The brain undergoes changes throughout life (Eriksson et al., 1998), with intervals of modest change punctuated by periods of more rapid transformation (Spear, 2000). Periods of more dramatic change include not only pre-and early postnatal eras but also adolescence (Spear, 2000). Rakic, Bourgeios, and Goldman-Rakic (1994) estimate that up to 30,000 cortical synapses are lost every second during portions of the pubertal period in nonhuman primates, resulting in a decline of nearly 50% in the average number of synaptic contacts per neuron, compared with the number prior to puberty. There is a similar loss of synapses in the human brain betwen 7 and 16 years of age (Huttenlocher, 1979), but the scarcity of human postmortem tissue makes it difficult to provide a more detailed description of this phenomenon. Although the implications of the massive pruning remain speculative, it is likely that it reflects active restructuring of connections and the promotion of more mature patterns. Some forms of mental retardation are associated with unusually high density of synapses (Goldman-Rakic, Isseroff, Schwartz, & Bugbee, 1983).

The elimination of synapses that are presumed to be excitatory, accompanied by a reduction in brain energy utilization, transform the adolescent brain into one that is more efficient and less energy consuming (Chugani, 1996; Rakic et al., 1994). These changes could permit more selective reactions to stimuli that in younger children activate broader cortical regions (Casey, Geidd, & Thomas, 2000).

Adolescence is also marked by changes in the relative volume and level of activity in different brain regions. For example, there is an increase in cortical white matter density (reflected in myelinated fiber tracts) and a corresponding decrease in gray matter, especially in frontal and prefrontal regions (Giedd et al., 1999; Sowell et al., 1999a, 1999b). The overall result of these varied changes is net decrease in the volume of the prefrontal cortex (Sowell et al., 1999b, van Eden, Kros, & Uylings, 1990). In the hippocampus and the amygdala however, gray matter volumes continue to increase during late childhood and adolescence (Giedd et al., 1997; Yurgelun-Todd, Killgrove, & Cintron, 2003).

There are also developmental shifts in patterns of innervation, including the circuits involved in the recognition and expression of fear, anxiety, and other emotions (Charney & Deutsch, 1996). The responsiveness of the cortical gamma-aminobutyric acid (GABA)–benzodiazepine receptor complex to challenge increases as animals approach puberty (Kellogg, 1998), and there are maturational changes in the hippocampus in humans as well as in animals (Benes, 1989; Wolfer & Lipp, 1995), especially increases in GABA transmission (Nurse & Lacaille, 1999). Further, pubescent animals show lower utilization rates of serotonin in the nucleus accumbens than younger or older animals (Teicher & Andersen, 1999).

Developmental increases in amygdala–prefrontal cortex (PFC) connectivity are seen during adolescence in work conducted in laboratory animals (Cunningham, Bhattacharyya, & Benes, 2002), along with alterations in amygdala activation (Terasawa & Timiras, 1968) and the processing of emotional and stressful stimuli. Lesions of the amygdala have opposite effects on fearfulness to social stimuli when those lesions are in infant versus adult monkeys (Prather et al., 2001). Although levels of negative affect and anxiety have been correlated with amygdalar activity in adults (Davidson, Abercrombie, Nitschke, & Putnam, 1999), recent studies using functional magnetic resonance imaging (fMRI) to examine amygdalar activation in response to emotionally expressive faces in younger individuals have yielded a varying mosaic of evidence (Killgore, Oki, & Yurgelun-Todd, 2001; Pine, Grun, et al., 2001).

Maturational changes in the cerebellum, and the circuitry connecting the cerebellum to the prefrontal cortex, continue through adolescence. Lesions of the adult cerebellum disrupt the regulation of emotion and interfere with performance on tasks requiring executive functions (Schmahmann & Sherman, 1998), although this is less apparent in those younger than 16 years

(Levisohn, Cronin-Golomb, & Schmahmann, 2000).

One consequence of this restructuring of the brain during adolescence is that early developmental compromises might be exposed. That is, brain regions vulnerable to dysfunction, due either to genetics or to adverse early experience, might be unmasked by the combination of brain restructuring and stressful life experiences (Goldman-Rakic et al., 1983; Hughes & Sparber, 1978).

Many, but not all, adults with anxiety disorders show abnormalities of autonomic regulation, especially lability of the cardiovascular system. This feature is most common among adults with panic disorder, social anxiety, and GAD (Gorman & Sloan, 2000). These abnormalities occur in both the sympathetic and parasympathetic systems and probably contribute to the association between anxiety disorder and cardiovascular mortality (Gorman & Sloan, 2000). Although children at risk for one or more anxiety disorders, because of a temperamental bias, show high sympathetic tone in the cardiovascular system (Kagan, Snidman, McManis, & Woodward, 2001), this relation is not robust and children with different disorders often display similar autonomic profiles (Pine et al., 1998). One mechanism that ties autonomic regulation to psychology is the result of peripheral feedback from the cardiovascular system to the brain. If this so

matic activity pierces consciousness, the person might conclude that a threat is imminent (Moss & Damasio, 2001). Perturbations in respiratory function are characteristic of panic disorder (Pine, 1999) and lead panic patients to experience a heightened feeling of anxiety (Coryell, Fyer, Pine, Martinez, & Arndt, 2001; Pine et al., 2000).

Patients with an anxiety disorder often show perturbations in the HPA axis. Further, both rodents and nonhuman primates show changes in the hypothalamic–pituitary–adrenal (HPA) axis during acute stress, as well as after a stress experienced early in life (Essex, Klein, Cho, & Kalin, 2002; Kaufman, Plotsky, Nemeroff, & Charney, 2000; Meaney, 2001; Monk, Pine, & Charney, 2002). The strongest association between activation in the HPA axis and anxiety disorder is seen in PTSD (Bremner, 1999; Bremner et al., 1999; Yehuda, 2002). Although enhanced feedback sensitivity in the HPA axis is often associated with an anxiety disorder, unfortunately, some children with an anxiety disorder exhibit the opposite pattern of reduced feedback sensitivity (Coplan et al., 2002; De Bellis, 2001; Heim & Nemeroff, 2002).

Brain chemistry can affect the excitability of a particular brain region in diverse ways. Neurochemical regulation in adult anxiety disorders is studied most often with pharmacological challenges, positron emission tomography, or measurement of peripheral neurochemical metabolites. Because the first two techniques are invasive, data on adolescents are restricted primarily to peripheral measures.

Adults with anxiety often show enhanced activity in the neurons of the locus ceruleus (Coplan et al., 1997; Sullivan, Coplan, & Gorman, 1998; Sullivan, Coplan, Kent, & Gorman, 1999). For example, adults with panic disorder and children with separation anxiety disorder show an abnormal response to the administration of yohimbine (Sallee, Sethuraman, Sine, & Liu, 2000). However, children and adults with a diagnosis of OCD show an abnormal, neurohormonal response to clonidine (Sallee et al., 1998). There is also an association between environmental stress and a prolactin response to serotonergic probes (Heim & Nemeroff, 2002), and adults with anxiety disorders show abnormalities in serotonergic regulation.

A dramatic indication of a relation between immunology and anxiety disorder comes from studies of children with OCD. Earlier work had found a specific association between OCD and neurological conditions affecting the basal ganglia, including pediatric Snydenham's chorea. This discovery led to the recognition of a specific form of OCD, called Pediatric Autoimmune Neuropsychiatric Disorder Associated with Streptococcus (PANDAS; Swedo, 2002), marked by anxiety, OCD, and motor tics that emerge following infection with group A ß-hemolytic streptococcus. This syndrome reflects an immunological reaction within underlying fronto-striatal-thalamo-cortical-circuitry. It may be relevant that the offspring of adults with panic disorder show selected allergic disorders reflecting anomalies in the immune system (Kagan et al., 2001; Slattery et al., 2002).

A variety of techniques have been used to study anxiety disorder. These include MRI, fMRI, magnetic resonance spectroscopy (MRS), and electrophysiology.

Morphometric MRI evidence, which provides information on brain structure, reveals that OCD adults have abnormalities in the circuit involving the prefrontal cortex, basal ganglia, and thalamus (Rauch, Savage, Alpert, Fischman, & Jenicke, 1997). Some of these abnormalities have been observed in children and adolescents with OCD (Rosenberg & Hanna, 2000; Rosenberg, MacMillan, & Moore, 2001). Adults with PTSD have reduced volume in the hippocampus; but children with PTSD do not show these specific reductions, even though they have a smaller brain volume (De Bellis et al., 1999). Children with GAD show increased volume of the amyg

dala and superior temporal gyrus of the right hemisphere (De Bellis et al., 2002).

Functional magnetic resonance imaging quantifies brain activity. Despite these advantages it relies on measures of blood flow and therefore is an indirect index of neuronal events. Moreover, fMRI does not measure absolute amount of blood flow, but differences in changes in blood flow during an experimental task compared with a control task.

Despite these caveats, adults with PTSD show enhanced amygdalar activation (Rauch et al., 2000), and children with anxiety disorders show activation to faces with fearful expressions (Thomas et al., 2001a, 2001b). However, this latter response could be due to the surprise of seeing fearful faces, because healthy children show enhanced amygdalar activation to neutral faces (Thomas et al., 2001a, 2001b).

Magnetic resonance spectroscopy (MRS) is a noninvasive technique that can reveal aspects of brain neurochemistry. One study with MRS found a reduction in levels of N-acetylaspartate in the cingulate gyrus of children who had PTSD (De Bellis, Keshavan, Spencer, & Hall, 2000).

The electroencephalogram (EEG) represents the synchronized activity of large numbers of cortical pyramidal neurons which, at any moment, have a dominant frequency of oscillation at particular sites. A state of mental and physical relaxation is usually but not always associated with more power in the alpha frequency band (8–13 Hz) in frontal areas. A state of psychological arousal is associated with greater power in the higher-frequency beta band (14–30 Hz). The change to higher frequencies could be the result of more intense volleys from the amygdala to the cortex.

In addition, there are usually small hemispheric differences in the amount of alpha power on the right, compared with the left, side at frontal and parietal sites. Because alpha frequencies are associated with a relaxed psychological state, the less alpha power at a particular site, the more likely that site is neuronally active. The technical term for loss of alpha power is desynchronized, and investigators assume that desyn chronization of alpha frequencies is a sign that the individual has moved to a more aroused state.

Subjects reporting higher anxiety tend to have greater activation in the right frontal area than the left, whereas normal controls show more activation in the left frontal area. A preference for display of right versus left frontal activation could reflect either a stable trait or a transient state. It appears that a stable preference for right or left frontal activation can be influenced by an individual's temperament and, therefore, could reflect a stable property (Fox, Henderson, Rabin, Caikins, & Schmidt, 2001). McManis, Kagan, Snidman, and Woodward (2002) have found that 11-year-old children who had been highly reactive infants and fearful toddlers were likely to show right frontal activation under resting conditions. However, an asymmetry of activation can also reflect a transient state. Infants watching the approach of a stranger showed greater right frontal activation during that brief period of time (Fox & Bell, 1990). Hagemann and colleagues, who gathered EEG data on four separate occasions on a sample of 59 adults, concluded that 60% of the variance in asymmetry of activation reflected a stable trait while 40% was attributable to the specific occasion of testing (Hagemann, Naumann, Thayer, & Bartussek, 2002).

The event-related potential is a time-locked, post-synaptic potential generated by large numbers of cortical pyramidal neurons to a specific stimulus. The first waveform that represents the detection of a discrepancy is called N2 because it usually peaks at about 200 msec to an unexpected event. The two most frequently studied waveforms, P3 and N4, appear a bit later with peak voltages at about 400 msec, and are prominent at frontal sites when the subject is passive and has no task to perform. Kagan and colleagues have unpublished data indicating that 11-year-old children who had been highly reactive infants and fearful toddlers showed a larger negative waveform at 400 msec to nonthreatening discrepant scenes. Although this research is preliminary, it suggests that future investigators should examine EEG profiles and event-related potentials in their study of anxiety and anxiety disorders.