Visual representations are essential for communicating ideas in the science classroom; however, the design of such representations is not always beneficial for learners. This paper presents instructional design considerations providing empirical evidence and integrating theoretical concepts related to cognitive load. Learners have a limited working memory, and instructional representations should be designed with the goal of reducing unnecessary cognitive load. However, cognitive architecture alone is not the only factor to be considered; individual differences, especially prior knowledge, are critical in determining what impact a visual representation will have on learners' cognitive structures and processes. Prior knowledge can determine the ease with which learners can perceive and interpret visual representations in working memory. Although a long tradition of research has compared experts and novices, more research is necessary to fully explore the expertnovice continuum and maximize the potential of visual representations.

Neuronal cultures in vitro readily oxidized both D-[ 14 C]glucose and L-[14C]lactate to 14 CO2, whereas astroglial cultures oxidized both substrates sparingly and metabolized glucose predominantly to lactate and released it into the medium. [ 14 C]Glucose oxidation to 14 CO2 varied inversely with unlabeled lactate concentration in the medium, particularly in neurons, and increased progressively with decreasing lactate concentration. Adding unlabeled glucose to the medium inhibited [ 14 C]lactate oxidation to 14 CO2 only in astroglia but not in neurons, indicating a kinetic preference in neurons for oxidation of extracellular lactate over intracellular pyruvate͞ lactate produced by glycolysis. Protein kinase-catalyzed phosphorylation inactivates pyruvate dehydrogenase (PDH), which regulates pyruvate entry into the tricarboxylic acid cycle. Dichloroacetate inhibits this kinase, thus enhancing PDH activity. In vitro dichloroacetate stimulated glucose and lactate oxidation to CO2 and reduced lactate release mainly in astroglia, indicating that limitations in glucose and lactate oxidation by astroglia may be due to a greater balance of PDH toward the inactive form. To assess the significance of astroglial export of lactate to neurons in vivo, we attempted to diminish this traffic in rats by administering dichloroacetate (50 mg͞kg) intravenously to stimulate astroglial lactate oxidation and then examined the effects on baseline and functionally activated local cerebral glucose utilization (lCMRglc). Dichloroacetate raised baseline lCMRglc throughout the brain and decreased the percent increases in lCMRglc evoked by functional activation. These studies provide evidence in support of the compartmentalization of glucose metabolism between astroglia and neurons but indicate that the compartmentalization may be neither complete nor entirely obligatory.G lucose is an essential and normally almost exclusive substrate for cerebral energy metabolism (1). As in other tissues, it is metabolized in brain in two sequential pathways, first to pyruvate͞lactate by glycolysis in cytosol, followed by oxidation in mitochondria to CO 2 and H 2 O. It was recently proposed that the glycolytic and oxidative components of glucose and glycogen metabolism are compartmentalized not only between cytosol and mitochondria but also between astroglia and neurons, i.e., glucose and glycogen metabolism in astroglia to lactate, which is then exported to neurons where it is oxidized to provide the ATP needed for neuronal function (2, 3). Arguments in support of this hypothesis are: (i) capillaries in brain are largely enveloped by astroglial processes that present a barrier to the transport of glucose from blood to neurons; (ii) glycogen in brain is confined almost entirely to astrocytes; (iii) astrocytes in culture readily metabolize glucose to lactate and release it into the medium (4, 5); and (iv) glutamate, the most prevalent excitatory neurotransmitter in brain, stimulates aerobic glycolysis in cultured astrocytes (6, 7). Much of the evidence suppo...

Cerebral auditory areas were delineated in the awake, passively listening, rhesus monkey by comparing the rates of glucose utilization in an intact hemisphere and in an acoustically isolated contralateral hemisphere of the same animal. The auditory system defined in this way occupied large portions of cerebral tissue, an extent probably second only to that of the visual system. Cortically, the activated areas included the entire superior temporal gyrus and large portions of the parietal, prefrontal, and limbic lobes. Several auditory areas overlapped with previously identified visual areas, suggesting that the auditory system, like the visual system, contains separate pathways for processing stimulus quality, location, and motion.

Functional brain mapping based on changes in local cerebral blood flow (lCBF) or glucose utilization (lCMRglc) induced by functional activation is generally carried out in animals under anesthesia, usually ␣-chloralose because of its lesser effects on cardiovascular, respiratory, and reflex functions. Results of studies on the role of nitric oxide (NO) in the mechanism of functional activation of lCBF have differed in unanesthetized and anesthetized animals. NO synthase inhibition markedly attenuates or eliminates the lCBF responses in anesthetized animals but not in unanesthetized animals. The present study examines in conscious rats and rats anesthetized with ␣-chloralose the effects of vibrissal stimulation on lCMRglc and lCBF in the whisker-to-barrel cortex pathway and on the effects of NO synthase inhibition with N G -nitro-L-arginine methyl ester (L-NAME) on the magnitude of the responses. Anesthesia markedly reduced the lCBF and lCMRglc responses in the ventral posteromedial thalamic nucleus and barrel cortex but not in the spinal and principal trigeminal nuclei. L-NAME did not alter the lCBF responses in any of the structures of the pathway in the unanesthetized rats and also not in the trigeminal nuclei of the anesthetized rats. In the thalamus and sensory cortex of the anesthetized rats, where the lCBF responses to stimulation had already been drastically diminished by the anesthesia, L-NAME treatment resulted in loss of statistically significant activation of lCBF by vibrissal stimulation. These results indicate that NO does not mediate functional activation of lCBF under physiological conditions. whisker-to-barrel cortex pathway ͉ cerebral glucose utilization ͉ deoxy[ 14 C]glucose ͉ functional brain imaging ͉ iodo[ 14 C]antipyrine N euronal functional activation is normally associated with increases in local cerebral glucose utilization (lCMR glc ) and blood flow (lCBF) in anatomic units of the activated neural pathways. These associations are now widely exploited to map regions of the brain involved in specific neural and cognitive processes. The mechanisms underlying the functional activation of lCMR glc are reasonably well understood. Glucose utilization is increased by functional activation in direct proportion to the increases in spike frequency in the afferent inputs to the activated areas, and the increases are localized in neuropil and not perikarya (1, 2). The increases in lCMR glc appear to result mainly from activation of Na ϩ ,K ϩ -ATPase activity (2, 3), needed to restore ionic gradients in the neuronal elements degraded by the spike activity. Neuropil contains not only axonal and dendritic processes but also astroglial processes, and glutamate stimulates glucose utilization in astroglia, also due in part to activation of Na ϩ ,K ϩ -ATPase activity by coupled uptake of Na ϩ ions with the glutamate released during functional activation (4, 5). Glucose utilization is also increased to support the ATPdependent conversion to glutamine of the glutamate taken up by the astroglia (6).The mechan...