Patterns of growth in Darwin’s finches

Abstract

Measurements of threshold visibility were made as a function of duration of stimulus exposure for small moving dot targets, drifting sinusoidal gratings and moving patches of sinusoidal gratings, to investigate how the human visual nervous system summates over time signals arising from stimuli in motion. At image speeds of less that 16 deg/s, temporal summation is as strong and as extended for moving as for stationary dots (total summation over to about 100 ms). This summation is about twice that which would be expected from separate consideration of the regions of spatial and temporal integration. Measurements with sinusoidal gratings reveal that the nature of the summation depends critically on the spatial frequency of the stimulus: gratings of low spatial frequency summate well when in motion (and only when in motion), whereas those of high spatial frequency summate well only when stationary or in very slow motion. An analogue simulation with electronic filters showed that these psychophysical results are directly predictable from the known transfer characteristics of the human visual system (with the additional assumption of probability summation at threshold). Finally, with small patches of sinusoidal grating, it was established that translation per se across the retina has little effect on temporal summation. This suggests that the results obtained with sinusoidal gratings of large extent are also relevant to small moving stimuli, allowing the summation results obtained with dot stimuli to be discussed in terms of the temporal transfer properties of spatially selective visual detectors. On the basis of these results it is proposed that the extended temporal summation observed for dots in motion results from summation of energy of low spatial frequency present in these stimuli.