The Stiles-Crawford Effect in the Eye of the Blowfly, Calliphora erythrocephala

Stiles-Crawford-like effects (that is, directional sensitivity of the retina) were investigated in the fly’s eye. Intracellular recordings from the visual sense cells were made, and the radiation patterns emerging from the photoreceptors with antidromic light were photographed, and evaluated with a microdensitometer. The measurements from both methods agree well, and can be satisfactorily described by a theoretical model based on waveguide theory. Clear radiation patterns from the first and second order modes were observed at the level of the cornea. As in the vertebrate eye, the photoreceptors are aligned towards the center of the lens, a phenomenon for which a theoretical explanation is proposed.

Directional Tuning Curves, Elementary Movement Detectors, and the Estimation of the Direction of Visual Movement

Both the insect brain and the vertebrate retina detect visual movement with neurons having broad, cosine-shaped directional tuning curves oriented in either of two perpendicular directions. This article shows that this arrangement can lead to isotropic estimates of the direction of movement: for any direction the estimate is unbiased (no systematic errors) and equally accurate (constant random errors). A simple and robust computational scheme is presented that accounts for the directional tuning curves as measured in movement sensitive neurons in the blowfly. The scheme includes movement detectors of various spans, and predicts several phenomena of movement perception in man.

Fast temporal adaptation of on-off units in the first optic chiasm of the blowfly

We recorded from spiking units in the first optic chiasm between lamina and medulla in the brain of the blowfly (Calliphora vicina). Both previously characterized neuron types, on-off units and sustaining units, were encountered. On-off units had a temporal frequency response with a lower cut-off frequency than blowfly photoreceptors. This low cut-off frequency is related to a fast temporal adaptation of the on-off units to trains of short light pulses. Temporal adaptation occurred independently for short on- and off-pulses. On-off units only responded to stimuli of relatively large contrast. Contrasts of less than 10% gave little or no response.

Butterfly Optics Exceed the Theoretical Limits of Conventional Apposition Eyes

Optical experiments on butterfly compound eyes show that they have angular sensitivities narrower than expected from conventional apposition eyes. This superior performance is explained by a theoretical model where the cone stalk is considered as a modecoupling device. In this model the Airy diffraction pattern of the corneal facet excites a combination of the two waveguide modes LP01 and LP02. When the two modes propagate through the cone stalk the power of LP02 is transferred to LP01 alone which is supported by the rhabdom. This mechanism produces a higher on-axis sensitivity and a narrower angular sensitivity than conventional apposition optics. Several predictions of the model were confirmed experimentally.