Mechanisms of motion perception: Investigating perceptual interactions between different varieties of motion in human vision
Movement is a ubiquitous feature of visual experience. Not only do objects in the world around us move but also as observers we are constantly moving our eyes, heads and bodies actively to explore the environment. Consequently elucidating the properties of the mechanisms that encode motion is a central goal of visual neuroscience. Moving objects can differ from their backgrounds along a wide range of visual dimensions including luminance (“first-order motion”) and more complex, textural properties such as the contrast, orientation and size of surface markings (“second-order motion"). Each of these cues provides the observer with a potentially independent and useful source of information concerning an object’s direction and speed. However it is not known if specialised motion detectors exist for encoding movement conveyed by each of these different visual attributes. Indeed the nature, and even the number, of motion-detection mechanisms that are actually used in human vision is still unresolved. In this talk I will describe the results of some of my recent research that aims to address this important issue. In a series of psychophysical experiments adult observers were asked to make simple forced-choice judgements of the appearance (e.g. drift direction or identity) of a range of qualitatively different motion stimuli to investigate whether the same or separate (specialised) mechanisms are involved in the detection of different varieties of first-order and second-order motion. The findings support the notion that first-order and second-order motion are initially encoded separately in human vision. Moreover, they demonstrate that although some second-order patterns are processed by a common mechanism, this is not true of all types of second-order stimuli. As such, when considered in terms of their perceptual properties, second-order motion patterns should not be considered an homogenous class of visual stimuli as is often the case. These results place important constraints on current theories of motion processing by the human visual system.