LARGE- AND SMALL-SCALE FUNCTIONAL ORGANIZATION OF VISUAL FIELD REPRESENTATION IN THE HUMAN VISUAL CORTEX

A fundamental characteristic of the human visual cortex is its retinotopic organization. Taking advantage of the systematic association between cortical position and visual field position, many important aspects of visual processing have been revealed by functional brain imaging. We have investigated, visualized, and characterized retinotopic organization using fMRI, in conjunction with several novel methods of analysis. In this chapter, we describe the methodology used and present findings on the basic functional organization of the visual cortex from two interlocking large- and small-scale perspectives. By larger-scale analyses of retinotopic organization, we have been able to delineate hierarchically organized visual areas (Vl, V2, V3, V3A, V3B, V4v, V8, LOc, and MT+) for ten hemispheres and investigated their individual variability in size and location using a probabilistic approach, in which probability maps of the visual areas were created. With smaller-scale analyses of retinotopy, we obtained two basic factors of visual field representation within each area (cortical magnification factor and average receptive field size), and with these factors estimated the cortical point spread of fMRI activity. We found that point spread is nearly constant across eccentricities and increases as one ascends the visual cortical hierarchy. Knowledge of retinotopic organization is important not only in itself; it also provides essential information for analysis and interpretation of functional activity in visual cortex. As representative examples, we present our recent findings on visual functions involving contextual effects. The present findings on the large- and small-scale functional organization of the human visual cortex shed new light on the relationship between functional segregation and cortical processing hierarchy in the visual system

Toward a common circle: interhemispheric contextual modulation in human early visual areas.

Humans can readily and effortlessly perceive a rich, stable, and unified visual world from a complex visual scene. Yet our internal representation of a visual object appears to be sparse and fragmented. How and where in the brain are such fragmented representations organized into a whole percept? Recent studies have accumulated evidence that some global feature integration is mediated at the early stage of visual processing. However, the spatial operating range of the integration still remains unclear. The present human functional magnetic resonance imaging study provides support that the global integration process in early visual areas, including even the primary visual area V1, is mediated beyond the separated projection of visual hemifields from right and left sides of the fixation to the visual cortex of the contralateral cerebral hemisphere. Retinotopic neural responses corresponding to a visual target were significantly enhanced when another target was simultaneously presented at the point-symmetrical position in the nonassociated visual field quadrant. The result makes a convincing case that the contextual effects involve feedback from higher areas, because there are no direct callosal connections that allow such interhemispheric contextual modulation. This enhancement from the ipsilateral hemifield may help rapid position-and-size-invariant detection of a circular pattern, which may be special among visual structures because of its ubiquity in natural scenes. Early visual areas as well as higher ones may play a more essential role in perceiving the unity of the real world than previously thought

Inconsistency and uncertainty of the human visual area loci following surface-based registration: Probability and Entropy Maps.

Here we created two different multisubject maps (16 subjects) to characterize interindividual variability in the positions of human visual areas (V1, dorsal and ventral parts of V2/3, V3A, V3B, V7, LOc, MT+, and hV4 [or V4v and V8]), which were localized using fMRI and coregistered using a surface-based method. The first is a probability map representing the degree of alignment inconsistency for each area, in which each point in space is associated with the probability affiliated with a given area. The second, a novel map termed an entropy map in which each point is associated with Shannon entropy computed from the probabilities, represents the degree of uncertainty regarding the area that resides there, and is maximal when all areas are equally probable. The overall average probability and entropy values were about 0.27 and 1.15 bits, respectively, with dependencies on the visual areas. The probability and entropy maps generated here will benefit any application which requires predictions of areas that are most likely present at an anatomical point and know the uncertainty associated with such predictions

Preparation of polymer-based multimodal imaging agent to visualize the process of bone regeneration.

The objective of this study is to design a new multimodal imaging system for the evaluation of bone regeneration process. Pamidronate (PA) of bisphosphonates with a high affinity for hydroxyapatite, was introduced to pullulan with different molecular weights (PA-pullulan). Then, two probes for fluorescence and magnetic resonance (MR) imagings were introduced into the PA-pullulan to prepare the PA-pullulan conjugates containing both the imaging probes (PA-pullulan-F/M). The PA-pullulan-F/M conjugates had an affinity for hydroxyapatite. A gelatin hydrogel incorporating bone morphogentic protein (BMP)-2 was prepared and implanted subcutaneously into mice to obtain an animal model of bone regeneration. When intravenously injected into mice with the bone tissue ectopically formed by the BMP-2-incorporated hydrogel to fluorescently evaluate their body distribution, the PA-pullulan-F/M conjugates were accumulated in the bone tissue regenerated. The time profile of fluorescent intensity well corresponded with that of calcium amount in the bone tissue newly formed. In addition, the PA-pullulan-F/M conjugates showed an MR ability similar to Gd-DTPA (gadopentetate dimeglumine) of a MR imaging agent clinically used. The MR signal around the bone tissue newly formed was enhanced for mice injected with PA-pullulan-F/M prepared from pullulan with the molecular weight of 6000. It is concluded that the PA-pullulan-F/M conjugate is a useful multimodal agent of polymeric delivery system to evaluate the process of bone regeneration

SYMPATHETIC NERVOUS CONTROL OF THE PULMONARY VESSELS IN EXPERIMENTAL PULMONARY EMBOLISM

この論文は国立情報学研究所の学術雑誌公開支援事業により電子化されました。In the acute stage of the pulmonary embolism, so-called Bezold-Jarisch-like reflexes are brought about which include bradycardia, systemic hypotension, pulmonary hypertension and hyperventilation following hypoventilation. There have been many works which dealt with pulmonary hypertension induced by experimental pulmonary embolism that are ascribed to the mechanical obstruction of the pulmonary vascular beds. But recently many investigators have revealed the possibility of concomitant pulmonary vasoconstriction along with the mechanical obstruction. Dexter et al showed in 1963 that arteriolar embolization caused active vasoconstriction in opposition to prearteriolar embolization by comparing the embolized vascular areas with grades of pulmonary hypertension when different sizes of emboli were injected into the pulmonary arteries. Bernthal and Aviado utilized the perfusion technique with constant pressure and constant flow respectively in order to measure the active vasoconstriction and emphasized that an intrinsic or sympathetic regulation of the vasoconstriction plays an important role in pulmonary embolism. The possibility of vasoconstriction in pulmonary embolism has been re-examined in this study by the method of constant flow perfusion of the left lower lobe and the rapid freezing method

Neuronal response to Shepard\u27s tones. an auditory fMRI study using multifractal analysis

Shepard\u27s tones are a typical example for auditory illusion. They consist in a series of computer generated tones, which prohibit relative pitch discrimination. As a result, when repetitively played in sequence, the illusion of an ever-ascending scale is evoked. In order to investigate this aural phenomenon, fMRI time series were acquired during presentation of a conventional block-designed paradigm as well as during continuous presentation of Shepard\u27s tones. With respect to the different setups of the two experiments, two fundamentally different methods were applied in order to conduct data analysis. Common Statistical Parameter Mapping served to evaluate the time series obtained with the block-designed paradigm. For the continuous experiment, a novel wavelet-based multifractal analysis was used, recently proposed as a classification tool for fMRI time series. This approach applies the wavelet transform to extract multifractal spectra from time-signals. For reasons of quantification, we introduced an ameliorated method for visual inspection of the multifractal properties. The results proved existence of characteristic neural responses to continuously presented Shepard\u27s tones. Interestingly, the same was not restricted to the auditory cortex, but also involved areas of the visual cortex. Related impact on the imaged cognitive areas, primary motor cortex, and primary sensory cortex could not be observed. We further provide evidence that pitch misjudgment does not occur in temporal concurrence with the repetition of the whole scale, but according to whether the main perceived frequency is located in the sensitive range of auditory perception or not. We remark that this is the first time, continuously stimulated brain areas could be detected by means of fMRI

Neural Correlates of Color-Selective Metacontrast in Human Early Retinotopic Areas

Metacontrast is a visual illusion in which the visibility of a target stimulus is virtually lost when immediately followed by a nonoverlapping mask stimulus. For a colored target, metacontrast is color-selective, with target visibility markedly reduced when the mask and target are the same color, but only slightly reduced when the colors differ. This study investigated neural correlates of color-selective metacontrast for cone-opponent red and green stimuli in the human V1, V2, and V3 using functional magnetic resonance imaging. Neural activity was suppressed when the target was rendered less visible by the same-colored mask, and the suppression was localized in the cortical region retinotopically representing the target, correlating with the perceptual topography of visibility/invisibility rather than the physical topography of the stimulus. Retinotopy-based group analysis found that activity suppression was statistically significant for V2 and V3 and that its localization to the target region was statistically significant for V2. These results suggest that retinotopic color representations in early visual areas, especially in V2, are closely linked to the visibility of color

Preparation of polymer-based multimodal imaging agent to visualize the process of bone regeneration.

The objective of this study is to design a new multimodal imaging system for the evaluation of bone regeneration process. Pamidronate (PA) of bisphosphonates with a high affinity for hydroxyapatite, was introduced to pullulan with different molecular weights (PA–pullulan). Then, two probes for fluorescence and magnetic resonance (MR) imagings were introduced into the PA–pullulan to prepare the PA–pullulan conjugates containing both the imaging probes (PA–pullulan–F/M). The PA–pullulan–F/M conjugates had an affinity for hydroxyapatite. A gelatin hydrogel incorporating bone morphogentic protein (BMP)-2 was prepared and implanted subcutaneously into mice to obtain an animal model of bone regeneration. When intravenously injected into mice with the bone tissue ectopically formed by the BMP-2-incorporated hydrogel to fluorescently evaluate their body distribution, the PA–pullulan–F/M conjugates were accumulated in the bone tissue regenerated. The time profile of fluorescent intensity well corresponded with that of calcium amount in the bone tissue newly formed. In addition, the PA–pullulan–F/M conjugates showed an MR ability similar to Gd-DTPA (gadopentetate dimeglumine) of a MR imaging agent clinically used. The MR signal around the bone tissue newly formed was enhanced for mice injected with PA–pullulan–F/M prepared from pullulan with the molecular weight of 6000. It is concluded that the PA–pullulan–F/M conjugate is a useful multimodal agent of polymeric delivery system to evaluate the process of bone regeneration