation The automatic process by which the eyes adjust their focus when the gaze is shifted from one point to another at a different distance.So, if the near point of an emmetrope is at 25 cm from the spectacle plane, the amplitude of accommodation is equal to − [−1/(25 ✕ 10)] = 4 D.The amplitude of accommodation declines from about 14 D at age 10 to about 0.5 D at age 60 (although the measured value is usually higher due to the depth of focus of the eye).This is accomplished by dynamic retinoscopy, by autorefractors or by visually evoked cortical potentials.