Time-course of cortical networks involved in working memory.

Working memory (WM) is one of the most studied cognitive constructs. Although many neuroimaging studies have identified brain networks involved in WM, the time course of these networks remains unclear. In this paper we use dense-array electroencephalography (dEEG) to capture neural signals during performance of a standard WM task, the n-back task, and a blend of principal components analysis and independent components analysis (PCA/ICA) to statistically identify networks of WM and their time courses. Results reveal a visual cortex centric network, that also includes the posterior cingulate cortex, that is active prior to stimulus onset and that appears to reflect anticipatory, attention-related processes. After stimulus onset, the ventromedial prefrontal cortex, lateral prefrontal prefrontal cortex, and temporal poles become associated with the prestimulus network. This second network appears to reflect executive control processes. Following activation of the second network, the cortices of the temporo-parietal junction with the temporal lobe structures seen in the first and second networks re-engage. This third network appears to reflect activity of the ventral attention network involved in control of attentional reorientation. The results point to important temporal features of network dynamics that integrate multiple subsystems of the ventral attention network with the default mode network in the performance of working memory tasks.

Reentrant processing in intuitive perception.

The process of perception requires not only the brain's receipt of sensory data but also the meaningful organization of that data in relation to the perceptual experience held in memory. Although it typically results in a conscious percept, the process of perception is not fully conscious. Research on the neural substrates of human visual perception has suggested that regions of limbic cortex, including the medial orbital frontal cortex (mOFC), may contribute to intuitive judgments about perceptual events, such as guessing whether an object might be present in a briefly presented fragmented drawing. Examining dense array measures of cortical electrical activity during a modified Waterloo Gestalt Closure Task, results show, as expected, that activity in medial orbital frontal electrical responses (about 250 ms) was associated with intuitive judgments. Activity in the right temporal-parietal-occipital (TPO) region was found to predict mOFC (approximately 150 ms) activity and, in turn, was subsequently influenced by the mOFC at a later time (approximately 300 ms). The initial perception of gist or meaning of a visual stimulus in limbic networks may thus yield reentrant input to the visual areas to influence continued development of the percept. Before perception is completed, the initial representation of gist may support intuitive judgments about the ongoing perceptual process.

Selection of saphenous vein conduit in varicose vein disease.

BACKGROUND: Limbs with varicose veins are difficult to assess as a source of saphenous vein conduit. Anatomic, histologic, and ultrasound studies demonstrate two types of longitudinal veins in the lower extremities. The great saphenous vein is deep to the saphenous fascia. Accessory saphenous veins are superficial to this layer and have thin walls with diminished muscle cells and elastic fiber. Accessory saphenous veins dilate and form varicosities. Segments of great saphenous veins are often suitable as coronary conduits. No studies have assessed the suitability of saphenous veins as coronary artery conduits in patients with varicose vein disease. METHODS: Intraoperative high-resolution ultrasound studies were performed in coronary artery bypass graft procedures to assess lower extremity venous morphology in limbs of 77 patients without known venous disease, in 19 limbs with venous telangiectases, and in 23 limbs with varicose veins. RESULTS: Dilated great saphenous vein segments were identified in 6% of normal limb venous segments compared with 21% of segments in limbs with telangiectases (p = 0.027) and 22% of segments in limbs with varicosities (p = 0.012). The incidence of absent or hypoplastic great saphenous vein segments is increased in limbs with varicosities (35%) compared with normal limbs (21%; p = 0.032). In the calf, at least one great saphenous vein segment suitable for coronary artery bypass grafting is present in 70% of limbs with varicosities and in 89% of limbs with telangiectases. CONCLUSIONS: Ultrasound studies document that varicose veins are limited to accessory saphenous veins. Great saphenous vein conduits, identified by ultrasonography, are available in limbs with varicose vein disease.

Accessory and great saphenous veins as coronary artery bypass conduits.

Anatomic, histologic and ultrasound studies demonstrate two distinct types of longitudinal veins in the lower extremities. The great saphenous vein is deep to the saphenous fascia. Accessory saphenous veins are superficial to this layer, have thin walls with diminished muscle cells and elastic fibers. Vein characteristics may affect long-term graft patency. This study assesses the incidence of accessory saphenous veins in various patient groups and evaluates issues related to their use as coronary conduits. Ultrasound imaging assessed great and accessory saphenous veins in 476 normal limbs, in 42 patients post-saphenous vein harvesting and in 75 patients undergoing coronary revascularization. Accessory saphenous veins are found in 67% of normal subjects. Intraoperative ultrasound identifies accessory saphenous veins in 54% of limbs, most in the proximal calf where 42% of all accessory vein segments are located. Great saphenous vein segments are more common in males while accessory veins are more common in females. In 54 limbs following saphenectomy, patent great saphenous vein segments are demonstrated in 20%, adjacent to incision sites, indicating use of accessory saphenous veins in these patients. Ultrasound studies at two medical centers document the common occurrence of accessory saphenous veins. Additional studies are required to determine the efficacy of utilizing accessory saphenous vein segments for coronary artery bypass conduits.

Optimizing saphenous vein site selection using intraoperative venous duplex ultrasound scanning.

BACKGROUND: Saphenous vein is the most common conduit utilized for coronary artery bypass. However, preoperative noninvasive venous studies to evaluate saphenous vein morphology are not commonly performed due to limited logistical support. A prospective, nonrandomized study was developed to assess the utility of intraoperative saphenous vein duplex ultrasound studies in optimizing saphenous vein site selection. METHODS: Intraoperative saphenous vein duplex scanning was performed in 58 consecutive patients undergoing coronary artery bypass surgery utilizing two-dimensional ultrasound monitoring equipment. Following anesthetic intubation, studies were performed by one of the surgeons. Most scans were completed in less than 8 minutes. RESULTS: Findings demonstrate at least 1 venous abnormality in 31 of 116 (26.7%) above knee saphenous veins and 59 of 116 (50.9%) below knee veins. In 38 of 58 patients (65.5%), duplex ultrasound scanning proved beneficial in surgical site selection. Most abnormalities are related to major branches and bifurcations except in the lower calf where small lumen caliber is the most common abnormal finding. Additional beneficial findings include identifying abnormal vein course, identifying suitable conduit in reoperative procedures and precise localization of vein segments for endoscopic surgery. CONCLUSIONS: Intraoperative saphenous vein duplex scanning is rapidly and easily accomplished with available operating room resources. Study information allows optimal surgical site selection, avoiding unnecessary surgical dissection, time delays, vein wastage and potential for wound complications. Optimizing incision site selection eliminates blind exploration for vein conduit, improves conduit planning, and expedites surgical dissection during endoscopic vein harvest.