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The amplitude of the naturally occurring 2-Hz lens fluctuations at levels of accommodative response ranging from 1 to 4 D were measured in absolute terms (i.e., in diopters) and compared with the overall variability of accommodation (in the time domain) over the same range of accommodative output levels. The amplitude of the 2-Hz component was found to be positively correlated with the mean level of the accommodative response. It was as small as 0.02 D (mean to peak) when the mean response level was 1 D, and it was no larger than 0.1 D (mean to peak) when the mean response level was 4 D. Furthermore, the overall variability of accommodation (in the time domain) was also found to be positively correlated with the mean response level, having its minimum when the accommodative response level was least rather than at the resting focus of accommodation (when neurologic activity is least). This suggests that the observed increase in accommodative noise with dc (response) level is more likely attributed to plant noise than to neurologic (controller) noise.

A study is reported which shows that accommodation can be stimulated by a blur stimulus which is below the threshold for visual perception. It is also shown that perceptual fading of the target, caused by stabilization of the retinal image (Troxler phenomenon), can eliminate the accommodative response causing it to default to its resting level. The first finding suggests a way in which the visual system can filter the percept of blur out of our conscious awareness and still effectively utilize the blur as a steady-state error for the accommodative control system. The second finding is consistent with a locus for the Troxler phenomenon in the early afferent part of the visual pathway, ie the retinal ganglion cells.

A mathematical model is proposed for the error detector of the human visual accommodative system. The model supposes that the accommodative error detector derives both the direction and the magnitude of the accommodative error from naturally-occurring oscillations of the lens and their effects on retinal-image contrast. Differential operators take the first derivatives of two time varying functions: lens power and retinal-image contrast. Directional information is obtained by comparing the signs of these two derivatives and magnitude information is obtained by comparing their amplitudes.

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