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Quantitative Account (quantitative + account)

Distribution by Scientific Domains
Life Sciences50%

Selected Abstracts

Computational motor control: feedback and accuracy

EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 4 2008
Emmanuel Guigon
Abstract Speed/accuracy trade-off is a ubiquitous phenomenon in motor behaviour, which has been ascribed to the presence of signal-dependent noise (SDN) in motor commands. Although this explanation can provide a quantitative account of many aspects of motor variability, including Fitts' law, the fact that this law is frequently violated, e.g. during the acquisition of new motor skills, remains unexplained. Here, we describe a principled approach to the influence of noise on motor behaviour, in which motor variability results from the interplay between sensory and motor execution noises in an optimal feedback-controlled system. In this framework, we first show that Fitts' law arises due to signal-dependent motor noise (SDNm) when sensory (proprioceptive) noise is low, e.g. under visual feedback. Then we show that the terminal variability of non-visually guided movement can be explained by the presence of signal-dependent proprioceptive noise. Finally, we show that movement accuracy can be controlled by opposite changes in signal-dependent sensory (SDNs) and SDNm, a phenomenon that could be ascribed to muscular co-contraction. As the model also explains kinematics, kinetics, muscular and neural characteristics of reaching movements, it provides a unified framework to address motor variability. [source]

Dynamics of a Transgene Expression in Acute Rat Brain Slices Transfected with Adenoviral Vectors

EXPERIMENTAL PHYSIOLOGY, Issue 4 2003
C. E. L. Stokes
We present a quantitative account of the expression dynamics of a transgene (enhanced green fluorescent protein, EGFP) in acute brain slices transfected with an adenoviral vector (AVV) under control of the human cytomegalovirus (HCMV) promoter. Micromolar concentrations of EGFP could be detected in brainstem and hippocampal slices as early as 7 h after in vitro transfection with a viral titre of 4.4 × 109 plaque-forming units (pfu) ml,1. Although initially EGFP appeared mainly in glia, it could be detected in neurones with longer incubation times of 10-12 h. However, fluorescence was never detected within some populations of neurones, such as hippocampal pyramidal cells, or within the hypoglossal motor nucleus. The density of cells expressing EGFP peaked at 10 h and then decreased, possibly suggesting that high concentrations of EGFP are toxic. The age of the animal significantly affected the speed of EGFP accumulation: after 10 h of incubation in 30-day-old rats only 4.88 ± 0.51 cells/10 000 ,m2 were fluorescent compared to 7.28 ± 0.39 cells/10 000 ,m2 in 12-day-old rats (P < 0.05). HCMV promoter-driven transgene expression depended on the activity of protein kinase A, and was depressed with a cAMP/protein kinase A antagonist (20 ,M Rp-cAMPS; P < 0.0005). This indicates that expression of HCMV-driven constructs is likely to be skewed towards cellular populations where cAMP-dependent signalling pathways are active. We conclude that acute transfection of brain slices with AVVs within hours causes EGFP expression in micromolar concentrations and that such transfected cells may remain viable for use in physiological experiments. [source]

Neural mechanisms of chromatic and achromatic vision

COLOR RESEARCH & APPLICATION, Issue 6 2008
Arne Valberg
Abstract Building upon electrophysiological recordings from the lateral geniculate nucleus (LGN) of the macaque monkey, we describe a model for neural processing of color and brightness/lightness information that starts in the cone receptors and continues in the opponent cells of the retina, LGN, and visual cortex. The excitation of the three cone types to direct stimulation by light is modified in accordance with a hyperbolic response function before providing inputs to retinal ganglion cells. Using weighted differences of such cone outputs, we simulate the responses of common types of opponent ganglion and geniculate cells to light modulation along the chromatic and luminance dimensions. Extrapolating the results of the simulation, we suggest a way in which the brain might combine inputs from the geniculate to obtain correlates of chromatic and achromatic color vision and of brightness/lightness perception. In particular, we demonstrate for the first time how combinations of "L,M" and "M,L" parvocellular ON- and OFF-opponent-cells may lead to a quantitative account of brightness and blackness scaling. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 433,443, 2008 [source]

An Activation-Based Model of Sentence Processing as Skilled Memory Retrieval

COGNITIVE SCIENCE - A MULTIDISCIPLINARY JOURNAL, Issue 3 2005
Richard L. Lewis
Abstract We present a detailed process theory of the moment-by-moment working-memory retrievals and associated control structure that subserve sentence comprehension. The theory is derived from the application of independently motivated principles of memory and cognitive skill to the specialized task of sentence parsing. The resulting theory construes sentence processing as a series of skilled associative memory retrievals modulated by similarity-based interference and fluctuating activation. The cognitive principles are formalized in computational form in the Adaptive Control of Thought,Rational (ACT,R) architecture, and our process model is realized in ACT,R. We present the results of 6 sets of simulations: 5 simulation sets provide quantitative accounts of the effects of length and structural interference on both unambiguous and garden-path structures. A final simulation set provides a graded taxonomy of double center embeddings ranging from relatively easy to extremely difficult. The explanation of center-embedding difficulty is a novel one that derives from the model' complete reliance on discriminating retrieval cues in the absence of an explicit representation of serial order information. All fits were obtained with only 1 free scaling parameter fixed across the simulations; all other parameters were ACT,R defaults. The modeling results support the hypothesis that fluctuating activation and similarity-based interference are the key factors shaping working memory in sentence processing. We contrast the theory and empirical predictions with several related accounts of sentence-processing complexity. [source]