Abstract
An object in outer space is weightless due to the absence of gravity, but astronauts can still judge whether one object is heavier than another one by accelerating the object. How heavy an object feels depends on the exploration mode: an object is perceived as heavier when holding it against the pull of gravity than when accelerating it. At the same time, perceiving an object’s size influences the percept: small objects feel heavier than large objects with the same mass (size–weight illusion). Does this effect depend on perception of the pull of gravity? To answer this question, objects were suspended from a long wire and participants were asked to push an object and rate its heaviness. This way the contribution of gravitational forces on the percept was minimised. Our results show that weight is not at all necessary for the illusion because the size–weight illusion occurred without perception of weight. The magnitude of the illusion was independent of whether inertial or gravitational forces were perceived. We conclude that the size–weight illusion does not depend on prior knowledge about weights of object, but instead on a more general knowledge about the mass of objects, independent of the contribution of gravity. Consequently, the size–weight illusion will have the same magnitude on Earth as it should have on the Moon or even under conditions of weightlessness.
Highlights
The size-weight illusion is the well-known effect that large objects are perceived to be lighter than small objects of the same weight [1]
The size–weight illusion occurs without any haptic or visual information about gravitational forces acting on the object
We have shown that perception of weight is not at all necessary for the size–weight illusion to occur
Summary
The size-weight illusion is the well-known effect that large objects are perceived to be lighter than small objects of the same weight [1]. In the present study we investigated whether the size-weight illusion depends on perceiving the pull of gravity, i.e. whether it is caused by a prior for weight or a by more general prior for the mass of an object To this end we investigated whether the size– weight illusion occurs in the absence of weight through perception of inertial forces only. One reason for not expecting the illusion to occur in the absence of gravitational forces is that in daily life we rarely experience the heaviness of an object without perceiving gravitational forces This means that the prior for larger objects being heavier may be limited to perception of gravitational forces, i.e. weight. This allowed us to test whether the magnitude of the illusion as obtained through perception of inertial forces differed from the illusion obtained in the traditional way
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