The Simon Effect
The Simon effect refers to the finding that people are faster and more accurate when
responding to stimuli that occur in the same relative location as the response, even
though the location information is irrelevant to the actual task (Simon, 1969). Studying
the Simon effect gives us insight into a stage of decision making called “response
selection.” According to information-processing theory, there are three stages of
decision making: stimulus identification, response selection, and response execution
or the motor stage.
Superficially, the Simon effect may seem similar to the Stroop effect. However, it is
generally accepted that the interference that occurs in the Stroop effect comes from
the stimulus identification, while the interference that occurs in the Simon effect occurs
in the response-selection stage. During response selection, a person uses a rule to
translate the relevant stimulus dimension, usually shape or color, to the correct left or
right response. However, the location dimension of the stimulus (its position on the
screen) overlaps with the relevant stimulus dimension (left or right). Because of this,
the irrelevant location dimension of the stimulus activates the corresponding response
and interferes with making a response to the non-corresponding side. As a result,
same-side responses are faster and more accurate than opposite-side responses.
In the real world, the Simon effect has important implications. Primarily, it shows that
location information cannot be ignored and will affect decision making, even if the user
knows that the information is irrelevant. The Simon effect (and related phenomena)
must be taken into account in design of man-machine interfaces. Good interfaces
display information in ways that match the types of responses people should make.
For example, imagine that you are flying a plane, and the left engine has a problem.
The indicator for that engine should be to the left of a corresponding indicator for the
right engine. If it is the other way around, you may respond incorrectly to the indicator
and adjust the wrong engine. That could be problematic.
What methods did we employ in this experiment?
On each trial, a red or green square was shown to the left or right of fixation. Your task
was to press the key on the left side of the keyboard (or tap a button on the left side
of the screen) if the square was green and to press a key on the right side of the
keyboard (or tap a button on the right side of the screen) if the square was red. Trials
with incorrect responses were repeated.
The independent variable in this experiment was the location of the square (left or
right). The dependent variable was the time (response time) between the appearance
of the square and your key-press.
What do we predict participants will do? Why?
Although the task requires you to attend only to the color of the square, people are
almost invariably influenced by the location of the square as well. When the location
of the square and the location of the matching key press have the same relative
position (e.g., a green square on the left side of the screen), the response time tends
to be faster than when they have different relative positions (e.g., a red square on the
left side of the screen). The difference between response times is the Simon effect.
How robust is this effect? Are there limits to this effect?
The Simon effect is generally quite small (tens of milliseconds). Nevertheless, it is
found in many different situations. It might seem that such a small effect can hardly be
important. However, there are many situations in which a person has to respond to a
small flashing light. The Simon effect can be repeated thousands of times a day and
lead to more significant durations.
Also, there are some special situations in which even a few milliseconds can make a
difference. Many emergency situations in aircraft, for example, are indicated by the
sudden appearance of a light. A pilot must be able to respond quickly to such an
indicator, and accounting for the Simon effect can play an important role in the design
of a cockpit.