Documents

one_person_one_neuron.pdf

Categories
Published
of 6
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Description
76 SCI ENTI FI C AMERI CAN MI ND Febr uar y/Mar ch 2006 C R E D I T www. sci ammi nd. com SCI ENTI FI C AMERI CAN MI ND 77 I M A G E S . C O M C o r b i s Nerve cells devoted to recognizing Halle Berry or Bill Clinton? Absurd. That’s what most neuroscientists thought —until recently By Katja Gaschler One Person, One Neuron? T hink of the hundreds of people you can remember ever having met. Add those individuals—such as celebrities, politicians and other famous figures— whose fac
Transcript
  76  SCIENTIFIC AMERICAN MIND February/March 2006    C   R   E   D   I   T  www.sciammind.com SCIENTIFIC AMERICAN MIND 77    I   M   A   G   E   S .   C   O   M        C     o     r      b      i     s Nerve cells devoted to recognizing Halle Berry or Bill Clinton? Absurd. That’s what most neuroscientists thought —until recently By Katja Gaschler One  Person, One  Neuron? T hink of the hundreds of people you can remember ever having met. Add those individuals — such as celebrities, politicians and other famous figures — whose faces you know well only from movies, TV and photographs. Is it possible that each of those individuals, along with thousands of other objects you can easily recognize from earlier encounters, could be captured in your memory by its own personal brain cell?  Perhaps. A recent study published in the jour-nal Nature by scientists at the California Insti-tute of Technology and the University of Califor-nia, Los Angeles, suggests that our brains use far fewer cells to interpret any given image than pre-viously believed. For instance, researchers dis-covered a “Bill Clinton cell” that responds al-most exclusively to the former president. Anoth-er neuron fires only when the actor Halle Berry comes into view.Exactly how the brain recognizes images has been a matter of debate. Two wildly divergent theories exist. In one, millions of neurons work together to create a cohesive picture. In the ex-treme version of the other, the brain contains a separate neuron for each individual object and person. In 1967 Polish neurophysiologist Jerzy Konorski described his theory of “gnostic neu-rons” — derived from  gnosis,  Greek for “rec og-nition.” According to this theory, the activity of one or several nerve cells determines whether someone thinks of his boss, wife or grandmoth-er. Jerome Lettvin, then a neuroscientist at the Massachusetts Institute of Technology, thus dubbed the neurons “grandmother cells,” and the name stuck.Many researchers immediately criticized the theory: Wouldn’t such one-to-one congruence take up too much space? Opinions were still much the same two decades later. “It’s very hard to take the grandmother cell theory seriously,” commented neurobiologist and Nobel Prize lau-reate David H. Hubel in the 1980s.Back then, it was not even clear how to go about exploring the entire problem of the neuro-nal foundations of consciousness. Using elec-trodes, neurophysiologists at the time had man-aged to trace the activity of individual neurons in the brains of monkeys and cats. But animal subjects cannot discuss their thoughts with us, making experiments on consciousness and per-ception more than a little difficult. Analogous tests on human beings had not yet been under-taken because of the obvious risks of inserting electrodes into the brain. Surprise Volunteers In recent years, however, a set of human vol-unteers unexpectedly emerged: patients suffer-ing from forms of epilepsy that cannot be treated with medication. In the early 1990s a number of patients were slated to undergo brain surgery to remove the zone in their brain responsible for the onsets of their seizures. Sometimes techniques such as electroencephalography and magnetic resonance imaging cannot locate the zone pre-cisely enough. In such cases, neurosurgeons may implant as many as 10 thin electrodes in the brain. These fine sensors monitor neuronal ac-tivity day and night on a continuous basis until the seizure-onset zone can be localized with suf-ficient precision and can then be removed by the neurosurgeon.Researchers realized that this procedure of-fered a unique opportunity to study the activities of individual cells. This fact led neurosurgeon Itzhak Fried of U.C.L.A., one of the principal investigators in the current research, to design a study as early as 1992 and then invite otherwise untreatable epileptics to participate in this basic neural research. The grandmother cell study, carried out with bioengineer Rodrigo Quian Quiroga of the University of Leicester in Eng-land as chief experimentalist, was rather simple. Test subjects lay in bed watching while photo-graphs flashed on a computer screen at one-sec-ond intervals. At the same time, Quian Quiroga monitored the electrical signals coming from the “attached” neurons.One of the first gnostic neurons discovered using this method was the Bill Clinton cell, lo- 78  SCIENTIFIC AMERICAN MIND February/March 2006 FAST FACTS Grandmother Cells 1 >> The controversial grandmother cell theory holds that the firing of individual nerve cells can represent the abstract concept of a specific thing or person. 2 >> Recent research has confirmed the existence of such neurons in the medial temporal lobe. Neuroscientists have found nerve cells in the brains of epilepsy patients that respond to images of specific persons — independent of the type of image. 3 >> The brain presumably generates this kind of sparse representation within a small group of neurons, par-ticularly for persons and things we encounter frequently. The neuron responded  to three different pictures of Clinton but not of other American presidents. ( )  www.sciammind.com SCIENTIFIC AMERICAN MIND 79 cated deep inside one female patient’s amygda-la — the almond-shaped region of the brain in-volved in emotions. The neuron responded to three different pictures of Clinton: a drawing, a painting and a group portrait with other politi-cians. When the patient looked at photographs of other American presidents, from George Washington to George H. W.   Bush, the cell re-mained silent.Shortly thereafter, Fried’s team found similar selective nerve cells in other patients in the me-dial temporal lobe that responded to the Beatles, the TV cartoon Simpsons family and one neuron that was galvanized into action only at the sight of Jennifer Aniston. In another test subject, one nerve cell in the right hippocampus fired as soon as Halle Berry appeared on the screen — even when she was in a Catwoman costume and her face was masked. Apparently, the cell responded to the idea of her as a person, not just to a view of her face: the caption “Halle Berry” was enough to get the neuron going.Quian Quiroga and his co-workers were fas-cinated. They theorized that the specialized nerve cells were crucial to the process of recogni-tion. Their locations were in the hippocampus, entorhinal cortex, parahippocampal gyrus and amygdala — all structures in the medial temporal lobe known to be involved in long-term memory. But how are we to conceive of a single neuron capable of representing something as complex as the identity of Bill Clinton?From the point of view of information theo-ry, this question is not hard to answer, according to computational neuroscientist Christof Koch of Caltech, who was also involved in the study and has been working with Fried’s team since 1998. In his book The Quest for Consciousness  (Roberts & Company Publishers, 2004)  , Koch illustrates this premise with an analogy. When we turn on the TV, the screen presents us with an explicit — that is, immediate — pattern of mul-ticolored pixels distributed over the monitor. Yet implicitly concealed within this pattern is spe-cific information, such as data about Bill Clin-ton’s face.Let us assume that a robot is tasked with de-termining whether the ex-president’s image is currently on the screen. Its electronic brain has to expend enormous computational resources to extract the concealed information from the array of pixels. The computation involves many itera-tions, with some level of screening for Clinton-like information going on at each one, and each iteration involves a more and more sophisticated Clinton search through a smaller and smaller set of screened data. Whereas the initial mass of data shrinks with each computational step, the “logi-cal depth of processing” increases steadily. In the end, a minute quantity of information — one bit — remains, indicating explicitly whether Clinton is present or not: 1 (Bill) or 0 (no Bill).According to a theory of consciousness de-    S   T   A   N   H   O   N   D   A    A   F   P   G  e   t   t  y   I  m  a   g  e  s    (    l  e   f   t    )  ;   U   N   D   E   R   W   O   O   D   &   U   N   D   E   R   W   O   O   D    C  o  r   b   i  s    (   r   i   g   h   t    ) (The Author) KATJA GASCHLER has a Ph.D. in biology and is an editor at Gehirn & Geist.   Former president Bill Clinton’s image ( left  ) excited a single neuron in a female patient’s brain; a cell in another person responded to the Beatles ( right  ).
Search
Tags
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks