The relationship between visual acuity and illumination in the fly, lucilia sericata.

The variation in visual acuity with illumination has been studied by monitoring the extracellular response of direction sensitive motion detecting neurons to a drifting sine wave grating displayed upon an oscilloscope spreen. Acuity reaches a maximum value of 0.46 cycles/degree at luminances above 1.0 cd/m2 and decreases gradually over a 3.8 log unit attenuation in intensity to a minimum value of 0.05 cycles/degree. The results have been compared with theoretical acuity curves for the coupound eye with various dark adaptation mechanisms. The analysis indicates that a major strategy of dark adaptation in the fly is a process involving intensity-dependent neural summation of signals from photoreceptors having different visual axes.