ADAPTIVE PSYCHOPHYSICAL PROCEDURES AND IMBALANCE IN THE PSYCHOMETRIC FUNCTION

One class of adaptive psychophysical procedures was studied, using sim ulated and human observers. These procedures are those which require a n increase in stimulus intensity after an incorrect response, and a de crease after k successive correct responses. This paper analyzes how s tep size and the value of k affect the mean and standard deviation of threshold estimates based on a k-down 1-up adaptive procedure. Compute r 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 sti mulus either up or down at equal stimulus distances from the equilibri um point. These imbalance functions can be used to understand the thre shold biases obtained in the computer simulations. The computer simula tions also show that the average number of reversals obtained per tria l is dependent on different values of k, but are largely independent o f step size. The standard error of the threshold estimates, however, v aries systematically with step size, but are nearly independent of k. Finally, we compare the stability of threshold estimates for human lis teners 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 st ep size. (C) 1996 Acoustical Society of America.