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News story    
Single neurons track your conscious vision

A human brain has about 100 billion neurons, each firing away many times per second. It is very unusual to observe single neurons working in the human brain. Yet individual cells are the basic unit of brain function. They are the “atoms” of the nervous system --- miniscule, complex, and rich in numbers and connections. But most of what we know about neurons in conscious vision comes from studying monkeys and cats. Now a new study by Kreiman and colleagues at Caltech has taken a step toward understanding how single brain cells behave in human conscious vision.  

Normally it is not ethical to insert electrodes into living human brains.  However, in patients with intractable epilepsy, recording the activity of specific neurons is the best way to find out which ones start the electrical brainstorms that trigger epileptic fits. The authors received permission to study patients with uncontrollable seizures. While the patients were conscious, up to 10 tiny needle electrodes were inserted in different parts of the inner temporal lobes. Since there are no pain receptors in the brain itself, it does not hurt people to be conscious during brain surgery. In fact, it is vital to ask conscious patients what they experience, to ensure that essential brain tissues are not damaged.

"Electrodes are inserted under general anesthesia based on clinical needs of the patient", says Dr. Fried, one of the co-authors on the paper. "Monitoring is done later on the ward at UCLA over a period of several days at least, so that seizure focus can be identified. Other methods such as electrical stimulation are done sometime during surgery under local anesthesia. Number of electrode inserted is usually about 10, and each electrode has several microwires."

How do we find out about conscious vision in this unique  situation? In the last dozen years a method for specifically studying conscious brain events has emerged. It consists of comparing neural events that are reportable as conscious to similar events that are not reportable, so that the effect of consciousness “as such” can be teased out. After all, the brain does a dizzying variety of things without consciousness. Mere brain activity by itself does not prove anything about the conscious aspects of the brain.

Many methods have been used to compare conscious and unconscious brain events, but Kreiman et al chose a novel one called flash suppression. Suppose you see two images, one in each eye. Your left eye has a face, while your right eye has a chessboard pattern. This is the standard phenomenon of binocular rivalry phenomenon found in the 19th century. You can show it simply by holding two different objects, one in front of each eye. But now make a small change: For one second only you see the face in  your left eye only. Under those conditions the NEW picture in the right eye will become conscious, and the face will disappear from consciousness. The face is suppressed “in a flash,” even though it is still there in front of your left eye.   

This method allows us to look for the neurons that respond to the conscious picture, and those that respond to the unconscious one. Kreiman at all found something that can be summed up in two sentences. Two-thirds of the neurons they recorded fired to the conscious picture. Not one of the neurons responded to the unconscious input.

So consciousness DOES SOMETHING ACTIVE in the brain, even at the level of single neurons. That doesn’t mean that it only activates single cells. Probably whatever we are conscious of involves billions of neurons and tens of billions of interactions between them. But we can now trace some our personal experience to the activity of the smallest biological unit, the single cell.

Bernard J. Baars

© B.J. Baars, 2002.


Gabriel Kreiman, Itzhak Fried, Christof Koch (2002) Single neuron correlates of subjective vision in the human medial temporal lobe.  Proceedings of the National Academy of Sciences, USA. 99 (12), 8378-8383.

Visual information from the environment is transformed into perceptual sensations through several stages of neuronal processing. Flash suppression constitutes a striking example where the same retinal input can give rise to two different conscious visual percepts. We directly recorded the responses of individual neurons during flash suppression in the human amygdala, entorhinal cortex, hippocampus and parahippocampal gyrus, allowing us to explore for the first time the neuronal responses in untrained subjects at a high spatial and temporal resolution in the medial temporal lobe. Subjects were patients with pharmacologically intractable epilepsy implanted with depth electrodes to localize the seizure onset focus. We observed that the activity of two-thirds of all visually selective neurons followed the perceptual alternations rather than the retinal input. None of the selective neurons responded to a perceptually suppressed stimulus. Therefore, the activity of most individual neurons in the medial temporal lobe of naive human subjects directly correlates with the phenomenal visual experience.


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