Perceptual detectability of ocular accommodation microfluctuations

When the eye fixates a stationary stimulus, the power of the lens is known to change rapidly and continuously. Although the basic characteristics of this fluctuating activity are known and the influencing factors have been identified, their exact role in the control of accommodation is uncertain. It is thought that, for the fluctuations to be useful, a detectable change in the retinal image is necessary, yet an accommodation response can be provoked by a stimulus change of only 0.1 D, well within the ocular depth of focus as conventionally measured. In this study we examine whether the fluctuations of accommodation are able to provide perceptual sign information or whether the presence of a sensorimotor mechanism utilizing subthreshold blur is required. Normal accommodation fluctuations for steady-state viewing were recorded using an infrared optometer and subsequently were used to drive a Badal stimulus optometer. The perceptual threshold of the fluctuation signal for an eye under cycloplegia was then determined. The threshold for the detection of the fluctuations at optimal focus was not significantly different from the root-mean-square value of the actual response. Hence a portion of the fluctuations more than spans the dead space (depth of focus) and is therefore capable of providing information to control accommodation without the need for a subthreshold mechanism.