RIKEN Brain Science Institute (RIKEN BSI) RIKEN BSI News No. 17 (Aug. 2002)

Columnar structures in the human cerebral cortex
successfully observed from outside of the head

Cognitive Brain Mapping Research Team
There are high expectations for the elucidation of higher human brain functions, as a result of accumulating experimental results from experimental animals and continuing developments in noninvasive means of taking readings of neural activity. However, the spatial precision of conventional non-invasive methods for recording readings hase been on the order of 5 mm. With this degree of spatial precision, it has been possible to determine where neural activity is raised to a higher position with various mental activities, but impossible to confirm the mechanism by which each of the positions in the brain achieves its respective functions. Our research team in the Cognitive Brain Mapping Laboratory achieved spatial accuracy of 0.5 mm through improvements in magnetic resonance imaging technique, and was successful in imaging the columnar cellular functional structure of the cerebral cortex.
The columns are small regions that extend in a direction perpendicular to the surface of the cerebral cortex, and in which neurons with similar properties are gathered. The breadth of the columns at the surface of the cerebral cortex is approximately 0.5 mm in cats and monkeys. In the present study, the research team imaged ocular dominance columns of the primary visual cortex, that is, the cells gathered there that mainly receive input from the left eye, comprising the left eye column, and the cells that mainly receive input from the right eye, comprising the right eye column.
Using magnetic resonance imaging equipment, the team implemented a method that determined heightened neuron activity by measuring increases in local blood flow (functional magnetic resonance imaging method). Local blood flow increases reflexively with increased neuron activity, and decreases levels of deoxidized hemoglobin in capillaries. The decline in levels of deoxidized hemoglobin delays the attenuation of proton magnetic resonance signals, thus increasing magnetic resonance signals overall. The limitations on the spatial precision of functional magnetic resonance imaging are ultimately determined by the space between capillaries (on the order of 50 microns); however, due to the poor signal-to-noise ratio of the readings, only highly imprecise results could in fact be obtained in the past. In order to improve the signal-to-noise ratio, we developed various technologies -for example, raising the static magnetic field to 4 Tesla, 2.5 times the level of conventional technologies - and consequently increased spatial precision.
The human primary visual cortex extends along the sulcus known as the calcarine sulcus, which stretches forward and back along the inner surface of the occipital lobe of the cerebral hemisphere. The detailed shape of the brain varies among individuals, but in most people the part of the cerebral cortex that extends along the upper and lower walls of the calcarine sulcus is relatively smooth. Accordingly, we adjusted the inclination and position of the imaging slice surfaces for individual test subjects, so that the slices completely overlapped the cerebral cortex over as wide a range as possible covering the upper or lower wall of the calcarine sulcus. For visual stimulation, we projected 8 black-white alternations per second, in a black and white checkerboard pattern, onto the retina of one eye at a time via an optic fiber bundle. A striped pattern was obtained by comparing the signal distribution occurring in left eye stimuli with that occurring in right eye stimuli (see Figure). The a striped pattern was similar to that of the ocular dominance columns of monkeys, and the long axes of the bands were roughly perpendicular to the boundary of the primary visual cortex (on the inner surface of the cerebral hemisphere), as in monkeys. However, the average width of a human column was 1 mm, about twice the width of monkey columns.
In the present study, we noted that structures known to be present in monkeys and cats are also present in humans. We plan to further improve the method, and apply it to areas associated with the cerebral areas that are involved in uniquely human higher brain functions. By identifying the stimuli and conditions that activate each column, we will be able to infer information expression at the cellular level; by comparing this with information expressed by adjacent columns, we will be able to infer the details of information processing performed through interactions between columns. This gives rise to the possibility of dramatically accelerating research into the mechanisms of higher human brain functions.

Pattern of ocular dominance columns imaged by 4T fMRI in human primary visual cortex
Neuron, Vol 32, 359-374, October 2001A aggregates in the presence or absence of zinc.


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