Measuring Reaction Time and Donders' Method of Subtraction

Overview

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University

The ambition of experimental psychology is to characterize the mental events that support the human ability to solve problems, perceive the world, and turn thoughts into words and sentences. But people cannot see or feel those mental events; they cannot be weighed, combined in test tubes, or grown in a dish. Wanting to study mental life, nonetheless, Franciscus Donders, a Dutch ophthalmologist in the early 1800s, came up with a property that he could measure—even back then: he measured the time it took for human subjects to perform simple tasks, reasoning that he could treat those measurements as proxies for the time it takes to complete the unobservable mental operations involved. In fact, Donders went one step further, developing a basic experimental paradigm known as the Method of Subtraction. It simply asks a researcher to design two tasks that are identical in nearly every way, excepting a mental operation hypothesized to be involved in one of the tasks and omitted in the other. The researcher then measures the time it takes to complete each task, and by subtracting the outcomes, he extracts an estimate of the time it takes to execute the one mental operation of interest. In this way, the method allows a researcher to isolate a mental operation. The time it takes to complete a task has become known as reaction time or latency. Even today, reaction time is by a wide margin the most prevalent dependent variable in experimental psychology.

This video will demonstrate the measurement of reaction time using Donders’ Method of Subtraction.

Procedure

1. Pick a task and material to implement it.

To use Donders’ Method of Subtraction, one first needs a mental operation of interest, and a pair of tasks thought to differ in terms of the operation. For current purposes, this video explores the ability to resolve conflicts between different sources of information—an important aspect of the ability to exert self-control on behavior. The Stroop task is a good basis for measuring the time it takes to resolve a conflict between information sources.

Results

It is hard to draw conclusions from a single subject, and so an experiment typically tests many subjects, aggregating their results to draw reliable conclusions. For this Stroop experiment, you would test 20 or so participants just the way you tested one. For each participant, you end up with two reaction times, one from the ‘Conflict’ and one from the ‘No Conflict’ condition (Table 1). These results can be summarized with a simple graph of the average reaction time across all par

Application and Summary

Donders’ Method of Subtraction can be used with reaction time measures in a variety of areas in experimental psychology, not just with Stroop or conflict paradigms. In addition, the Method of Subtraction underpins the basic logic for a wide array of approaches to experimental psychology with dependent variables beyond reaction time. These include measures as diverse as how long an infant glares at a stimulus, and the blood-oxygen-level-dependent (BOLD) response measured in the human brain by sophisticated fMRI mach