Proactive interference in a two-tone pitch-comparison task without additional interfering tones
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Two-tone pitch-comparison tasks typically comprise several successive pairs of successive tones separated by silent intervals. The serial occurrence of such pairs has been associated with degraded task performance, but the nature of this association is not fully understood. Human adult participants were presented with successive pairs of successive tones. The latter, to-be-compared tone of a pair could differ from the former, to-be-remembered tone of 1046.5 Hz by no more than ±15 Hz (25 cents). The direction of this difference was easier to identify when it was opposite to that of the preceding pair than when being the same. Merely responding accordingly (irrespectively of whether the response was correct or not) was found not to account for this finding. Our study demonstrates proactive interference in a two-tone pitch comparison task as the difficulty to remember when the first tone of the present pair occurred relative to the last tone of the immediately preceding pair.
The pitch of a tone becomes less accurately remembered as time passes. Partly this can be accounted for by the interference of other tones that may either precede (proactive interference) or follow (retroactive interference) a tone to be remembered (e.g., Keppel & Underwood, 1962; McGeoch, 1932). Both proactive and retroactive interference are viewed to reflect the difficulty to remember when the tone occurred relative to the other tones (Deutsch, 1972, 1984; see also Baddeley, 1976). This type of confusion is termed here as “inter-item confusion”.
Inter-item confusion is familiar from studies where subjects make pitch comparisons between two tones of their pairs, a standard tone and a comparison tone, respectively, while ignoring additional interfering tones that shortly precede (e.g., Ruusuvirta, 2000) or follow (e.g., Deutsch, 1972) the standard tone. The remembered pitch of the standard tone has then been found to be biased towards the frequencies of the interfering tones, as revealed by the way different frequency combinations of the standard, comparison, and interfering tones affect comparison accuracy (Deutsch, 1972, 1984; Ruusuvirta, 2000).
The use of additional interfering tones appears to be not necessary for at least some proactive effects to be observed in a two-tone pitch comparison task. It has been found that the shorter the non-stimulated interval is between the present pair (pair N) and the immediately preceding pair (pair N − 1) of tones relative to such an interval between the tones within a pair, the poorer is pitch comparison accuracy (Cowan, Saults, & Nugent, 1997). Findings like this could but may not necessarily originate from inter-item confusion for a number of reasons, however.
First, as long as the effect of the tones of pair N − 1 is not shown to be specific for specific physical features of the tones, it could be claimed to be caused by the tones merely as general auditory activations (for neurophysiological evidence of the feature-non-specific effect of a tone on the human cortical processing of the subsequent tone, see, Näätänen & Picton, 1987). Also, the temporally closer pair N − 1 is to pair N the temporally closer subjects’ response for pair N − 1 is to pair N, and the more likely the response itself is to generally affect the encoding of the standard tone of pair N.
Second, even if the interference of pair N − 1 with pitch comparisons for pair N shows frequency-specificity, it may not reflect inter-item confusion (confusion between consecutive tones; “Standard–Comparison, Standard–Comparison”) but another type of confusion that is here termed “inter-group confusion” (Ryan, 1969). This type of confusion, originally described between items in working memory (Baddeley, 1976), might take place between the standard tone of pair N and that of pair N − 1 (“Standard–Comparison, Standard–Comparison”). The fact that only one standard tone is held or even rehearsed (Keller, Cowan, & Saults, 1995) at a time in working memory (for pitch, see, Anourova, Rämä, Koivusalo, Alho, & Carlson, 1999) may not exclude this possibility. After all, no working memory appears to be necessary for the human brain to process the sequential order of tones of different frequencies in a pair in relation to that in the previous pair despite non-stimulated intervals separate the tones both within and between their pairs (Saarinen, Paavilainen, Schröger, Tervaniemi, & Näätänen, 1992).
Third, it is possible that even an inter-item-confusion-like pattern in the data actually reflects subjects’ effort to reason the right response. Simply by adopting a strategy to purposefully alternate “the comparison tone is higher pitched than the standard tone” and “the comparison tone is lower pitched than the standard tone” judgments between pair N − 1 and pair N (as duped by the gambler’s fallacy; Jarvik, 1946), participants would selectively and artificially increase the likelihood of correct judgment (hits) for pairs that conform to this strategy.
By controlling for these alternative explanations, we explored whether inter-item confusion could truly account for the interference of pair N − 1 with pitch comparisons for pair N. To reach this aim, we slightly varied the frequency of only the comparison tone so that it could be in either direction from the standard tone. We predicted inter-item confusion (between the standard tone of pair N and the comparison tone of pair N − 1) to facilitate the identification of frequency differences that were in opposite directions relative to those that were in the same direction between pair N − 1 and pair N. Inter-group confusion (between the standard tone of pair N and that of pair N − 1), in turn, could be expected not to have such an effect. We also predicted that findings of inter-item confusion, if occurring, could not be explained by a specific pattern of subjects’ judgments over consecutive pairs irrespectively of the correctness of the judgments.
Fourteen students and personnel from University of Jyväskylä (five male, nine female, age range 19–34 years) with self-reported normal hearing participated in the study. An informed consent was obtained after the nature of the experiment was explained to the participants.
Stimuli and apparatus
Sinusoidal tones (about 70 dB SPL, 30 ms duration, including 5-ms rise/fall times) were binaurally delivered via earphones (Sennheiser, HD25-1). They were presented in pairs consisting of a standard tone and a comparison tone, respectively. The frequency of the standard tone was constant 1046.5 Hz. The frequency of the comparison tone was –15 Hz (−25 cents), −10 Hz (−16.6 cents), −5 Hz (−8.3 cents), 0 Hz, +5 Hz (8.3 cents), +10 Hz (16.5 cents), or +15 Hz (24.6 cents) from the standard tone at random.
The participants were seated comfortably at the computer used in data gathering and familiarized with the task before the experiment. They were instructed to judge whether the comparison tone was lower, same, or higher in pitch than the standard tone in each pair. The participants responded by pressing the left, down, or right arrow key on the keyboard, depending on their judgments. Although they were encouraged to respond as quickly as possible (within about 2 s, as demonstrated by the experimenter), accuracy was stressed. The participants were informed that the magnitude of pitch difference, if present, in each pair was assigned on a random basis. To avoid extensive numbers of judgments indicating identicalness due to small frequency differences between the tones in their pairs, the participants were also informed that, on average, tones in their pairs were identical in only one out of seven cases. Throughout the instructions, non-correlation between the types of consecutive pairs was emphasized. This was to lessen the likelihood of the participants to fall in the gambler’s fallacy (Jarvik, 1946) to alternate between rather than to repeat specific judgments in a row in belief of a correlation between random events.
A silent interval between the participants’ judgment (key press) about pair N − 1 and the standard tone of pair N was 350, 650, or 950 ms at random. A silent interval between the standard and comparison tone was 270, 570, or 870 ms at random. Three hundred and sixty trials were gathered from each participant. Judgments made 0.1–3 s from the onset of the comparison tone for both pair N and pair N − 1 were selected for the statistical analyses (there were only up to a few rejected judgments per each participant) in which Greenhouse-Geisser-adjusted degrees of freedom for the averaged tests of significance were used in repeated measures analyses of variance (ANOVA) whenever the sphericity assumption was violated (P-values reported accordingly). All factors were treated as within-subject factors.
Stimulus effects of pair N − 1
Judgment effects of pair N − 1
We found that pair N − 1 affected pitch comparisons for pair N in a frequency specific fashion. Namely, the direction of the difference between the standard tone and the comparison tone was easier to identify when it was opposite than the same between pair N − 1 and pair N (Fig. 2). The use of the constant standard tone assured that this effect could be only of inter-item confusion. Inter-group confusion could not have this effect because it would have been between physically identical stimuli. It was, thus, the sequential order of the standard tone of pair N and the comparison tone of pair N − 1 that appeared to be difficult to remember at the occurrence of the comparison tone of pair N (Baddeley, 1976; Cowan et al., 1997; Deutsch, 1972, 1984).
One may suggest that our finding did not reflect simple inter-item confusion but the grouping of both tones of pair N − 1 with the standard tone of pair N. This suggestion gets its basis from neurophysiological findings that the human brain can automatically anticipate future stimuli with the use of linear melodic trajectories it extracts from consecutive tones at least when such trajectories are illusorily created by Shepard tones (Tervaniemi, Maury, & Näätänen, 1994; for a review, see Näätänen, Tervaniemi, Sussman, Paavilainen, & Winkler, 2001). The logic follows that if such anticipation was drawn from the comparison tone of pair N − 1 and the standard tone of pair N, the comparison tone of pair N that matched the anticipation could have been more easily discriminated from the standard tone than the one that did not. This was unlikely the case, however. In a previous study (Ruusuvirta, 2000) in which a standard tone was very shortly preceded by three additional interfering tones of different frequencies, ordering the tones into linear melodic trajectories was not found to have such an effect (for Shepard tones, however, see, Giangrande, Kelson, & Tuller, 2003).
The inter-item confusion account was further supported by our failure to observe an alternation of “the comparison tone is higher pitched than the standard tone” and “the comparison tone is lower pitched than the standard tone” judgments themselves (i.e., irrespectively of whether they were correct or erroneous) from pair N − 1 to pair N. Such response alternation could itself have accounted for our findings, just as any systematic pattern of responding over consecutive tone pairs would have favored correct responses (hits) for pairs obeying this particular pattern. We did observe a statistical trend for such response alternation but only when judgments about pair N − 1 of non-identical tones were correct (Fig. 3), which again implied inter-item confusion rather than any response-based effect to be involved.
Our finding of inter-item confusion between tones of their consecutive pairs was surprising considering that the overall frequency variation of tones was less than 3% and that the tones were very easy to perceive as pairs by a response required after each pair. It shows that the role of remembering the timing of a tone in relation to other tones may play a greater role in short-term forgetting and that the memory representation of the frequency of a tone, as revealed by its proactive interference, may survive accurately longer than could have been thought possible previously.
In sum, we showed proactive interference in a two-tone pitch-comparison task and its origin in the difficulty to remember when the first tone of the present pair occurred in relation to the last tone of the immediately preceding pair.
This work was supported by the Academy of Finland (73038). The authors thank Christian Kaernbach and Virpi Kalakoski for constructive comments on an earlier version of this manuscript.
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