Movement detection in the posterolateral eyes of jumping spiders (Evarcha arcuata, Salticidae)

Abstract

1. Jumping spiders react to a single moving object in the visual field of the posterolateral eyes with an accurate turn of the prosoma which brings the object into the visual field of the large frontal eyes. However, it was not possible to elicit optomotor reactions to moving stripe patterns nor turning reactions to random patterns consisting of moving black squares. 2. By systematically “diluting” the random square pattern in a drum apparatus, the optical parameters which lead to the suppression of a turning reaction in situations where several stimuli interact could be determined. The behavioral results fit the hypothesis of a regular arrangement of neurons exhibiting a receptive field with an excitatory centre and an inhibitory surround with maximum inhibition at a distance of approximately 20° from the centre (Fig. 10). It is assumed that the role of these neurons is to filter out irrelevant background movements. 3. The minimal input parameters for movement detection have been investigated by evoking turning reactions with real and apparent motion stimuli: Almost exclusively the off-components of sequentially presented point stimuli elicit a turning reaction (Fig. 7). Stimulation of two receptors at a distance of up to 15° is sufficient for the reaction. Exceptional spiders reacted to single off-stimuli subtending as little as 0.3°. 4. A hypothetical model for movement detecting neurons is proposed (Fig. 9) exhibiting the following characteristics: elliptical receptive field (approximately 20°×30°, Fig. 8); no directional selectivity; fast adapting and localizing response to off-stimulation of one receptor after lateral excitation (or disinhibition) by off-stimulation of another receptor within the surrounding receptive field. 5. The behavioral experiments suggest the localization of the “off-pass” filter is in the peripheral optic ganglia, whereas the movement detecting system as well as the background filtering units are placed more centrally. 6. From a comparison with other arthropod visual systems in combination with present neuroanatomical investigation of the salticid side eyes, it is concluded that in salticids only the peripheral basic steps in movement detection are realized. Local answers lead to exact position commands, but they are not transformed into a rotational velocity command. An analogue to the insect lobula containing integrative large field neurons seems to be lacking in the posterolateral eyes of salticids.