In a first-ever demonstration, UCLA School of
Medicine and Caltech researchers have shed new light on how the
"mind's eye" works, uncovering evidence that single neurons --
individual cells in the brain -- are involved in recalling specific
visual images to mind.
The study, published in the Nov. 16 issue of the
journal Nature, further defines the role that individual neurons
play in the brain during imagery and builds upon previous UCLA
findings.
"Our research helps clarify how the mind's eye
works," said lead author and UCLA neurosurgeon Dr. Itzhak Fried.
"Visual images can be generated in our mind's eye in the absence of
actually looking at the image. Our study reveals that the same brain
cells that fire when a person looks at a picture of the Mona Lisa
are, in fact, the same neurons that excite when that person is asked
to imagine the Mona Lisa."
To investigate neuronal activity related to
encoding and retrieval of visual images, the researchers recorded
the activity of 276 single neurons during several patient sessions.
In this study, researchers recorded the electrical activity of
several brain cells in nine patients with severe epilepsy who had
tiny microelectrodes implanted in their brains to find the focus of
their seizures.
Researchers implanted the electrodes in areas of
the brain involved in memory and social behavior. They recorded
impulses from single neurons in the human medial temporal lobe while
the subjects were asked to imagine previously viewed images. They
found that single neurons in certain areas of the brain - the
hippocampus, amygdala, entorhinal cortex and parahippocampal gyrus -
selectively altered their firing rates depending on the stimulus the
subjects imagined.
A series of images was shown on a monitor and each
picture was repeated several times. Subjects viewed images of faces,
household objects, spatial layouts, cars, animals, food, drawings
and photos of famous people and complex patterns. Subsequently,
subjects were asked to imagine previously viewed images.
The researchers found that when patients were
imagining the images, the firing rate of the brain cells was almost
as high as when the study subjects were actually looking at the
photos.
"When you look at something, it's really vivid and
when you close your eyes to imagine it, the image is not so vivid,"
said Fried. "So, we were surprised that the brain cells fired at
almost the same intensity."
Of the neurons that fired selectively during both
vision and imagery, the majority – 88 percent – had identical
selectivity, suggesting that the brain processes incoming visual
information and visual recall in a similar manner.
One example of a neuron that showed selective
changes in firing rate during vision and visual recall was a neuron
in the amygdala portion of the brain that showed an increased firing
rate when the subject saw pictures of animals and when she formed
mental images of the same pictures, but not during vision or recall
of other stimuli like faces.
The researchers observed three different types of
selective neurons. Some neurons responded during the processing of
incoming visual information, but not during imagery. There also were
neurons activated only during visual recall, which may be involved
in retrieval mechanisms not associated with vision. Finally, some
neurons responded selectively during both vision and imagery.
"Our results provide a rare opportunity to
directly observe the activity of the human brain in a pure mental
state without visual stimulation from the outside world," Fried
said. "This activity may represent the retrieval of the picture
information from memory or the maintenance of the visual image
during imagination. The firing of these neurons could represent a
correlation common to vision and imagery. Given the prominent role
of the medial temporal lobe in memory, it also seems possible that
these neurons could be activated during storage of incoming visual
inputs, and later reactivated during the mnemonic retrieval process
required for imagery."
These findings build upon previous UCLA research
that found evidence that single neurons in the human brain are
involved in memory and can respond selectively to a wide variety of
facial expressions and emotions. Most recently, Fried and his team
found evidence that single neurons in the human brain can
differentiate between separate categories of visual images, ranging
from animals to caricatures of famous people to photos of
celebrities.
"Obviously, the brain listens to the input of
millions of neurons during the process of cognition, memory and
behavior," Fried said. "Understanding how this happens is the
ultimate goal of brain researchers everywhere. This study in another
step in that direction."
In addition to Fried, Professor Christof Kock and
first author Gabriel Kreiman, both of California Institute of
Technology's Computation and Neural Systems Program, contributed to
the research.
Support for the study was provided by grants from
the National Institutes of Health, the Keck Foundation and the
Center for Consciousness Studies, University of Arizona.
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