Showing papers by "Roger B. H. Tootell published in 1995"
TL;DR: Cortical magnification factor curves for striate and extrastriate cortical areas were determined, which showed that human visual areas have a greater emphasis on the center-of-gaze than their counterparts in monkeys.
Abstract: The borders of human visual areas V1, V2, VP, V3, and V4 were precisely and noninvasively determined. Functional magnetic resonance images were recorded during phase-encoded retinal stimulation. This volume data set was then sampled with a cortical surface reconstruction, making it possible to calculate the local visual field sign (mirror image versus non-mirror image representation). This method automatically and objectively outlines area borders because adjacent areas often have the opposite field sign. Cortical magnification factor curves for striate and extrastriate cortical areas were determined, which showed that human visual areas have a greater emphasis on the center-of-gaze than their counterparts in monkeys. Retinotopically organized visual areas in humans extend anteriorly to overlap several areas previously shown to be activated by written words.
2,590 citations
TL;DR: The lateral occipital complex (LO) showed preferential activation to images of objects, compared to a wide range of texture patterns as mentioned in this paper, suggesting that objects varying widely in their recognizability (e.g., famous faces, common objects, and unfamiliar three-dimensional abstract sculptures) activated it to a similar degree.
Abstract: The stages of integration leading from local feature analysis to object recognition were explored in human visual cortex by using the technique of functional magnetic resonance imaging. Here we report evidence for object-related activation. Such activation was located at the lateral-posterior aspect of the occipital lobe, just abutting the posterior aspect of the motion-sensitive area MT/V5, in a region termed the lateral occipital complex (LO). LO showed preferential activation to images of objects, compared to a wide range of texture patterns. This activation was not caused by a global difference in the Fourier spatial frequency content of objects versus texture images, since object images produced enhanced LO activation compared to textures matched in power spectra but randomized in phase. The preferential activation to objects also could not be explained by different patterns of eye movements: similar levels of activation were observed when subjects fixated on the objects and when they scanned the objects with their eyes. Additional manipulations such as spatial frequency filtering and a 4-fold change in visual size did not affect LO activation. These results suggest that the enhanced responses to objects were not a manifestation of low-level visual processing. A striking demonstration that activity in LO is uniquely correlated to object detectability was produced by the "Lincoln" illusion, in which blurring of objects digitized into large blocks paradoxically increases their recognizability. Such blurring led to significant enhancement of LO activation. Despite the preferential activation to objects, LO did not seem to be involved in the final, "semantic," stages of the recognition process. Thus, objects varying widely in their recognizability (e.g., famous faces, common objects, and unfamiliar three-dimensional abstract sculptures) activated it to a similar degree. These results are thus evidence for an intermediate link in the chain of processing stages leading to object recognition in human visual cortex.
1,697 citations
TL;DR: FMRI activity in human MT does in fact decrease at and near individually measured equiluminance, and area MT has a much higher contrast sensitivity than that in several other areas, including primary visual cortex (V1).
Abstract: Using noninvasive functional magnetic resonance imaging (fMRI) technique, we analyzed the responses in human area MT with regard to visual motion, color, and luminance contrast sensitivity, and retinotopy. As in previous PET studies, we found that area MT responded selectively to moving (compared to stationary) stimuli. The location of human MT in the present fMRI results is consistent with that of MT in earlier PET and anatomical studies. In addition we found that area MT has a much higher contrast sensitivity than that in several other areas, including primary visual cortex (V1). Functional MRI half-amplitudes in V1 and MT occurred at approximately 15% and 1% luminance contrast, respectively. High sensitivity to contrast and motion in MT have been closely associated with magnocellular stream specialization in nonhuman primates. Human psychophysics indicates that visual motion appears to diminish when moving color-varying stimuli are equated in luminance. Electrophysiological results from macaque MT suggest that the human percept could be due to decreases in firing of area MT cells at equiluminance. We show here that fMRI activity in human MT does in fact decrease at and near individually measured equiluminance. Tests with visuotopically restricted stimuli in each hemifield produced spatial variations in fMRI activity consistent with retinotopy in human homologs of macaque areas V1, V2, V3, and VP. Such activity in area MT appeared much less retinotopic, as in macaque. However, it was possible to measure the interhemispheric spread of fMRI activity in human MT (half amplitude activation across the vertical meridian = approximately 15 degrees).
1,365 citations
TL;DR: A clear increase in activity is demonstrated in human cortical area MT when subjects viewed a stationary stimulus undergoing illusory motion, following adaptation to stimuli moving in a single local direction.
Abstract: Functional magnetic resonance imaging (fMRI) was used to measure local haemodynamic changes (reflecting electrical activity) in human visual cortex during production of the visual motion aftereffect, also known as the waterfall illusion. As in previous studies, human cortical area MT (V5) responded much better to moving than to stationary visual stimuli. Here we demonstrate a clear increase in activity in MT when subjects viewed a stationary stimulus undergoing illusory motion, following adaptation to stimuli moving in a single local direction. Control stimuli moving in reversing, opposed directions produced neither a perceptual motion aftereffect nor elevated fMRI levels postadaptation. The time course of the motion aftereffect (measured in parallel psychophysical tests) was essentially identical to the time course of the fMRI motion aftereffect. Because the motion aftereffect is direction specific, this indicates that cells in human area MT are also direction specific. In five other retinotopically defined cortical areas, similar motion-specific aftereffects were smaller than those in MT or absent.
632 citations
TL;DR: Human visual cortex stained for myelin, cytochrome oxidase, and the monoclonal antibody CAT-301 revealed a distinctive staining topography in several cortical areas, including areas V1, V2, MT, PX, and VX.
Abstract: We stained human visual cortex for myelin, cytochrome oxidase, and the monoclonal antibody CAT-301 in an attempt to demonstrate and map MT (V5) and other visual cortical areas in humans. Both flattened and unflattened cortical tissue was examined. A likely candidate for area MT (V5), which we refer to as MT, was demonstrated using all three stains. Myelin and CAT-301 labels for MT were demonstrated to be coincident by comparing results from the two stains in adjacent sections. In all three stains, MT was an oval area approximately 1.2 x 2.0 cm, located 5-6 cm anterior and dorsal to the foveal V1-V2 border. The position and size of MT as defined by the present anatomy are consistent with MT (V5) as defined by functional measures in humans. In addition, flattened cortical tissue stained for cytochrome oxidase revealed a distinctive staining topography in several cortical areas, including areas V1, V2, MT, PX, and VX. Similar studies in flattened cortex of macaque and green monkeys demonstrated distinctive dark cytochrome oxidase staining in MT, PX, MTc, and V3.
265 citations
TL;DR: A technique for automatic motion detection and correction which is based on the ratio‐variance minimization algorithm to the fMRI subject motion problem performed well in the range of image characteristics common for fMRI experiments, reducing motion to under 0.5 mm, and removed significant motion‐induced artifacts while restoring true regions of activation.
Abstract: Subject motion present during the time course of functional activation studies is a pervasive problem in mapping the spatial and temporal characteristics of brain activity. In functional MRI (fMRI) studies, the observed signal changes are small. Therefore, it is crucial to reduce the effect of subject motion during the acquisition of image data in order to differentiate true brain activation from artifactual signal changes due to subject motion. We have adapted a technique for automatic motion detection and correction which is based on the ratio-variance minimization algorithm to the fMRI subject motion problem. This method was used for retrospective correction of subject motion in the acquired data and resulted in improved functional maps. In this paper we have designed and applied a series of tests to evaluate the performance of this technique which span the classes of image characteristics common to fMRI. These areas include tests of the accuracy and range of motion as well as measurement of the effect of image signal to noise ratio, focal activation, image resolution, and image coverage on the motion detection system. Also, we have evaluated the amount of residual motion remaining after motion correction, and the ability of this technique to reduce the motion-induced artifacts and restore regions of activation lost due to subject motion. In summary, this method performed well in the range of image characteristics common for fMRI experiments, reducing motion to under 0.5 mm, and removed significant motion-induced artifacts while restoring true regions of activation. Motion correction is expected to become a routine requirement in the analysis of fMRI experiments. © 1995 Wiley-Liss, Inc.
194 citations