THE EFFECT OF KNOWLEDGE OF OBJECT DISTANCE ON ACCOMMODATION DURING INSTRUMENT VIEWING

We present data in which instrument accommodation was measured while k nowledge of object distance was varied. The accommodative feedback loo p was 'semiopen'-an intermediate state between the closed-loop and ope n-loop conditions of previous experiments. The semi-open-loop situatio n mimicked the degraded-image conditions which are frequently encounte red during instrument viewing. The results show that for some subjects knowledge of object distance is a more powerful cue for instrument ac commodation than is the optical distance of the object; however, for t he majority of subjects this is not the case. We also found that subje cts whose accommodation is influenced by knowledge of object distance tend to have a more proximal dark focus than those whose accommodation is independent of knowledge of object distance. We propose that the M andelbaum effect, in which involuntary accommodation occurs when a tra nsparency is superimposed between the observer and the object of regar d, could account for the accommodative behavior of all subjects. Howev er, the Mandelbaum effect would have to be interpreted more broadly th an before. In the broader interpretation, the transparency could be co gnitive (ie known distance) rather than physical.