A practical perspective on how to develop, implement, execute, and reproduce high-resolution respirometry experiments: The physiologist's guide to an Oroboros O2k.

The development of high-resolution respirometry (HRR) has greatly expanded the analytical scope to study mitochondrial respiratory control relative to specific tissue/cell types across various metabolic states. Specifically, the Oroboros Oxygraph 2000 (O2k) is a common tool for measuring rates of mitochondrial respiration and is the focus of this perspective. The O2k platform is amenable for answering numerous bioenergetic questions. However, inherent variability with HRR-derived data, both within and amongst users, can impede progress in bioenergetics research. Therefore, we advocate for several vital considerations when planning and conducting O2k experiments to ultimately enhance transparency and reproducibility across laboratories. In this perspective, we offer guidance for best practices of mitochondrial preparation, protocol selection, and measures to increase reproducibility. The goal of this perspective is to propagate the use of the O2k, enhance reliability and validity for both new and experienced O2k users, and provide a reference for peer reviewers.

Rapid runtime learning by curating small datasets of high-quality items obtained from memory.

We propose the "runtime learning" hypothesis which states that people quickly learn to perform unfamiliar tasks as the tasks arise by using task-relevant instances of concepts stored in memory during mental training. To make learning rapid, the hypothesis claims that only a few class instances are used, but these instances are especially valuable for training. The paper motivates the hypothesis by describing related ideas from the cognitive science and machine learning literatures. Using computer simulation, we show that deep neural networks (DNNs) can learn effectively from small, curated training sets, and that valuable training items tend to lie toward the centers of data item clusters in an abstract feature space. In a series of three behavioral experiments, we show that people can also learn effectively from small, curated training sets. Critically, we find that participant reaction times and fitted drift rates are best accounted for by the confidences of DNNs trained on small datasets of highly valuable items. We conclude that the runtime learning hypothesis is a novel conjecture about the relationship between learning and memory with the potential for explaining a wide variety of cognitive phenomena.

Implications of capacity-limited, generative models for human vision.

Although discriminative deep neural networks are currently dominant in cognitive modeling, we suggest that capacity-limited, generative models are a promising avenue for future work. Generative models tend to learn both local and global features of stimuli and, when properly constrained, can learn componential representations and response biases found in people's behaviors.

High-intensity interval exercise reduces tolerance to a simulated haemorrhagic challenge in heat-stressed individuals.

NEW FINDINGS: What is the central question of this study? In heat-stressed individuals, does high-intensity interval exercise reduce tolerance to a simulated haemorrhagic challenge (lower body negative pressure, LBNP) relative to steady state exercise? What is the main finding and its importance? LBNP tolerance was lower in heat-stressed individuals following high-intensity interval exercise relative to steady state exercise. This was likely owing to the greater cardiovascular strain required to maintain arterial blood pressure prior to and early during LBNP following high-intensity interval exercise. These findings are of importance for individuals working in occupations in which combined heat stress and intense intermittent exercise are common and where the risk of haemorrhagic injury is increased. ABSTRACT: This study investigated whether tolerance to a simulated haemorrhagic challenge (lower body negative pressure, LBNP) was lower in heat-stressed individuals following high-intensity interval exercise relative to steady state exercise. Nine healthy participants completed two trials (Steady State and Interval). Participants cycled continuously at ∼38% (Steady State) or alternating between 10 and ∼88% (Interval) of the maximal power output whilst wearing a hot water perfused suit until core temperatures increased ∼1.4°C. Participants then underwent LBNP to pre-syncope. LBNP tolerance was quantified as cumulative stress index (CSI; mmHg min). Mean skin and core temperatures were elevated in both trials following exercise prior to LBNP (to 38.1 ± 0.6°C and 38.3 ± 0.2°C, respectively, both P < 0.001 relative to baseline) but not different between trials (both P > 0.05). In the Interval trial, heart rate was greater (122 ± 12 beats min-1 ) prior to LBNP, relative to the Steady State trial (107 ± 8 beats min-1 , P < 0.001) while mean arterial pressure was similarly reduced in both trials prior to LBNP (from baseline 89 ± 5 to 77 ± 7 mmHg; P = 0.001) and at pre-syncope (to 62 ± 9 mmHg, P < 0.001). CSI was lower in the Interval trial (280 ± 194 vs. 550 ± 234 mmHg min; P = 0.0085). In heat-stressed individuals, tolerance to a simulated haemorrhagic challenge is reduced following high-intensity interval exercise relative to steady state exercise.

Optimal attentional allocation in the presence of capacity constraints in uncued and cued visual search.

The vision sciences literature contains a large diversity of experimental and theoretical approaches to the study of visual attention. We argue that this diversity arises, at least in part, from the field's inability to unify differing theoretical perspectives. In particular, the field has been hindered by a lack of a principled formal framework for simultaneously thinking about both optimal attentional processing and capacity-limited attentional processing, where capacity is limited in a general, task-independent manner. Here, we supply such a framework based on rate-distortion theory (RDT) and optimal lossy compression. Our approach defines Bayes-optimal performance when an upper limit on information processing rate is imposed. In this article, we compare Bayesian and RDT accounts in both uncued and cued visual search tasks. We start by highlighting a typical shortcoming of unlimited-capacity Bayesian models that is not shared by RDT models, namely, that they often overestimate task performance when information-processing demands are increased. Next, we reexamine data from two cued-search experiments that have previously been modeled as the result of unlimited-capacity Bayesian inference and demonstrate that they can just as easily be explained as the result of optimal lossy compression. To model cued visual search, we introduce the concept of a "conditional communication channel." This simple extension generalizes the lossy-compression framework such that it can, in principle, predict optimal attentional-shift behavior in any kind of perceptual task, even when inputs to the model are raw sensory data such as image pixels. To demonstrate this idea's viability, we compare our idealized model of cued search, which operates on a simplified abstraction of the stimulus, to a deep neural network version that performs approximately optimal lossy compression on the real (pixel-level) experimental stimuli.

The importance of constraints on constraints.

The "resource-rational" approach is ambitious and worthwhile. A shortcoming of the proposed approach is that it fails to constrain what counts as a constraint. As a result, constraints used in different cognitive domains often have nothing in common. We describe an alternative framework that satisfies many of the desiderata of the resource-rational approach, but in a more disciplined manner.

Abstract Representations of Object-Directed Action in the Left Inferior Parietal Lobule.

Prior neuroimaging and neuropsychological research indicates that the left inferior parietal lobule in the human brain is a critical substrate for representing object manipulation knowledge. In the present functional MRI study we used multivoxel pattern analyses to test whether action similarity among objects can be decoded in the inferior parietal lobule independent of the task applied to objects (identification or pantomime) and stimulus format in which stimuli are presented (pictures or printed words). Participants pantomimed the use of objects, cued by printed words, or identified pictures of objects. Classifiers were trained and tested across task (e.g., training data: pantomime; testing data: identification), stimulus format (e.g., training data: word format; testing format: picture) and specific objects (e.g., training data: scissors vs. corkscrew; testing data: pliers vs. screwdriver). The only brain region in which action relations among objects could be decoded across task, stimulus format and objects was the inferior parietal lobule. By contrast, medial aspects of the ventral surface of the left temporal lobe represented object function, albeit not at the same level of abstractness as actions in the inferior parietal lobule. These results suggest compulsory access to abstract action information in the inferior parietal lobe even when simply identifying objects.

Can multisensory training aid visual learning? A computational investigation.

Although real-world environments are often multisensory, visual scientists typically study visual learning in unisensory environments containing visual signals only. Here, we use deep or artificial neural networks to address the question, Can multisensory training aid visual learning? We examine a network's internal representations of objects based on visual signals in two conditions: (a) when the network is initially trained with both visual and haptic signals, and (b) when it is initially trained with visual signals only. Our results demonstrate that a network trained in a visual-haptic environment (in which visual, but not haptic, signals are orientation-dependent) tends to learn visual representations containing useful abstractions, such as the categorical structure of objects, and also learns representations that are less sensitive to imaging parameters, such as viewpoint or orientation, that are irrelevant for object recognition or classification tasks. We conclude that researchers studying perceptual learning in vision-only contexts may be overestimating the difficulties associated with important perceptual learning problems. Although multisensory perception has its own challenges, perceptual learning can become easier when it is considered in a multisensory setting.

Adaptive allocation of human visual working memory capacity during statistical and categorical learning.

Human brains are finite, and thus have bounded capacity. An efficient strategy for a capacity-limited agent is to continuously adapt by dynamically reallocating capacity in a task-dependent manner. Here we study this strategy in the context of visual working memory (VWM). People use their VWM stores to remember visual information over seconds or minutes. However, their memory performances are often error-prone, presumably due to VWM capacity limits. We hypothesize that people attempt to be flexible and robust by strategically reallocating their limited VWM capacity based on two factors: (a) the statistical regularities (e.g., stimulus feature means and variances) of the to-be-remembered items, and (b) the requirements of the task that they are attempting to perform. The latter specifies, for example, which types of errors are costly versus irrelevant for task performance. These hypotheses are formalized within a normative computational modeling framework based on rate-distortion theory, an extension of conventional Bayesian approaches that uses information theory to study rate-limited (or capacity-limited) processes. Using images of plants that are naturalistic and precisely controlled, we carried out two sets of experiments. Experiment 1 found that when a stimulus dimension (the widths of plants' leaves) was assigned a distribution, subjects adapted their VWM performances based on this distribution. Experiment 2 found that when one stimulus dimension (e.g., leaf width) was relevant for distinguishing plant categories but another dimension (leaf angle) was irrelevant, subjects' responses in a memory task became relatively more sensitive to the relevant stimulus dimension. Together, these results illustrate the task-dependent robustness of VWM, thereby highlighting the dependence of memory on learning.

Small reductions in skin temperature after onset of a simulated hemorrhagic challenge improve tolerance in exercise heat-stressed individuals.

We investigated whether small reductions in skin temperature 60 s after the onset of a simulated hemorrhagic challenge would improve tolerance to lower body negative pressure (LBNP) after exercise heat stress. Eleven healthy subjects completed two trials (High and Reduced). Subjects cycled at ~55% maximal oxygen uptake wearing a warm water-perfused suit until core temperatures increased by ~1.2°C before lying supine and undergoing LBNP to presyncope. LBNP tolerance was quantified as cumulative stress index (CSI; product of each LBNP level multiplied by time; mmHg·min). Skin temperature was similarly elevated from baseline before LBNP and remained elevated 60 s after the onset of LBNP in both High (37.72 ± 0.52°C) and Reduced (37.95 ± 0.54°C) trials (both P < 0.0001). At 60%CSI skin temperature remained elevated in the High trial (37.51 ± 0.56°C) but was reduced to 34.97 ± 0.72°C by the water-perfused suit in the Reduced trial ( P < 0.0001 between trials). Cutaneous vascular conductance was not different between trials [High: 1.57 ± 0.43 vs. Reduced: 1.39 ± 0.38 arbitrary units (AU)/mmHg; P = 0.367] before LBNP but decreased to 0.67 ± 0.19 AU/mmHg at 60%CSI in the Reduced trial while remaining unchanged in the High trial ( P = 0.002 between trials). CSI was higher in the Reduced (695 ± 386 mmHg·min) relative to the High (441 ± 290 mmHg·min; P = 0.023) trial. Mean arterial pressure was not different between trials at presyncope (High: 62 ± 10 vs. Reduced: 62 ± 9 mmHg; P = 0.958). Small reductions in skin temperature after the onset of a simulated hemorrhagic challenge improve LBNP tolerance after exercise heat stress. This may have important implications regarding treatment of an exercise heat-stressed individual (e.g., soldier) who has experienced a hemorrhagic injury.

Twenty-eight days of exposure to 3454 m increases mitochondrial volume density in human skeletal muscle.

The role of hypoxia on skeletal muscle mitochondria is controversial. Studies superimposing exercise training on hypoxic exposure demonstrate an increase in skeletal muscle mitochondrial volume density (Mito(VD)) over equivalent normoxic training. In contrast, reductions in both skeletal muscle mass and Mito(VD) have been reported following mountaineering expeditions. These observations may, however, be confounded by negative energy balance, which may obscure the results. Accordingly we sought to examine the effects of high altitude hypoxic exposure on mitochondrial characteristics, with emphasis on Mito(VD), while minimizing changes in energy balance. For this purpose, skeletal muscle biopsies were obtained from nine lowlanders at sea level (Pre) and following 7 and 28 days of exposure to 3454 m. Maximal ergometer power output, whole body weight and composition, leg lean mass and skeletal muscle fibre area all remained unchanged following the altitude exposure. Transmission electron microscopy determined that intermyofibrillar (IMF) Mito(VD) was augmented (P = 0.028) by 11.5 ± 9.2% from Pre (5.05 ± 0.9%) to 28 Days (5.61 ± 0.04%). In contrast, there was no change in subsarcolemmal (SS) Mito(VD). As a result, total Mito(VD) (IMF + SS) was increased (P = 0.031) from 6.20 ± 1.5% at Pre to 6.62 ± 1.4% at 28 Days (7.8 ± 9.3%). At the same time no changes in mass-specific respiratory capacities, mitochondrial protein or antioxidant content were found. This study demonstrates that skeletal muscle Mito(VD) may increase with 28 days acclimation to 3454 m.

Multisensory Part-based Representations of Objects in Human Lateral Occipital Cortex.

The format of high-level object representations in temporal-occipital cortex is a fundamental and as yet unresolved issue. Here we use fMRI to show that human lateral occipital cortex (LOC) encodes novel 3-D objects in a multisensory and part-based format. We show that visual and haptic exploration of objects leads to similar patterns of neural activity in human LOC and that the shared variance between visually and haptically induced patterns of BOLD contrast in LOC reflects the part structure of the objects. We also show that linear classifiers trained on neural data from LOC on a subset of the objects successfully predict a novel object based on its component part structure. These data demonstrate a multisensory code for object representations in LOC that specifies the part structure of objects.

Adaptations of skeletal muscle mitochondria to exercise training.

Mitochondrial volume density (Mito(VD)) is composed of two distinct mitochondrial subpopulations--intermyofibrillar mitochondria (Mito(IMF)) and subsarcolemmal mitochondria (Mito(SS)). With exercise training, Mito(VD) may increase by up to 40% and is, for the most part, related to an increase in Mito(IMF). Exercise-induced adaptations in mitochondrial function depend on the intensity of training and appear to be explained predominately by an increased expression of mitochondrial enzymes that facilitate aerobic metabolism. Although mitochondrial content often increases with training, it seems that mitochondrial adaptations are not needed to facilitate maximal oxygen uptake, whereas such adaptations are of greater importance for endurance capacity.

From Sensory Signals to Modality-Independent Conceptual Representations: A Probabilistic Language of Thought Approach.

People learn modality-independent, conceptual representations from modality-specific sensory signals. Here, we hypothesize that any system that accomplishes this feat will include three components: a representational language for characterizing modality-independent representations, a set of sensory-specific forward models for mapping from modality-independent representations to sensory signals, and an inference algorithm for inverting forward models-that is, an algorithm for using sensory signals to infer modality-independent representations. To evaluate this hypothesis, we instantiate it in the form of a computational model that learns object shape representations from visual and/or haptic signals. The model uses a probabilistic grammar to characterize modality-independent representations of object shape, uses a computer graphics toolkit and a human hand simulator to map from object representations to visual and haptic features, respectively, and uses a Bayesian inference algorithm to infer modality-independent object representations from visual and/or haptic signals. Simulation results show that the model infers identical object representations when an object is viewed, grasped, or both. That is, the model's percepts are modality invariant. We also report the results of an experiment in which different subjects rated the similarity of pairs of objects in different sensory conditions, and show that the model provides a very accurate account of subjects' ratings. Conceptually, this research significantly contributes to our understanding of modality invariance, an important type of perceptual constancy, by demonstrating how modality-independent representations can be acquired and used. Methodologically, it provides an important contribution to cognitive modeling, particularly an emerging probabilistic language-of-thought approach, by showing how symbolic and statistical approaches can be combined in order to understand aspects of human perception.

Haematological rather than skeletal muscle adaptations contribute to the increase in peak oxygen uptake induced by moderate endurance training.

It remains unclear whether improvements in peak oxygen uptake (V̇(O2peak)) following endurance training (ET) are primarily determined by central and/or peripheral adaptations. Herein, we tested the hypothesis that the improvement in V̇(O2peak) following 6 weeks of ET is mainly determined by haematological rather than skeletal muscle adaptations. Sixteen untrained healthy male volunteers (age = 25 ± 4 years, V̇(O2peak) = 3.5 ± 0.5 l min(-1)) underwent supervised ET (6 weeks, 3-4 sessions per week). V̇(O2peak), peak cardiac output (Q̇(peak)), haemoglobin mass (Hb(mass)) and blood volumes were assessed prior to and following ET. Skeletal muscle biopsies were analysed for mitochondrial volume density (Mito(VD)), capillarity, fibre types and respiratory capacity (OXPHOS). After the post-ET assessment, red blood cell volume (RBCV) was re-established at the pre-ET level by phlebotomy and V̇(O2peak) and Q̇(peak) were measured again. We speculated that the contribution of skeletal muscle adaptations to the ET-induced increase in V̇(O2peak) would be revealed when controlling for haematological adaptations. V̇(O2peak) and Q̇(peak) were increased (P < 0.05) following ET (9 ± 8 and 7 ± 6%, respectively) and decreased (P < 0.05) after phlebotomy (-7 ± 7 and -10 ± 7%). RBCV, plasma volume and Hb(mass) all increased (P < 0.05) after ET (8 ± 4, 4 ± 6 and 6 ± 5%). As for skeletal muscle adaptations, capillary-to-fibre ratio and total Mito(VD) increased (P < 0.05) following ET (18 ± 16 and 43 ± 30%), but OXPHOS remained unaltered. Through stepwise multiple regression analysis, Q̇(peak), RBCV and Hb(mass) were found to be independent predictors of V̇(O2peak). In conclusion, the improvement in V̇(O2peak) following 6 weeks of ET is primarily attributed to increases in Q̇(peak) and oxygen-carrying capacity of blood in untrained healthy young subjects.

Phlebotomy eliminates the maximal cardiac output response to six weeks of exercise training.

With this study we tested the hypothesis that 6 wk of endurance training increases maximal cardiac output (Qmax) relatively more by elevating blood volume (BV) than by inducing structural and functional changes within the heart. Nine healthy but untrained volunteers (Vo2max 47 ± 5 ml·min(-1)·kg(-1)) underwent supervised training (60 min; 4 times weekly at 65% Vo2max for 6 wk), and Qmax was determined by inert gas rebreathing during cycle ergometer exercise before and after the training period. After the training period, blood volume (determined in duplicates by CO rebreathing) was reestablished to pretraining values by phlebotomy and Qmax was quantified again. Resting echography revealed no structural heart adaptations as a consequence of the training intervention. After the training period, plasma volume (PV), red blood cell volume (RBCV), and BV increased (P < 0.05) by 147 ± 168 (5 ± 5%), 235 ± 64 (10 ± 3%), and 382 ± 204 ml (7 ± 4%), respectively. Vo2max was augmented (P < 0.05) by 10 ± 7% after the training period and decreased (P < 0.05) by 8 ± 7% with phlebotomy. Concomitantly, Qmax was increased (P < 0.05) from 18.9 ± 2.1 to 20.4 ± 2.3 l/min (9 ± 6%) as a consequence of the training intervention, and after normalization of BV by phlebotomy Qmax returned to pretraining values (18.1 ± 2.5 l/min; 12 ± 5% reversal). Thus the exercise training-induced increase in BV is the main mechanism increasing Qmax after 6 wk of endurance training in previously untrained subjects.

Assessing exogenous carbohydrate intake needed to optimize human endurance performance across sex: insights from modeling runners pursuing a sub-2-h marathon.

Carbohydrate (CHO) availability sustains high metabolic demands during prolonged exercise. The adequacy of current CHO intake recommendations, 30-90 g·h-1 dependent on CHO mixture and tolerability, to support elite marathon performance is unclear. We sought to scrutinize the current upper limit recommendation for exogenous CHO intake to support modeled sub-2-h marathon (S2M) attempts across elite male and female runners. Male and female runners (n = 120 each) were modeled from published literature with reference characteristics necessary to complete a S2M (e.g., body mass and running economy). Completion of a S2M was considered across a range of respiratory exchange rates, with maximal starting skeletal muscle and liver glycogen content predicted for elite male and female runners. Modeled exogenous CHO bioavailability needed for male and female runners were 93 ± 26 and 108 ± 22 g·h-1, respectively (P < 0.0001, d = 0.61). Without exogenous CHO, males were modeled to deplete glycogen in 84 ± 7 min, females in 71 ± 5 min (P < 0.0001, d = 2.21) despite higher estimated CHO oxidation rates in males (5.1 ± 0.5 g·h-1) than females (4.4 ± 0.5 g·h-1; P < 0.0001, d = 1.47). Exogenous CHO intakes ≤ 90 g·h-1 are insufficient for 65% of modeled runners attempting a S2M. Current recommendations to support marathon performance appear inadequate for elite marathon runners but may be more suitable for male runners in pursuit of a S2M (56 of 120) than female runners (28 of 120).NEW & NOTEWORTHY This study scrutinizes the upper limit of exogenous carbohydrate (CHO) recommendations for elite male and female marathoners by modeling sex-specific needs across an extreme metabolic challenge lasting ∼2 h, a sub-2-h marathon. Contemporary nutritional guidelines to optimize marathon performance appear inadequate for most elite marathon runners but appear more appropriate for males over their female counterparts. Future research examining possible benefits of exogenous CHO intakes > 90 g·h-1 should prioritize female athlete study inclusion.

Non-transgenic guinea pig strains exhibit divergent age-related changes in hippocampal mitochondrial respiration.

AIM: Alzheimer's disease (AD) is the most common form of dementia. However, while 150+ animal models of AD exist, drug translation from preclinical models to humans for treatment usually fails. One factor contributing to low translation is likely the absence of neurodegenerative models that also encompass the multi-morbidities of human aging. We previously demonstrated that, in comparison to the PigmEnTed (PET) guinea pig strain which models "typical" brain aging, the Hartley strain develops hallmarks of AD like aging humans. Hartleys also exhibit age-related impairments in cartilage and skeletal muscle. Impaired mitochondrial respiration is one driver of both cellular aging and AD. In humans with cognitive decline, diminished skeletal muscle and brain respiratory control occurs in parallel. We previously reported age-related declines in skeletal muscle mitochondrial respiration in Hartleys. It is unknown if there is concomitant mitochondrial dysfunction in the brain. METHODS: Therefore, we assessed hippocampal mitochondrial respiration in 5- and 12-month Hartley and PET guinea pigs using high-resolution respirometry. RESULTS: At 12 months, PETs had higher complex I supported mitochondrial respiration paralleling their increase in body mass compared to 5 months PETs. Hartleys were also heavier at 12 months compared to 5 months but did not have higher complex I respiration. Compared to 5 months Hartleys, 12 months Hartleys had lower complex I mitochondrial efficiency and compensatory increases in mitochondrial proteins collectively suggesting mitochondrial dysfunction with age. CONCLUSIONS: Therefore, Hartleys might be a relevant model to test promising therapies targeting mitochondria to slow brain aging and AD progression.

Lactate oxidation in human skeletal muscle mitochondria.

Lactate is an important intermediate metabolite in human bioenergetics and is oxidized in many different tissues including the heart, brain, kidney, adipose tissue, liver, and skeletal muscle. The mechanism(s) explaining the metabolism of lactate in these tissues, however, remains unclear. Here, we analyze the ability of skeletal muscle to respire lactate by using an in situ mitochondrial preparation that leaves the native tubular reticulum and subcellular interactions of the organelle unaltered. Skeletal muscle biopsies were obtained from vastus lateralis muscle in 16 human subjects. Samples were chemically permeabilized with saponin, which selectively perforates the sarcolemma and facilitates the loss of cytosolic content without altering mitochondrial membranes, structure, and subcellular interactions. High-resolution respirometry was performed on permeabilized muscle biopsy preparations. By use of four separate and specific substrate titration protocols, the respirometric analysis revealed that mitochondria were capable of oxidizing lactate in the absence of exogenous LDH. The titration of lactate and NAD(+) into the respiration medium stimulated respiration (P ≤ 0.003). The addition of exogenous LDH failed to increase lactate-stimulated respiration (P = 1.0). The results further demonstrate that human skeletal muscle mitochondria cannot directly oxidize lactate within the mitochondrial matrix. Alternately, these data support previous claims that lactate is converted to pyruvate within the mitochondrial intermembrane space with the pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized.

Acute and chronic elevation of erythropoietin in the brain improves exercise performance in mice without inducing erythropoiesis.

Application of recombinant human erythropoietin (rhEpo) improves exercise capacity by stimulating red blood cell production that, in turn, enhances oxygen delivery and utilization. Apart from this, when applied at high doses, rhEpo crosses the blood-brain barrier, triggering protective neuronal effects. Here we show a fundamental new role by which the presence of Epo in the brain augments exercise performance without altering red blood cell production. Two different animal models, the transgenic mouse line Tg21, which constitutively overexpresses human Epo exclusively in the brain without affecting erythropoiesis, and wild-type mice treated with a single high dose of rhEpo, demonstrate an unexpected improvement in maximal exercise performance independent of changes in total hemoglobin mass, as well as in whole blood volume and cardiovascular parameters. This novel finding builds a more complete understanding regarding the central effects of endogenously produced and exogenously applied Epo on exercise performance.