Psychology 403.
Laboratory in Cognitive Psychology
Experiment 2.
Fall, 2003
Mental Rotation
Overview
Participants are presented pictures of a familiar object (R,2) or its mirror image. The
object may be in its normal orientation or rotated. Participants must respond whether the object is
a familiar object or its mirror image. The speed of responding and accuracy will be examined as a
function of the difference in orientation between the presented object and its normal orientation.
One issue that has been the motivation for many experiments has been the desire to
characterize the form in which information is represented in mental operations. One contrast that
has been useful to understanding a number of phenomena has been between verbal and pictorial
representations (also referred to as propositional vs. image). A classic experiment by Shepard
and Metzler (1971) has been interpreted to provide supported for the hypothesis that some visual
information is represented by an image or picture-like code. The important characteristic of this
kind of representation is that it preserves spatial information, making performance of tasks that
depend upon spatial information much easier than they would be if the representation were in a
more verbal or propositional form.
In their original experiment Shepard and Metzler (1971) created stimuli that were two
perspective line drawings of three-dimensional objects. Participants were presented with pictures
of a pair of objects that could either be the same object or two different objects that were mirror
images of each other. The objects could be in the same orientation or differing in orientation.
Participants had to respond as quickly as possible whether the objects were the same or mirror
images, and they measured reaction time and accuracy. Their principal finding was that the
reaction time to respond correctly increased linearly as a function of the difference in angle of
orientation of the two objects. This pattern of results was interpreted to support the idea that the
objects were represented in a pictorial or image-like code, and that to compare the two objects the
participants had to transform the representations of the two objects until they represented objects
in the same orientation. Then they were able to compare them. If they matched it meant that they
were the same object, and if they did not match it meant that they were mirror images. The fact
that the time to respond correctly increased linearly as a function of the difference in angle
between the objects was interpreted to mean that transforming the representations of the objects
was like rotating real objects in space. In order to get an object from one orientation to another it
is necessary to move it through a sequence of intermediate orientations. In order to transform the
representation of one object into the representation of an object in the same orientation as the
other object, it is necessary to create the representations of the object in orientations intermediate
to those of the two objects. Because this transformation process takes time, and because the
number of intermediate orientations increases in proportion to the difference in orientations of the
two objects, the reaction time increases in proportion to the difference in orientations of the two
objects.
Method
A participant will initiate a trial by pressing the space bar on the computer. A plus sign
serving as a fixation target will appear for 500 milliseconds and then disappear, followed by a test
stimulus that will remain on the screen until the participant responds. The test stimulus will be
either the digit 2 or the letter R, or the mirror image of either of these. The stimulus will appear
either in its normal, upright position, or rotated clockwise by 45, 90, 135, or 180 degrees. If the
stimulus is a normal letter the participant is instructed to press the 4 key and if it is a mirror image
to press the 6 key. Reaction time and accuracy will be measured. Following successive blocks of
10 practice trials until the participant attains an accuracy of at least 80%, participants will carry out
20 trails at each orientation, half with the normal target and half with the mirror image. Mirror
image and normally oriented stimuli will occur equally often for each stimulus at each orientation.
REFERENCES
ShepardD, R.N., & Metzler, J. (1971) Mental rotation of three-dimensional objects. Science, 171,
701-703.
Psy403F03MentalRotation.doc