Further evidence for four mechanisms mediating vision at threshold: sensitivities to complex gratings and aperiodic stimuli.

It is now generally accepted that several mechanisms mediate contrast detection at threshold. Many attempts to measure the bandwidths of these mechanisms using subthreshold summation have yielded estimates suggesting very narrow frequency tuning. We have measured thresholds for three classes of stimuli: gratings containing two sinusoidal components, sums of "difference of Gaussian" (DOG) patterns, and square waves. The results of the subthreshold summation experiments with gratings are consistent with those of other studies. All of these thresholds can be predicted by a model requiring only four center-surround-type mechanisms at each retinal locus. These mechanisms have a full bandwidth at a half-height of about 1.75 octaves. Quantitative prediction of sensitivities depends critically upon spatial probability summation. It is this spatial nonlinearity that yields these somewhat conterintuitive results. The much narrower bandwidths inferred by others are due to the assumption of linearity used in their analyses.