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Visual Regions (visual + regions)
Selected AbstractsEarly processing in the human lateral occipital complex is highly responsive to illusory contours but not to salient regionsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 10 2009Marina Shpaner Abstract Human electrophysiological studies support a model whereby sensitivity to so-called illusory contour stimuli is first seen within the lateral occipital complex. A challenge to this model posits that the lateral occipital complex is a general site for crude region-based segmentation, based on findings of equivalent hemodynamic activations in the lateral occipital complex to illusory contour and so-called salient region stimuli, a stimulus class that lacks the classic bounding contours of illusory contours. Using high-density electrical mapping of visual evoked potentials, we show that early lateral occipital cortex activity is substantially stronger to illusory contour than to salient region stimuli, whereas later lateral occipital complex activity is stronger to salient region than to illusory contour stimuli. Our results suggest that equivalent hemodynamic activity to illusory contour and salient region stimuli probably reflects temporally integrated responses, a result of the poor temporal resolution of hemodynamic imaging. The temporal precision of visual evoked potentials is critical for establishing viable models of completion processes and visual scene analysis. We propose that crude spatial segmentation analyses, which are insensitive to illusory contours, occur first within dorsal visual regions, not the lateral occipital complex, and that initial illusory contour sensitivity is a function of the lateral occipital complex. [source] Functional neuroanatomy of the human near/far response to blur cues: eye-lens accommodation/vergence to point targets varying in depthEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 10 2004Hans O. Richter Abstract The purpose of this study was to identify the networks involved in the regulation of visual accommodation/vergence by contrasting the cortical functions subservient to eye-lens accommodation with those evoked by foveal fixation. Neural activity was assessed in normal volunteers by changes in rCBF measured with PET. Thirteen right-handed subjects participated in three monocular tasks: (i) resting with eyes closed; (ii) sustained foveal fixation upon a LED at 1.2 m (0.83 D); and (iii) accommodating alternately on a near (24 cm, 4.16 D) vs. a far (3.0 m, 0.33 D) LED alternately illuminated in sequential 2 s epochs. The contrast between the conditions of near/far accommodation and of constant foveal fixation revealed activation in cerebellar hemispheres and vermis; middle and inferior temporal cortex (BA 20, 21, 37); striate cortex and associative visual areas (BA 17/18). Comparison of the condition of constant fixation with the condition of resting with closed eyes indicated activation of cerebellar hemispheres and vermis; visual cortices (BA 17/18); a right hemisphere dominant network encompassing prefrontal (BA 6, 9, 47), superior parietal (BA 7), and superior temporal (BA 40) cortices; and bilateral thalamus. The contrast between the conditions of near/far accommodation with closed-eye rest reflected an incremental summation of the activations found in the previous comparisons (i.e. activations associated with constant fixation). Neural circuits activated selectively during the near/far response to blur cues over those during constant visual fixation, occupy posterior structures that include occipital visual regions, cerebellar hemispheres and vermis, and temporal cortex. [source] Detecting functional nodes in large-scale cortical networks with functional magnetic resonance imaging: A principal component analysis of the human visual systemHUMAN BRAIN MAPPING, Issue 9 2007Christine Ecker Abstract This study aimed to demonstrate how a regional variant of principal component analysis (PCA) can be used to delineate the known functional subdivisions of the human visual system. Unlike conventional eigenimage analysis, PCA was carried out as a second-level analysis subsequent to model-based General Linear Model (GLM)-type functional activation mapping. Functional homogeneity of the functional magnetic resonance imaging (fMRI) time series within and between clusters was examined on several levels of the visual network, starting from the level of individual clusters up to the network level comprising two or more distinct visual regions. On each level, the number of significant components was identified and compared with the number of clusters in the data set. Eigenimages were used to examine the regional distribution of the extracted components. It was shown that voxels within individual clusters and voxels located in bilateral homologue visual regions can be represented by a single component, constituting the characteristic functional specialization of the cluster(s). If, however, PCA was applied to time series of voxels located in functionally distinct visual regions, more than one component was observed with each component being dominated by voxels in one of the investigated regions. The model of functional connections derived by PCA was in accordance with the well-known functional anatomy and anatomical connectivity of the visual system. PCA in combination with conventional activation mapping might therefore be used to identify the number of functionally distinct nodes in an fMRI data set in order to generate a model of functional connectivity within a neuroanatomical network. Hum Brain Mapp, 2006. © 2006 Wiley-Liss, Inc. [source] Neurogenic development of the visual areas in the Chinese softshell turtle (Pelodiscus sinensis) and evolutionary implicationsJOURNAL OF ANATOMY, Issue 5 2008Chao Xi Abstract To characterize the neurogenic development of the visual areas of the turtle (Pelodiscus sinensis) during embryogenesis, a single dose of [3H]-thymidine (10 µCi) was injected into egg yolks from stages S11~12 to S21. At hatching, localization of [3H]-thymidine incorporation was examined, and led to three main observations. (1) Neurogenesis occurred in the stratum griseum centrale of the tectum opticum from S11~12 to S16 with a peak at S12. No obvious gradients of neurogenesis were observed. (2) Neurogenesis in the nucleus rotundus (Rot) and in the dorsal lateral geniculate nucleus (GLd) occurred from S11~12 to S15. Gradients of neurogenesis were detected along ventral,dorsal and lateral,medial axes in the Rot, but only the latter neurogenic gradient occurred in the GLd. (3) In the visual region of the dorsal ventricular ridge, neurogenesis lasted from S11~12 to S16. Similarly, neurogenesis occurred from S11~12 to S16~17 in the dorsal cortex, with a peak at S12 for both telencephalic visual regions. Neurogenesis followed a ventrolateral to dorsomedial gradient in the visual region of the dorsal ventricular ridge, and a superficial to deep gradient in the caudal dorsal cortex. A significant number of neurons in the rostral dorsal cortex followed a deep (earlier arising) to superficial (later arising) pattern of neurogenesis, similar to that in the avian Wulst or in the mammalian isocortex. Finally, we compared the timing and development of neurogenesis in the turtle with birds and mammals to understand the evolutionary implications of these processes. [source] | ||||||||||