Fig. 3. Target detection. Signal detection analysis of predicted target responses at small target widths (large distances) in 7 male and 7 female photoreceptors. The signal-to-noise ratio, d', measures mean response amplitude divided by photoreceptor noise S.D. after filtering responses to suppress noise. Values are means ± S.D. Also plotted is an error line that shows the value of d' (3.29) at which an ideal observer would incorrectly identify signal-present and signal-absent trials 5% of the time in an experiment in which half the trials do not contain a target and when the noise is Gaussian.