Non-linearities in response properties of insect visual cells: an analysis in time and frequency domain

Intracellularly recorded voltage responses of the visual cells of the blowfly (Calliphora erythrocephala) were analysed in the time and frequency domains. The photoreceptors were stimulated with pulse (impulse), sine, sine-sweep and pseudorandomly (white noise) modulated green light. The blowfly photoreceptor responses, as analysed from the linear transfer functions, seem to arise from a system similar to that of cascaded low-pass filters, with a corner frequency at about 63 Hz (SD +/- 12 Hz). The system is likely to have at least five poles, including one linear second order term, and a pure delay element. Arising from the non-linearities a second harmonic can be seen in the power spectra of responses elicited by sine modulated light. This non-linearity is at least partly explained by the self-shunting property of the membrane voltage response. Light adaptation increases the non-linearities in frequencies lower than 20 Hz, as seen in the decrease of the coherence function with the signal-to-noise ratio remaining constant. Light adaptation also accelerates the transduction process and it appears in the linear transfer function in a form typical to negative feedback. With low stimulus frequencies it causes a 'phase lead'-type non-linearity. In addition, the sine-sweep responses show quite different frequency characteristics in respect of depolarization and repolarization. Lateral inhibition between photoreceptor responses recorded from retinular cell axons in the lamina appears as a drop in gain and as an increasing phase-lag in frequencies from 30 Hz upwards in linear transfer functions. The source of this capacitive-like coupling can be considered to be in the high resistance barriers compartmentalizing the second optic ganglion into discrete anatomical units.