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Knowledge of brain circuitry is critical for understanding the organization, function, and evolution of central nervous systems. Most commonly, brain connections have been elucidated using histological and experimental methods that require animal sacrifice. On the other hand, magnetic resonance diffusion tensor imaging and associated tractography have emerged as a preferred method to noninvasively visualize brain white matter tracts. However, existing studies have primarily examined large, heavily myelinated fiber tracts. Whether tractography can visualize fiber bundles that contain thin and poorly myelinated axons is uncertain. To address this question, the midbrain auditory pathway to the thalamus was investigated in Alligator. This species was chosen because of its evolutionary importance as it is the reptilian group most closely related to birds and because its brain contains many thin and poorly myelinated tracts. Furthermore, this auditory pathway is well documented in other reptiles, including a related crocodilian. Histological observations and experimental determination of anterograde connections confirmed this path in Alligator. Tractography identified these tracts in Alligator and provided a 3-dimensional picture that accurately identified the neural elements of this circuit. In addition, tractography identified one possible unrecognized pathway. These results demonstrate that tractography can visualize circuits containing thin, poorly myelinated fibers. These findings open the door for future studies to examine these types of pathways in other vertebrates.

The purpose of this study was to investigate the brain circuitry involved in the processing of both positive and negative emotions in normal healthy subjects.we have recruited 15 healthy volunteers (9 males and 6 females, age range 30-60). In this block-design fMRI study, we compared the blood oxygen level dependant (BOLD) signal change as response to pleasant and unpleasant IAPS pictures, each compared to a neutral condition.Pleasant pictures versus neutral condition contrast demonstrated significant activation (p(FDRcorrected)0.05) in bilateral pre-frontal cortex (PFC), anterior and posterior cingulate gyri and temporal lobe. Unpleasant pictures relative to neutral condition exhibit significant activation (p(FDRcorrected)0.05) in amygdala, hippocampus, parahippocampal gyri, temporal lobe, visual cortex, fusiform gyri, PFC and anterior cingulate gyrus.Amygdala is mainly involved in the processing of negative emotions. Although an overlap in regions involved in the processing of pleasant and unpleasant IAPS pictures exists, the neural network for each is unique.