In vivo functional localization of the human visual cortex using positron emission tomography and magnetic resonance imaging.

Positron emission tomography (PET) and magnetic resonance imaging (MRI) are two recently developed methods for imaging the human brain in vivo. One application of PET measures stimulus-evoked changes in cerebral blood flow while MRI provides a detailed anatomical map of the brain. Here we report the combined application of these two techniques in the same human subject. Subtracted PET scans of a brain receiving visual stimulation were superimposed upon MRI images of the same brain. The PET scans were converted into the MRI coordinate space before superposition, which allowed for a more precise correlation between MRI anatomical data and PET physiological data. Responses were localized in striate and extrastriate visual areas as well as in the posterior thalamus.

Patterns in the distribution of REM sleep in normal human sleep.

Results are reported of four analyses of the distribution of REM sleep across nights of two subjects who slept for 50 consecutive nights on a regimented, but normal, sleep schedule. We found (a) a strong phase-setting effect of sleep onset on the distribution of REM sleep within nights, (b) no systematic change in the phase of the distribution of REM sleep across nights, and (c) a relationship between the nocturnal temperature minimum and the distribution of REM sleep within nights. Our results are consistent with the notion that the nightly distribution of REM sleep may be determined by an oscillatory process, the phase of which is reset at sleep onset, but which may be subject to other influences, such as the circadian temperature rhythm. These results are in general agreement with those found by investigators studying subjects on free-running or other abnormal sleep schedules.

Three-dimensional illusory contours and surfaces.

Under general viewing conditions, objects are often partially camouflaged, obscured or occluded, thereby limiting information about their three-dimensional position, orientation and shape to incomplete and variable image cues. When presented with such partial cues, observers report perceiving 'illusory' contours and surfaces (forms) in regions having no physical image contrast. Here we report that three-dimensional illusory forms share three fundamental properties with 'real' forms: (1) the same forms are perceived using either stereo or motion parallax cues (cue invariance); (2) they retain their shape over changes in position and orientation relative to an observer (view stability); and (3) they can take the shape of general contours and surfaces in three dimensions (morphic generality). We hypothesize that illusory contours and surfaces are manifestations of a previously unnoticed visual process which constructs a representation of three-dimensional position, orientation and shape of objects from available image cues.

Computational methods for reconstructing and unfolding the cerebral cortex.

We describe computational methods for constructing three-dimensional models and unfolded, two-dimensional maps of the cerebral cortex. These methods consist of four procedures, including (1) sampling of a surface within the cortex, (2) reconstruction of a three-dimensional model of that surface, (3) unfolding of the surface to generate a two-dimensional cortical map, and (4) visualization of data on the model and the map. These methods produce structurally accurate representations of the cortex and have practical advantages over previous manual and automated approaches for flattening the cortex. We illustrate the application of these methods to neuroanatomical data obtained from histological sections of cerebral cortex in the macaque monkey. The approach should be equally useful for structural and functional studies in other species, including humans.

Mapping striate and extrastriate visual areas in human cerebral cortex.

Functional magnetic resonance imaging (fMRI) was used to identify and map the representation of the visual field in seven areas of human cerebral cortex and to identify at least two additional visually responsive regions. The cortical locations of neurons responding to stimulation along the vertical or horizontal visual field meridia were charted on three-dimensional models of the cortex and on unfolded maps of the cortical surface. These maps were used to identify the borders among areas that would be topographically homologous to areas V1, V2, V3, VP, and parts of V3A and V4 of the macaque monkey. Visually responsive areas homologous to the middle temporal/medial superior temporal area complex and unidentified parietal visual areas were also observed. The topography of the visual areas identified thus far is consistent with the organization in macaque monkeys. However, these and other findings suggest that human and simian cortical organization may begin to differ in extrastriate cortex at, or beyond, V3A and V4.