In this paper, we describe a novel approach to the study of problem solving involving the detailed analysis of natural scanning eye movements during the "one-touch" Tower-of-London (TOL) task. We showed subjects a series of pictures depicting two arrangements of colored balls in pockets within the upper and lower halves of a computer display. The task was to plan (but not to execute) the shortest movement sequence required to rearrange the balls in one half of the display (the Workspace) to match the arrangement in the opposite half (the Goalspace) and indicate the minimum number of moves required for problem solution. We report that subjects are more likely to look towards the Goalspace in the initial period after picture presentation, but bias gaze towards the Workspace during the middle of trials. Towards the end of a trial, subjects are once again more likely to fixate the Goalspace. This pattern is found regardless of whether the subjects solve problems by rearranging the balls in the lower or upper visual fields, demonstrating that this strategy correlates with discrete phases in problem solving. A second experiment showed that efficient planners direct their gaze selectively towards the problem critical balls in the Workspace. In contrast, individuals who make errors spend more time looking at irrelevant items and are strongly influenced by the movement strategy needed to solve the preceding problem. We conclude that efficient solution of the TOL requires the capacity to generate and flexibly shift between control sets, including those underlying ocular scanning. The role of working memory and the prefrontal cerebral cortex in the task are discussed.
BackgroundUse of blood oxygenation level-dependent cardiovascular magnetic resonance (BOLD-CMR) to assess perfusion in the lower limb has been hampered by poor reproducibility and a failure to reliably detect post-revascularization improvements in patients with critical limb ischemia (CLI).ObjectivesThis study sought to develop BOLD-CMR as an objective, reliable clinical tool for measuring calf muscle perfusion in patients with CLI.MethodsThe calf was imaged at 3-T in young healthy control subjects (n = 12), age-matched control subjects (n = 10), and patients with CLI (n = 34). Signal intensity time curves were generated for each muscle group and curve parameters, including signal reduction during ischemia (SRi) and gradient during reactive hyperemia (Grad). BOLD-CMR was used to assess changes in perfusion following revascularization in 12 CLI patients. Muscle biopsies (n = 28), obtained at the level of BOLD-CMR measurement and from healthy proximal muscle of patients undergoing lower limb amputation (n = 3), were analyzed for capillary-fiber ratio.ResultsThere was good interuser and interscan reproducibility for Grad and SRi (all p < 0.0001). The ischemic limb had lower Grad and SRi compared with the contralateral asymptomatic limb, age-matched control subjects, and young control subjects (p < 0.001 for all comparisons). Successful revascularization resulted in improvement in Grad (p < 0.0001) and SRi (p < 0.0005). There was a significant correlation between capillary-fiber ratio (p < 0.01) in muscle biopsies from amputated limbs and Grad measured pre-operatively at the corresponding level.ConclusionsBOLD-CMR showed promise as a reliable tool for assessing perfusion in the lower limb musculature and merits further investigation in a clinical trial.
Downloaded from Bajwa et al Assessment of Lower Limb Perfusion 837when compared with normal limbs and cannot quantify the severity of perfusion deficit. 12 More recent studies show that LDF is a good discriminator of skin flap perfusion and can predict the likelihood of healing after limb amputation.13 LDI is used in preclinical animal models of limb ischemia (eg, the murine hindlimb ischemia model) for high throughput testing of interventions, including angiogenic therapies. The technique allows longitudinal acquisition of data but is limited by motion artifact, ambient temperature variations, and perfusion measurements, which are weighted toward the larger vessels that are shallow and reside on the surface of the limb musculature. 14 Transcutaneous Oxygen PressureThe measurement of transcutaneous oxygen pressure (TcPo 2 ) is a noninvasive technique that allows longitudinal assessment of local oxygen diffusion from the capillary bed through the skin epidermis and provides an indication of the amount of oxygen delivered to the tissue. TcPo 2 measurements can reflect the severity of PAD, 15 assess ulcer healing potential, 16 and determine the optimal level for limb amputation. 17 A TcPo 2 <20 to 30 mm Hg is consistent with CLI, 2,15,18 and levels <40 mm Hg are associated with poor healing after amputation. 18 Inhalation of 100% oxygen would normally result in TcPo 2 >100 mm Hg, but in patients with CLI the TcPo 2 does not rise >30 mm Hg. 18 Oxygen therapy resulting in <10 mm Hg rise in limb TcPo 2 is associated with an >60% chance of an amputation at that level not healing. 18 A rise of >40 mm Hg in TcPo 2 after limb revascularization is considered significant and increases the chances of tissue healing. 15,18 TcPo 2 measurements vary in different tissues and under different physiological conditions. Barriers to diffusion, such as edema, increased oxygen consumption secondary to inflammation, and vasoconstriction, may lead to falsely low TcPo 2 readings falsely. Readings are also affected by temperature, with a 1°C reduction in the thermistor temperature resulting in a 2% to 3% lower TcPo 2 reading. 18 Indocyanine Green Fluorescence AngiographyThis technique has been used for many years to image retinal vessels, 19 and more recently in the assessment of microanastomoses in surgical flap reconstruction surgery, detection of arteriovenous malformations, and measurement of hepatic blood flow. 20 Indocyanine green is a fluorescent dye that is activated by near-infrared laser light at ≈780 nm. 21 This allows a penetrance of 3 mm from the surface of the skin to assess the subdermal microcirculation. 20,22 A semiquantitative measure of perfusion is obtained by measuring pixel fluorescence intensity in different areas of the image from the limb. Perfusion measured by indocyanine green fluorescence angiography correlates with LDI in the hindlimb ischemia model 23 and predicts the likelihood of amputation healing in man. 21 This technique can identify patients with peripheral arterial occlusions who have extensive collater...
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