Figure 6.

Receiver operating characteristic (ROC) analysis and the relationship between signal discriminability, Δm, and stimulus intensity for cone detection at absolute threshold. A) Typical ROC curves for subject 2. B) ROC curves for the same subject now on a double probability plot and with a sample estimation of discriminability from linear fits to the low intensity curve - Δm equals z(S|n) on the ROC curve where z(S|s) equals zero (dashed gray lines). On double probability plots slopes (σ_n/σ_sn) < 1 imply unequal variance in the noise and signal plus noise distributions; slopes for our subjects are uniformly < 1 (see also Table 3). C) Log Δm vs log stimulus energy for subjects 1 (blue circles), 2 (red triangles), 3 (green squares) and 4 (purple diamonds). Photons at the cornea for subject 4 are relative, all others are absolute. Datasets 4-6 for subject 1, where only relative thresholds were obtained, are not included but are included in Table 3 below. Unit slope, as expected for Poisson noise-limited detection, is shown for comparison (gray dashed line). All subjects demonstrate an acceleration of discriminability with signal strength in cone detection that is inconsistent with a linear Poisson noise-limited process.