A complex physiological system, composed of central and peripheral signals, balances energy intake and expenditure. Among these signals, the enteric and orexigenic hormone ghrelin is a regulator of energy balance with several uncharacterized functions. Although much research has accumulated regarding ghrelin's effects on metabolic parameters, little is known about its other behavioural and cognitive effects. Consequently, this study, using functional magnetic resonance imaging, showed that ghrelin administered intravenously to healthy volunteers increased the neural response to food pictures, as well as faces of fear and disgust, in brain areas regulating the hedonic and incentive evaluation of visual stimuli, such as the amygdala. Moreover, ghrelin exhibited memory enhancing effects for both food pictures and faces of fear and disgust. These findings suggest that ghrelin's activation of the amygdala may serve as a magnitude signal for value judgments of visually-presented food and non-food stimuli, thus engaging critical feeding, emotional and cognitive responses.

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Feeding behavior is a complex phenomenon involving homeostatic signals, and non-homeostatic inputs such as visual cues. In primates, exposure to food-related sensory cues has been shown to elicit cephalic phase responses as well as trigger central appetitive processing, in a motivationally-dependent manner. Neural structures consistently implicated in such responses and/or in the regulation of ingestive behavior in general, in both monkeys and in humans, include the amygdala, insula, striatum, hypothalamus, and frontal and occipital cortices. In humans however, the cerebral response to visual food stimulation remains minimally explored. / Functional Magnetic Resonance Imaging (fMRI) provides information about state-dependent changes in local neuronal activity in vivo. Using fMRI, the present dissertation examined changes in human brain activity to food and nonfood pictures following the pharmacological induction of hunger with the orexigenic hormone ghrelin (Study 1), and following manipulation of the cognitive state of food expectation (Study 2). / Our data reinforce the involvement of a distributed frontal-limbic-paralimbic circuit in the central processing of food imagery, under both experimental conditions. The first study revealed that intravenous ghrelin administration potently modulated food-associated neural responses III areas involved in reward, motivation, memory, and attention (amygdala, insula, orbitofrontal cortex, striatum, hippocampus, midbrain, visual areas). This suggests that metabolic signals such as ghrelin may promote food consumption by enhancing the appetitive response to food cues via engagement of the hedonic network. / The second study revealed that brain regions activated in the 'expectant' state (i.e. when subjects were anticipating food reward) were at least partially dissociable from those in the 'not expectant' state. In particular, recruitment of the dorsolateral prefrontal cortex, a principal component in the cognitive control network, exclusively in the 'not expectant' condition, may signal an attempt to suppress appetite in the absence of food expectation. Areas of convergence were observed in the amygdala and insula. / Obesity is rapidly becoming the major cause of excess mortality worldwide; therefore, understanding how the central nervous system controls appetite and nutrient consumption is of considerable interest. The projects in this thesis offer significant insights regarding the effects two select factors (one intrinsic and the other extrinsic) on the neural reaction to visual food stimuli, in healthy male participants.

This study investigated the neural correlates of memory for human nonlinguistic emotional vocalizations as a function of individual differences in trait anxiety and depression. 20 healthy subjects (female; aged 18-30) free from neurological impairments or psychiatric illness underwent MRI scanning to obtain T1 structural images of their brain, and participated in a subsequent behavioral memory task outside the scanner. Volumetry of the hippocampus and amygdala was performed using a validated protocol. We found emotional vocalizations were better remembered than neutral ones, with performance for negative better than positive. Memory performance for emotional items was associated with hippocampal volume, with no association between memory and I amygdala volume detected. Differences in anxiety or depression had no influence on memory or volume. These results lay the groundwork for future functional neuroimaging work to investigate the neural correlates of memory, personality, and brain structure volume in healthy and clinical populations.

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