Mental Chronometry: Subtractive and Additive Factors Method

Mental Chronometry: Subtractive and Additive Factors Method

He, Jibo

(Department of Psychology, University of Illinois, Urbana/Champaign, IL, 61801, US)

Reaction time (RT) is one of the best quantitative measures in psychology, which is objective and could be compared easily among diversified tasks. RT is not only a good measure of task performance, but also can be used to probe into the mental process. Mental chronometry uses RT to confirm the existence and quantify the duration of a specific mental process. The widely use of mental chronometry benefits from the methodological breakthroughs, that is, the invention of subtractive and additive factors methods. In this reaction paper to the two materials (Johnson & Proctor, 2004; Wichens, &Holland, 1992), I will briefly summarize the two methods and comment on their usage and relative merits.

Subtractive method measures the duration of mental process by deleting a mental operation entirely from the RT task (Wichens, &Holland, 1992). Subtractive uses go-no-go task paradigm. The duration of a mental operation is the time difference of a go-response and no-go response. However, subtractive method cannot be widely used because of the dependencies of mental process with its proceeding procedures. We can use go-no-go paradigm to measure the duration of a response execution, because the deletion of response execution does not hinder other mental process as it is the end of a RT task. We cannot delete other mental process without disturbing following processes, such as perceptual encoding and response selection. The difficulty in deleting mental process should be one of the reasons why subtractive method is not widely used as the additive factors method.

Additive factors method is used to identify the existence of a proposed mental process by manipulating variables only influencing on a specific mental process. Additive factors method assumes that mental processes are executed in serial without dependencies and parallel processing. By adding variables to influencing on a target mental process, the experiment manipulation will only change this mental process without disturbing other mental process. If two manipulations influence two different mental processes, the additive effect on RT should be larger than the condition that the two manipulations operate on the same mental processes. If two operations operate on the same mental process, it will cause interactive effect on the total RT. If two operations operate on different mental processes, it will cause additive effect on the total RT. Therefore, if the time of the interactive RT task and the additive RT task differs, a specific mental process should exist, and the difference of the time for interactive RT and additive RT is the duration of this mental process.

However, despite of the success of the subtractive and additive factors methods, both of the two methods can not estimate the duration of mental process accurately enough, because the basic assumptions are not held for every RT tasks. Both of the two methods assume that mental processes are executed in serial. We can measure the time for a specific mental process by subtracting or adding/changing a mental process. However, the assumption of serial processing is not always true. Many of our mental operations are processed in parallel benefiting from the functions are allocated to independent parts of the brain or modalities of our organs. For example, we could speak while listen, smile while think etc. The estimation of RT based on the serial assumption of mental processes should over estimate the total RT for a task since some of the mental processes are executed in parallel. Another limitation of the subtractive and additive factors methods is that they assume the independence of different mental processes. That is, we can influence on only a specific mental process without disturbing other mental processes. However, actually, this assumption does not hold. The firing of a mental process usually depends on its proceeding process. For instance, we cannot carry out the process of response selection without perceptual encoding first. Therefore, if we manipulate on the stage of perceptual encoding, we do not change the duration of perceptual encoding only, we are likely to change the following response selection too. The failure of perceptual encoding will cause us fail to come up with relevant response too, which will definitely lengthen the duration of response selection.

To sum up, the invention of subtractive and additive factors methods contribute to mental chronometry, which can identify a mental process and measure its duration. But we should also be aware of its limitation in assuming serial processing and independent of different mental processes.

REFERENCES

Johnson, Addie & Robert W. Proctor. "Information Processing and the Study of Attention (excerpt)." Attention: theory and practice. Sage, 2004. 32-37.

Wickens, C.D., Hollands, J. G. (1992). Engineering Psychology and Human Performance (2^nd Edition). Published by Harper Collins. Pages: 335-339.