Localization of Function • 31(The dose is low enough that it is not harmful to the person.) The person’s brain is then scanned by
the PET apparatus, which measures the signal from the tracer at each location in the brain. Higher
signals indicate higher levels of brain activity (● Figure 2.8).
PET enabled researchers to track changes in blood fl ow, and thus to determine which brain
areas were being activated. To use this tool, researchers developed the subtraction technique.
Brain activity is measured fi rst in a “control state,” before stimulation is presented, and again
while the stimulus is presented. For example, in a study designed to determine which areas
of the brain are activated when a person manipulates an object, activity generated by simply
placing the object in the hand would be measured fi rst. This is the control state (● Figure 2.9a).
Then activity is measured as the person manipulates the object. This is the stimulation state
(Figure 2.9b). Finally, the activity due to manipulation is determined by subtracting the control
activity from the stimulation activity (Figure 2.9c).(a)Percent Activation–1 0 +1 +2
(b)
● FIGURE 2.8 (a) Person in a brain scanner. (b) In this cross section of the brain, areas
of the brain that are activated are indicated by the colors. Increases in activation are
indicated by red and yellow, decreases by blue and green. (Source: Part b from Alumit Ishai,
Leslie G. Ungerleider, Alex Martin, James V. Haxby, “The Representation of Objects in the Human Occipital
and Temporal Cortex,” Journal of Cognitive Neuroscience, 12:2, 2000, pp. 35–51. © 2000 by the Massachusetts
Institute of Technology.)Jupiter Images● FIGURE 2.9 The subtraction technique used to interpret the results of brain imaging
experiments. (a) Colored area indicates activation when a person is holding a small object.
(b) Colored areas indicate activation when the person begins manipulating the object.
(c) Subtracting the activation in (a) from the activation in (b) indicates the activation
due to manipulation of the object. (Source: B. Goldstein, Sensation and Perception, 8th ed., Fig. 4.16,
p. 83. Copyright © 2010, Wadsworth, a part of Cengage Learning. Reproduced with permission. http://www.cengage
.com/permissions.)(a) Initial condition—
hold object(b) Test condition—
manipulate object(c) Activity associated with
manipulating objectCopyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.