A theoretical analysis of the visual accommodation system in humans. NASA CR-606.

This study has attempted to model the human visual-accommodation system, starting directly with the retinal image. The models that are developed are reasonably consistent with existing data and offer a certain degree ot understanding of certain features of the data. The modeling is in three stages. Starting at the retina we ask: (1) What portion of the retinal picture is involved in accommodation control, (2) How that portion of the picture is processed to derive a measure of defocus, and (3) How that signal in turn is used to control the ciliary muscles. We tentatively conclude that the relevant portion of the retina is a central region of the fovea, having a diameter of some 30 minutes of arc, or 6 mils-the diameter of a coarse human hair. As for processing of the retinal image, it is shown how neural circuits based on lateral inhibition can yield a measure of defocus that is consistent with experimental data over several orders of magnitude of object size and illumination. It is also shown how interaction between three such overlapping receptor regions could account for certain chromatic effects in accommodation control. As for the control system, we tentatively propose an intermittent control model in which accommodation correction cycles may be initiated by relatively abrupt changes in the retinal pattern, caused for example by involuntary eye-movement saccads or certain target movements. The case is argued for a control cycle involving a sampling of the accommodation error followed by a ballistic corrective movement. In terms of thiscontrol model the elusive lens “vibrations” appear to be no more than normal accommodation correction cycles, what we have termed “accommodation saccads.” Apart from its role in accommodation control it is also noted how these lens vibrations could possibly increase the depth of field for strong accommodation. The models predict significant interaction between accommodation control and eye-movements. A number of experiments are proposed which would help elucidate the nature of this interaction.

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