jects' incorrect reproduction ofthe internal temporal ratios ofthe stimulus patterns. The mecha- nism of assimilation (of a central tendency) led subjects to enforce ...
Perception & Psychophysics 1994, 55 ~), 204-217
Sensorimotor synchronization: Motor responses to pseudoregular auditory patterns MAREK FRANEK Academy of Sciences of the Czech Republic, Prague, Czech Republic
JIiH MATES Ludwig-Maximilians University, Munich, Germany and Academy of Sciences of the Czech Republic, Prague, Czech Republic TOMAs RADIL Academy of Sciences of the Czech Republic, Prague, Czech Republic and KARIN BECK and ERNST POPPEL Ludwig-Maximilians University, Munich, Germany Musically trained and untrained subjects (N = 30) were asked to synchronize their finger tapping with stimuli in auditory patterns. Each pattern comprised six successive tonal stimuli of the same duration, the first of which was accented by a different frequency. The duration of interstimulus onset intervals (ISIs) gradually increased or decreased in constant steps toward the end of the patterns. Four values of such steps were used in different trials: 20, 30, 45, and 60 msec. Various time-control mechanisms are hypothesized as being simultaneously responsible for subjects' incorrect reproduction ofthe internal temporal ratios ofthe stimulus patterns. The mechanism of assimilation (of a central tendency) led subjects to enforce a regular (isochronous) structure on the patterns. The influence of other time-control mechanisms (distinction, subjective expression of an accent, sequential transfer) was expressed mainly in differences between intertap onset intervals (ITIs) and the corresponding ISIs at the beginning of the patterns. The duration of the first two ITIs was in the majority of the trials in an inverse ratio to the ratio of the respective ISIs. The distortions resulting from the timing mechanisms concerned were more pronounced in the performance of nonmusicians than in that of musicians.
The precision and accuracy of the motor reproduction of rhythmic patterns is influenced by several aspects of stimuli, such as the internal temporal structure (interstimuIus onset intervals, ISIs), the type of tone accentuation, or the duration of the patterns. A difference in performance has been found between two basic groups of stimulus patterns-regular and irregular ones (Povel, 1981). Regular patterns are based on repetition of temporal units
This work was supported by Grant PO 121/13 from the Deutsche Forschungsgemeinschaft (E.P., principal investigator) and by an Alexander von Humboldt Foundation fellowship granted to J.M. The final revision of the text was done while J.M.'s research was being supported by a fellowship from the Max Planck Society, at the Max-Planck-Institute for Psychological Research, Munich, Germany. M.F. is in the Institute of Musicology in the Czech Academy of Sciences; J.M. is in the Institute of Medical Psychology at Ludwig-Maximilians University and the Institute of Physiology in the Czech Academy of Sciences. T.M. is also in the Institute of Physiology. K.B. and E.P. are in the Institute of Medical Psychology at Ludwig-Maximilians University. Correspondence should be addressed to J. Mates, Ludwig-Maximilians University, Goethestrasse 31, D-80336 Munich 2, Germany. -Accepted by previous editor, Charles W. Eriksen
Copyright 1994 Psychonomic Society, Inc.
of equal duration (isochronous structure), whereas irregular patterns are based on temporal intervals of arbitrary durations (nonisochronous structure). The latter patterns are usually hard to estimate correctly, and their correct reproduction is difficult or even impossible. Fraisse (1946-1947) found that subjects had no problems in creating a rhythm with time interval ratios 1: 1 or 2: 1 (and patterns derived from these ratios, such as 3: 1), but that other interval ratios caused great difficulty. It has been shown (Franek, RadiI, & Indra, 1988) that subjects are not able to create rhythmic patterns with the ratios 1:0.7, 1:0.8, 1; 1.1, and 1: 1.4, even under continuous control of acoustical feedback. Fraisse (1947) suggested two basic mechanisms of timing and time perception: (1) assimilation, expressing a central tendency (Helson, 1964); and (2) distinction, expressing a tendency toward contrast. Assimilation means minimizing small differences of stimuli. Distinction means that irregularities that are too large to be assimilated are overestimated in order to stress differences. Both tendencies express subjects' effort to transform temporal ratios in irregular patterns toward more acceptable (regular) ratios. Supporting evidence comes from experiments by Es-
204
SENSORIMOTOR SYNCHRONIZATION sens and Povel (1985), Summers (1975), and Summers, Sargent, and Hawkins (1984). Povel (1981) suggested a •'beat-based model," which describes a mechanism of the perception and internal representation of rhythmic patterns. According to this model, a subject perceives and estimates the structure of rhythmic patterns by measuring a temporal unit of constant duration (the beat interval). The only rhythmic patterns that can be successfully represented internally are those that are measurable by means of a beat interval of constant duration. Each rhythmic pattern is based on a hierarchical structure of relationships between an accented element and unaccented ones. It has been found that accents at the beginning of a pattern also cause timing changes of certain elements-that is, of intertap onset intervals (ITls) and duration of taps or key-touching times (KTs), even in stimulus patterns with an isochronous structure (i.e., with constant lSI durations; Franek, Mates, RadiI, Beck, & Poppel, 1991b; Franek, RadiI, & Indra, 1990). Two types of subjective expression of an accent at the beginning of a rhythmic group (in the time domain) have been detected. Subjects made ITI I longer than ITI 2 (timing behavior long-short, or L-S) or produced ITI I shorter than ITI 2 (timing behavior short-long, or S-L). In the timing behavior L-S, the effect of an accent is reflected in a prolongation of the first element (see, e.g., Clarke, 1985). The timing behavior S-L reflects a different mechanism of accentuation-to tap sharply, shortly, and (probably) more intensively. The first element was therefore shorter than the second. This type of timing behavior was also combined with shorter KTs than was the type L-S. In synchronization with rhythmically unstructured stimulus sequences with random fluctuations ofISI durations, the simple strategy in which the ITI copies the preceding lSI was considered by Michon (1967) as possibly one of the subjects' synchronization strategies. The feedback correction of the "reference interval," which Hary and Moore (1987) used in their model of performance in synchronization tasks for placement of the next response, expresses the same phenomenon to some extent. Therefore, in the synchronization with nonisochronous patterns, it is also necessary to take into account a possible influence of the duration of the preceding lSI on the duration of the actual m. We denote this sequential influence a mechanism of sequential transfer. On the basis of these findings, it can be concluded that the timing of simple rhythmic patterns, especially those with a nonisochronous structure, represents in fact a very complex task, which probably involves various timing mechanisms with presumably simultaneous influences on subjects' performance. In the present study, these mechanisms are considered and examined: (1) central tendency (assimilation) and distinction; (2) the phenomena observed as a result of stimulus accentuation and its subjective expressions; and (3) the mechanism of sequential transfer. It will be shown later that in a synchronization paradigm, distinction can be operative only together with assimila-
205
tion. Thus, we will handle these two timing mechanisms together. The present experiment was designed to investigate whether these mechanisms, observed in other synchronization tasks, would also influence subjects' performance in a task in which an isochronous and rhythmic structure of stimuli was disrupted to some degree in a systematic (predictable) way. Repetitive stimulus sequences of pseudoregular patterns were presented-that is, sequences that combined certain features of regular temporal structure (rhythmic pattern repetition) with a predictable irregularity (regular temporal changes inside the patterns). Such a structure represents an intermediate level in the transition from rhythmic sequences of regular patterns toward purely random sequences. We consider linear gradual acceleration or deceleration within the patterns to be one of the simplest and reproducible types of irregularity. A synchronization paradigm was used to enable the subjects to correct their performance on the basis of instantaneously available information about the correspondence or the lack of correspondence between the perceived and produced patterns. The predictability and repetitive presentation of a stimulus pattern enabled subjects to build up an internal template of the pattern, but its reproduction could be subjected to a joint influence of the mechanisms above. Such an influence could explain the difficulties in reproducing irregular patterns with various interval ratios already mentioned. Two types of six-tone patterns were considered: (1) I patterns, the duration of ISIs successively increased from an initial value ISII by a constant temporal step d, according to the relation ISIj = ISII
+
d
= (ISII-d)
* (j-l) +
d *j;j = 1,2, ... ,6,
(1)
where j denotes the position of an lSI in the pattern; and (2) D patterns, the duration ofISls successively decreased toward a final value ISIF by a constant temporal step d, according to the relation ISlj
= ISIF + d * (6-j) = (ISIF + 6 * d) - d * j; j =
1, 2, ... , 6.
(2)
This procedure enabled us to create nonisochronous patterns with regular and predictable temporal differences between neighboring ISIs. A schematic representation of the temporal structure of the stimulus patterns described is shown in Figure 1. In the analysis of the structure of perceived rhythmic patterns and in proper rhythm performance, musical practice represents an important factor. Not only are musicians more skilled performers in timing tasks, but they often use different, more sophisticated perceptual and timing strategies than nonmusicians do (Franek, Mates, RadiI, Beck, & Poppel, 1991a; Povel, 1981; Smith, 1983). Therefore, both groups of subjects participated in the present experi-
206
FRANEK, MATES, RADIL, BECK, AND POPPEL I-pattern
151 1 151 2
ISI 3
151 4
ITIj+l,AV for the I patterns, or for which ITIj,AV < ITIj+l,AV for the D patterns, the response was classified as incorrect. For incorrect responses, the local effect of subjective expression of the accent at the beginning of the patterns was investigated. As already mentioned, this effect is mainly observable in different timing of the first two ITls (timing behavior L-S, S-L). The timing behavior L-S (i.e., the prolongation of ITI, in comparison with ITI 2 ) dominated in the sequences with the I patterns. In the sequences with the D patterns, the timing behavior S-L (i.e., the prolongation of ITI z in comparison with ITI,) was observed more often (see Table 3). There were also subjects who did not produce significant differences between ITI, and ITI 2 • We denote this type of behavior as equalequal (E-E). The average response patterns for the timing groups introduced are shown in Figure 6. For every stimulus pattern, besides the corresponding main type of timing behavior, the other types of consistent timing occurred, although less frequently (cf. Table 3). The most frequent type of timing for each stimulus pattern is reflected by the average values plotted in Figure 3. For all the classifications above, a paired t test was used for each subject to compare ITls that did not fit in the correct temporal structure. A nonparametric sign test of ITI differences was also used to avoid any unreliability of results that might arise from incidentally nonnormal distributed measurements. The results of both tests were equivalent. An ANOVA was not used, because of likely significant correlations between ITls within the patterns already mentioned.
Table 2
= ITIj+' - ITlj (i.e., Differences Between Successive Intertap Onset Intervals in the Patterns) With the Step ~ (in Milliseconds; Positive for the I Patterns and Negative for the D Patterns) in Interstimulus Onset Intervals T Statistics From the Comparison of the Differences dj
~
20 j
1 2 3 4 5
Mus 5.30t 0.59 0.98 0.85 0.82
45
30 NMus 8.00t 7.72t 1.49 I. 71 1.63
60
Mus
NMus
Mus
NMus
Mus
NMus
2.21 t 3.06t 0.40 1.96 0.43
I Patterns 2.12 5.65t 0.36 2.16t 0.14 0.32 1.60 3.06* 0.29 0.58
6.42t 1.25 0.06 4.49* 1.32
3.36t 1.80 1.09 1.08 0.87
7.20t 0.41 0.20 4.20* 2.13
D Patterns 3.05t 1.66 6.64t 1.41 2.64t 2.36t 1 3.05t 5.77t 1.90 2.37t 2.78t 2.26t 0.35 3.35t 0.87 2 3.55t 0.39 1.97 0.61 2.24* 0.16 0.89 0.36 3 0.67 2.47* 2.30* 1.33 1.16 1.03 2.03 0.16 1.27 4 0.02 1.69 2.33t 0.54 0.12 0.30 0.41 0.81 5 Note-The degrees of freedom were 15 for musicians and 13 for nonmusicians in all cases. Mus, musicians; NMus, nonmusicians. *p < .05. tp < .05, but in the direction opposite to the changes of interstimulus onset intervals.
SENSORIMOTOR SYNCHRONIZATION 135
211
r--.,-----.--,-----,-----r---,
90
+
-------~~~~~,+.:~'"'.~~"',.~"'"-.-..-------90 -135 '---'------'-----'---'------'---' 135 . - - r - - . , - - - - - . - - , - - - - - , - - - - ,
90 45
o
-45
-----------------------,...,,~-±:::~:-l----- t--~~t·-·· +.
bee,,!:::~~_t':::~:f:"'~4-------
---------------
-90 -135 '---'------'-----'---'------'---' 135 .----,r-~-----.--,----r---,
j
'6
-45
-90
j
'6
-45
-90
............. Nmus
-135 1-----''----'2----'-3---'-4--5-'-----'
2
Difference positIOn
3
M.Ja
5
4
Difference position
Figure 5. Average values and between-trial-blocks standard errors of the mean of differences between successive intertap onset intervals (lTls) in the patterns (lTlj+l - ITlj, wherej = 1, 2, ... , 5 is the position of an ITI in the pattern). Results are given separately, for each value of the step .0. (from the top to the bottom, .0. = 20, 30, 45, and 60 msec), for both the I patterns (left panel) and D patterns (right panel), and for musicians (Mus) and nonmusicians (NMus). Dashed straight lines represent the respective value of .0. (positive for the I patterns and negative for the D patterns)-that is, a difference that would be observed in a perfect performance. (Tick marks on the abscissas are not a scale; points are connected with lines to aid the eye.)
Although each subject maintained the same type of timing behavior during the tapping of a sequence, the subjects did not tap in the same way for all eight sequences presented; they produced different types of timing behavior in different sequences. This disagreement may have occurred because of the difference between the character of particular stimulus patterns, and especially because of the difference between the two basic structural types of patterns-with successively increasing ISIs and with successively decreasing ISIs. Therefore, consistency of the two dominant types of timing behavior was investigated for the groups of I patterns and D patterns separately. The number of subjects with the same type of timing behavior in different trials is given in Table 4. However, the consistency was disrupted, especially in the D patterns, mainly by the results obtained in patterns with the step of 45 msec.
When the musical practice of the subjects was taken into consideration, the timing mechanisms mentioned were involved differently in the performance of the two groups of subjects. Nonmusicians produced the prevailing type Table 3 Occurrence of Type of Timing Behavior in the Stimulus Patterns I Patterns .0.
Correct
L-S
S-L
20 30 45 60
0 4 3 1
25 18 15 21
3 5 4 8
D Patterns E-E
Correct
L-S
S-L
E-E
18 6 4 15 13 4 1 18 Note-.o., step of interstimulus onset interval changes (in milliseconds). Values give number of subjects who produced particular type of timing behavior (correct or incorrect, the latter divided into Subgroups L-S, S-L, E-E). N = 30 subjects. For explanation of timing behavior, see text. 2 3 8 0
2 3 2 6
4 8 11 5
212
FRANEK, MATES, RAOIL, BECK, AND POPPEL 820 ,--.----,----,-,-----,-..,...----,
o
B; 750
.E.
.. ~ ..
. . .' . ' l
470
.
... ~.", ~ '~ __
..•
.;
--
~ ~' ~ ~ ~
...'
L-----'-_-"-_L.-----'-_-'-------'_-l
2
345
6
2
3
4
5
6
I-pattern. 30 msec
I-pattern. 20 msec
820 ,------.--,-----,--,--,---.--,
o
~ 750
c
/
o 680 ~
'5
D
Q)
.. -•.
.E.
/e
/// /
•. - -.- _. - -.... - . - -e- -
610
Ol
