Are perceptual and motor inhibition processes really dissociated? A comment on Nassauer and Halperin (2003)

Abstract

The Journal of the International Neuropsychological Society published recently a paper that should become influential shortly. However, given its potential impact, this study has to be examined carefully.According to Nassauer and Halperin (2003), their new computerized tasks would be able to show that perceptual inhibition and motor inhibition are dissociated, that is, independent mechanisms. The authors designed a set of six subtests by crossing two dichotomic variables, namely, absence vs. presence of a perceptual conflict, and absence vs. presence of a motor conflict, which led to four experimental conditions. The logical rationale underlying this study was the additive-factor method offered by Sternberg (1969) in the field of mental chronometry. According to this framework, if the two kinds of inhibition recruit separate mental resources, their simultaneous involvement in a given task will lead to an increase of response latency corresponding to the addition of both kinds of inhibition; conversely, if the two kinds of inhibition are not dissociated but share common resources, the resulting increase of latency should be higher than an addition, the supplementary cost resulting from the need for resources sharing. Concretely, in the former case, one should observe a significant main effect of each kind of inhibition but no interaction while, in the latter case, the interaction should be significant. Nassauer and Halperin (2003) claimed that their results support the dissociation thesis.Three qualifications apply. First, the dissociation thesis relies on the prediction of a null effect (of the interaction). Now, the meaning of an absence of effect is generally overestimated, as it can result from a lot of factors not theoretically relevant, such as the power of the tests, the perceptiveness of the tools (floor and ceiling effects), and so on. To really argue such absence of effect, Nassauer and Halperin (2003) should report a control task where a significant interaction would be predicted and verified.Second, the reported measures of main effects and interaction are not really convincing. Indeed, given the rationale of the ANOVA, the main effect of perceptual inhibition is assessed for the two conditions of motor conflict (conflict 1 no conflict) pooled, and the main effect of motor inhibition is assessed for the two conditions of perceptual conflicts (conflict 1 no conflict) pooled. And, accordingly, there was no significant interaction, with a perceptual inhibition of 170 ms (calculated from the means mentioned in the paper), a motor inhibition of 114 ms (calculated from the means mentioned in the paper), and a combined effect of about 280 ms (deduced from the inspection of Figure 2), that is, an additive effect. Now, it seems to me that the effect of each conflict should be assessed "purely", that is, in the absence of the other conflict. By means of such a procedure and by estimating the not reported means of the four subconditions, I note a perceptual inhibition of about 215 ms, a motor inhibition of about...

Detecting Age Differences in Resistance to Perceptual and Motor Interference

“…A related issue is the independence of the two inhibition scores. An issue already raised by Bruyer (Bruyer, 2003) by pointing out that the demonstration of independence of Nassauer and Halperin (Nassauer & Halperin, 2003) rests on additive factor assumptions and the absence of a statistical interaction. Indeed, individual differences yielded a correlation between perceptual and motor inhibition both in our sample and in Nassauer and Halperin's (Nassauer & Halperin, 2003) when data were not stratified by age.…”