No abstract

Tractography based on non-invasive diffusion imaging is central to the study of human brain connectivity. To date, the approach has not been systematically validated in ground truth studies. Based on a simulated human brain data set with ground truth tracts, we organized an open international tractography challenge, which resulted in 96 distinct submissions from 20 research groups. Here, we report the encouraging finding that most state-of-the-art algorithms produce tractograms containing 90% of the ground truth bundles (to at least some extent). However, the same tractograms contain many more invalid than valid bundles, and half of these invalid bundles occur systematically across research groups. Taken together, our results demonstrate and confirm fundamental ambiguities inherent in tract reconstruction based on orientation information alone, which need to be considered when interpreting tractography and connectivity results. Our approach provides a novel framework for estimating reliability of tractography and encourages innovation to address its current limitations.

The prefrontal cortex and the amygdala have synergistic roles in regulating purposive behavior, effected through bidirectional pathways. Here we investigated the largely unknown extent and laminar relationship of prefrontal input-output zones linked with the amygdala using neural tracers injected in the amygdala in rhesus monkeys. Prefrontal areas varied vastly in their connections with the amygdala, with the densest connections found in posterior orbitofrontal and posterior medial cortices, and the sparsest in anterior lateral prefrontal areas, especially area 10. Prefrontal projection neurons directed to the amygdala originated in layer 5, but significant numbers were also found in layers 2 and 3 in posterior medial and orbitofrontal cortices. Amygdalar axonal terminations in prefrontal cortex were most frequently distributed in bilaminar bands in the superficial and deep layers, by columns spanning the entire cortical depth, and less frequently as small patches centered in the superficial or deep layers. Heavy terminations in layers 1−2 overlapped with calbindin positive inhibitory neurons. A comparison of the relationship of input to output projections revealed that among the most heavily connected cortices, cingulate areas 25 and 24 issued comparatively more projections to the amygdala than they received, whereas caudal orbitofrontal areas were more receivers than senders. Further, there was a significant relationship between the proportion of 'feedforward' cortical projections from layers 2−3 to 'feedback' terminations innervating the superficial layers of prefrontal cortices. These findings indicate that the connections between prefrontal cortices and the amygdala follow similar patterns as corticocortical connections, and by analogy suggest pathways underlying the sequence of information processing for emotions.The amygdala and the prefrontal cortex have synergistic roles in regulating purposive behavior [(Schoenbaum et al., 2000;Izquierdo and Murray, 2005); reviewed in (Barbas, 2000;Bechara et al., 2000)]. The amygdala appears to extract the affective significance of stimuli, and the prefrontal cortex guides goal-directed behavior (Damasio, 1994;Petrides, 1996;Roberts and Wallis, 2000;Levy and Goldman-Rakic, 2000;Fuster, 2000;Barbas et al., 2002). Communication between the amygdala and the prefrontal cortex is bidirectional [e.g., (Nauta, 1961;Pandya et al., 1973;Jacobson and Trojanowski, 1975;Aggleton et al., 1980;Porrino et al., e Current address: Department of Molecular Biomedical Sciences, School of Veterinary Medicine, North Carolina State University, Raleigh, NC. * Corresponding author. Dept. Health Sciences, Boston University, 635 Commonwealth Ave. Room 431, Boston, MA 02215. E-mail address: barbas@bu.edu Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is p...