New research by scientists at the University of Minnesota and the University of California, San Diego has shown that neurons in the human visual cortex, a brain center that processes visual information, can respond to patterns of lines too fine for subjects to resolve.
The work reveals that some types of visual information, while not consciously perceived, are still conveyed closer to the brain's center(s) of consciousness than was previously thought.
The discovery contributes to the understanding of vision and has implications for the age-old question of consciousness. The work is published in today's issue of Nature.
"This is probably the first demonstration that visual cortical neurons are capable of resolving fine lines past the limits previously thought to exist," said Sheng He, assistant professor of psychology at the University of Minnesota and first author of the Nature paper. His coauthor is Donald MacLeod, a psychologist at UC San Diego.
Everyone with normal vision can perceive patterns of lines up to a certain point. But when the spacing of lines becomes too fine, the lines seem to disappear and only a uniform blur is seen.
Previously, vision researchers thought this limitation was due to optical blurring in the eye -- that is, a failure of the retina to resolve the lines. Now, at least some of the blurring has been shown to occur in the visual cortex, said He.
The researchers studied the responses of two subjects to patterns of lines projected directly onto their retinas by lasers. The lines were either horizontal or vertical.
It has been known for some time that certain neurons in the visual cortex respond preferentially to either vertical lines, horizontal lines or in-between orientations. With enough neurons responding to every conceivable orientation, humans can perceive lines that run in any direction.
It has also been known that when human subjects are shown a pattern of vertical or horizontal lines for several seconds and then shown a second grid of lines, they are better able to perceive the orientation of the second grid if it is perpendicular to the original one. The reason is that the neurons responding to, say, a vertical grid become fatigued and have trouble perceiving a second vertical grid. But the neurons that respond to horizontal lines are fresh and respond strongly.
He and MacLeod observed this phenomenon in their subjects. But when they projected lines so fine that they were slightly, but definitely, past the subjects' ability to resolve, the subjects exhibited the same difficulty perceiving a second grid of clearly visible lines with the same orientation.
This, said He, is evidence that the cortical neurons geared to that orientation were perceiving the lines the first time, when the lines were invisible to the subjects. Therefore, the subjects' inability to see the too-fine lines must be due to a blurring that occurs after the visual cortex receives input.
The visual cortex lies in the rear of the cerebrum. He said that researchers have hypothesized that people cannot become aware of optical information unless it reaches the frontal area of the cortex. The work of He and MacLeod is consistent with this theory, He said.
The blurring of lines appears to be due to processes inside the visual cortex that prevent some information from other cortical areas and consciousness.
"This suggests that not everything in the cortex can become conscious knowledge," said He. "Your visual cortex isn't telling you everything."
The work was supported by the National Institutes of Health and the Alfred P. Sloan Foundation.