Single neurons track your conscious vision
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
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
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
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.
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.
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.