ABSTRACT
This chapter challenges that entire set of popular theories for the moon illusion which are based on the 900-year-old idea that the apparent size of the moon is derived by compensating for its apparent distance. When apparent distance to an "artificial moon", as perceived through stereopsis, is greatly altered experimentally, the associated change in perceived size is negligibly small. It thus seems probable that the kind of apparent "size" involved in the moon illusion refers not to dimensions in ordinary units of length or area, but to the apparent visual angle subtended by the moon, for which apparent distance is an irrelevant issue. The fundamental idea embodied in the alternative, oculomotor hypothesis proposed here is that the moon illusion represents a particular instance of those effects of oculomotor adjustments on perceived angular size that have previously been referred to as "convergence micropsia" and "accommodative micropsia". It is by no means obvious, however, that the near-horizon moon would evoke a different state of convergence or accommodation than the zenith moon. The substance of this contribution is a combined theoretical and experimental demonstration that such a connection between "micropsia" and the moon illusion can be formulated in a way that is consistent with a broad spectrum of empirical evidence. The theoretical foundation of this approach involves a proposed model for the coupling between the oculomotor system (vergence, accommodation and pupil diameter) and perception of apparent size (perceived angular subtense of a target), with "near-triad zooming" leading to perceived enlargement or shrinkage (micropsia). The experimental evidence includes the demonstration that relatively modest oculomotor adjustments can produce impressive changes in apparent size of a moonlike stimulus; and the demonstration that several different aspects of the natural stimulus situations associated with the near-horizon moon and the near-zenith moon induce
INTRODUCTION
For thousands of years, the moon illusion has been an object of wonder, of speculation, and-in the last couple of centuries-of experimentation. Despite this sustained interest, there has been no clear trend toward unanimity in the interpretation of the various observations and experiments; even in the recent literature, disagreements and contradictory outcomes from seemingly comparable experiments are conspicuous. Given this state of affairs, it may seem premature, if not presumptuous, to propose a unified explanation for the moon illusion; and it is even conceivable that the moon illusion is due to many different factors, in which case a unified theory would seemingly be impossible. Nevertheless, this chapter represents an attempt to bring order to that chaotic body of observations by means of a novel and general explanatory scheme, a unified hypothesis, and to present the available evidence that supports this interpretation. The proposal is that whereas there are indeed many components of the natural stimulus situation that contribute to and can enhance the moon illusion, all these factors have their effect because of the way in which they influence the oculomotor system.
