Chapter 12

Chapter summary

Many organisms detect image motion across the retina using specialised neural circuits called motion detectors. Psychophysical evidence for neural motion detectors in the human visual system includes the motion after-effect, direction-specific threshold elevation, and studies of random dot kinematograms. Ambiguities in detector outputs are resolved by integrating across the outputs of motion detectors, and by tracking spatial features in moving patterns. It is still not clear how speed is analysed in the visual system.

The “outflow” signals which control eye movements are used to attribute retinal motion signals to eye movements rather than to external motion. Optic flow caused by self-motion provides information about the observer’s heading direction and speed, and is analysed by neurons in area MST.

Motion detector responses can support the computation of 3-D object structure and kinetic depth, as well as judgements about the gender, mood and identity of moving human figures. Responses to visual motion also contribute to the perception of causality and agency in dynamic displays. Although motion analysis is considered to be part of the dorsal processing stream, several lines of evidence including motion streaks and biological motion show that there are extensive interactions with the ventral stream at all levels.