Areas of the brain involved in visual awareness localised

[Angel]

Areas of the brain involved in visual awareness localised
Category: Neurology/Neuroscience News, Date: 13 Nov 2005

Dartmouth professor of psychological and brain sciences Peter Tse has published new results in his on-going investigation of the brain and how it transforms visual stimuli into conscious experience. His paper, "Visibility, visual awareness, and visual masking of simple unattended targets are confined to areas in the occipital cortex beyond human V1/V2," is available in the Nov. 8 issue of the weekly journal, The Proceedings of the National Academy of Sciences. Tse's findings help to explain what part of the brain is at work in turning sight into understanding.

Tse, who is currently on sabbatical in Regensburg, Germany as the recipient of the prestigious Friedrich Wilhelm Bessel Research Award, conducted experiments using the phenomenon of "masking." Masking occurs when "a quickly flashed object seems to vanish because it is flanked by subsequently presented objects," said Tse. Using Functional Magnetic Resonance Imaging (fMRI), Tse said he and his team, "looked for areas of the brain where neuronal activity decreased when the object was made invisible. These areas are arguably the areas of the brain where the correlates of visual consciousness lie."

Tse's findings may advance the understanding of the brain's interaction with the eye by identifying the neural basis of conscious experience, a relationship important to the fields of medicine, neurology and psychology.

Tse and his co-authors, Susana Martinez-Conde, Alexander A. Schlegel and Stephen L. Macknik of the Barrow Neurological Institute, found that, "early areas in the visual processing hierarchy respond the same whether or not objects are visible to us or invisible in the context of visual masking." That is, some parts of the brain respond to visual stimuli regardless of whether the conscious mind "sees" them or not. However, Tse and his team found that, "neural activity in areas beyond visual area 2 appear to correlate with perception." They also found that the areas of the brain related to visual perception appear to reside exclusively in the occipital lobe (at the back of the head.) Tse's team concluded that, "the neural correlates of conscious visual visibility for masking stimuli lie in the occipital lobe, but after visual area 2."

Genevieve Haas, genevieve.haas@dartmouth.edu
Dartmouth College, http://www.dartmouth.edu

[Rajiv]

Thanx, this one is also interesting(source as above):

Visual information is processed even when the visual cortex is temporarily shut down

Visual information can be processed unconsciously when the area of the brain that records what the eye sees is temporarily shut down, according to research at Rice University in Houston.

The research, published the week of Oct. 31 in the Proceedings of the National Academy of Sciences' (PNAS) online Early Edition, suggests the brain has more than one pathway along which visual information can be sent.

For the study, the researchers induced temporary, reversible blindness lasting only a fraction of a second in nine volunteers with normal vision. Transcranial magnetic stimulation (TMS), a harmless noninvasive technique using brief magnetic pulses, was applied to the volunteers' visual cortex -- the area at the back of the brain that processes what the eye sees - to interrupt the normal visual pathway. The volunteers looked at a computer screen, and during their momentary blindness, either a horizontal or a vertical line or a red or a green dot flashed on the screen.

Researchers then asked the study participants whether they had seen a horizontal or a vertical line; because their primary visual pathway had been shut down, the participants reported that they saw nothing. However, when forced to guess which line had appeared on their computer screen, the participants gave the correct answer 75 percent of the time. When the participants had to guess whether a red or a green dot had flashed on the screen, they gave the correct answer with 81 percent accuracy.

"This high degree of accuracy for both the directional orientation and color tasks was significantly above chance," said Tony Ro, associate professor of psychology and principal investigator for the study. "Even though the human primary visual cortex activity was temporarily shut down, it's clear that detailed visual information was still being processed unconsciously."

Because only a certain region of the thalamus - the area of the brain where all sensory information is relayed -- can process color, the study provides evidence that there must be a pathway that goes through this region of the thalamus to the higher visual centers of the brain, Ro said.

"In addition to providing direct evidence that unconscious processing takes place within the brain - a controversial claim that was advanced by the likes of Sigmund Freud and William James - our results suggest that multiple pathways relay visual input into the central nervous system for different types of processing," Ro said. "And our study also begins to shed light on the brain structures that are necessary for consciousness, with the primary visual cortex playing an essential role for visual awareness."

The phenomenon of "blindsight" has been reported in patients with brain damage who report not seeing something but correctly identify the shape and location when forced to guess. Ro noted that his study demonstrates that TMS can be used successfully to induce blindsight in people with normal vision.

Ro's co-authors on the PNAS paper were graduate student Jennifer Boyer and Stephanie Harrison, a summer intern.

B.J. Almond
balmond@rice.edu
713-348-6770
Rice University
http://media.rice.edu