Demany and Ramos [(2005). J. Acoust. Soc. Am. 117, 833-841] found that it is possible to hear an upward or downward pitch change between two successive pure tones differing in frequency even when the first tone is informationally masked by other tones, preventing a conscious perception of its pitch. This provides evidence for the existence of automatic frequency-shift detectors (FSDs) in the auditory system. The present study was intended to estimate the magnitude of the frequency shifts optimally detected by the FSDs. Listeners were presented with sound sequences consisting of (1) a 300-ms or 100-ms random “chord” of synchronous pure tones, separated by constant intervals of either 650 cents or 1000 cents; (2) an interstimulus interval (ISI) varying from 100 to 900 ms; (3) a single pure tone at a variable frequency distance (Delta) from a randomly selected component of the chord. The task was to indicate if the final pure tone was higher or lower than the nearest component of the chord. Irrespective of the chord’s properties and of the ISI, performance was best when Delta was equal to about 120 cents (1/10 octave). Therefore, this interval seems to be the frequency shift optimally detected by the FSDs.
If you present 5 tones simultaneously, people cannot tell whether a subsequently presented tone was one of the 5 tones, or lay in the middle between any 2 of the 5 tones. On the other hand, people can judge whether a subsequently presented tone lay above or below any one of the 5 tones. This paper investigates the dependency of this effect on how much the subsequent tone lay above or below one of the 5 (here actually 6) tones (frequency shift), on how much the 6 tones were separated (Iv) and on the interstimulus interval (ISI) between the first set of tones and the subsequent tone. The authors replicated the mentioned previous findings and presented data suggesting that there is an optimal frequency shift at which subjects performed best in the task. They argue that this is at roughly 120 cents.
I have several remarks about the analysis. First of all, the number of subjects in the two experiments is very low (7 and 4, each including the first author). While in experiment 1 the curves of d-prime over subjects look relatively consistent, this is not the case for larger ISIs in experiment 2. The main flaw of the analysis is that their suggestion of an optimal frequency shift of 120 cents is based on curve fitting of an exponential function to 4,5, or 6 data points where they also add an artificial baseline data point at d-prime=0 for frequency shift=0. The data point as such makes sense as the judgement of a subject whether the shift was up or down must be random when the true shift was actually 0. Still, it feels wrong to include an artificial data point in the analysis. In the end, especially for large ISIs the variability of thus estimated optimal frequency shifts for individual subjects is so variable that it seems pointless to conclude anything about the mean over (4) subjects.
Sam actually tried to replicate the original finding on which this paper is based. He commented that it was hard to replicate it in a large group of subjects and he found differences between musicians and non-musicians (which shouldn’t be true for something that belongs to really basic hearing abilities). He also noted that subjects were generally quite bad in this task and that he found it to be impossible to make the task easier, when one wants to maintain that the 6 initial tones cannot be perceived individually.
The authors of the paper seem to use subjects, which perform particularly well in these tasks, repeatedly in their experiments.
It has been noted in the groupmeeting that this research could be linked better to, e.g., the mismatch negativity literature which is also concerned with detection of deviations. Sam pointed to the publication containing the original findings in response.