Until recently, most psychophysical experiments have studied sensory mechanisms involved in processing signals that vary along a single dimension. Under ordinary circumstances, however, the sensory system must operate in an environment where incoming signals vary along many different dimensions and often must deal with signals occurring simultaneously in different sensory modalities. In this report, we explore the question of how an O's performance on a set of simple discrimination tasks is affected by requiring the 0 to perform a number of these tasks at the same time.

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Observers performed several different detection tasks using both the PEST adaptive psychophysical procedure and a fixed-level (method of constant stimuli) psychophysical procedure. In two experiments, PEST runs targeted at P (C) = 0.80 were immediately followed by fixed-level detection runs presented at the difficulty level resulting from the PEST run. The fixed-level runs yielded P (C) about 0.75. During the fixed-level runs, the probability of a correct response was greater when the preceding response was correct than when it was wrong. Observers, even highly trained ones, perform in a nonstationary manner. The sequential dependency data can be used to determine a lower bound for the observer's "true" capability when performing optimally; this lower bound is close to the PEST target, and well above the forced choice P (C). The observer's "true" capability is the measure used by most theories of detection performance. A further experiment compared psychometric functions obtained from a set of PEST runs using different targets with those obtained from blocks of fixed-level trials at different levels. PEST results were more stable across observers, performance at all but the highest signal levels was better with PEST, and the PEST psychometric functions had shallower slopes. We hypothesize that PEST permits the observer to keep track of what he is trying to detect, whereas in the fixed-level method performance is disrupted by memory failure. Some recently suggested "more virulent" versions of PEST may be subject to biases similar to those of the fixed-level procedures.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of pre- and post-stimulus cues on detection were studied using 2-alternative forced-choice tasks. The stimuli used were tones presented to Ss through earphones and dots presented on TV monitors. The tones varied in pitch and intensity and the dots varied in displacement both horizontally and vertically. Thus Ss could be required to make discrimination in 4 dimensions. Performance on 1-dimensional input—1-dimensional output was superior to that for 4-dimensional input—1-dimensional output even when the stimulus for which a response was required was cued 4.0 sec. before its presentation. The duration of the post-stimulus cue delay, 0.10 sec. to 4.0 sec., had no differential effect on performance levels. These results are quite different from those reported for absolute judgment tasks.

Human observers were given a monaural detection task. Information in the form of a "cue" derived in some manner from the signal was presented simultaneously to the other ear. The signal and the cue always had the same timing and spectral characteristics and were samples of low-pass filtered noise. The cue was always presented in each possible signal interval, whether or not the signal itself was presented. Different cue conditions were used, in which the cue bore different amounts of information relevant to the signal. When the cue and signal were independently generated but identically timed and filtered, performance was indistinguishable from the no-cue control condition, and was like that of an inefficient energy detector using a filter matched to the signal bandwidth. When the cue was an identical replica of the signal, performance was much better than that of an ideal energy detector and, for bandwidths less than 1600 Hz, approached that of an equally inefficient likelihood-ratio detector for signal known exactly. Efficiency in both cases was of the order of 25%-30%. When the cue was derived from the signal but passed through a wide-band 90 ø phase shifter, it provided an intermediate amount of information, and detection performance was also at an intermediate level. The results are taken to show that when relevant information is available to the detection mechanism, it can be used; and that the usual finding of energy-detector-like monaural performance is due to lack of more precise information about the signal at the detector.

The frequency resolution of the human auditory system at high frequencies has been difficult to measure because of the nonlinear response of earphones on real ears at frequencies above approximately 4000 cps. Variation of this response with frequency is sufficiently great that amplitude rather than frequency differences may govern discrimination. An experiment with random amplitude signals [Henning, J. Acoust. Soc. Am. 39, 336–339 (1966)] indicated that frequency discrimination at high frequencies is almost an order of magnitude poorer than previous measures indicate. This finding is confirmed in experiments in which the amplitudes of the signals of different frequency to be discriminated are equalized under the earphones by means of a small probe microphone.

The PEST psychophysical method [Taylor and Creelman, J. Acoust. Soc. Am. (to be published)] was used to determine the signal burst intensity giving 80% correct responses in a 2AFC detection task. Each PEST run was immediately followed by fixed level trials at that level. For the fixed level trials, P(C) averaged only 0.75. However, the probability of a correct response conditional on a correct preceding response, P(C/C−1), averaged 0.765. Assuming the apparent correlation to result from averaging results over periods of good and poor detection, the worst possible performance during “good” periods is about 0.80. These results suggest that PEST may be more accurate than are the conventional fixed level methods. PEST makes the task easier when the subject “loses track” of what he is trying to detect; whereas in a fixed level experiment the subject has little opportunity to refresh his memory for the signal to be detected. In computer simulation, PEST has proved to be quick and unbiased.

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