Adaptive psychophysical procedures and imbalance in the psychometric function.

One class of adaptive psychophysical procedures was studied, using simulated and human observers. These procedures are those which require an increase in stimulus intensity after an incorrect response, and a decrease after k successive correct responses. This paper analyzes how step size and the value of k affect the mean and standard deviation of threshold estimates based on a k-down 1-up adaptive procedure. Computer simulations are used to study the bias in threshold estimates, which are most evident when larger step size and small values of k are used. The adaptive procedure can be characterized by a function called the imbalance of the track, the relative probability of adjusting the stimulus either up or down at equal stimulus distances from the equilibrium point. These imbalance functions can be used to understand the threshold biases obtained in the computer simulations. The computer simulations also show that the average number of reversals obtained per trial is dependent on different values of k, but are largely independent of step size. The standard error of the threshold estimates, however, varies systematically with step size, but are nearly independent of k. Finally, we compare the stability of threshold estimates for human listeners using two very different sets of parameters: a very large step size (approximately half the range of the psychometric function) with k = 4, and the conventional k = 3 with an initial 4-dB and a final 2-dB step size.