title: The slope of the psychometric function and non-stationarity of thresholds in spatiotemporal contrast vision.
abstract: The slope of the two-interval, forced-choice psychometric function (e.g. the Weibull parameter, β) provides valuable information about the relationship between contrast sensitivity and signal strength. However, little is known about how or whether β varies with stimulus parameters such as spatiotemporal frequency and stimulus size and shape. A second unresolved issue concerns the best way to estimate the slope of the psychometric function. For example, if an observer is non-stationary (e.g. their threshold drifts between experimental sessions), β will be underestimated if curve fitting is performed after collapsing the data across experimental sessions. We measured psychometric functions for 2 experienced observers for 14 different spatiotemporal configurations of pulsed or flickering grating patches and bars on each of 8 days. We found β≈3 to be fairly constant across almost all conditions, consistent with a fixed nonlinear contrast transducer and/or a constant level of intrinsic stimulus uncertainty (e.g. a square law transducer and a low level of intrinsic uncertainty). Our analysis showed that estimating a single β from results averaged over several experimental sessions was slightly more accurate than averaging multiple estimates from several experimental sessions. However, the small levels of non-stationarity (SD≈0.8dB) meant that the difference between the estimates was, in practice, negligible.
authors.affiliation: School of Life & Health Sciences, Aston University, Birmingham B4 7ET, UK. s.a.wallis2@aston.ac.uk
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Wallis
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Baker
authors.forename: Tim S
authors.initials: TS
authors.lastname: Meese
authors.forename: Mark A
authors.initials: MA
authors.lastname: Georgeson
mesh_terms: D002755:Choice Behavior; D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D011594:Psychometrics / Q000379:methods*; D012684:Sensory Thresholds / Q000502:physiology*; D035501:Uncertainty