Saccadic countermanding: a comparison of central and peripheral stop signals

AbstractWe compared the effectiveness of central and peripheral targets in a saccadic countermanding task. Stop-signal reaction times (SSRTs) do not differ significantly for central and peripheral stop signals. Further, when central and peripheral stop signals are presented together, SSRTs behave as expected of independent processes in parallel. A linear rise-to-threshold race model (LATER) with independent go and stop processes describes the behavioural data successfully, predicting not only the latency distribution of saccades that escaped inhibition, but also the probability of successful countermanding. Central and peripheral stop signals appear to act independently and with equal effectiveness

Does the Instantaneous Wave-Free Ratio Approximate the Fractional Flow Reserve?

Objectives This study sought to examine the clinical performance of and theoretical basis for the instantaneous wave-free ratio (iFR) approximation to the fractional flow reserve (FFR). Background Recent work has proposed iFR as a vasodilation-free alternative to FFR for making mechanical revascularization decisions. Its fundamental basis is the assumption that diastolic resting myocardial resistance equals mean hyperemic resistance. Methods Pressure-only and combined pressure-flow clinical data from several centers were studied both empirically and by using pressure-flow physiology. A Monte Carlo simulation was performed by repeatedly selecting random parameters as if drawing from a cohort of hypothetical patients, using the reported ranges of these physiologic variables. Results We aggregated observations of 1,129 patients, including 120 with combined pressure-flow data. Separately, we performed 1,000 Monte Carlo simulations. Clinical data showed that iFR was +0.09 higher than FFR on average, with +/- 0.17 limits of agreement. Diastolic resting resistance was 2.5 +/- 1.0 times higher than mean hyperemic resistance in patients. Without invoking wave mechanics, classic pressure-flow physiology explained clinical observations well, with a coefficient of determination of >0.9. Nearly identical scatter of iFR versus FFR was seen between simulation and patient observations, thereby supporting our model. Conclusions iFR provides both a biased estimate of FFR, on average, and an uncertain estimate of FFR in individual cases. Diastolic resting myocardial resistance does not equal mean hyperemic resistance, thereby contravening the most basic condition on which iFR depends. Fundamental relationships of coronary pressure and flow explain the iFR approximation without invoking wave mechanics. (J Am Coll Cardiol 2013;61:1428-35) (C) 2013 by the American College of Cardiology Foundatio