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Temporal Delays (temporal + delay)
Selected AbstractsAnalysis and use of FMRI response delaysHUMAN BRAIN MAPPING, Issue 2 2001Ziad S. Saad Abstract In this study, we implemented a new method for measuring the temporal delay of functional magnetic resonance imaging (fMRI) responses and then estimated the statistical distribution of response delays evoked by visual stimuli (checkered annuli) within and across voxels in human visual cortex. We assessed delay variability among different cortical sites and between parenchyma and blood vessels. Overall, 81% of all responsive voxels showed activation in phase with the stimulus while the remaining voxels showed antiphase, suppressive responses. Mean delays for activated and suppressed voxels were not significantly different (P < 0.001). Cortical flat maps showed that the pattern of activated and suppressed voxels was dynamically induced and depended on stimulus size. Mean delays for blood vessels were 0.7,2.4 sec longer than for parenchyma (P < 0.01). However, both parenchyma and blood vessels produced responses with long delays. We developed a model to identify and quantify different components contributing to variability in the empirical delay measurements. Within-voxel changes in delay over time were fully accounted for by the effects of empirically measured fMRI noise with virtually no measurable variability associated with the stimulus-induced response itself. Across voxels, as much as 47% of the delay variance was also the result of fMRI noise, with the remaining variance reflecting fixed differences in response delay among brain sites. In all cases, the contribution of fMRI noise to the delay variance depended on the noise power at the stimulus frequency. White noise models significantly underestimated the fMRI noise effects. Hum. Brain Mapping 13:74,93, 2001. © 2001 Wiley-Liss, Inc. [source] Changing climate and historic-woodland structure interact to control species diversity of the ,Lobarion' epiphyte community in ScotlandJOURNAL OF VEGETATION SCIENCE, Issue 5 2007Christopher J. Ellis Abstract Question: How will changing climate and habitat structure interact to control the species diversity of lichen epiphytes? Location: Scotland. Method: Species richness (=diversity) of the epiphyte lichen community known as Lobarion (named after Lobaria pulmonaria) was quantified for 94 Populus tremula stands across Scotland, and compared in a predictive model to seven climate variables and eight measures of woodland structure. An optimum model was selected and used to project Lobarion diversity over the geographic range of the study area, based on IPCC climate change scenarios and hypothetical shifts in woodland structure. Results: Species diversity of the Lobarion community was best explained by three climate variables: (1) average annual temperature; (2) autumn and winter precipitation; in combination with (3) historic-woodland extent. Projections indicate a positive effect of predicted climate change on Lobarion diversity, consistent with the physiological traits of cyanobac-terial lichens comprising the Lobarion. However, the general response to climate is modified significantly by the effect on diversity of historic-woodland extent. Conclusions: Historic-woodland extent may exert an important control over local climate, as well as impacting upon the metapopulation dynamics of species in the Lobarion. In particular, a temporal delay in the response of Lobarion species to changed woodland structure is critical to our understanding of future climate change effects. Future Lobarion diversity (e.g. in the 2050s) may depend upon the interaction of contemporary climate (e.g. 2050s climate) and historic habitat structure (e.g. 1950s woodland extent). This is supported by previous observations for an extinction debt amongst lichen epiphytes, but suggests an extension of simple climate-response models is necessary, before their wider application to lichen epiphyte diversity. [source] Paclitaxel pharmacodynamics: application of a mechanism-based neutropenia modelBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 6 2001Gerald J. Fetterly Abstract Antineoplastic agents exert adverse effects that impact both dose and scheduling of drug administration. Our objective was to develop a quantitative relationship between paclitaxel (taxol) exposure and pharmacodynamic endpoints, such as neutropenia or body weight loss. Paclitaxel in liposomes or Cremophor EL was administered to rats at doses of 20 or 40 mg/kg. Body weight and absolute neutrophil count were determined daily. The decrease in body weight was greater for paclitaxel in Cremophor EL than for liposomal paclitaxel, but hematological toxicity was similar. The hematological data was fit using a pharmacodynamic model to investigate the temporal delay between drug exposure and neutropenia. From the model, the lifespan of neutrophils (TN), of surviving precursor cells in bone marrow (TP), and a killing rate constant (K) were determined. The values of TN, TP, and K for liposomal paclitaxel were 95 h, 82 h, and 0.735 (,M h),1, respectively, and for paclitaxel in Cremophor EL, 86 h, 78 h, and 0.475 (,M h),1, respectively. Simulations of various doses indicated a dependency of the neutropenia time course on paclitaxel exposure. The entire time course of changes in neutrophil count is more informative than a single measurement if myelosuppression is prolonged and at a level associated with increased incidence of clinical adverse effects. Copyright © 2001 John Wiley & Sons, Ltd. [source] Subjective neuronal coding of reward: temporal value discounting and riskEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 12 2010Wolfram Schultz Abstract A key question in the neurobiology of reward relates to the nature of coding. Rewards are objects that are advantageous or necessary for the survival of individuals in a variety of environmental situations. Thus reward appears to depend on the individual and its environment. The question arises whether neuronal systems in humans and monkeys code reward in subjective terms, objective terms or both. The present review addresses this issue by dealing with two important reward processes, namely the individual discounting of reward value across temporal delays, and the processing of information about risky rewards that depends on individual risk attitudes. The subjective value of rewards decreases with the temporal distance to the reward. In experiments using neurophysiology and brain imaging, dopamine neurons and striatal systems discount reward value across temporal delays of a few seconds, despite unchanged objective reward value, suggesting subjective value coding. The subjective values of risky outcomes depend on the risk attitude of individual decision makers; these values decrease for risk-avoiders and increase for risk-seekers. The signal for risk and the signal for the value of risky reward covary with individual risk attitudes in regions of the human prefrontal cortex, suggesting subjective rather than objective coding of risk and risky value. These data demonstrate that important parameters of reward are coded in a subjective manner in key reward structures of the brain. However, these data do not rule out that other neurons or brain structures may code reward according to its objective value and risk. [source] Mechanisms of time-based figure,ground segregationEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 10 2003Farid I. Kandil Abstract Figure,ground segregation can rely on purely temporal information, that is, on short temporal delays between positional changes of elements in figure and ground (Kandil, F.I. & Fahle, M. (2001) Eur. J. Neurosci., 13, 2004,2008). Here, we investigate the underlying mechanisms by measuring temporal segregation thresholds for various kinds of motion cues. Segregation can rely on monocular first-order motion (based on luminance modulation) and second-order motion cues (contrast modulation) with a high temporal resolution of ,,20 ms. The mechanism can also use isoluminant motion with a reduced temporal resolution of 60 ms. Figure,ground segregation can be achieved even at presentation frequencies too high for human subjects to inspect successive frames individually. In contrast, when stimuli are presented dichoptically, i.e. separately to both eyes, subjects are unable to perceive any segregation, irrespective of temporal frequency. We propose that segregation in these displays is detected by a mechanism consisting of at least two stages. On the first level, standard motion or flicker detectors signal local positional changes (flips). On the second level, a segregation mechanism combines the local activities of the low-level detectors with high temporal precision. Our findings suggest that the segregation mechanism can rely on monocular detectors but not on binocular mechanisms. Moreover, the results oppose the idea that segregation in these displays is achieved by motion detectors of a higher order (motion-from-motion), but favour mechanisms sensitive to short temporal delays even without activation of higher-order motion detectors. [source] Role of videofluorography in assessing functional abnormalities in patients of head and neck cancer treated with chemoradiotherapyASIA-PACIFIC JOURNAL OF CLINICAL ONCOLOGY, Issue 4 2009Punita LAL Abstract Aim: The major toxicity following treatment for head neck cancer is swallowing dysfunction which can be easily assessed by videofluorography (VFG), allowing documentation of the site and extent of abnormality thereby facilitating directed management. Methods: Between October 2003 and January 2007, 56 patients with locally advanced head and neck cancer were treated by an accelerated radiotherapy schedule with concurrent weekly cisplatin chemotherapy. Three months following treatment, these patients were locally disease free clinically, but complained of varying degrees of dysphagia and were subjected to a VFG evaluation. Results: This group comprised 52 men and four women with a median age of 56 years. The primary site distribution was: oral cavity (9), oropharynx (22), larynx (19), hypopharynx (5) and unknown primary (1). Swallowing function abnormalities in the form of structural displacement and temporal delays were documented and recorded as weakness of the tongue musculature (n = 6), palatal kink (n = 8), premature leak into the oropharynx (n = 20), impaired hyoid elevation (n = 23), impaired epiglottic tilt (n = 26), unilateral pharyngeal wall impairment (n = 16), residuum in vallecula or pyriform fossa (n = 30), aspiration in trachea (n = 29) and loss of nasopharyngeal seal (n = 7). Multiple abnormalities of different sub-sites were seen in each patient. Conclusion: VFG can document dysmotility disorders of upper aero-digestive tract like dysfunction of the base of tongue, larynx and pharyngeal musculature leading to stasis of the bolus and vallecular residuum, epiglottis dysmotility resulting in silent aspirations, and inadequate nasopharyngeal seal leading to nasal regurgitation. A clinical correlation alongwith quantification of VFG findings is required. [source] | |||||||||||||