323,533 research outputs found
Understanding the rotational vestibular ocular reflex: From differential equations to Laplace transforms
No full textThe description of a biological system using a mathematical model is the most effective means to specify the functioning of a quantitative hypothesis, providing at the same time the ability to make predictions that can be further tested experimentally. The Vestibulo-Ocular Reflex (VOR), and more generally the ocular motor control system has been one of the first biological systems to be modeled mathematically and, thanks to contributions from anatomy, biology, biochemistry and information technology it is now the best understood sensory-motor system in humans. Basic science has made it possible to write the differential equations describing the functioning of the semicircular canals, of the otoliths, and of the ocular motor plant at different scales: from models explaining neurotransmitter behavior, to cell membranes and ionic currents, to individual neurons and entire populations, to those describing muscle contractions and eye movements. The differential equations are frequently represented in terms of Laplace transforms and provide a description of the input-output behavior of the system being considered as a function of frequency. Here we will review the input-output behavior of the rotational VOR to exemplify its mathematical modeling as a linear time-invariant dynamic system being stimulated by head rotations and producing eye movements as an output
Gli avverbi monolessicali di luogo, tempo e maniera nelle lingue indoeuropee antiche. Verso un approccio tipologico
No full textA novel expression cassette for the efficient visual selection of transformed tissues in florists' chrysanthemum (Chrysanthemum morifolium Ramat.).
No full textConstructs carrying visual reporter genes coupled with efficient promoters could facilitate the process of identification and selection of stable transformants in recalcitrant crops. Here, a novel construct utilizing a ribulose-1,5-bisphosphate carboxylase (RbcS) promoter combined with the green fluorescent protein (GFP) reporter gene to initiate very high expression of GFP in florist's chrysanthemum (Chrysanthemum morifolium Ramat.) was described. Based on this expression cassette, a new regeneration protocol using leaf discs as explants was developed for the Agrobacterium-mediated transformation of Chrysanthemum genotype ‘1581’, and a transformation efficiency of 7% was obtained. The expression of two different GFP constructs targeted to either cytosol or plastids was compared in transgenic lines. Both GFP constructs were expressed at such a high level that the green fluorescence dominated red fluorescence in the leaf tissues, allowing easy observation and microdissection of transformed tissues even without a GFP filter. Under normal light, plants with GFP targeted to plastids had a light green phenotype deriving from the high GFP expression. Quantitative reverse transcriptional PCR analysis showed that the plastid targeted construct with intron had significantly higher steady state transcript levels of GFP mRNA. This novel expression cassette may allow direct visual selection of transformed tissues independent of antibiotic selection in a wide range of plant specie
Velocity storage in the human vertical rotational vestibulo-ocular reflex.
No full textHuman horizontal rotational vestibulo-ocular reflex (rVOR) has been extensively investigated: the horizontal semicircular canals sense yaw rotations with high-pass filter dynamics and a time constant (TC) around 5 s, yet the rVOR response shows a longer TC due to a central processing stage, known as velocity storage mechanism (VSM). It is generally assumed that the vertical rVOR behaves similarly to the horizontal one; however, VSM processing of the human vertical rVOR is still to be proven. We investigated the vertical rVOR in eight healthy human subjects using three experimental paradigms: (1) per- and post-rotatory around an earth-vertical axis (ear down rotations, EDR), (2) post-rotatory around an earth-horizontal axis with different stopping positions (static otolith stimulation), (3) per-rotatory around an earth-horizontal axis (dynamic otolith stimulation). We found that the TC of vertical rVOR responses ranged 3-10 s, depending both on gravity and on the direction of rotation. The shortest TC were found in response to post-rotatory earth-horizontal stimulation averaging 3.6 s, while they were prolonged in EDR stimulation, i.e. when the head angular velocity vector is aligned with gravity, with a mean value of about 6.0 s. Overall, the longest TC were observed in per-rotatory earth-horizontal stimulation, averaging 7.8 s. The finding of longer TC in EDR than in post-rotatory earth-horizontal stimulation indicates a role for the VSM in the vertical rVOR, although its contribution appears to be weaker than on the horizontal rVOR and may be directionally asymmetric. The results from per-rotatory earth-horizontal stimulation, instead, imply a role for the otoliths in controlling the duration of the vertical rVOR response. We found no reorientation of the response toward earth horizontal, indicating a difference between human and monkey rVOR
The translational vestibulo ocular reflex (TVOR) in humans: responses to abrupt, high-acceleration stimuli
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Human responses to vestibular and visual stimuli moving in depth: similarities and differences
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Ocular motor responses to abrupt interaural head translation in normal humans
No full textWe characterized the interaural translational vestibulo-ocular reflex (tVOR) in 6 normal humans to brief (∼200 ms), high-acceleration (0.4-1.4g) stimuli, while they fixed targets at 15 or 30 cm. The latency was 19 ± 5 ms at 15-cm and 20 ± 12 ms at 30-cm viewing. The gain was quantified using the ratio of actual to ideal behavior. The median position gain (at time of peak head velocity) was 0.38 and 0.37, and the median velocity gain, 0.52 and 0.62, at 15- and 30-cm viewing, respectively. These results suggest the tVOR scales proportionally at these viewing distances. Likewise, at both viewing distances, peak eye velocity scaled linearly with peak head velocity and gain was independent of peak head acceleration. A saccade commonly occurred in the compensatory direction, with a greater latency (165 vs. 145 ms) and lesser amplitude (1.8 vs. 3.2 deg) at 30- than 15-cm viewing. Even with saccades, the overall gain at the end of head movement was still considerably undercompensatory (medians 0.68 and 0.77 at 15- and 30-cm viewing). Monocular viewing was also assessed at 15-cm viewing. In 4 of 6 subjects, gains were the same as during binocular viewing and scaled closely with vergence angle. In sum the low tVOR gain and scaling of the response with viewing distance and head velocity extend previous results to higher acceleration stimuli. tVOR latency (∼20 ms) was lower than previously reported. Saccades are an integral part of the tVOR, and also scale with viewing distance
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