Visual sensitivity in the light- and dark-adapted compound eye of the desert locust

Abstract

Spike discharges in response to switching on and off a light source, recorded electrophysiologically from the thoracic region of the ventral nerve cord, have been used to establish visual thresholds for the dark- and light-adapted compound eye of Schistocerca gregaria. Sensitivity increases 5 to 10 times after 15 min dark adaptation. Frontal and lateral sensitivity for the two environmental conditions have been compared. The dark-adapted eye has a similar level of sensitivity at the two sites of sampling; whereas the light-adapted eye has a more sensitive frontal region. These sensitivity variations could relate to behaviour. The problems of this experimental approach are discussed. Potential changes during 5 min periods of light and dark adaptation were recorded in the retinula cell region of the compound eye. There was a rapid negative deflexion at ‘on’ and a positive deflexion at ‘off’. During steady illumination the potential diminished and reached a steady level after 5 min. The potential changes have two stable states, one in darkness and the other in light, each reached after a pronounced but transient overshoot. It is suggested that these potential levels are related to the measured visual thresholds for the light- and dark-adapted eye. The threshold changes during early light adaptation have been shown to follow a similar time course to that of the potential changes. During dark adaptation correlation is not as exact. It is suggested that the potential changes cause the threshold changes and thus are related to the sensitivity of the eye directly. The similarity between visual adaptation in human and insect eyes is discussed. Control of sensitivity appears to consist of neural (both peripheral and central) as well as photochemical factors; and it is further suggested that an investigation of the potential changes may provide information regarding the neural control of visual sensitivity.