“oddball” paradigm using event-related potentials) to study the development of explicit memory (see Bachevalier, 1992; Bachevalier et al., 1991, 1993; Nelson, C.A., 1994, 1995, 1996). Finally, Luciana and colleagues (e.g., Luciana and Nelson, 1998) have used an extensive battery of tasks to examine a range of cognitive behaviors.

The use of neuropsychological tools have several advantages over the other approaches discussed below: (a) they are completely noninvasive, (b) they can be used across the lifespan, (c) parallel studies can be conducted across species, and (d) they can provide insight into specific behaviors. The neuropsychological approach also has shortcomings: (a) these tools only indirectly couple brain structure and function (i.e., because no direct measures of the brain are taken) and thus may lack precision with regard to this relation; (b) when adopting such tools from the animal literature, it is important to consider whether both species are responding to the tasks the same way; (c) caution must be exercised when generalizing from clinical to normative samples; and (d) when used with the lesion method (i.e., the population under study has experienced a lesion to a particular part of the brain), it is important to be aware that the mapping of specific lesion to specific function may be less than one to one (i.e., a lesion in a particular area could affect the function of surrounding areas as well).

METABOLIC PROCEDURES

This class of tools depends on the ability to track various metabolic functions as they occur in real time. These include positron emission tomography and functional magnetic resonance imaging, each of which is described below.

Positron Emission Tomography

Positron emission tomography (PET) scanning typically involves the injection of a natural substance such as oxygen or glucose that has been made radioactive. In so doing one is able to track the metabolism of this substance by those regions of the brain calling for its use. Positrons are emitted as the radioactive substance decays, and these positrons can be measured using a positron detector (i.e., PET scanner). The detector, in turn, computes the point of origin of these positrons, and thus localizes in the brain (within centimeters of resolution) the source of neural activity.

A good example of this work comes from studies conducted by Chugani and his colleagues. Here a form of radioactive glucose (FDG) has been used in infants and children to infer the development of synapses (i.e., synapse formation requires energy and thus glucose can be used as an indirect marker for synaptogenesis; see Chugani, 1994; Chugani and Phelps, 1986;



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