Visual Motion In Animals. (Neuroscience) Essay

2973 words - 12 pages

Visual motion information plays a crucial part in the lives of animals and insects. It is essential in order to perform successful navigation, avoid predators and catch prey. An animal receives motion information all the time it perceives an object in its field of vision, this however is not the only source of motion information, motion stimuli are in fact processed all the time during self-motion. This produces a self-generated retinal flowfield, which gets continually displaced according to the animal's trajectory and the three-dimensional structure of the visual environment (Egelhaaf et al, 1992).There are three main types of visual flow patterns, which encode three possible motion situations. A rotatory large-field motion will signal to the animal any unintended deviation from its own course and trigger a negative feedback response to compensate for these deviations, for example for hovering behaviour bees. Image expansion signals that the animal is heading towards an obstacle, this can be useful both to avoid obstacles and to correctly time a smooth landing. The third type of flowfield is produced by discontinuities in the retinal motion field and small field motion which indicate the presence of a stationary or moving object (Egelhaaf, 1992).There are four good reasons for researchers to analyse the mechanisms underlying motion detection in the fly (fig 1):Fig1: A picture showing the compound eyes of a fly· Flies have relatively "compressed" and simplified neural circuitries· They also have relatively large and fixed-focused compound eyes· they have acquired a highly specialised visual system for motion vision (due to their poor visual resolution)· The nervous system of at least the relatively big blowfly Calliphora erythrocephala is amenable to an analysis on the basis of morphologically distinct nerve cells that can easily be identified individually (Egelhaaf & Borst, 1992).Anatomical and behavioural crossed analyses carried out in recent years, with the aid of computer modelling, have uncovered the neuronal computation that underlie visuo-motor processing, in flies. On the basis of this research, it is now possible to model this kind of neural computations at the level of the synaptic interactions and transmitters being released, which had previously been defined exclusively in formal terms. Outputs from the model were consequently monitored when specific components were functionally disabled. This allowed scientists to elaborate an accurate picture of the different computations performed by the fly in vivo as well as enabling us to identify and characterise basic local motion detectors. These are individually responsible for processing motion information from discrete areas of the visual field and are organised in two-dimensional retinotopic arrays which cover the entire area of the animal's visual field (Egelhaaf et al, 1992).In the 1950s Reichardt and Hassenstein proposed a model, based on the...

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