Motion perception

Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult problem from a computational perspective, and extraordinarily difficult to explain in terms of neural processing. Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult problem from a computational perspective, and extraordinarily difficult to explain in terms of neural processing. Motion perception is studied by many disciplines, including psychology (i.e. visual perception), neurology, neurophysiology, engineering, and computer science. The inability to perceive motion is called akinetopsia and it may be caused by a lesion to cortical area V5 in the extrastriate cortex. Neuropsychological studies of a patient who could not see motion, seeing the world in a series of static 'frames' instead, suggested that visual area V5 in humans is homologous to motion processing area MT in primates. Two or more stimuli that are switched on and off in alternation can produce two different motion percepts. The first, demonstrated in the figure to the right is 'Beta movement', often used in billboard displays, in which an object is perceived as moving when, in fact, a series of stationary images is being presented. This is also termed 'apparent motion' and is the basis of movies and television. However, at faster alternation rates, and if the distance between the stimuli is just right, an illusory 'object' the same colour as the background is seen moving between the two stimuli and alternately occluding them. This is called the phi phenomenon and is sometimes described as an example of 'pure' motion detection uncontaminated, as in Beta movement, by form cues. This description is, however, somewhat paradoxical as it is not possible to create such motion in the absence of figural percepts. The phi phenomenon has been referred to as 'first-order' motion perception. Werner E. Reichardt and Bernard Hassenstein have modelled it in terms of relatively simple 'motion sensors' in the visual system, that have evolved to detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighbouring point on the retina after a short delay. Sensors that are proposed to work this way have been referred to as either Hassenstein-Reichardt detectors after the scientists Bernhard Hassenstein and Werner Reichardt, who first modelled them, motion-energy sensors, or Elaborated Reichardt Detectors. These sensors are described as detecting motion by spatio-temporal correlation and are considered by some to be plausible models for how the visual system may detect motion. (Although, again, the notion of a 'pure motion' detector suffers from the problem that there is no 'pure motion' stimulus, i.e. a stimulus lacking perceived figure/ground properties). There is still considerable debate regarding the accuracy of the model and exact nature of this proposed process. It is not clear how the model distinguishes between movements of the eyes and movements of objects in the visual field, both of which produce changes in luminance on points on the retina. Second-order motion has been defined as motion in which the moving contour is defined by contrast, texture, flicker or some other quality that does not result in an increase in luminance or motion energy in the Fourier spectrum of the stimulus. There is much evidence to suggest that early processing of first- and second-order motion is carried out by separate pathways. Second-order mechanisms have poorer temporal resolution and are low-pass in terms of the range of spatial frequencies to which they respond. (The notion that neural responses are attuned to frequency components of stimulation suffers from the lack of a functional rationale and has been generally criticized by G. Westheimer (2001) in an article called 'The Fourier Theory of Vision.') Second-order motion produces a weaker motion aftereffect unless tested with dynamically flickering stimuli.