Preferred rates of repetitive tapping and categorical time production.

In a constrained finger-tapping task, in which a subject attempts to match the rate of tapping responses to the rate of a pacer stimulus, interresponse interval (IRI) was a nonlinear function of interstimulus interval (ISI), in agreement with the results of Collyer, Broadbent, and Church (1992). In an unconstrained task, the subjects were not given an ISI to match, but were instructed to tap at their preferred rate, one that seemed not too fast or too slow for comfortable production. The distribution of preferred IRIs was bimodal rather than unimodal, with modes at 272 and 450 msec. Preferred IRIs also tended to become shorter over successive sessions. Time intervals that were preferred in the unconstrained task tended to be intervals that were overproduced (IRI > ISI) when they were used as ISIs in the constrained task. A multiple-oscillator model of timing developed by Church and Broadbent (1990) was used to simulate the two tasks. The nonlinearity in constrained tapping, termed the oscillator signature, and the bimodal distribution in unconstrained tapping were both exhibited by the model. The nature of the experimental results and the success of the simulation in capturing them both provide further support for a multiple-oscillator view of timing.

Categorical time production: evidence for discrete timing in motor control.

Subjects performed a repetitive manual tapping task, attempting to match a given rate of auditory stimulus pulses, first with the pulses audible (synchronization) and then with the pulses turned off (continuation). In different sessions, the interstimulus interval (ISI) was selected from the range 175 to 825 msec in steps of 25 msec, with different ISI values presented in a random order. Across this range of ISI conditions, interresponse intervals (IRIs) exhibited alternating positive bias (too slow) and negative bias (too fast). We interpret this pattern of bias in terms of a discrete, or categorical, timing mechanism in motor timing. Categorical time production can be viewed as extending our conception of the timekeeper in Wing's (Wing & Kristofferson, 1973a, 1973b) two-process model of motor timing and may be related to the system of multiple clocks proposed by Kristofferson (1980) to explain a categorical pattern of variability measures in duration discrimination.

Psychology of the scientist: LXIII. Perceiving scattergrams: is visual line fitting related to estimation of the correlation coefficient?

Visual line fitting and direct estimation of the correlation coefficient were carried out by 50 subjects using computer-generated scattergrams as stimuli. In visual line fitting, slopes of visual lines were generally greater than the corresponding regression slopes, in agreement with the hypothesis that visual lines are placed so as to bisect the cloud of displayed points at the cloud's major axis rather than to approximate a regression line. Subjects tended to underestimate the correlation coefficient, scaling their judgments of linear structure somewhat more as if they were judging the coefficient of determination. With the actual degree of linear structure partialed out, there were no strong relationships between measures of visual line fitting and measures of estimation. While both of these tasks offer quickly-obtained correlates of linear structure in scattergrams, users should be aware of their biases. We suggest that visual lines do not approximate regression lines very well and estimates of correlation do not approximate the correlation coefficient very well, because the perceptual processes involved perform operations other than regression and correlation. In the present data, these operations appeared to be independent of each other.