Fast and accurate measurement of taste and smell thresholds using a maximum-likelihood adaptive staircase procedure

This paper evaluates the use of a maximum-likelihood adaptive staircase psychophysical procedure (ML-PEST), originally developed in vision and audition, for measuring detection thresholds in gustation and olfaction. The basis for the psychophysical measurement of thresholds with the ML-PEST procedure is developed. Then, two experiments and four simulations are reported. In the first experiment, ML-PEST was compared with the Wetherill and Levitt up-down staircase method and with the Cain ascending method of limits in the measurement of butyl alcohol thresholds. The four Monte Carlo simulations compared the three psychophysical procedures. In the second experiment, the test-retest reliability of ML-PEST for measuring NaCl and butyl alcohol thresholds was assessed. The results indicate that the ML-PEST method gives reliable and precise threshold measurements. Its ability to detect malingerers shows considerable promise. It is recommended for use in clinical testing.     

Tracking of nociceptive thresholds using adaptive psychophysical methods

Psychophysical thresholds reflect the state of the underlying nociceptive mechanisms. For example, noxious events can activate endogenous analgesic mechanisms that increase the nociceptive threshold. Therefore, tracking thresholds over time facilitates the investigation of the dynamics of these underlying mechanisms. Threshold tracking techniques should use efficient methods for stimulus selection and threshold estimation. This study compares, in simulation and in human psychophysical experiments, the performance of different combinations of adaptive stimulus selection procedures and threshold estimation methods. Monte Carlo simulations were first performed to compare the bias and precision of threshold estimates produced by three different stimulus selection procedures (simple staircase, random staircase, and minimum entropy procedure) and two estimation methods (logistic regression and Bayesian estimation). Logistic regression and Bayesian estimations resulted in similar precision only when the prior probability distributions (PDs) were chosen appropriately. The minimum entropy and simple staircase procedures achieved the highest precision, while the random staircase procedure was the least sensitive to different procedure-specific settings. Next, the simple staircase and random staircase procedures, in combination with logistic regression, were compared in a human subject study (n = 30). Electrocutaneous stimulation was used to track the nociceptive perception threshold before, during, and after a cold pressor task, which served as the conditioning stimulus. With both procedures, habituation was detected, as well as changes induced by the conditioning stimulus. However, the random staircase procedure achieved a higher precision. We recommend using the random staircase over the simple staircase procedure, in combination with logistic regression, for nonstationary threshold tracking experiments.     

Microcomputer-based estimation of psychophysical thresholds: The Best PEST

A new, maximally efficient technique for measuring psychophysical thresholds (Pentland, 1980) has been implemented on the microcomputer. This PEST (parameter estimation by sequential testing) technique is the most efficient sequential parameter estimation technique possible, given that the form of the psychometric function is known. The technique is similar to but faster and more accurate than other staircase procedures and may be applied whenever staircase techniques are applicable. The “Best PEST” is easily implemented on the micro-computer; a BASIC program for the Apple II which does so is presented. The Best PEST is compared with other staircase procedures, including one recently implemented on a micro-computer (Corwin, Kintz, & Beaty, 1979).     

Comparison of cupulometric and psychophysical thresholds for perception of rotation and the oculogyral illusion

The purpose of this study was to compare thresholds for angular acceleration derived by subjective cupulometry and by a staircase method. Thresholds for the perception of rotation and the oculogyral illusion were determined for 10 Os who were rotated about their vertical axis. The cupulometric thresholds were significantly higher, more variable, and not predictable from the staircase thresholds. Furthermore, cupulometry failed to distinguish between the thresholds for the perception of rotation and the oculogyral illusion, although both indicators functioned according to the prediction of the underlying linear model. Individual differences supported the general conclusion that cupulometric thresholds bear no relationship to the sensory threshold derived in a classical psychophysical manner.     

A SIMULTANEOUS DISCRIMINATION PROCEDURE FOR THE MEASUREMENT OF VISION IN NONVERBAL CHILDREN1

Few nonverbal developmentally disabled children ever receive adequate vision assessment because of their limited language skills. The present study details a simultaneous discrimination procedure for measuring subjective visual acuity in such children. A stimulus fading procedure was used to train a discrimination between Snellen Es differing in orientation, and a psychophysical tracking method was used to determine acuity thresholds. The procedure was tested with 11 nonverbal autistic and schizophrenic children and validated with four nonpsychotic children. Eight of the psychotic children were successfully examined in one to three sessions. Two of these children were identified as having significant acuity losses. The validity assessment showed that the experimental procedure resulted in thresholds equal to or slightly lower than those obtained with the Illiterate E chart.     

A comparison of three adaptive psychophysical procedures using inexperienced listeners

Thresholds for a masked 1-kHz tone were obtained in single sessions from 60 inexperienced listeners. A two-alternative forced-choice procedure was used in conjunction with one of three adaptive psychophysical techniques. These techniques comprised two staircase techniques targeting either 70.7% or 79.4% correct detection (Staircase-71 and Staircase-79), and parameter estimation by sequential testing (PEST), targeting 80% correct detection. Listeners were provided with a rationale for maintaining concentration at weak signal levels. Similar threshold values were obtained from the Staircase-79 and PEST groups in equal numbers of trials. The degree of oscillation in the level of the signal around the value finally chosen as the threshold was comparable for both staircase techniques. Subsequent fixed-level testing did not provide a true indication of the subjects' capabilities. The amount by which percent correct in fixed-level testing differed from expectations based on adaptive testing varied among the techniques. Additional thresholds were obtained in a second session from 30 of the original subjects. Thresholds with both staircase techniques improved by about 1 dB on retest, while thresholds with PEST were constant across sessions. The variability of the data compared well with that from studies involving experienced listeners.     

Detecting malingerers by means of response-sequence analysis

The feigning of sensory loss (malingering) poses a challenge for psychophysicists. To uncover malingerers by means of psychophysical testing, we combined measures of response-sequence randomness with the maximum-likelihood adaptive-staircase procedure used to measure sensory detection thresholds. The two-alternative, forced-choice maximum-likelihood adaptive-staircase procedure calculates an estimate of the threshold after each trial and also recommends the stimulus concentration for the next trial. Olfactory detection thresholds for butyl alcohol were measured in 7 normals, 6 anosmics, and 6 malingerers. Each participant was tested for 20 trials. A discriminant analysis, using threshold concentration and probability of being correct over the 20 trials, could correctly classify only 68% of the malingerers and anosmics. Correct classification of anosmics and malingerers rose to 100% when statistical measures of randomness in the response sequences were included in the discriminant analysis. We conclude that the maximum-likelihood adaptive-staircase procedure, combined with response-sequence analysis, is a powerful addition to the arsenal of techniques for detecting malingerers in the evaluation of sensory ability.     

A comparison of sniff bottle staircase and olfactometer-based threshold tests

Olfactometers have been gaining popularity as research tools, but they have yet to replace established testing procedures in a variety of laboratory and clinical settings, including absolute threshold tests. In this research, we designed and operated a simple olfactometer with which to assess threshold. To do this, we used a method-of-adjustment test that was compared to the three-alternative forced choice ascending sniff bottle staircase method, which is currently a standard threshold test procedure. We found that the olfactometer threshold test correlated highly with the staircase method, and that it possessed suitable test–retest reliability. The advantages of the olfactometer threshold test include faster test time and reduced cleaning and reassembly demands. Future use of the olfactometer in olfactory identification and/or detection thresholds amongst odors is also outlined.     

Psychophysics with children: Investigating the effects of attentional lapses on threshold estimates

When assessing the perceptual abilities of children, researchers tend to use psychophysical techniques designed for use with adults. However, children’s poorer attentiveness might bias the threshold estimates obtained by these methods. Here, we obtained speed discrimination threshold estimates in 6- to 7-year-old children in UK Key Stage 1 (KS1), 7- to 9-year-old children in Key Stage 2 (KS2), and adults using three psychophysical procedures: QUEST, a 1-up 2-down Levitt staircase, and Method of Constant Stimuli (MCS). We estimated inattentiveness using responses to “easy” catch trials. As expected, children had higher threshold estimates and made more errors on catch trials than adults. Lower threshold estimates were obtained from psychometric functions fit to the data in the QUEST condition than the MCS and Levitt staircases, and the threshold estimates obtained when fitting a psychometric function to the QUEST data were also lower than when using the QUEST mode. This suggests that threshold estimates cannot be compared directly across methods. Differences between the procedures did not vary significantly with age group. Simulations indicated that inattentiveness biased threshold estimates particularly when threshold estimates were computed as the QUEST mode or the average of staircase reversals. In contrast, thresholds estimated by post-hoc psychometric function fitting were less biased by attentional lapses. Our results suggest that some psychophysical methods are more robust to attentiveness, which has important implications for assessing the perception of children and clinical groups.

     

Errors of the standard in the horizontal-vertical illusion

Three experiments were conducted to test the claim that the magnitude of the horizontal-vertical illusion in an L figure varies as a function of which component is used as standard. No difference was found in Experiment 1, in which a staircase procedure was used to establish the PSE. However, by the use of a method of adjustment, the magnitude of illusion was affected by the component used as standard in Experiment 2. The results of Experiment 3, in which a staircase procedure was used with S in upright and recumbent body postures, confirmed those of Experiment 1. It was concluded that earlier differences associated with the standard were a methodological accompaniment of the psychophysical technique used.     

Odor magnitude: Coarse vs. fine grain

Despite a dismal reputation, olfaction’s resolving power actually appears to be keen, especially when the contaminating influence of noise in the stimulus is discounted from psychophysical performance. In a previous investigation, subjects were able to resolve a change of 4% in the concentration of butyl alcohol. In the present experiment, these same subjects resolved a change of 9% in the concentration of amyl butyrate. The data imply that resolving power varies among odorants. The growth of odor intensity assessed by magnitude estimations of easily discriminable (i.e., grossly different) concentrations also varies among odorants. These observations prompt the question: Does an odorant for which resolution is relatively difficult lead to slower growth of perceived intensity with concentration than an odorant for which resolution is easy? In a scaling experiment, amyl butyrate indeed led to slower growth of intensity than butyl alcohol. Nevertheless, amyl alcohol, which permitted poorer resolution than butyl alcohol, led to approximately the same rate of growth as butyl alcohol. Accordingly, growth of odor intensity with concentration may correlate only weakly with resolving power.     

Frequency recognition in temporal interference tasks: A comparison among four psychophysical procedures

The frequency discrimination of a 20-msec test tone was measured for the same subjects in four psychophysical paradigms: a single-interval procedure, a two-alternative forced-choice procedure, a same-different procedure, and a method-of-adjustment procedure. In each paradigm, the 20-msec test tone was preceded or followed by a 500-msec, 800-Hz interfering tone. The interfering tone occurred 50 or 5 msec before the onset of the test tone (forward interference) or 100 or 5 msec after the offset of the test tone (backward interference). In each of the four paradigms and for each of the interference conditions the value of Δf for the test tone was varied symetrically around 800 Hz to obtain an estimate of the frequency discrimination thresholds. In all psychophysical paradigms except for the single-interval procedure, there were small or no significant differences in observers’ frequency discrimination behavior among the interference conditions. The thresholds for Δf from these conditions were approximately the same as those obtained without an interfering tone. In the single-interval task, when the test tone preceded the interfering tone (backward interference) by 5 msec, an increase in the value of Δf required for discrimination over that required in the other conditions was measured. These results suggest that the effect of backward interference on a target tone does depend on psychophysical procedure.     

Time discrimination in Columba livia and Homo sapiens.

Pigeons' ability to discriminate stimulus duration, focusing on stimuli less than 1 s in duration, was evaluated in 4 experiments. In Experiment 1, the performances of pigeons and humans were compared with a staircase technique, and in Experiment 2, the method of constant stimuli was used. Both experiments produced similar results: The pigeon and human data were well described by the generalized form of Weber's law (Getty, 1975). Experiment 3 demonstrated that the birds did not use perceived brightness to mediate the discrimination of brief visual durations. Experiment 4 used a modified staircase procedure that yielded a continuous measure of discrimination from absolute threshold (0 s) to about 1 s. The difference thresholds were constant over a considerable range, similar to findings reported by Kristofferson (1980) for human timing.     

Statistical properties of staircase estimates

The bias and variability of staircase estimators were studied by means of repeated computer simulations of staircase runs. Both forced-choice and yes-no staircases were simulated. The influence of the shape of the psychometric function, the location and spacing of the stimuli, the number of trials in a run, and the method of deriving the estimate from the data are discussed. The forced-choice staircase is compared to the yes-no staircase, and the limitations of the simulation procedure are outlined.     

On the limitations of fixed‐step‐size adaptive methods with response confidence

The family of (non‐parametric, fixed‐step‐size) adaptive methods, also known as ‘up–down’ or ‘staircase’ methods, has been used extensively in psychophysical studies for threshold estimation. Extensions of adaptive methods to non‐binary responses have also been proposed. An example is the three‐category weighted up–down (WUD) method (Kaernbach, 2001) and its four‐category extension (Klein, 2001). Such an extension, however, is somewhat restricted, and in this paper we discuss its limitations. To facilitate the discussion, we characterize the extension of WUD by an algorithm that incorporates response confidence into a family of adaptive methods. This algorithm can also be applied to two other adaptive methods, namely Derman's up–down method and the biased‐coin design, which are suitable for estimating any threshold quantiles. We then discuss via simulations of the above three methods the limitations of the algorithm. To illustrate, we conduct a small scale of experiment using the extended WUD under different response confidence formats to evaluate the consistency of threshold estimation.     

Differential aversive learning enhances orientation discrimination

A number of recent studies have documented rapid changes in behavioural sensory acuity induced by aversive learning in the olfactory and auditory modalities. The effect of aversive learning on the discrimination of low-level features in the visual system of humans remains unclear. Here, we used a psychophysical staircase procedure to estimate discrimination thresholds for oriented grating stimuli, before and after differential aversive learning. We discovered that when a target grating orientation was conditioned with an aversive loud noise, it subsequently led to an improvement of discrimination acuity in nearly all subjects. However, no such change was observed in a control group conditioned to an orientation shifted by ±90° from the target. Our findings cannot be explained by contextual learning or sensitisation factors. The results converge with those reported in the olfactory modality and provide further evidence that early sensory systems can be rapidly modified by recently experienced reinforcement histories.     

Comparison of detection threshold values determined using glass sniff bottles and plastic squeeze bottles

Olfactory threshold measures are influenced by such factors as odorant species, diluent type, psychophysical paradigm, and stimulus-presentation procedure. In this study, we compared phenyl ethyl alcohol odor-detection thresholds obtained using 120-ml glass sniff bottles to those obtained using 120-ml plastic squeeze bottles. Although these presentation media are commonly employed in published studies, there has never been a formal comparison of values obtained using them. 10 male and 10 female subjects were tested on two threshold test sessions, one for each type of bottle. Order of sessions was systematically counterbalanced and completed on the same day for a given subject, with a minimum of 30 min. elapsing between sessions. A seven-reversal, single-staircase threshold procedure was employed. Although the threshold values were similar for the two procedures, slightly lower thresholds were obtained using the glass sniff bottles [respective M (SEM) log vol/vol values = -6.61 (.20) and -6.13 (.24)]. These data suggest that, while threshold values using these two presentation procedures can be roughly compared across studies, accurate comparisons may require a slight mathematical adjustment.     

Using the standard staircase to measure the point of subjective equality: A guide based on computer simulations

In experiments that measure a point of subjective equality, it is necessary to employ a psychophysical technique that measures the 50% point (P50) on the psychometric function. The standard staircase is an attractive candidate, because its simplicity makes it easy to understand and implement. However, Pentland (1980, Perception & Psychophysics, 28, 377-379) found his own maximum likelihood method (Best PEST) to be considerably less variable than the standard staircase. Here, computer simulations were used to study the effects of manipulating several parameters of the standard staircase in order to try to find a variant that would be competitive with the performance of Best PEST. A graphical summary of some of the standard staircase implementations that were found to be most stable is presented in order that investigators may choose easily the parameters best suited to their requirements. A comparison with an implementation of Pentland’s Best PEST failed to replicate Pentland’s finding, suggesting that investigators should have no misgivings about the standard staircase in experiments that measure a point of subjective equality.     

The haptic perception of curvature

Ninety-two subjects, schoolchildren and undergraduate and postgraduate students, took part in a series of experiments on the haptic perception of curvature. A graded series of surfaces was produced using piano-convex lenses masked off to produce curved strips that could be explored without using arm movements. Thresholds were measured using the constant method and a staircase procedure. Experiments 1 and 2 yielded data on the absolute and difference thresholds for curvature. Experiment 3 demonstrated that the effective stimulus for curvature is represented by the overall gradient of a curved surface. Using this measure, it was shown that the present absolute thresholds for curvature are lower than those previously reported. In Experiment 4, absolute thresholds were compared using spherical and cylindrical curves: the results showed that, at least with the narrow strips used, the type of curvature does not exert a significant influence on performance. In Experiment 5, the subjective response to curvature was assessed using a rating procedure. Power functions are reported, although the relationship between stimuli and responses had a strong linear component. This suggests that haptically perceived curvature may be a metathetic rather than a prothetic continuum.     

Spatial-frequency discrimination, brain lateralisation, and acute intake of alcohol

The effect of alcohol (breath-alcohol level of 0.1%) on perceptual discrimination of low (1.5 cycles deg-1) and high (8 cycles deg-1) spatial frequencies in the left and right visual field was measured in eighteen right-handed males, in a double-blind, balanced placebo design. Discrimination thresholds for briefly (180 ms) presented sinusoidal gratings were determined by two-alternative forced-choice judgments with four interleaving psychophysical staircases providing random trial-to-trial variation of reference spatial frequency and visual field, in addition to a random (+/- 10%) jitter of reference spatial frequency. Alcohol produced overall higher discrimination thresholds but did not alter the visual-field balance: no main effect of visual field was observed, but in both placebo and alcohol conditions spatial frequency interacted with visual field in the direction predicted by the spatial-frequency hypothesis of hemispheric asymmetry in visual-information processing, with left-visual-field/right-hemisphere superiority in discrimination of low spatial frequencies and right-visual-field/left-hemisphere superiority in discrimination of high spatial frequencies.