In rabbit reticulocytes there is formed 4–16 times as much 14CO2 from tracer amounts of [U‐14C]glutamine than from [U‐14C]glutamate. 14CO2 formation from substrate amounts of [U‐14C]glutamine and [U‐14C]glutamate is concentration dependent according to a Michaelis‐Menten relation. The apparent Km is 2.0 mM for glutamine and 40 mM for glutamate. The maximal value of the relative 14CO2 formation (14CO2 formed as a percentage of O2 consumed) reaches 80% for both glutamine and glutamate. Under physiological conditions, i.e. at its average plasma concentration, glutamine furnishes 15–50% of the respiratory CO2 formed by the reticulocyte. It is the most important exogenous substrate. The kinetic data are consistent with the assumption that the CO2 formation from glutamine proceeds via glutamate. The large quantitative difference between glutamine and glutamate may be explained by the vastly greater permeation of glutamine as compared with that of glutamate.
When we imagine an object and when we actually see that object, similar brain regions become active. Yet, the time course of neurocognitive mechanisms that support imagery is still largely unknown. The current view holds that imagery does not share early perceptual mechanisms, but starts with high-level visual representations. However, evidence of early shared mechanisms is difficult to obtain because imagery and perception tasks typically differ in visual input. We therefore tracked electrophysiological brain responses while fully controlling visual input, (1) comparing imagery and perception of objects with varying amounts of associated knowledge, and (2) comparing the time courses of successful and incomplete imagery. Imagery and perception were similarly influenced by knowledge already at early stages, revealing shared mechanisms during low-level visual processing. It follows that imagery is not merely perception in reverse; instead, both are active and constructive processes, based on shared mechanisms starting at surprisingly early stages. Keywords: mental imagery, early visual processing, event-related potentials, semantic knowledge, P1 component MENTAL IMAGERY: TIME COURSE, COGNITIVE MECHANISMS 3 Time course and shared neurocognitive mechanisms of mental imagery and visual perception A growing body of research suggests that seeing something with the mind's eyemental imagery-may not be all that different from seeing something with one's physical eyes. Indeed, imagery and perception recruit overlapping neural circuits, including primary visual areas 1-8 , and the vividness of imagination correlates with the similarity of brain activities accompanying imagery and perception 9 . Predictive processing accounts posit that perception arises from hierarchical Bayesian predictions-essentially imaginations-that are constrained by bottom-up sensory input 10-14 .This theoretical framework is neurally plausible 15-20 and supported by evidence that even early stages of perception are subject to top-down influences 15,[21][22][23][24][25][26][27][28][29][30][31] . This suggests that initial aspects of imagery could be fast enough to generate early top-down effects.This suggestion contrasts with alternative accounts assuming that perception first runs through a strictly hierarchical succession of increasingly complex visual representations, with early stages mainly driven by bottom-up sensory processes. At later stages, recurrent feedback from higher-level brain areas is assumed to enable stabilization of visual representations and, eventually, conscious access 32,33 . Based on this account of perception, recent work has mapped out how visual imagery could follow a reverse hierarchy of activation compared to perception 6,7,[34][35][36] . Under these assumptions, imagery would not rely on early perceptual mechanisms like feature processing but start relatively late, with entire visual representations that bring several levels of the visual hierarchy into concert 6,7,35 . In support of this idea, Dijkstra, et al. 35 found ...
I.It is shown by means of experiments on intact cells with 14C-labelled substrates that the first step in the oxidative metabolism of glutamate in rabbit-reticulocytes is a transamination with oxaloacetate to form aspartate. 2.The aspartate is metabolized further in such a manner that CO, is formed from all C-atoms. With tracer amounts of glutamate no accumulation of aspartate occurs, in contrast to isolated mitochondria. The mechanism of the utilization of aspartate appears to be a decarboxylation of oxaloacetate to pyruvate, which is the main reaction of oxaloacetate under conditions of loading with glutamate.3. On account of the minute contribution of the amino group of glutamate to the ammonia formation, which in itself is smaller than expected from the CO, formation, an acceptor for the amino group of amino acids is postulated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.