title: Visual discrimination of direction changes based upon two types of angular motion.
abstract: We address the question of how the visual system analyses changes in direction. Using plaid stimuli, we define type O direction changes which entail a change in the orientations of the plaid components, and type V direction changes in which the orientations of the components remain constant, relative to the observer but their relative speeds change. Lower thresholds for discriminating type O and type V direction changes were compared. Type O thresholds for clockwise/anticlockwise direction change were very low (0.2-0.5 degree), were resistant to directional noise, and showed a low-pass relationship with drift velocity. Type V thresholds on the other hand were higher (1-5 degrees), and exhibited a bandpass relationship with drift velocity. Type O direction changes gave low thresholds at short inter-stimulus intervals (ISI) (< 160 ms) and higher thresholds (successive orientation discrimination) at long ISI (240 ms-12.8 s). Type V thresholds, on the other hand, exhibited no short-range process and performance at short ISI, was no better than for successive direction discrimination at long ISI. A two-stage rotary motion model is sufficient to explain the discrimination of type O direction changes and results rule out a model based on velocity discrimination. For type V direction changes, a two-stage mechanism is insufficient and results are consistent with a minimum of three computational stages.
authors.affiliation: Department of Human Sciences, Brunel University, Uxbridge, UK. michael.wright@brunel.ac.uk
authors.forename: M J
authors.initials: MJ
authors.lastname: Wright
authors.forename: K N
authors.initials: KN
authors.lastname: Gurney
mesh_terms: D006801:Humans; D008297:Male; D009039:Motion Perception / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D012399:Rotation; D012684:Sensory Thresholds / Q000502:physiology; D013997:Time Factors