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Ben M Harvey - One of the best experts on this subject based on the ideXlab platform.

  • radial asymmetries in population receptive field size and cortical Magnification Factor in early visual cortex
    NeuroImage, 2018
    Co-Authors: Maria De Fátima C. Silva, Jan W Brascamp, Miguel Castelobranco, Ben M Harvey, Sónia Ferreira, Serge O Dumoulin
    Abstract:

    Abstract Human visual cortex does not represent the whole visual field with the same detail. Changes in receptive field size, population receptive field (pRF) size and cortical Magnification Factor (CMF) with eccentricity are well established, and associated with changes in visual acuity with eccentricity. Visual acuity also changes across polar angle. However, it remains unclear how RF size, pRF size and CMF change across polar angle. Here, we examine differences in pRF size and CMF across polar angle in V1, V2 and V3 using pRF modeling of human fMRI data. In these visual field maps, we find smaller pRFs and larger CMFs in horizontal (left and right) than vertical (upper and lower) visual field quadrants. Differences increase with eccentricity, approximately in proportion to average pRF size and CMF. Similarly, we find larger CMFs in the lower than upper quadrant, and again differences increase with eccentricity. However, pRF size differences between lower and upper quadrants change direction with eccentricity. Finally, we find slightly smaller pRFs in the left than right quadrants of V2 and V3, though this difference is very small, and we find no differences in V1 and no differences in CMF. Moreover, differences in pRF size and CMF vary gradually with polar angle and are not limited to the meridians or visual field map discontinuities. PRF size and CMF differences do not consistently follow patterns of cortical curvature, despite the link between cortical curvature and polar angle in V1. Thus, the early human visual cortex has a radially asymmetric representation of the visual field. These asymmetries may underlie consistent reports of asymmetries in perceptual abilities.

  • the relationship between cortical Magnification Factor and population receptive field size in human visual cortex constancies in cortical architecture
    The Journal of Neuroscience, 2011
    Co-Authors: Ben M Harvey, Serge O Dumoulin
    Abstract:

    Receptive field (RF) sizes and cortical Magnification Factor (CMF) are fundamental organization properties of the visual cortex. At increasing visual eccentricity, RF sizes increase and CMF decreases. A relationship between RF size and CMF suggests constancies in cortical architecture, as their product, the cortical representation of an RF (point image), may be constant. Previous animal neurophysiology studies of this question yield conflicting results. Here, we use fMRI to determine the relationship between the population RF (pRF) and CMF in humans. In average and individual data, the product of CMF and pRF size, the population point image, is near constant, decreasing slightly with eccentricity in V1. Interhemisphere and subject variations in CMF, pRF size, and V1 surface area are correlated, and the population point image varies less than these properties. These results suggest a V1 cortical processing architecture of approximately constant size between humans. Up the visual hierarchy, to V2, V3, hV4, and LO1, the population point image decreases with eccentricity, and both the absolute values and rate of change increase. PRF sizes increase between visual areas and with eccentricity, but when expressed in V1 cortical surface area (i.e., corticocortical pRFs), they are constant across eccentricity in V2/V3. Thus, V2/V3, and to some degree hV4, sample from a constant extent of V1. This may explain population point image changes in later areas. Consequently, the constant Factor determining pRF size may not be the relationship to the local CMF, but rather pRF sizes and CMFs in visual areas from which the pRF samples.

Myung-hyun Kim - One of the best experts on this subject based on the ideXlab platform.

  • Fatigue Life Estimation for HFMI-Treated Weldments Considering Weld Toe Magnification Factors
    Journal of Offshore Mechanics and Arctic Engineering, 2020
    Co-Authors: Dong Yub Kim, Myung-hyun Kim
    Abstract:

    Abstract High-frequency mechanical impact (HFMI) post-treatment is a proven method to improve the fatigue life of welded structures. The positive effects of HFMI treatment are influenced by the weld toe geometries and residual stresses. This study investigates the effect of geometric and mechanical improvements by HFMI treatment on fatigue strength with explicit consideration of the weld toe Magnification Factor. General fatigue life estimation method is based on experiment data, and residual stress may be considered in addition. In terms of HFMI-treated structures, geometric improvement also affects the fatigue life. Thus, a more efficient method is suggested by considering the weld toe Magnification Factor to assess the effects of HFMI treatment. First, the weld toe Magnification Factor in HFMI-treated conditions is calculated to consider the geometrical effect of HFMI treatment at the weld toe region. Second, a stress ratio model is introduced to consider the compressive residual stress by HFMI treatment based on the Paris equation. The results were validated by comparing them with the estimated fatigue life from previous studies on HFMI-treated welded specimens.

  • Fatigue Life Estimation for HFMI Treated Weldments Considering Weld Toe Magnification Factor
    Volume 4: Materials Technology, 2019
    Co-Authors: Dong Yub Kim, Myung-hyun Kim
    Abstract:

    Abstract HFMI (High-Frequency Mechanical Impact) post-treatment is a statistically proven method to significantly improve fatigue life of welded structures. Positive effects of improved fatigue life by the HFMI treatment are attributed to the weld toe geometry and the compressive residual stress. This study investigates the effect of geometric and mechanical effects by HFMI treatment on fatigue life with an explicit consideration of the weld Magnification Factor Mk. The conventional fatigue life estimation method to assess the effect of HFMI treatment by an experiment and analysis involves significant expense and time. In this regard, this study suggests an improved method by using Mk weld Magnification Factor and the stress ratio to assess the effect of HFMI post-treatment in more efficient manner. First step of this method is to calculate Mk weld Magnification Factor in HFMI treated condition to consider the geometrical effect by HFMI treatment at weld toe region. Second, the stress ratio model is introduced to consider the compressive residual stress by HFMI treatment based on Paris equation. The result is obtained by Paris-Erdogan equation for a S355 material. The result from this study is validated by a comparison with a previous fatigue test of HFMI treated welded specimen.

Serge O Dumoulin - One of the best experts on this subject based on the ideXlab platform.

  • radial asymmetries in population receptive field size and cortical Magnification Factor in early visual cortex
    NeuroImage, 2018
    Co-Authors: Maria De Fátima C. Silva, Jan W Brascamp, Miguel Castelobranco, Ben M Harvey, Sónia Ferreira, Serge O Dumoulin
    Abstract:

    Abstract Human visual cortex does not represent the whole visual field with the same detail. Changes in receptive field size, population receptive field (pRF) size and cortical Magnification Factor (CMF) with eccentricity are well established, and associated with changes in visual acuity with eccentricity. Visual acuity also changes across polar angle. However, it remains unclear how RF size, pRF size and CMF change across polar angle. Here, we examine differences in pRF size and CMF across polar angle in V1, V2 and V3 using pRF modeling of human fMRI data. In these visual field maps, we find smaller pRFs and larger CMFs in horizontal (left and right) than vertical (upper and lower) visual field quadrants. Differences increase with eccentricity, approximately in proportion to average pRF size and CMF. Similarly, we find larger CMFs in the lower than upper quadrant, and again differences increase with eccentricity. However, pRF size differences between lower and upper quadrants change direction with eccentricity. Finally, we find slightly smaller pRFs in the left than right quadrants of V2 and V3, though this difference is very small, and we find no differences in V1 and no differences in CMF. Moreover, differences in pRF size and CMF vary gradually with polar angle and are not limited to the meridians or visual field map discontinuities. PRF size and CMF differences do not consistently follow patterns of cortical curvature, despite the link between cortical curvature and polar angle in V1. Thus, the early human visual cortex has a radially asymmetric representation of the visual field. These asymmetries may underlie consistent reports of asymmetries in perceptual abilities.

  • the relationship between cortical Magnification Factor and population receptive field size in human visual cortex constancies in cortical architecture
    The Journal of Neuroscience, 2011
    Co-Authors: Ben M Harvey, Serge O Dumoulin
    Abstract:

    Receptive field (RF) sizes and cortical Magnification Factor (CMF) are fundamental organization properties of the visual cortex. At increasing visual eccentricity, RF sizes increase and CMF decreases. A relationship between RF size and CMF suggests constancies in cortical architecture, as their product, the cortical representation of an RF (point image), may be constant. Previous animal neurophysiology studies of this question yield conflicting results. Here, we use fMRI to determine the relationship between the population RF (pRF) and CMF in humans. In average and individual data, the product of CMF and pRF size, the population point image, is near constant, decreasing slightly with eccentricity in V1. Interhemisphere and subject variations in CMF, pRF size, and V1 surface area are correlated, and the population point image varies less than these properties. These results suggest a V1 cortical processing architecture of approximately constant size between humans. Up the visual hierarchy, to V2, V3, hV4, and LO1, the population point image decreases with eccentricity, and both the absolute values and rate of change increase. PRF sizes increase between visual areas and with eccentricity, but when expressed in V1 cortical surface area (i.e., corticocortical pRFs), they are constant across eccentricity in V2/V3. Thus, V2/V3, and to some degree hV4, sample from a constant extent of V1. This may explain population point image changes in later areas. Consequently, the constant Factor determining pRF size may not be the relationship to the local CMF, but rather pRF sizes and CMFs in visual areas from which the pRF samples.

T. Brandt - One of the best experts on this subject based on the ideXlab platform.

  • dependence of visual stabilization of postural sway on the cortical Magnification Factor of restricted visual fields
    Experimental Brain Research, 1994
    Co-Authors: Anne Straube, Stephan Krafczyk, Walter Paulus, T. Brandt
    Abstract:

    Monocular visual stabilization of fore-aft and lateral body sway was tested posturographically in normal subjects (wearing visual field blinds) as a function of visual field size and location of the visual field on the central or peripheral retina. Body sway applied to a force- measuring platform is less with central (foveal) vision when central and peripheral visual fields have the same area. If, however, the peripheral field size is corrected by the cortical Magnification Factor of the retina in the primary cortex, body sway is stabilized by the peripheral retina to the same extent. Thus, there is no functional specialization of central and peripheral retina with respect to balance control. Visual stabilization of upright stance is a function of field size and cortical representation of the retina. The central and the peripheral retina have different thresholds to detect motion; this was surprisingly not reflected in measurements of normal fore-aft and lateral body sway.

Jan W Brascamp - One of the best experts on this subject based on the ideXlab platform.

  • radial asymmetries in population receptive field size and cortical Magnification Factor in early visual cortex
    NeuroImage, 2018
    Co-Authors: Maria De Fátima C. Silva, Jan W Brascamp, Miguel Castelobranco, Ben M Harvey, Sónia Ferreira, Serge O Dumoulin
    Abstract:

    Abstract Human visual cortex does not represent the whole visual field with the same detail. Changes in receptive field size, population receptive field (pRF) size and cortical Magnification Factor (CMF) with eccentricity are well established, and associated with changes in visual acuity with eccentricity. Visual acuity also changes across polar angle. However, it remains unclear how RF size, pRF size and CMF change across polar angle. Here, we examine differences in pRF size and CMF across polar angle in V1, V2 and V3 using pRF modeling of human fMRI data. In these visual field maps, we find smaller pRFs and larger CMFs in horizontal (left and right) than vertical (upper and lower) visual field quadrants. Differences increase with eccentricity, approximately in proportion to average pRF size and CMF. Similarly, we find larger CMFs in the lower than upper quadrant, and again differences increase with eccentricity. However, pRF size differences between lower and upper quadrants change direction with eccentricity. Finally, we find slightly smaller pRFs in the left than right quadrants of V2 and V3, though this difference is very small, and we find no differences in V1 and no differences in CMF. Moreover, differences in pRF size and CMF vary gradually with polar angle and are not limited to the meridians or visual field map discontinuities. PRF size and CMF differences do not consistently follow patterns of cortical curvature, despite the link between cortical curvature and polar angle in V1. Thus, the early human visual cortex has a radially asymmetric representation of the visual field. These asymmetries may underlie consistent reports of asymmetries in perceptual abilities.