Reaching for a Cup: The Interaction Between Perceiving and Taking Action • 73other involving the parietal lobe, is neuropsychology—studying the behavior
of people with brain damage. One of the central procedures in neuropsychol-
ogy is determining dissociations.METHOD Dissociations in Neuropsychology
One of the basic principles of neuropsychology is that we can understand the eff ects
of brain damage by studying dissociations—situations in which one function is
absent while another function is present. There are two kinds of dissociations: single
dissociations, which can be studied in one person, and double dissociations, which
require two or more people.
To illustrate a single dissociation, let’s consider a woman, Alice, who has suf-
fered damage to her temporal lobe. She is shown an object, then asked to name the
object and indicate where it is on the table by pointing to it. When given this task,
Alice can’t name the object, but she can reach to where it is located on the table
(● Figure 3.36a). Alice demonstrates a single dissociation—one function is absent
(naming objects) and another is present (locating objects). From a single dissociation such as
this, in which one function is lost while another function remains, we can conclude that the two
functions (in this example, naming and locating objects) involve diff erent mechanisms, although
they may not operate totally independently of one another.
We can illustrate a double dissociation by fi nding another person who has one function pres-
ent and another absent, but in a way opposite to Alice. For example, Bert, who has parietal lobe
damage, can identify objects but can’t tell exactly where they are located (Figure 3.36b). The key
to understanding the cases of Alice and Bert is that they are both given the same two tasks, but
Alice can do one task (reaching) and not the other (naming) while the opposite result occurs for
Bert. The cases of Alice and Bert, taken together, represent a double dissociation. Establishing a
double dissociation enables us to conclude that two functions are served by diff erent mechanisms
and that these mechanisms operate independently of one another.The method of determining dissociations was used by Milner and
Goodale (1995) to study D.F., a 34-year-old woman who suffered dam-
age to her temporal lobe from carbon monoxide poisoning caused by a
gas leak in her home. One result of the brain damage was revealed when
D.F. was asked to match the orientation of a card held in her hand to dif-
ferent orientations of a slot (● Figure 3.37a). She was unable to do this,
as shown in the left circle in Figure 3.37b. Each line in the circle indicates
how D.F. adjusted the card’s orientation. Perfect matching performance
would be indicated by a vertical line for each trial, but D.F.’s responses
are widely scattered. The right circle shows the accurate performance of
the normal controls.
Because D.F. had trouble orienting a card to match the orientation of
the slot, it would seem reasonable that she would also have trouble plac-
ing the card through the slot because to do this she would have to turn
the card so that it was lined up with the slot. But when D.F. was asked
to “mail” the card through the slot (● Figure 3.38a), she could do it, as
indicated by the results in Figure 3.38b. Even though D.F. could not turn
the card to match the slot’s orientation, once she started moving the card
toward the slot, she began rotating it to match the orientation of the slot.
Thus, D.F. performed poorly in the static orientation-matching task but
did well as soon as action was involved (Murphy, Racicot, & Goodale,
1996). Milner and Goodale interpreted D.F.’s behavior as showing that
there is one mechanism for judging orientation and another for coordi-
nating vision and action.● FIGURE 3.36 (a) Alice can’t name objects
but can accurately reach for them; (b) Bert
can name objects, but has trouble accurately
reaching for them. Alice and Bert together
illustrate a double dissociation.
(a) Alice
Name object Accurately reachfor object(b) Bert Ye s
NoNoYe s● FIGURE 3.37 (a) D.F.’s orientation task;
(b) results for the orientation task.
(b) Results of orientation matching(a) Task: Match orientationDF ControlCopyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.