Publications (48)169.11 Total impact
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Article: Dips and bumps: On Bloch's law and the Broca-Sulzer phenomenon.
Proceedings of the National Academy of Sciences 03/2013; · 9.68 Impact Factor -
Article: A simpler structure for local spatial channels revealed by sustained perifoveal stimuli.
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ABSTRACT: A new evaluation of the local structure of spatial channels with sustained stimuli in perifoveal retina employs the masking sensitivity approach to minimize analytic assumptions. The stimuli were designed to address the range of channel tunings of the predominantly sustained response system in the near periphery. Under these conditions, the range of identifiable channels spanned a narrow range of spatial frequencies, from roughly 2-8 cpd at 2° eccentricity to 1-4 cpd at 8° eccentricity. The analysis showed that there are no sustained channels tuned below 2 cpd for the central visual field. This two-octave range of channel tuning is much narrower than is conventionally assumed. For local sustained stimuli, human peripheral spatial processing therefore appears to be based on a simpler channel structure than is often supposed.Journal of Vision 01/2013; 13(1). · 3.38 Impact Factor -
Article: Depth structure from asymmetric shading supports face discrimination.
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ABSTRACT: To examine the effect of illumination direction on the ability of observers to discriminate between faces, we manipulated the direction of illumination on scanned 3D face models. In order to dissociate the surface reflectance and illumination components of front-view face images, we introduce a symmetry algorithm that can separate the symmetric and asymmetric components of the face in both low and high spatial frequency bands. Based on this approach, hybrid faces stimuli were constructed with different combinations of symmetric and asymmetric spatial content. Discrimination results with these images showed that asymmetric illumination information biased face perception toward the structure of the shading component, while the symmetric illumination information had little, if any, effect. Measures of perceived depth showed that this property increased systematically with the asymmetric but not the symmetric low spatial frequency component. Together, these results suggest that (1) the asymmetric 3D shading information dramatically affects both the perceived facial information and the perceived depth of the facial structure; and (2) these effects both increase as the illumination direction is shifted to the side. Thus, our results support the hypothesis that face processing has a strong 3D component.PLoS ONE 01/2013; 8(2):e55865. · 4.09 Impact Factor -
Article: Brain mapping: the (un)folding of striate cortex.
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ABSTRACT: Neurons in the primary visual cortex of mammals form an orderly representation of the visual field. A recent study shows that the cortical folding pattern of the human brain accurately predicts not only the extent of this area, but also the location of cells that represent different points of visual space, leading to further considerations of the cortical mapping principles operating in related species.Current biology: CB 12/2012; 22(24):R1051-3. · 10.99 Impact Factor -
Article: Analysis of human vergence dynamics.
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ABSTRACT: Disparity vergence is commonly viewed as being controlled by at least two mechanisms, an open-loop vergence-specific burst mechanism analogous to the ballistic drive of saccades, and a closed-loop feedback mechanism controlled by the disparity error. We show that human vergence dynamics for disparity jumps of a large textured field have a typical time course consistent with predominant control by the open-loop vergence-specific burst mechanism, although various subgroups of the population show radically different vergence behaviors. Some individuals show markedly slow divergence responses, others slow convergence responses, others slow responses in both vergence directions, implying that the two vergence directions have separate control mechanisms. The faster time courses usually had time-symmetric velocity waveforms implying open-loop burst control, while the slow response waveforms were usually time-asymmetric implying closed-loop feedback control. A further type of behavior seen in a distinct subpopulation was a compound anomalous divergence response consisting of an initial convergence movement followed by a large corrective divergence movement with time courses implying closed-loop feedback control. For slow responses to disparity steps, the closed-loop response exhibited pronounced oscillations in the velocity trace, implying the involvement of a sampled-data system with a rate of about three samples/s. This analysis of the variety of human vergence responses thus contributes substantially to the understanding of the oculomotor control mechanisms underlying the generation of vergence movements.Journal of Vision 01/2012; 12(11). · 3.38 Impact Factor -
Article: Collinear facilitation over space and depth.
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ABSTRACT: The detection threshold of a Gabor target can be reduced by the presence of collinear flanking Gabors but is disrupted when the target and the flankers have different disparity. Here, we further investigated whether it is the depth or surface difference between the target and the flanker that causes the abolition of collinear facilitation. The target and the flankers were 1.6 cycle per degree vertical Gabor patches with a separation of three wavelength units between them. There were six viewing conditions: target and flankers were set (A) in the same frontoparallel plane in a collinear configuration, (B) at different disparities but embedded in the same slanted plane, (C) at different disparities in different frontoparallel planes (flankers occupied at the same depth), (D) at different disparities in different frontoparallel planes (flankers occupied at different depth), (E) in the same frontoparallel plane in a noncollinear configuration, and (F) at the same disparity but locally slanted. We measured the target contrast detection threshold with and without the flankers present with a temporal 2AFC paradigm with the Ψ staircase method. Strong collinear facilitation was observed when the target and the flankers were either in the same frontoparallel plane or embedded in the same slanted surface even though the target and the flankers were at different disparities. Our results suggest that it is the difference in surface assignment, not the difference in disparity per se, that causes the disruption of collinear facilitation.Journal of Vision 01/2012; 12(2). · 3.38 Impact Factor -
Article: The role of the visual arts in the enhancing the learning process.
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ABSTRACT: With all the wealth of scientific activities, there remains a certain stigma associated with careers in science, as a result of the inevitable concentration on narrow specializations that are inaccessible to general understanding. Enhancement of the process of scientific learning remains a challenge, particularly in the school setting. While direct explanation seems the best approach to expedite learning any specific subject, it is well known that the ability to deeply absorb facts and concepts is greatly enhanced by placing them in a broader context of relevance to the issues of everyday life and to the larger goals of improvement of the quality of life and advancement to a more evolved society as a whole. If the sciences can be associated with areas of artistic endeavor, they may be viewed as more accessible and favorable topics of study. There is consequently an urgent need for research in the relationship between learning and experience in the arts because both art education and scientific literacy remain at an inadequate level even in economically advanced countries. The focus of this review is the concept that inspiration is an integral aspect of the artistic experience, both for the artist and for the viewer of the artwork. As an integrative response, inspiration involves not only higher cortical circuitry but its integration with the deep brain structures such as limbic system and medial frontal structures, which are understood to mediate the experience of emotions, motivational rewards, and the appreciation of the esthetic values of the impinging stimuli. In this sense, inspiration can turn almost any occupation in life into an avocation, a source of satisfaction in achieving life goals. Conversely, when inspiration is lacking, the motivation to learn, adapt, and prosper is impeded. Thus, inspiration may be viewed as a potent aspect of human experience in linking art and science.Frontiers in Human Neuroscience 01/2012; 6:8. · 2.34 Impact Factor -
Article: Estimating neural signal dynamics in the human brain.
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ABSTRACT: Although brain imaging methods are highly effective for localizing the effects of neural activation throughout the human brain in terms of the blood oxygenation level dependent (BOLD) response, there is currently no way to estimate the underlying neural signal dynamics in generating the BOLD response in each local activation region (except for processes slower than the BOLD time course). Knowledge of the neural signal is critical if spatial mapping is to progress to the analysis of dynamic information flow through the cortical networks as the brain performs its tasks. We introduce an analytic approach that provides a new level of conceptualization and specificity in the study of brain processing by non-invasive methods. This technique allows us to use brain imaging methods to determine the dynamics of local neural population responses to their native temporal resolution throughout the human brain, with relatively narrow confidence intervals on many response properties. The ability to characterize local neural dynamics in the human brain represents a significant enhancement of brain imaging capabilities, with potential applications ranging from general cognitive studies to assessment of neuropathologies.Frontiers in Systems Neuroscience 01/2011; 5:33. -
Article: Modeling magnification and anisotropy in the primate foveal confluence.
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ABSTRACT: A basic organizational principle of the primate visual system is that it maps the visual environment repeatedly and retinotopically onto cortex. Simple algebraic models can be used to describe the projection from visual space to cortical space not only for V1, but also for the complex of areas V1, V2 and V3. Typically a conformal (angle-preserving) projection ensuring local isotropy is regarded as ideal and primate visual cortex is often regarded as an approximation of this ideal. However, empirical data show systematic deviations from this ideal that are especially relevant in the foveal projection. The aims of this study were to map the nature of anisotropy predicted by existing models, to investigate the optimization targets faced by different types of retino-cortical maps, and finally to propose a novel map that better models empirical data than other candidates. The retino-cortical map can be optimized towards a space-conserving homogenous representation or a quasi-conformal mapping. The latter would require a significantly enlarged representation of specific parts of the cortical maps. In particular it would require significant enlargement of parafoveal V2 and V3 which is not supported by empirical data. Further, the recently published principal layout of the foveal singularity cannot be explained by existing models. We suggest a new model that accurately describes foveal data, minimizing cortical surface area in the periphery but suggesting that local isotropy dominates the most foveal part at the expense of additional cortical surface. The foveal confluence is an important example of the detailed trade-offs between the compromises required for the mapping of environmental space to a complex of neighboring cortical areas. Our models demonstrate that the organization follows clear morphogenetic principles that are essential for our understanding of foveal vision in daily life.PLoS Computational Biology 01/2010; 6(1):e1000651. · 5.22 Impact Factor -
Article: Symmetry: modeling the effects of masking noise, axial cueing and salience.
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ABSTRACT: Symmetry detection is an interesting probe of pattern processing because it requires the matching of novel patterns without the benefit of prior recognition. However, there is evidence that prior knowledge of the axis location plays an important role in symmetry detection. We investigated how the prior information about the symmetry axis affects symmetry detection under noise-masking conditions. The target stimuli were random-dot displays structured to be symmetric about vertical, horizontal, or diagonal axes and viewed through eight apertures (1.2 degrees diameter) evenly distributed around a 6 degrees diameter circle. The information about axis orientation was manipulated by (1) cueing of axis orientation before the trial and (2) varying axis salience by including or excluding the axis region within the noise apertures. The percentage of correct detection of the symmetry was measured at for a range of both target and masking noise densities. The threshold vs. noise density function was flat at low noise density and increased with a slope of 0.75-0.8 beyond a critical density. Axis cueing reduced the target threshold 2-4 fold at all noise densities while axis salience had an effect only at high noise density. Our results are inconsistent with an ideal observer or signal-to-noise account of symmetry detection but can be explained by a multiple-channel model is which the response in each channel is the ratio between the nonlinear transform of the responses of sets of early symmetry detectors and the sum of external and intrinsic sources of noise.PLoS ONE 01/2010; 5(4):e9840. · 4.09 Impact Factor -
Article: An algebra for the analysis of object encoding.
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ABSTRACT: The encoding of the objects from the world around us is one of the major topics of cognitive psychology, yet the principles of object coding in the human brain remain unresolved. Beyond referring to the particular features commonly associated with objects, our ability to categorize and discuss objects in detailed linguistic propositions implies that we have access to generic concepts of each object category with well-specified boundaries between them. Consideration of the nature of generic object concepts reveals that they must have the structure of a probabilistic list array specifying the Bayesian prior on all possible features that the object can possess, together with mutual covariance matrices among the features. Generic object concepts must also be largely context independent for propositions to have communicable meaning. Although, there is good evidence for local feature processing in the occipital lobe and specific responses for a few basic object categories in the posterior temporal lobe, the encoding of the generic object concepts remains obscure. We analyze the conceptual underpinnings of the study of object encoding, draw some necessary clarifications in relation to its modality-specific and amodal aspects, and propose an analytic algebra with specific reference to functional Magnetic Resonance Imaging approaches to the issue of how generic (amodal) object concepts are encoded in the human brain.NeuroImage 12/2009; 50(3):1243-50. · 5.89 Impact Factor -
Article: The foveal confluence in human visual cortex.
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ABSTRACT: The human visual system devotes a significant proportion of its resources to a very small part of the visual field, the fovea. Foveal vision is crucial for natural behavior and many tasks in daily life such as reading or fine motor control. Despite its significant size, this part of cortex is rarely investigated and the limited data have resulted in competing models of the layout of the foveal confluence in primate species. Specifically, how V2 and V3 converge at the central fovea is the subject of debate in primates and has remained "terra incognita" in humans. Using high-resolution fMRI (1.2 x 1.2 x 1.2 mm(3)) and carefully designed visual stimuli, we sought to accurately map the human foveal confluence and hence disambiguate the competing theories. We find that V1, V2, and V3 are separable right into the center of the foveal confluence, and V1 ends as a rounded wedge with an affine mapping of the foveal singularity. The adjacent V2 and, in contrast to current concepts from macaque monkey, also V3 maps form continuous bands (approximately 5 mm wide) around the tip of V1. This mapping results in a highly anisotropic representation of the visual field in these areas. Unexpectedly, for the centermost 0.75 degrees, the cortical representations for both V2 and V3 are larger than that of V1, indicating that more neuronal processing power is dedicated to second-level analysis in this small but important part of the visual field.Journal of Neuroscience 08/2009; 29(28):9050-8. · 7.11 Impact Factor -
Article: Excitatory and inhibitory interaction fields of flankers revealed by contrast-masking functions.
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ABSTRACT: To study spatial interactions corresponding to the non-classical receptive field organization for human vision, we used a dual-masking paradigm to measure how target contrast discrimination can be affected by the relative location of the flankers. The observers' task was to detect a 4 cycle/deg vertical Gabor superimposed on a matching Gabor pedestal in the presence of vertical Gabor flankers. The flankers were either (i) collinear with the target and varying in distance or (ii) at a fixed distance from the target but with varying in location relative to the vertical axis. Compared with the no-flanker condition, the collinear flankers decreased target threshold at low pedestal contrasts (facilitation) and increased threshold at high contrasts (suppression). The low contrast facilitation increased with distance up to 4 wavelengths and decreased beyond that. Both facilitative and suppressive flanker effects were greatest at the collinear location and decreased monotonically as flanker location deviated from the collinear axis. These flanker effects are modeled with our sensitivity modulation model, which suggests that the flanker effects are multiplicative terms applied to both the excitatory and inhibitory terms of a divisive inhibition response function. The model parameters show that the facilitative flanker effect is narrowly tuned in space. The data are not compatible with a model of additive normalization by the pedestal contrast or with the uncertainty model.Journal of Vision 02/2008; 8(4):10.1-14. · 3.38 Impact Factor -
Article: Stereomotion processing in the human occipital cortex.
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ABSTRACT: Although a high proportion of the motion selective cells in primate motion area, MT, are disparity-selective, there is no convincing evidence for cells in this area specific to stereomotion-in-depth and the neural basis of stereomotion processing remains obscure. With functional magnetic resonance imaging (fMRI), we identify an occipito-temporal region activated by cyclopean stereomotion-in-depth stimulation, centered anterior to the human motion complex hMT+. This discovery suggests a reconceptualization of the architecture of the motion complex to incorporate the processing of motion in the third dimension.NeuroImage 12/2007; 38(2):293-305. · 5.89 Impact Factor -
Article: Face configuration processing in the human brain: the role of symmetry.
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ABSTRACT: Symmetry is an important cue in face perception. We manipulated symmetry and other configurational variables to study their role in face processing in the human brain. We employed 2 types of symmetry: image symmetry (where one part of the image is defined as the mirrored transform of the other part about an axis) and object symmetry (where the spatial relationships among the image components are interpreted as parts of a symmetric 3-dimensional object). We compared blood oxygenation level dependent responses in healthy human observers for upright front-view faces with responses to different symmetry-controlled images. The cortical areas activated by the face images, relative to Fourier-matched scrambled images, were the fusiform (FFA) and occipital (OFA) face areas, the middle occipital gyri (MOG), and areas around the superior temporal and intraoccipital sulci (IOS). Contrasting faces and their image-symmetric scrambled versions showed a similar activation pattern except in the right OFA, suggesting an involvement in facial symmetry processing. The upright versus inverted faces (with the same image symmetry but unfamiliar object identity) showed robust differential activation in the FFA, OFA, MOG, IOS, and precuneus. The response to frontal-view versus 3/4-view faces (having the same object symmetry but disrupted image symmetry) showed little differential activation in the FFA or the OFA but strong responses in the MOG and IOS, suggesting that face processing in the FFA and the OFA is holistic and viewpoint invariant.Cerebral Cortex 07/2007; 17(6):1423-32. · 6.54 Impact Factor -
Article: Two-dimensional mapping of the central and parafoveal visual field to human visual cortex.
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ABSTRACT: Primate visual cortex contains a set of maps of visual space. These maps are fundamental to early visual processing, yet their form is not fully understood in humans. This is especially true for the central and most important part of the visual field--the fovea. We used functional magnetic resonance imaging (fMRI) to measure the mapping geometry of human V1 and V2 down to 0.5 degrees of eccentricity. By applying automated atlas fitting procedures to parametrize and average retinotopic measurements of eight brains, we provide a reference standard for the two-dimensional geometry of human early visual cortex of unprecedented precision and analyze this high-quality mean dataset with respect to the 2-dimensional cortical magnification morphometry. The analysis indicates that 1) area V1 has meridional isotropy in areal projection: equal areas of visual space are mapped to equal areas of cortex at any given eccentricity. 2) V1 has a systematic pattern of local anisotropies: cortical magnification varies between isopolar and isoeccentricity lines, and 3) the shape of V1 deviates systematically from the complex-log model, the fit of which is particularly poor close to the fovea. We therefore propose that human V1 be fitted by models based on an equal-area principle of its two-dimensional magnification. 4) V2 is elongated by a factor of 2 in eccentricity direction relative to V1 and has significantly more local anisotropy. We propose that V2 has systematic intrinsic curvature, but V1 is intrinsically flat.Journal of Neurophysiology 07/2007; 97(6):4284-95. · 3.32 Impact Factor -
Article: Some principles of spatial organization in art.
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ABSTRACT: Rules of composition in paintings form a rich probe into the principles of perceptual processing that have been discussed for centuries. These principles can be studied by controlled scientific experiments, but an alternative approach is to use the art works themselves as a database for direct analysis. This paper focuses on the analysis of composition in relation to the canvas frame. An underlying principle is the compositional pyramid rising from the bottom of the frame to a center of consciousness high on the midline, which also finds its expression in the configuration of portrait paintings. The analyses presented reveal a dominant positioning principle for one eye in a portrait to lie on the vertical axis with an unbiased accuracy of the order of +/-5%. Analysis of the vertical location shows that the dominant height is at or above the Golden Section level on the vertical axis. In general, the layout of the portrait follows the principle of the compositional pyramid, with a center of consciousness at its apex, but there are many other compositional principles at work in the corpus of portraits in general. Analysis of the portraits of particular artists reveals that special features of their work must be considered in order to identify those that do and do not conform to the eye-centering principle.Spatial Vision 02/2007; 20(6):509-30. · 1.04 Impact Factor -
Article: Crowding: a neuroanalytic approach.
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ABSTRACT: The mechanisms underlying crowding are analyzed in terms of explicit neural processes mediating its perceptual characteristics as originally described by W. Korte (1923). A full understanding of crowding in letter recognition requires a detailed conceptualization of the process of recognition among large numbers of alternatives. The observed masking properties suggest the operation of recursive inhibition from V3 to V1 as a component of the crowding effect. The plausibility of six accounts of the neural basis of crowding (the template matching, feature integrator, attentional feature conjunction, propositional enumeration, attentional tracking, and relaxation network concepts) is then assessed in relation to the task of encoding the spatial structure of the letter forms. We conclude that the relaxation network approach is the most plausible hypothesis to account for the full-spectrum letter recognition performance.Journal of Vision 02/2007; 7(2):16.1-9. · 3.38 Impact Factor -
Article: Evidence for elongated receptive field structure for mechanisms subserving stereopsis.
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ABSTRACT: To study the spatial extent and shape of the binocular disparity mechanisms subserving depth perception, we employ the spatial summation paradigm of contrast threshold for front/back depth discrimination at a fixed binocular disparity. The stimuli were Gabor patches with disparity set at either 4 or 8 arcmin and spatial frequency set at an optimal value of 4 cy/deg. Contrast threshold was measured as a function of length and width of the Gabor patches to determine the aspect ratio of greatest efficiency. The space constant of the Gaussian envelope varied between 0.0375 degrees and 0.9 degrees in either vertical or horizontal directions, or both simultaneously. For vertical elongation of the Gabor patches, discrimination sensitivity improved by 4-6 dB for a doubling of the length of the Gabor patches, then reduced more slowly as the length further increased. However, extending the Gabor patches horizontally across cycles produced little or no sensitivity improvement. Instead, discrimination performance collapsed in a fashion that is incompatible with many models of disparity processing. The results imply that the main mechanisms subserving stereoscopic depth discrimination are vertically elongated for vertical-bar Gabors and encounter special difficulties integrating horizontal disparity information. Disparity discrimination sensitivity for very small targets was also much greater than predicted by the single-mechanism fit, implying the presence of a second, independent mechanism with a very small summation field, which may underlie the fine stereoscopic processing system.Vision Research 10/2006; 46(17):2691-702. · 2.41 Impact Factor -
Article: The specificity of cortical region KO to depth structure.
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ABSTRACT: Functional MRI studies have identified a cortical region designated as KO between retinotopic areas V3A/B and motion area V5 in human cortex as particularly responsive to motion-defined or kinetic borders. To determine the response of the KO region to more general aspects of structure, we used stereoscopic depth borders and disparate planes with no borders, together with three stimulus types that evoked no depth percept: luminance borders, line contours and illusory phase borders. Responses to these stimuli in the KO region were compared with the responses in retinotopically defined areas that have been variously associated with disparity processing in neurophysiological and fMRI studies. The strongest responses in the KO region were to stimuli evoking perceived depth structure from either disparity or motion cues, but it showed negligible responses either to luminance-based contour stimuli or to edgeless disparity stimuli. We conclude that the region designated as KO is best regarded as a primary center for the generic representation of depth structure rather than any kind of contour specificity.NeuroImage 04/2006; 30(1):228-38. · 5.89 Impact Factor
Top co-authors
Top Journals
- Journal of Vision (9)
- Vision Research (7)
- NeuroImage (5)
- PLoS ONE (2)
- Journal of Neurophysiology (1)
Institutions
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2013
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French National Centre for Scientific Research
Lyon, Rhone-Alpes, France
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2007–2013
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National Taiwan University
- Department of Psychology
Taipei, Taipei, Taiwan
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1994–2013
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Smith-Kettlewell Eye Research Institute
San Francisco, CA, USA
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