18 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 and accuracies 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 precise for our purposes.
18.1 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. The cost of making a simple two-alternative choice is typically in the range of 75 to 200 ms. Given that RTs in a simple reaction time task are typically between 200 and 350 ms, this represents a substantial cost.
This idea was introduced by Franciscus Donders more than 150 years ago in one of the most influential publications in the history of psychology (Donders, 1969)2:
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
We have implemented a version of the simple and choice reaction time task for you to try out. It takes about 2 minutes to complete. Try to be as quick as possible to make it to the top of the leaderboard!
Click on this link to run the experiment. (This link opens in a new browser tab or window.)
Once the experiment is complete, you will be automatically forwarded to an interactive app where you can view the leaderboard, the group results and individual results. Note that in this example, response time is our DV and type of task (simple vs choice) is our IV.
Reference
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.↩︎
This is the 100th anniversary reprint. The original was published in 1869.↩︎