Psychology 403.
Laboratory in Cognitive Psychology
Experiment 1.
Fall, 2003
Discrimination of Line Length Differences
Overview
In this experiment subjects must judge which of two lines is longer, signaling their response by
pressing one of two keys to indicate whether the top or bottom line looked longer. From trial to trial one of
two different differences in line length will occur, one smaller and more difficult than the other. In two
separate blocks of trials the instructions will vary. In one block subjects will be told to respond rapidly,
while in the other block they will be told to respond accurately. The data will be examined to understand
how the combination of speed and accuracy must be used together to assess the relative difficulty of
different tasks.
This experiment is an introduction to the use of reaction time procedures to study cognitive
processes. Under a variety of conditions, reaction time procedures are used to assess the relative
difficulty of performance across different experimental conditions. Commonly, the participant is required
to make a decision and indicate it by a key press (or in some cases a spoken response detected by an
electronic circuit called a “voice key” which detects the signal coming from a microphone.) Two
dependent measures are recorded, the reaction time, the time from the presentation of the stimulus
material to the response, and the accuracy of the response. It is usually assumed that greater task
difficulty will be reflected in a slower reaction time and/or lower accuracy.
We will examine the use of a reaction time procedure to assess task difficulty under conditions
where the relative difficulty of two different stimulus conditions is quite clear. Participants will have to
decide which of two lines is longer under two different experimental conditions. On some trials the
difference in the lengths of the two lines will be smaller than on other trials. The more similar the line
lengths, the more difficult the discrimination of which line is longer.
In comparing two conditions, we usually wish to determine which is more difficult. However,
performance is measured by two dependent measures, reaction time (or its inverse, speed) and accuracy.
Faster reaction times or greater speed indicate that a task is easier. However, the measures of reaction
time and accuracy are not independent of each other. Participants often may improve accuracy by
slowing down, or respond faster by becoming less accurate. For this reason, just observing reaction time
or just observing accuracy across two or more levels of an independent variable is not necessarily an
accurate assessment of the relative difficulty of the different levels of the independent variable. If a
participant is both faster and more accurate in one condition than another, it is easy to conclude that
condition is easier than the other. But, if a participant is faster, but less accurate in one condition
compared to another, then the difference in difficulty between the two conditions is ambiguous. The faster
reaction time suggests that the task is easier, but the lower accuracy suggests otherwise. It is possible
that there is no difference, and that the participant merely traded speed for accuracy in the two different
conditions. Without additional, parametric experiments, it is difficult to know how large a change in
accuracy will result from a specific change in speed. The trade-off which occurs between speed and
accuracy when the task difficulty remains fixed is called the speed-accuracy trade-off function.
In this experiment we will illustrate the use of reaction time procedures by examining performance
under different conditions. We will manipulate two different independent variables. The first will be the
difficulty of the task. Participants will be asked to determine which of two different lines is longer under
two different stimulus conditions that will differ in the size of the difference in length between the two lines.
We will assume (quite reasonably) that the task will be easier when the difference in the lengths of the two
lines is greater. We will observe how this difficulty is reflected in each of the two different dependent
measures; reaction time and accuracy. In addition, we will manipulate a second independent variable,
the instructions that either urge that participants perform as rapidly as possible, or as accurately as
possible. This instruction will differ for two blocks of trials. (In each block the same two line length
differences will be used.) Thus we will be able to observe within each block how speed and accuracy
reflect the difficulty of judging the two different pairs of lines. Comparing between blocks we will be able to
see how speed and accuracy vary when the same stimuli are judged under different instructions. Since
the instructions will not change the difficulty of the discrimination, performance under these two conditions
for a given stimulus pair will reflect the same difficulty. So we will be able to observe how speed can be
traded for accuracy, and vice versa.
Method
A trial will be initiated by the participant by pressing the space bar on the computer. A plus sign
will appear in the center of the screen, as a fixation target, for 500 msec. It will then disappear and the
test stimuli, a pair of lines of different length, will appear. The participant will indicate whether the longer
line was on the top (by pressing the "2" key" on the numberpad) or bottom (by pressing the "8" key). The
difference in the lengths of the lines will be longer in the easier condition, and shorter in the harder
condition. [The actual length of the lines on the screen will depend upon the specific computer and
monitor on which the experiment is run, although the relative lengths of the lines will be the same on all
computers.]
The two lines will be separated vertically from each other by a fixed distance, and horizontally by
one of two fixed distances. The vertical location of the pair of lines on the screen will be fixed, and will be
equidistant above and below the location of the fixation point. Finally, the pair of lines will appear at one of
4 locations horizontally across the screen, spaced equally across the screen. The order of trials within a
block will be randomized for each participant, and the order of blocks with different instructions will be
randomized across participants.
There will be three blocks of trials, a practice block, a block in which the instructions are to
respond as accurately as possible, and a block in which the instructions are to respond as quickly as
possible. The practice block will always occur first, and participants will be required to reach a criterion
level of performance before proceeding to the experimental conditions. The first practice block will be 14
trials, and subsequent blocks of 14 trials will be required until the criterion level of performance (80%
correct) has been reached, attempting to insure that the participants understand the task and have
mastered the required responses. The order of the experimental blocks will vary randomly across
subjects, with some receiving the instructions to be accurate first and others receiving the instructions to
respond rapidly first.
Each experimental block will consist of 96 trials, half with the easier discrimination and half with
the more difficult discrimination. In addition, the longer stimulus will appear on top half of the time, and the
shorter stimulus will be on top on the remainder of the trials.
Detailed instructions on how to run the experiment on the computer will be provided in class.
LineLengthF03.doc