How “Micro” is Microperimetry? Characterizing the Effect of Fundus Tracking on the Psychometric Function

Purpose To quantify whether fundus tracking in microperimetry improves psychometric parameter estimation ( in vivo demonstration of improved stimulus-delivery precision), and to derive a psychometrically grounded criterion intensity for suprathreshold (defect-mapping) microperimetry. Methods Twenty-five healthy volunteers underwent MAIA2-microperimetry at five loci: three outside and two inside the blind spot. Frequency-of-seeing (FoS) functions were measured in four blocks (2 tracking on; 2 tracking off). FoS-data were fit using cumulative-Gaussian psychometric functions estimating sensitivity parameters. Mixed-effect models assessed tracking effects, and posterior simulations defined the optimal criterion intensity for separating “seeing” from “non-seeing” loci. Results Tracking had little effect on threshold estimates at loci outside the blind spot, but lowered threshold estimates within the blind spot (posterior median difference PMD [95% CrI] of −1.46 dB [−2.30, −0.62] at locus 4, and −1.02 dB [−1.94, −0.08] at locus 5). Tracking was associated with steeper psychometric slope parameters at loci 1–3 (PMD of −0.14 dB [−0.29, 0.01], −0.27 dB [−0.43, −0.12], and −0.22 dB [−0.40, −0.04]). Without tracking, false-positive responses were more frequent when fixation shifts displaced stimuli toward the “seeing” retina. Simulation-based analysis identified 13 dB as nominally optimal criterion for suprathreshold microperimetry (Youden index: 0.76 [0.74, 0.79], comparable to 10 dB (0.74 [0.72, 0.76]). Conclusions Even in healthy volunteers with stable fixation, fundus tracking measurably reduced sensitivity estimates at “non-seeing” loci and sharpened FoS curves in the “seeing” retina. A criterion intensity of 10 to 13 dB is a defensible choice for separating “seeing” and “non-seeing” retina in suprathreshold (defect-mapping) perimetry paradigms.