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PubMed1999

Tactile perception in developmental dyslexia: a psychophysical study using gratings.

Multiple sensory abnormalities have been reported in individuals with developmental dyslexia, especially in the visual and auditory systems. We used gratings of alternating ridges and grooves to investigate tactile perception in this disorder using two tasks: spatial acuity-dependent discrimination of grating orientation and discrimination of gratings varying in ridge width. Compared to age-matched normal subjects, dyslexics were significantly impaired on grating orientation discrimination, with mean thresholds that were nearly twice normal. Unlike normal subjects, their performance on this task was slightly but significantly worse on the dominant hand than on the non-dominant hand. Dyslexics also showed a substantial but non-significant trend for impairment on grating ridge width discrimination. A group of subjects with attention deficit disorder did not differ significantly from normal on any of these measures. These findings expand the range of perceptual deficits reported in developmental dyslexia. Possible explanations for the results, including difficulties with temporal processing, abnormal lateral masking or parietal lobe dysfunction are discussed.

Adolescent↗

PubMed2003

Blobs strengthen repetition but weaken symmetry.

The human visual system is more sensitive to symmetry than to repetition. According to the so-called holographic approach [J. Math. Psychol. 35 (1991) 151; Psychol. Rev. 103 (1996) 429; Psychol. Rev. 106 (1999) 622], however, this perceptual difference between symmetry and repetition depends strongly on spatial scaling. This was tested in three experiments, using symmetry and repetition stimuli that consisted of black and white patches, with patch size as the critical variable. In Experiment 1, patch size was increased in the entire pattern, yielding fewer but larger patches (or blobs). This is known to have hardly any effect on symmetry but, as found now, it does have a strengthening effect on repetition. In the second experiment, we increased patch size in subpatterns only, yielding salient blob areas. This again strengthens repetition but, as double-checked in experiment 3, it can weaken symmetry. These results agree with the holographic approach, and enable an integration of computational, algorithmic, and implementational aspects of vision.

Adolescent↗

PubMed1996

The size of the horizontal cell receptive fields adapts to the stimulus in the light adapted goldfish retina.

In this study the dynamic properties of goldfish horizontal cell (HC) receptive fields were evaluated. The size of HC receptive fields increases up to about 60 msec after stimulus onset, and then reduces to a smaller end value. They can therefore not adequately be described by the cable equation. Estimates of the length constant of the HC network based on the sustained responses are about 43% smaller than those based on the initial part of the response. This difference can be accounted for by feedback connections from HCs to cones because negative feedback reduces the receptive field size. The implication is that HCs are strongly coupled when the retina is stimulated more or less homogeneously but that they partly uncouple from the rest of the HC network when they are stimulated differently than the rest of the retina. The HCs thus generate a feedback signal based on the "local" stimulus properties. The size of the HC receptive fields depends on the spatial detail of the stimulus.

Adaptation, Ocular↗

PubMed1999

Neuronal basis of contrast discrimination.

Psychophysical contrast increment thresholds were compared with neuronal responses, inferred from functional magnetic resonance imaging (fMRI) to test the hypothesis that contrast discrimination judgements are limited by neuronal signals in early visual cortical areas. FMRI was used to measure human brain activity as a function of stimulus contrast, in each of several identifiable visual cortical areas. Contrast increment thresholds were measured for the same stimuli across a range of baseline contrasts using a temporal 2AFC paradigm. FMRI responses and psychophysical measurements were compared by assuming that: (1) fMRI responses are proportional to local average neuronal activity; (2) subjects choose the stimulus interval that evoked the greater average neuronal activity; and (3) variability in the observer's psychophysical judgements was due to additive (IID) noise. With these assumptions, FMRI responses in visual areas V1, V2d, V3d and V3A were found to be consistent with the psychophysical judgements, i.e. a contrast increment was detected when the fMRI responses in each of these brain areas increased by a criterion amount. Thus, the pooled activity of large numbers of neurons can reasonably well predict behavioral performance. The data also suggest that contrast gain in early visual cortex depends systematically on spatial frequency.

Contrast Sensitivity↗

PubMed2006

Contribution of feedforward, lateral and feedback connections to the classical receptive field center and extra-classical receptive field surround of primate V1 neurons.

A central question in visual neuroscience is what circuits generate the responses of neurons in the primary visual cortex (V1). V1 neurons respond best to oriented stimuli of optimal size within their receptive field (RF) center. This size tuning is contrast dependent, i.e. a neuron's optimal stimulus size measured at high contrast (the high-contrast summation RF, or hsRF) is smaller than when measured using low-contrast stimuli (the low-contrast summation RF, or lsRF). Responses to stimuli in the RF center are usually suppressed by iso-oriented stimuli in the extra-classical RF surround. Iso-orientation surround suppression is fast and long range, extending well beyond the size of V1 cells' lsRF. Geniculocortical feedforward (FF), V1 lateral and extrastriate feedback (FB) connections to V1 could all contribute to generating the RF center and surround of V1 neurons. Studies on the spatio-temporal properties and functional organization of these connections can help disclose their specific contributions to the responses of V1 cells. These studies, reviewed in this chapter, have shown that FF afferents to V1 integrate signals within the hsRF of V1 cells; V1 lateral connections are commensurate with the size of the lsRF and may, thus, underlie contrast-dependent changes in spatial summation, and modulatory effects arising from the surround region closer to the RF center (the "near" surround). The spatial and temporal properties of lateral connections cannot account for the dimensions and onset latency of modulation arising from more distant regions of the surround (the "far" surround). Inter-areal FB connections to V1, instead, are commensurate with the full spatial range of center and surround responses, and show fast conduction velocity consistent with the short onset latency of modulation arising from the "far" surround. We review data showing that a subset of FB connections terminate in a patchy fashion in V1, and show modular and orientation specificity, consistent with their proposed role in orientation-specific center-surround interactions. We propose specific mechanisms by which each connection type contributes to the RF center and surround of V1 neurons, and implement these hypotheses into a recurrent network model. We show physiological data in support of the model's predictions, revealing that modulation from the "far" surround is not always suppressive, but can be facilitatory under specific stimulus conditions.

Animals↗

PubMed1999

Structural investigation of beta-lactoglobulin gelation in ethanol/water solutions.

The aggregation and gelation properties of beta-lactoglobulin (BLG), a globular protein from milk, was studied in hydro-ethanolic solutions (50/50% (v/v)) at room temperature. The phase state diagrams as a function of pH and ethanol concentration showed that a gel structure appeared after a period ranging from 1 min to 1 week depending on the physico-chemical conditions. The aggregation kinetics, studied by infrared spectroscopy and dynamical rheological measurements, highly depended upon the pH; the process being the fastest at pH 7. Alcohol-induced aggregation of BLG was characterized by the formation of intermolecular hydrogen bonded beta-sheet structures. Small angle neutron scattering indicated that the aggregates structures in the final gels were similar at pH 7, 8 and 9. Through the data obtained at the molecular and macroscopic levels, it can be concluded that the kinetics of gelation were pH dependent while the spatial arrangements of the aggregates were similar in the final structures. The heterogeneous structures formed in hydro-ethanolic gels could be analysed in terms of a phase separation, the syneresis being the final visible state.

Ethanol↗

PubMed2003

How accurate is magnetic resonance imaging of brain function?

Since it was introduced a decade ago, functional magnetic resonance imaging (fMRI) has come to dominate research on the human brain. However, fMRI maps are based on secondary metabolic and hemodynamic events that follow neuronal activity, and not on the electrical activity itself. Therefore, the representation provided by fMRI cannot be assumed a priori to be exact. The accuracy of these maps depends on the spatial extent of the metabolic and hemodynamic changes induced by neuronal activity, and the role played by the vasculature in converting these changes to signals detected by magnetic resonance imaging. Significant progress has been made in both areas, suggesting that it is possible to obtain both spatially accurate and quantitative data on brain function from magnetic resonance methodologies.

Brain↗

PubMed2003

Interpretation of out-diffusion experiments on crystalline rocks using random walk modeling.

Matrix diffusion in saturated rocks with very low permeability is one of the major mechanisms of solute transport. Laboratory out-diffusion experiments on rock samples may provide an estimate of the bulk diffusion coefficient. However, numerous results have shown that this average parameter does not really depict the complex mechanism of diffusion as a function of the internal heterogeneity of crystalline rocks. Two-dimensional images of the porosity distribution in a granite sample were obtained by impregnation with a radioactive resin and autoradiography. Some examples based on these images and synthetic images were used to perform numerical simulations of out-diffusion using two different random walk methods. The simulated shapes of the out-diffusion curves depend on the spatial distribution of the porosity and on the pore connectivity with the border of the sample. Such relations might explain the multiple nested slopes or the convex shapes often observed on real experimental curves.

Diffusion↗

PubMed2001

Paramecium--a model system for studying cellular graviperception.

Experiments under varied gravitational accelerations as well as in density-adjusted media showed that sensation of gravity in protists may be linked to the known principles of mechanosensation. Paramecium, a ciliate with clear graviresponses (gravitaxis and gravikinesis) is an ideal model system to prove this hypothesis since the ciliary activity and thus the swimming behaviour is controlled by the membrane potential. It has also been assumed that the cytoplasmic mass causes a distinct stimulation of the bipolarly distributed mechano-sensitive K+ and Ca2+ ion channels in the plasma membrane in dependence of the spatial orientation of the cell. In order to prove this hypothesis, different channel blockers are currently under investigation. Gadolinium did not inhibit gravitaxis in Paramecium, showing that it does not specifically block gravireceptors. Further studies concentrated on the question of whether second messengers are involved in the gravity signal transduction chain. Exposure to 5 g for up to 10 min led to a significant increase in cAMP.

Animals↗

PubMed2003

Temporal organization of covert motor processes during response selection and preparation.

Onset of a movement-related brain potential (lateralized readiness potential, LRP) was used to divide reaction time (RT) into two intervals: (1) stimulus onset to LRP onset, and (2) LRP onset to onset of the overt response. Effects on these intervals of advance information about the to-be-signaled response and of the mapping between fingers and response buttons were examined. These effects were used to reach conclusions about the organization of response preparation and about the identity of the processes influenced by advance information. In the absence of advance information, response preparation involved two steps. First, two of the four possible response alternatives were prepared, then one of these two was prepared further. Which pair of responses was prepared during the first step depended on the spatial arrangement of the fingers on the buttons, rather than on any common anatomical feature. Advance information about the upcoming response allowed the first step to be performed prior to the response signal, thus removing its contribution to RT. The second step, however, remained unaffected.

Arousal↗

PubMed1997

Rapid measurement of the spatial resolution of colour flow scanners.

An acoustic grid for rapid measurement of the spatial resolution of colour velocity images is described. The grid is composed of alternate widths of acoustic window and acoustic attenuator. Attenuation is provided by the use of a layer of waterproof paper, which is wrapped in close contact with a tube of a flow phantom. The colour image consists of several closely spaced lines. When the grid line spacing is less than the spatial resolution, the colour lines cannot be separately visualised. The use of several grids of different spacing provides a measure of the spatial resolution. It is shown that the spatial resolution is dependent on the position of the focal zone.

Blood Flow Velocity↗

PubMed2002

Unequal interval time-place learning.

In time-place learning tasks food availability depends upon both spatial and temporal variables. For example, food might be first available at location one, then location two, then location three, and finally location four. To date, the duration of food availability at each of the locations have been identical (e.g. for 4 min). The major purpose of the present experiment was to determine if rats can successfully learn a time-place task in which four locations provided food for different durations. Lever 1 intermittently produced food for 6 min, then Lever 2 produced food for 4 min. Lever 3 and 4 provided food for 2 and 8 min, respectively. Rats were able to learn this unequal interval time-place task. However, their behavior on this unequal interval time-place task was not in agreement with Scalar Expectancy Theory/Weber's Law.

Journal Article↗

PubMed2000

Spin echo MRI in the evaluation of hearts with a double outlet right ventricle: usefulness and limitations.

The surgical approach to a double outlet right ventricle (DORV) is dependent on the spatial relationship of the semilunar valves, outflow tracts and ventricular septal defect (VSD). The purpose of the study was to assess the value of MRI for the evaluation of cardiovascular anatomy in patients before and after surgery for DORV. Spin echo MRI was performed in 12 patients with DORV (eight preoperative and four postoperative patients). Thin-section MRI was performed in three orthogonal planes and selected angulated views were obtained. Conventional imaging by color Doppler echocardiography and cine-angiocardiography and surgical findings, when present, served as the reference standards. The results found that the spatial relationship between semilunar valves and VSD was accurately assessed by MRI in eight out of eight preoperative patients. In the four postoperative cases MRI depicted the morphology of both outflow tracts and provided adequate information on their patency. Of the eight preoperative patients, five have undergone corrective surgery and the MRI findings were confirmed. MRI provided additional information to conventional imaging preoperatively in three cases in which the VSD opened into the outlet portion of the DORV, without there being a direct relation to a semilunar valve. In two preoperative cases in which the VSD was directly committed to the aorta, conventional imaging was conclusive. MRI was unable to depict aberrant chordae tendineae in four out of four cases. We conclude that MRI provides accurate additional anatomic information in patients with DORV, which is helpful in presurgical planning as well as during follow-up. Spin echo MRI does not visualize aberrant chordae tendineae.

Adolescent↗

PubMed2003

Mechanical factors influence the expression of endostatin--an inhibitor of angiogenesis--in tendons.

Avascular zones of tendons are predisposed for degenerative changes and spontaneous rupture. Therefore, we analyzed the expression of the endogenous angiogenesis inhibiting factor endostatin in human fetal and adult tendons by immunohistochemical and biochemical methods. Moreover, to elucidate factors involved in the regulation of vascularity, we exposed primary cultures of rat tendon cells to intermittent hydrostatic pressure (0.2 MPa, 0.1 Hz for 24 h), and measured the endostatin content by ELISA and the effect of the conditioned medium to the proliferation of human umbilical vein endothelial cells (HUVEC). In fetal tendons high endostatin levels could be quantified by ELISA whereas low levels were found in adult tissue. In fetal tendons endostatin could also be immunostained in endothelial cells but mainly in fibroblasts. In adult Achilles tendons endostatin immunostaining was restricted to endothelial cells. In the tibialis posterior tendon--as an example for "wrap around"--endostatin immunostaining remained positive in the fibrocartilage adjacent to the medial malleolus. Fibrochondrocytes of the type II collagen positive fibrocartilage were intensively stained with the endostatin antibody. Factor VIII immunostaining showed that this region was largely avascular. Monolayer cultures of tendon cells released measurable amounts of endostatin into their culture supernatants. Application of intermittent hydrostatic pressure increased endostatin expression significantly. The conditioned media of tendon fibroblasts cultivated under intermittent hydrostatic pressure inhibited the proliferation of HUVEC in a dose dependent way. The spatial expression of endostatin in adult gliding tendons suggests that mechanical factors are involved in the regulation of this anti-angiogenic factor. In accordance, tendon cells exposed to intermittent hydrostatic pressure inhibit endothelial cell proliferation via humoral factors and produce endostatin. These findings support the view that the development and maintenance of avascular zones in tendons might be caused by a mechanically induced upregulation of anti-angiogenic factors.

Achilles Tendon↗

PubMed1999

Effects of tissue anisotropy and contrast acoustic properties on myocardial scattering in contrast echocardiography.

In this study we explored the potential effects that tissue anisotropy, in conjunction with the acoustic properties of contrast, may have on quantitative measurements of myocardial perfusion with the use of ultrasonic contrast agents. We used a computer simulation of the parasternal short-axis view, based on previously measured values for the anisotropy of backscatter and attenuation of myocardium, to predict the backscattered energy from 18 specific regions within the heart before and after myocardial contrast perfusion. Results demonstrated a regional variation of contrast enhancement in the short-axis view and variations caused by incremental increases in contrast level for specific myocardial regions. Thus quantitative assessment of myocardial perfusion with contrast echocardiography is influenced by the anisotropic properties of the myocardium, and the resulting postcontrast image will depend on the interaction between tissue properties and contrast acoustic properties. The degree of myocardial enhancement caused by the presence of contrast may depend on the spatial position of the specific region investigated with respect to the transducer and the amount of contrast in the myocardium.

Anisotropy↗

PubMed2002

Multisensory integration in cortex: shedding light on prickly issues.

Interactions between different sensory modalities can be observed in unimodal areas of the cortex, as revealed by recent neuroimaging studies. A new report by Macaluso and colleagues ( [this issue of Neuron]) shows that crossmodal effects of tactile stimulation in visual cortex critically depend on the spatial congruence of multisensory inputs. This work is discussed in relation to neural and computational models of multisensory integration.

Animals↗

PubMed1999

Human skeletal muscle architecture studied in vivo by non-invasive imaging techniques: functional significance and applications.

The internal architecture plays an essential role in determining the functional features of skeletal muscle. Both length-force and force-velocity relationships depend on the spatial arrangement of muscle fibres in skeletal muscle. The degree of muscle pennation determines both the amount of contractile tissue packed along the tendons and fibre length, and is reflected by the force-generating capacity and shortening velocity of the muscle and by the elastic properties of the muscle-tendon complex. Until recently, knowledge on human muscle architecture was based on measurements performed on cadavers, whose muscle fibres were often shrunk by the preserving medium and by age. With the introduction of non-invasive imaging techniques, it has become possible to study muscle architecture in vivo at rest and the changes thereof upon contraction. This paper discusses the applications of these techniques, namely ultrasonography and nuclear magnetic resonance imaging, and their relevance in physiology and biomechanics.

Animals↗

PubMed2003

Nitrogen lone pair interactions in organic molecules: a photoelectron spectroscopic study.

The electronic structure of several cyclic, saturated and unsaturated amines and imines has been investigated by UV photoelectron spectroscopy (UPS). The analysis of spectra has been performed with DFT and OVGF calculations and comparison with the UPS spectra of related compounds. The extent and type of nitrogen lone pair interactions is discussed because nitrogen lone pairs are the most important functional groups present in these molecules. The magnitude of interactions was found to depend on the spatial orientation and rigidity of mutual positions of the lone pairs, rather than on their spatial distance.

Chemistry, Physical↗