Intensity and structure of visually evoked neural activity: Rivals in modelling a visual system

Abstract Extracellular recordings have been made from the directionally selective movement detectors in the lobula complex of the blowfly. The intensity aspect of the spike activity—represented by the mean interspike interval—as well as the temporal structure aspect of the activity—represented by the interval distribution function and the correlation of adjacent intervals—are studied. Steady state stimuli appear to have no influence on the temporal structure of the spike activity: the stimulus tunes the firing frequency into a certain level, leaving the temporal structure—but for a change in mean interspike interval—unaffected. Dynamic stimuli, however, affect—besides their modulating influence on the spike activity—both aspects. The magnitude of the change in temporal structure is related to the excitatory resp. inhibitory character of the stimulus presented in the visual field of the detector. From the results presented, it follows that besides the intensity aspect of the spike activity, the structural aspect as well yields evident clues in modelling a visual system.