15 Psychological experiments
This week we are going to start having a closer look at experiments. As explained in detail in Beth’s book, by manipulating one or a few variables and by holding constant other variables, experiments allow us to draw causal inferences. If the experiment has a between-subjects design, random allocation to groups is another key characteristic of experiments.
Experiments are all about explaining an observed effect on the dependent variable (DV) as a result of the manipulation of the independent variable (IV). If the observed effect is actually the result of manipulating the IV, the experiment is referred to as internally valid (meaning that it appears there are no alternative explanations for the observed effect). Threats to internal validity are a key issue for experiments and are discussed in detail in Chapter 11 in Beth’s book.
Our focus in the labs will be on a number of experiments that measure response times (RTs)1 and accuracy. RTs have a number of advantages that make them suitable for our lab classes: they can be easily acquired using standard keyboards, they are objective and they are sufficiently accurate for our purposes.
15.1 A little bit of history
Experimental psychology evolved in 19th century Germany. Figure 15.1 shows three of the key scholars involved.
Why were they all men? Well, women were simply not allowed to study! In Germany, women were only allowed to matriculate at universities starting between 1900 and 1909 (depending on the Bundesland).2 In England, the first degrees for women were awarded by the University of London in 1878 (although Oxford only followed suit in 1920 and Cambridge in 1948!).
Before the advent of experimental psychology, psychological theorising had been the purview of philosophers. Unfortunately, while coming up with some ideas highly relevant to psychology (see, e.g., John Locke, David Hume and John Stuart Mill), they were not interested in testing their ideas empirically. In 19th century Germany, the physiologists mentioned above3, among others, took a different approach and started to perform experiments to empirically test ideas relevant to the nascent field of psychology. Among his many other contributions to science, in 1849 von Helmholtz measured neuronal conduction speeds and showed that, while neuronal conduction was fast (about 25 to 40 m/s in frog neurons), it was not so fast that it could not be measured.4 This insight opened the door to investigating the human mind by measuring the time it took to complete certain tasks. Fechner on the other hand, building on the work of Ernst Heinrich Weber, established the field of psychophysics. In 1879, Wundt founded the first psychological institute at the University of Leipzig. According to Boring (1950), he should be considered the first proper psychologist.
15.2 Simple vs choice RT
RTs and accuracies provide us with information about the cognitive and neuronal mechanisms taking place in your minds and brains. Any processing going on in our brains takes time. And more complicated things take more time (and typically increase error rates)! Let us take a look at what is perhaps the classic example of the idea that adding extra processing demands slows us down: The comparison of simple and choice reaction time. This idea was introduced by Franciscus Donders5 more than 150 years ago in one of the most influential publications in the history of psychology (Donders, 1969)6:
The idea occurred to me to interpose into the process of the physiological time [i.e., simple reaction time, J.D.] some new components of mental action. If I investigated how much this would lengthen the physiological time, this would, I judged, reveal the time required for the interposed term.
F. C. Donders (1869), On the speed of mental processes
Below, you will have the opportunity to find out how long it takes you to make very simple decisions! Please respond as quickly and as accurately as possible.
Simple reaction time task
Try out a simple reaction time task. There are 24 trials overall. Once finished, you will be presented with your mean RT for this task.
Choice reaction time task
Try out a choice reaction time task. Again, there will be 24 trials overall. You must get at least 21 out of 24 trials correct to obtain your mean RT for this task.
Results
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Note that the cost of making a choice is typically between 75 and 200 ms. Given that RTs in a simple reaction time task are typically between 200 and 350 ms, this represents a substantial cost.
References
Boring, E. G. (1950). A history of experimental psychology (2nd ed.). Appleton-Century-Crofts.
Cobb, M. (2020). The idea of the brain: A history. Profile Books.
Donders, F. C. (1969). On the speed of mental processes. Acta Psychologica, 30, 412–431.
The terms response time and reaction time are often used synonymously and refer to the length of time from stimulus onset (e.g., a picture on a screen) to movement offset (e.g., pressing down a key with a finger). Sometimes response time is defined as the sum of reaction time (i.e., time from stimulus onset to movement onset) plus movement time (i.e., time from movement onset to movement offset). Following this definition, in the above example reaction time would be measured from picture onset up to the point when the finger starts moving. Movement time on the other hand would be the length of time from when the finger starts moving to when the key is fully depressed. For simple responses (such as key presses), this distinction doesn’t usually matter. For more complex responses (such as, say, pointing responses involving whole arm movements), separating reaction and movement time can be of interest.↩︎
A notable, but also singular, exception was Dorothea Erxleben.↩︎
Notably, Fechner, von Helmholtz and Wundt had all studied medicine, not philosophy. That said, they were not just physiologists or psychologists. Boring (1950) notes that Fechner “was for seven years a physiologist (1817-1824); for fifteen a physicist (1824-1839); for a dozen years an invalid (1839 to about 1851); for fourteen years a psychophysicist (1851-1865); for eleven years an experimental estheticist (1865-1876); (…) recurrently and persistently a philosopher (1836-1879); and finally, during his last eleven years, an old man whose attention had been brought back (…) to psychophysics (1876-1887)” (p. 283). Von Helmholtz was also an eminent physicist. And Wundt was also a philosopher.↩︎
Even Johannes Müller, one of the most eminent physiologists of the time, was of the opinion that neuronal conduction speeds must be so high they cannot be measured (Cobb, 2020).↩︎
A Dutchman, but like the Germans mentioned before he had also studied medicine!↩︎
This is the 100th anniversary reprint. The original was published in 1869.↩︎