Speed tuning in elementary motion detectors of the correlation type

A prominent model of visual motion detection is the so-called correlation o r Reichardt detector. Whereas this model can account for many properties of motion vision, from humans to insects (review, Borst and Egelhaaf 1989), i t has been commonly assumed that this scheme of motion detection is not wel l suited to the measurement of image velocity. This is because the commonly used version of the model, which incorporates two unidirectional motion de tectors with opposite preferred directions, produces a response which varie s not only with the velocity of the image, but also with its spatial struct ure and contrast. On the other hand, information on image velocity can be c rucial in various contexts, and a number of recent behavioural experiments suggest that insects do extract velocity for navigational purposes (review, Srinivasan et al. 1996). Here we show that other versions of the correlati on model, which consists of a single unidirectional motion detector or inco rporates two oppositely directed detectors with unequal sensitivities, prod uce responses which vary with image speed and display tuning curves that ar e substantially independent of the spatial structure of the image. This sur prising feature suggests simple strategies of reducing ambiguities in the e stimation of speed by using components of neural hardware that are already known to exist in the visual system.