Lone cells reflect mind's eye
New research has shown that individual neurones in the brain are involved in recalling specific visual images to mind, with many of them being the same neurones that were originally involved in seeing the real thing.
The research, published in Nature, has found that a significant proportion of the same neurones fire when people recall objects, faces, patterns and so on, as fire when people actually see these things. For vision, as for recall, different neurones fire depending on what is being processed - a face, or a map, say.
"Visual images can be generated in our mind's eye in the absence of actually looking at the image." said lead author Dr Itzhak Fried of the University of California, Los Angeles. "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 neurones that excite when that person is asked to imagine the Mona Lisa."
To investigate neuronal activity the researchers used a rare opportunity with epilepsy patients who already had tiny microelectrodes implanted in their brains to find the focus of their seizures. They recorded impulses from several individual neurones in nine patients with epilepsy. By modifying these electrodes and adding recording wires, the researchers could 'plug in' to the specific neurones.
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.
They found that single neurones in certain areas of the brain selectively altered their firing rates depending on the stimulus the subjects imagined.
The researchers also 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."
Previous research by the UCLA researchers had found evidence that single neurones 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 neurones in the human brain can differentiate between separate categories of visual images, ranging from animals to caricatures of famous people to photos of celebrities.