Temporal properties of natural scenes

A major problem a visual system faces is how to fit the large intensity variation of natural image streams into the limited dynamic range of its neurons. One of the means to accomplish this is through the use of fast light adaptation of the photoreceptors. In order to investigate this, we measured first time series of natural intensities, and second responses of fly photoreceptors to these time series. Time series representative of what each photoreceptor of a real visual system would normally receive were measured with an optical system measuring the light intensity of a spot comparable to the field of view of single human foveal cones. This system was worn on a head-band by a freely walking person. Resulting time series have a high rms-contrast in the order of 1, and power spectra behaving approximately as 1/f (f: temporal frequency). Measured time series were subsequently presented to fly photoreceptors by playing them back on an LED. The results show that fast light adaptation indeed keeps the response within the dynamic range of the cells and that a large part of this range is actually needed for packing the information in natural time series.