A neuronal model of stroboscopic alternative motion

Abstract

A visual stimulus consisting of two alternating images, presented with an intermediate blank phase between each image, each showing a pair of illuminated spots located at opposite corners of an imaginary rectangle, will evoke the percept of either a horizontal or a vertical apparent motion, called ‘stroboscopic alternative motion’ (SAM). Psychological analyses have revealed that subjects can perceive only one of the two directions of motion at a time, favouring the direction along the shorter edge of the rectangle, but report spontaneous changes between the alternatives. If the stimulus configuration is gradually modified during stimulus presentation so that the preferred direction of motion changes from horizontal to vertical, or vice versa, subjects normally experience a corresponding change of their percept. This change, however, usually occurs with hysteresis, i.e. when the stimulus is significantly beyond its symmetric configuration. In this paper a microscopic neural model is presented which reproduces the main psychological findings. Its essential ingredients are simple motion detectors based on spatiotemporal receptive fields and an inhibition of detectors tuned to orthogonal directions of motion. The response of our network to the SAM stimulus is a high activity of either the detectors tuned to horizontal motion or those tuned to vertical motion, signalling a unique percept. In agreement with experimental studies, the preferred direction of motion is along the shorter edge of non-symmetrical stimuli and perceptual changes occur with hysteresis for a gradually changing stimulus configuration. We finish our argument by developing a mathematically tractable two-neuron model that captures the essentials of the above setup.