[Paleopsych] NYT: Improved Scanning Technique Uses Brain as Portal to Thought
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Mon Apr 25 19:07:41 UTC 2005
Improved Scanning Technique Uses Brain as Portal to Thought
By NICHOLAS WADE
By peering not into the eyes but into the brain, an improved scanning
technique has enabled scientists to figure out what people are looking
at - even, in some cases, when they are not aware of what they have
The advance, reported today, shows that the scanners may be better
able than previously supposed to probe the border between conscious
and unconscious thought and even, in certain circumstances, to read
people's state of mind.
The scanning technique, known as functional magnetic resonance
imaging, is a more powerful version of a technique widely used in
hospitals. It can show which regions of the brain are actively
performing some task, but until now has lacked the resolution to track
specific groups of neurons, as the functional units of the brain are
The improvement lies not in the scanners themselves but in a new
analytic technique developed by Dr. Frank Tong, a psychologist at
Vanderbilt University. In today's issue of the journal Nature
Neuroscience, he and a colleague, Dr. Yukiyasu Kamitani, report that
they were able with the scanner to distinguish the orientation of a
test pattern of lines being observed by their subjects.
The scanner was able to furnish the necessary data because it was
looking into a region of the brain known as the primary visual cortex,
where information from the eye is processed. One of the first relay
stations from the retina, an area of the visual cortex called V1,
holds columns of neurons that burst into activity when lines or edges
are perceived, with each column responding to a specific angle of
Dr. Tong set the scanner to monitor the orientation columns in V1.
Though the columns of neurons are too small for the scanner to see
directly, he found a way to infer statistically which columns were
active and hence which orientation the V1 area was responding to.
The existence of the orientation columns was discovered many years ago
in cats and monkeys by sticking electrodes directly into their brains.
But electrodes are too invasive for routine use in people. The new
scanning method makes the V1 columns in humans easily accessible to
researchers and should allow them to track visual information as it
crosses the border between unconscious and conscious thought.
Having established that the orientation columns could be monitored,
Dr. Tong had his subjects look at two superimposed grids of lines and
told them to focus first on one and then the other. Using the scanner,
he was able to tell which grid they were attending to, showing that he
could independently infer their state of mind. The finding also shows
that conscious attention can feed back into the visual processing
system at a very early stage and tell it what to focus on.
A team of researchers at University College London has used Dr. Tong's
analytical method with somewhat different results. Dr. John-Dylan
Haynes and Dr. Geraint Rees gave subjects a brief glimpse of a grid of
lines in various orientations and then masked the lines with a second
stimulus. Because of a visual illusion that occurs in these
conditions, subjects cannot describe the orientation of the grid they
first saw, and this was the case with the London subjects.
But the V1 area of their brains had nevertheless recorded the
information. Dr. Haynes and Dr. Rees report, also in today's Nature
Neuroscience, that the V1 columns had correctly detected the
orientation of the grid, even though the subjects were unable to say
what it was.
Does the conscious mind have access to the V1 neurons? Dr. Tong's
experiment might suggest that it does and Dr. Rees's that it does not.
Perhaps V1 lies in a borderland between the conscious and unconscious
mind. Taken together, Dr. Rees said in an e-mail message, the two
experiments show that neuronal activity in V1 is necessary but not
sufficient for the mind to be aware of the orientation data it holds.
Dr. Geoffrey Boynton, a neuroscientist at the Salk Institute in San
Diego, said the new technique "certainly opens the door to a lot of
questions," including V1's involvement with both conscious and
unconscious vision. Visual processing may not be a linear sequence of
steps, with V1 the first stop after the retina, but rather a
continuous loop in which timing is everything. At the stage explored
in Dr. Rees's experiment, the mind is unconscious of the orientation
information in V1. But Dr. Boynton suggested that one-fifth of a
second later - the stage explored by Dr. Tong - the conscious mind had
become aware of V1.
Magnetic resonance imaging, the scanning technique used by the two
research teams, does not directly measure the electrical activity of
neurons. Instead, it detects the flow of blood in the brain, down to
the tiny changes involved when a local group of active neurons demand
more oxygen. The imaging machines can monitor volumes of brain tissue
as small as three millimeters a side. But this is not small enough to
see the orientation columns, which are typically half a millimeter in
The achievement of Dr. Tong's statistical technique is that it allows
the scanners in effect to infer what is happening just below their
level of resolution. "The real breakthrough is getting down to the
resolution of the column," Dr. Boynton said, adding that it may
eventually be possible to apply the technique to the whole brain to
analyze patterns of thought.
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