Model Based or Model Free? Comparing Adaptive Methods for Estimating Thresholds in Neuroscience

The quantification of human perception through the study of psychometric functions Ψ is one of the pillars of experimental psychophysics. In particular, the evaluation of the threshold is at the heart of many neuroscience and cognitive psychology studies, and a wide range of adaptive procedures has been developed to improve its estimation. However, these procedures are often implicitly based on different mathematical assumptions on the psychometric function, and unfortunately, these assumptions cannot always be validated prior to data collection. This raises questions about the accuracy of the estimator produced using the different procedures. In the study we examine in this letter, we compare five adaptive procedures commonly used in psychophysics to estimate the threshold: Dichotomous Optimistic Search (DOS), Staircase, PsiMethod, Gaussian Processes, and QuestPlus. These procedures range from model-based methods, such as the PsiMethod, which relies on strong assumptions regarding the shape of Ψ, to model-free methods, such as DOS, for which assumptions are minimal. The comparisons are performed using simulations of multiple experiments, with psychometric functions of various complexity. The results show that while model-based methods perform well when Ψ is an ideal psychometric function, model-free methods rapidly outshine them when Ψ deviates from this model, as, for instance, when Ψ is a beta cumulative distribution function. Our results highlight the importance of carefully choosing the most appropriate method depending on the context.

Attention alters certainty but not appearance

Does paying attention to a stimulus change its appearance or merely influence the decision mechanisms involved in reporting it? Recently we proposed an uncertainty stealing hypothesis in which subjects, when uncertain about a perceptual comparison between a cued and uncued stimulus, tend to disproportionately choose the cued stimulus. The result is a psychometric function that mimics the results that would be measured if attention actually changed the appearance of the cued stimulus. In the present study, we measure uncertainty explicitly. In three separate experiments, subjects judged the relative appearance of two Gabor patches that differed in contrast. In the first two experiments, subjects performed a comparative judgment, reporting which stimulus had the higher contrast. In the third experiment, subjects performed an equality judgment, reporting whether the two stimuli had the same or different contrast. In the first comparative judgment experiment and in the equality judgment experiment, one of the two stimuli was pre-cued by an exogenous cue. In the second comparative judgment experiment, a decision bias was explicitly introduced: one stimulus was followed by a post-cue and the subjects were instructed, when uncertain, to choose the cued target. In all three experiments, subjects also indicated whether or not they were certain about each response. The results reveal that in the pre-cue comparative judgment, attention shifted the subjects’ uncertainty and made subjects more likely to report that the cued stimulus had higher contrast. In the post-cue biased comparative judgment, subjects also were more likely to report that the cued stimulus had higher contrast, but without a shift in uncertainty. In the equality judgment, attention did not affect the contrast judgment, and the subjects’ uncertainty remained aligned with their decision. We conclude that attention does not alter appearance but rather manipulates subjects’ uncertainty and decision mechanisms.

Imagery adds stimulus-specific sensory evidence to perceptual detection

Internally generated imagery and externally triggered perception rely on overlapping sensory processes. This overlap poses a challenge for perceptual reality monitoring: determining whether sensory signals reflect reality or imagination. In this study, we used psychophysics to investigate how imagery and perception interact to determine visual experience. Participants were instructed to detect oriented gratings that gradually appeared in noise while simultaneously either imagining the same grating, a grating perpendicular to the to-be-detected grating, or nothing. We found that, compared to both incongruent imagery and no imagery, congruent imagery caused a leftward shift of the psychometric function relating stimulus contrast to perceptual threshold. We discuss how this effect can best be explained by a model in which imagery adds sensory signal to the perceptual input, thereby increasing the visibility of perceived stimuli. These results suggest that, in contrast to changes in sensory signals caused by self-generated movement, the brain does not discount the influence of self-generated sensory signals on perception.

Psychometric function characteristics of Persian consonantvowel-consonant words

Background and Aim: Evaluation of word recognition score requires multiple lists that must be similar in terms of difficulty level. There is currently no such word lists for the Persian language. The aim of this study was to construct several lists of Persian monosyllabic words with psychometric homogeneity. Methods: The most common monosyllabic words were collected from a book of Persian word frequency. The selected monosyllabic Consonant-Vowel-Consonant (CVC) words were presented randomly to 30 normal hearing participants with the age range of 18 to 25 years. The presentation level was from 0 to 40 dB in 8 dB increments. The characteristics of psychometric function were determined for all words using the logistic regression. Results: The Persian CVC monosyllabic words have different difficulty levels with threshold varying from 2.8 to 37.2 dB HL and the slope from 2.3 to 16.4 %/dB. Conclusion: The final result of the present study is three full lists of monosyllabic words with CVC syllabic structure that have the same mean threshold and slope of psychometric function. The 25-word half-lists of each full list are similar in terms of psychometric characteristics. Keywords: Psychometric function; Persian monosyllabic words; speech audiometry

Chicken colour discrimination depends on background colour

ABSTRACTHow well can a bird discriminate between two red berries on a green background? The absolute threshold of colour discrimination is set by photoreceptor noise, but animals do not perform at this threshold; their performance can depend on additional factors. In humans and zebra finches, discrimination thresholds for colour stimuli depend on background colour, and thus the adaptive state of the visual system. We have tested how well chickens can discriminate shades of orange or green presented on orange or green backgrounds. Chickens discriminated slightly smaller colour differences between two stimuli presented on a similarly coloured background, compared with a background of very different colour. The slope of the psychometric function was steeper when stimulus and background colours were similar but shallower when they differed markedly, indicating that background colour affects the certainty with which the animals discriminate the colours. The effect we find for chickens is smaller than that shown for zebra finches. We modelled the response to stimuli using Bayesian and maximum likelihood estimation and implemented the psychometric function to estimate the effect size. We found that the result is independent of the psychophysical method used to evaluate the effect of experimental conditions on choice performance.

Asymmetries in visual acuity around the visual field

Human vision is heterogeneous around the visual field. At a fixed eccentricity, performance is better along the horizontal than the vertical meridian, and along the lower than the upper vertical meridian. These asymmetric patterns, termed performance fields, have been found in numerous visual tasks, including those mediated by contrast sensitivity and spatial resolution. However, it is unknown whether spatial resolution asymmetries are confined to the cardinal meridians or whether, and how far, they extend into the upper and lower hemifields. Here, we measured visual acuity at isoeccentric peripheral locations (10 deg eccentricity), every 15º of polar angle. On each trial, observers judged the orientation (±45º) of one out of four equidistant, suprathreshold grating stimuli varying in spatial frequency (SF). On each block, we measured performance as a function of stimulus SF at 4 out of 24 isoeccentric locations. We estimated the 75%-correct SF threshold, SF cutoff point (i.e., chance-level) and slope of the psychometric function for each location. We found higher SF estimates –i.e., better acuity– for the horizontal than the vertical meridian, and for the lower than the upper vertical meridian. These asymmetries were most pronounced at the cardinal meridians and decreased gradually as the angular distance from the vertical meridian increased. This gradual change in acuity with polar angle reflected a shift of the psychometric function without changes in slope. The same pattern was found under binocular and monocular viewing conditions. These findings advance our understanding of visual processing around the visual field and help constrain models of visual perception.