Temporal discrimination and a free-operant psychophysical procedure.

Pigeons were presented a series of keylight time periods (separated by blackouts) during which two response keys were lit, one by blue light and the other either by orange or green. Blue-key responses changed the color on the other key. Orange-key responses sometimes produced food during the first half of a time period; green-key responses sometimes produced food during the second half. In three experiments, the probability of a green-key response increased as a function of elapsed time. Experiment 1 compared performance when the duration of the keylight periods was varied across a wide range. Discrimination of performance was similar across the range of durations. Experiment 2 varied both relative reinforcement rate and the local reinforcement rate for orange-key and green-key responses. These manipulations produced changes in response bias but not discrimination sensitivity. Experiment 3 varied the local temporal placement of reinforcers within time periods and demonstrated that choice behavior was affected by differential reinforcement at different points during the time periods. The results were consistent with previous research on duration discrimination that used psychophysical trials procedures.     

An adaptive psychophysical method for subject classification

In psychophysical experiments, one’s goal is usually to measure some continuous parameter hypothesized to determine the statistical properties of a subject’s responses. Methods are well developed that adaptively manipulate stimulus characteristics in such a way that the reliability of the parameter estimate is maximized. However, such methods are inapplicable in situations in which the goal is to assign subjects to discrete categories, rather than to measure a continuous parameter. This paper introduces a technique that is directly applicable to efficient categorization and that adaptively manipulates stimulus characteristics in such a way that the information obtained from each trial is maximized. This technique is based on the principle of minimum estimated expected entropy, whereby stimulus parameters on each trial are chosen in order to minimize the estimated expected entropy of the a posteriori probability distribution that expresses how likely a subject is to belong to each of a group of mutually exclusive categories. A sample implementation of the technique—the classification of infant subjects according to their audiograms—is then described and evaluated via computer simulation.     

The step method: a new adaptive psychophysical procedure

A new adaptive psychophysical method, the step method, is introduced. Simulations show the method to be less biased and more efficient than constant stimuli or Pentland's adaptive method for fewer than 40 trials. An experiment using discrimination of dot number, however, failed to find any differences among the three methods in either bias or efficiency.     

An olfactory discrimination procedure for mice

This paper describes an olfactory discrimination procedure for mice that is inexpensively implemented and leads to rapid discrimination learning. Mice were first trained to dig in small containers of sand to retrieve bits of buried chocolate. For discrimination training, two containers were presented simultaneously for eight trials per session. One container held sand mixed with cinnamon, and the other held sand mixed with nutmeg. Both containers were baited with chocolate buried in the sand. One odor was designated S+, and mice were allowed to dig and retrieve the chocolate from this container. The other odor was S-, and both containers were removed immediately if subjects began to dig in an S- container. After meeting a two-session acquisition criterion, subjects were given a series of discrimination reversals. In Experiment 1, 12 Swiss-Webster mice (6 male and 6 female) acquired the olfactory discrimination in three to five sessions and completed 3 to 10 successive discrimination reversals within a 50-session testing limit. In Experiment 2, subjects were 14 Pah(enu2) mice, the mouse mutant for phenylketonuria; 7 were homozygotes in which the disorder was expressed (PKU), and 7 were heterozygotes with normal metabolism (non-PKU). Thirteen mice completed pretraining in four to seven sessions, acquisition required 3 to 12 sessions, and all mice completed at least three reversals. Learning rates were similar in PKU and non-PKU mice. We discuss issues related to implementation and several potentially useful procedural variations.     

Perception of psychophysical experiments

Summary Two experiments were conducted to determine perceived similarities among psychophysical tasks. In Exp. I Ss first participated in five brightness-perception tasks: category estimation, magnitude estimation, absolute judgment, method of constant stimuli, and binary choice. At the conclusion of this phase, they rated the similarity of each pair of tasks according to each of four judgment criteria: general similarity, visual discrimination, learning, and memory. The average ratings were scaled, yielding a one-dimensional solution; where absolute judgment and category estimation were grouped at one end, constant stimuli and binary choice at the other, and magnitude estimation in between. In Experiment II another group rated similarity of the tasks based only upon relevant instructions. (They did not actually judge brigthness.) Scaling results were in agreement with those obtained in Exp. I. Theoretical implications are examined and tested in the second part of this work (Baird & Szilagyi, 1977)The present article is the first of a two-part series. Some of the results reported here were first presented at the 1974 meetings of the Psychonomic Society in Boston     

An efficient procedure for selecting equal probability samples

FORTRAN code that implements an algorithm for selecting equal probability samples is given. The procedure is brief and decides to include or not include a record as it is called. Consequently, storage requirements are minimal. The procedure gives the exact number of values specified, with each record in the population having an equal probability for inclusion in the sample, including the last.     

Contrast gain control in psychophysical contrast discrimination

Contrast masking may in part reflect the operation of contrast gain control mechanisms in the visual system. Four experiments were conducted in order to examine contrast discrimination under conditions in which the base contrast was interrupted with various maskers. Interrupting the base contrast grating at 8 Hz—with a uniform field, with a randomly phase-shifted version of the same grating, or with a higher contrast version of the same grating—interfered with contrast discrimination. These effects occurred only when the background contrast was above its own threshold. These results may be explained by assuming that the networks responsible for contrast gain control operate by temporally averaging contrast over a period greater than 62.5 msec. When this time-averaged contrast is different from the base contrast, contrast discrimination suffers.     

An observable property equivalent to a choice model for discrimination experiments

The choice model considered by Luce in an earlier work is stated in terms of unobservable parameters. In this paper a consequence of the model, involving only observables, is shown to be equivalent to the model.This work was supported in part by National Science Foundation Grants NSF G-8864 and NSF G-17637 to the University of Pennsylvania.     

An adaptive (tracking) procedure for measuring visual search

PEST, an adaptive (tracking) procedure originally developed for sensory research, was modified for cognitive studies with the multiple sequential frames paradigm and a letter-search task. The program which controlled stimulus presentation was designed to track the frame duration required for successful search on 75% of trials (the duration threshold). For each of the three subjects used, the threshold increased as a linear function of the number of sequentially presented frames, which varied between 1 and 10. Each tracking run was immediately followed by 40 trials with frame-exposure time fixed at the computer-selected threshold. Performance on these fixed level trials was close to the expected 75% correct. The Discussion addresses issues related to the use of the threshold measure in basic and applied cognitive research, explores the present findings about performance on fixed level trials at the computer-selected threshold, and examines the increase in duration threshold with the number of sequentially presented frames.     

A contribution to the mathematical biophysics of psychophysical discrimination

The properties of a simple neural mechanism are developed theoretically and interpreted in terms of observable variables. The results are discussed in connection with experimental data in cases for which the latter are available.     

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.     

Accumulator and random-walk models of psychophysical discrimination: a counter-evaluation

In a recent assessment of models of psychophysical discrimination, Heath criticises the accumulator model for its reliance on computer simulation and qualitative evidence, and contrasts it unfavourably with a modified random-walk model, which yields exact predictions, is susceptible to critical test, and is provided with simple parameter-estimation techniques. A counter-evaluation is presented, in which the approximations employed in the modified random-walk analysis are demonstrated to be seriously inaccurate, the resulting parameter estimates to be artefactually determined, and the proposed test not critical. It is pointed out that Heath's specific application of the model is not legitimate, his data treatment inappropriate, and his hypothesis concerning confidence inconsistent with experimental results. Evidence from adaptive performance changes is presented which shows that the necessary assumptions for quantitative analysis in terms of the modified random-walk model are not satisfied, and that the model can be reconciled with data at the qualitative level only by making it virtually indistinguishable from an accumulator process. A procedure for deriving exact predictions for an accumulator process is outlined.     

Model discrimination through adaptive experimentation

An ideal experiment is one in which data collection is efficient and the results are maximally informative. This standard can be difficult to achieve because of uncertainties about the consequences of design decisions. We demonstrate the success of a Bayesian adaptive method (adaptive design optimization, ADO) in optimizing design decisions when comparing models of the time course of forgetting. Across a series of testing stages, ADO intelligently adapts the retention interval in order to maximally discriminate power and exponential models. Compared with two different control (non-adaptive) methods, ADO distinguishes the models decisively, with the results unambiguously favoring the power model. Analyses suggest that ADO's success is due in part to its flexibility in adjusting to individual differences. This implementation of ADO serves as an important first step in assessing its applicability and usefulness to psychology.     

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.     

A tracking procedure for determining the cat's frequency discrimination

A tracking procedure was used to investigate the time required to train cats to discriminate between a frequency-modulated tone and a steady tone. The animal was reinforced with food on a VR schedule only when the steady tone was present and the animal pressed the correct bar (one of two). After reinforcement, the steady tone usually changed to a frequency-modulated signal; by pressing the other bar, the tone could be changed to its steady state and the reinforcement then obtained as before. A major difficulty was the lack of control by the auditory stimulus on the cat's responses. This problem was solved by introducing interpress time outs which forced the animal to hesitate after every press. The use of light cues to signal the time outs and the correct bar to press accelerated the rate at which the training progressed.

With cats, this conditioning procedure apparently requires a much longer training period before the actual threshold determinations than the more commonly used avoidance conditioning procedures. However, when animals are to be tested repeatedly over a period of several months or longer, the procedure may prove the more desirable one because it reduces experimental neurosis.

     

Invariance in time series representations of 2-input 2-output psychophysical experiments

The dynamic representation of pairs of intensity judgement responses to two-component odour mixtures, composed of various concentrations of bergamote and amyl acetate, is explored using multivariate time series analysis and the associated root locus diagrams derived from factoring a polynomial determinant of the matrix polynomial z-transformation of the system transfer function. The topology of the root loci is examined for both pseudocoupled and really coupled conditions, to see how the component odourants interact in determining the psychophysics of odour mixture perception. It is shown that most subjects show a loss in gain for both components, but that they are not dynamically homogeneous. Third-order loop gain equations are a sufficient representation in nearly all cases.