Brain Physiology, Cognition and Consciousness: notice board entry
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Single Cell Firing Correlated With Consciousness
- Posted by:
- Alfredo Pereira Jr (group admin)
- Date:
- 20 February 2008
- Comments:
- 5 comments
Brain cells tied to consciousness reported found
Source: World Science
Feb. 19, 2008
Courtesy University of Leicester
and World Science staff
In a study billed as an exploration into the realm of “consciousness,” researchers claim to have found brain cells that become very busy only when something is consciously noticed.
Trying to understand what creates consciousness—the sense of being alive and aware—is one of the all-time most exasperating problems in science. The key stumbling block: even if one knew every brain mechanism underlying consciousness, there would still be no apparent way to see or measure the actual production of consciousness.
For now, many researchers figure they may as well just do the best they can in unraveling those physical mechanisms. The new study, led by Quian Quiroga of the University of Leicester, U.K., is among those designed to attack that question.
Volunteers were shown pictures on a computer screen very briefly—for a time just at the edge of being long enough to be noticeable. The participants were asked each time whether they saw the picture or not. Sometimes the exact same visual input was noticeable on one trial and not on another, for the same person, Quian Quiroga said.
The researchers examined what was happening in the brain during this. Certain neurons, or brain cells, “responded to the conscious perception in an ‘all-or-none’ way,” Quian Quiroga said: they dramatically changed their rate of firing signals, only when pictures were recognized. These neurons were in the medial temporal lobe, a region deep inside the brain often associated with memory.
For example, in one patient, a neuron in the hippocampus—a structure also in that area—“fired very strongly to a picture of the patient’s brother when recognized and remained completely silent when it was not,” Quian Quiroga said. “Another neuron behaved in the same manner with pictures of the World Trade Centre.” The volunteers were patients who had to undergo epilepsy surgery.
“Based on the firing of these neurons it was possible to predict far above chance whether a picture was recognized or not,” Quian Quiroga said. Also, “a picture flashed very briefly generated nearly the same response—if recognized—as when shown for much longer periods of time.”
The findings are to appear this week in the early online edition of the research journal Proceedings of the National Academy of Sciences.
Potential applications of the work include the development of “neural prosthetic” devices to be used by paralysed patients or amputees, Quian Quiroga said. A spinal injury patient, such as the late Christopher Reeve, can think about reaching a cup of tea, but the muscles don’t get the order. Neural prostheses are designed to read these commands directly from the brain and transmit them to bionic devices such as a robotic arm.
The findings, Quian Quiroga said, could also have implications treatment of patients with pathologies of the hippocampal formation, such as epilepsy, Alzheimer’s disease and schizophrenia.
The PubMed search below reveals that Quiroga collaborated with Cristof Koch, working on the hypothesis that consciousness is related with an increase of firing rates in sparse neurons (i.e., a few neurons spatially distributed in the brain, each small group responding to a specialized feature of the stimulus).
Abstract number 5 (a collaboration with T. Poggio instead of C. Koch) indicates that such an increase of firing is a consequence of the formation of Local Field Potentials. However, when Koch appear as co-author, the interpretation of results tend to emphasize the correlation of consciousness with an increase in spiking rates. This view has been advocated by Koch for a long time, since his association with Francis Crick.
Alfredo
1: Trends Cogn Sci. 2008 Feb 8 [Epub ahead of print]
Sparse but not ‘Grandmother-cell’ coding in the medial temporal lobe.
Quiroga RQ, Kreiman G, Koch C, Fried I.
Department of Engineering, University of Leicester, LE1 7RH, Leicester, UK;
Computation and Neural Systems, California Institute of Technology, 91125
Pasadena, CA, USA; Division of Neurosurgery, David Geffen School of Medicine and
Semel Institute for Neuroscience and Human Behavior, University of California Los
Angeles, 90095 Los Angeles, CA, USA.
Although a large number of neuropsychological and imaging studies have
demonstrated that the medial temporal lobe (MTL) plays an important role in human
memory, there are few data regarding the activity of neurons involved in this
process. The MTL receives massive inputs from visual cortical areas, and evidence
over the last decade has consistently shown that MTL neurons respond selectively
to complex visual stimuli. Here, we focus on how the activity patterns of these
cells might reflect the transformation of visual percepts into long-term
memories. Given the very sparse and abstract representation of visual information
by these neurons, they could in principle be considered as ‘grandmother cells’.
However, we give several arguments that make such an extreme interpretation
unlikely.
PMID: 18262826 [PubMed – as supplied by publisher]
2: J Neurophysiol. 2007 Oct;98(4):1997-2007. Epub 2007 Aug 1.
Decoding visual inputs from multiple neurons in the human temporal lobe.
Quiroga RQ, Reddy L, Koch C, Fried I.
Department of Engineering, University of Leicester, Leicester, UK.
rodri@vis.caltech.edu
We investigated the representation of visual inputs by multiple simultaneously
recorded single neurons in the human medial temporal lobe, using their firing
rates to infer which images were shown to subjects. The selectivity of these
neurons was quantified with a novel measure. About four spikes per neuron,
triggered between 300 and 600 ms after image onset in a handful of units (7.8 on
average), predicted the identity of images far above chance. Decoding performance
increased linearly with the number of units considered, peaked between 400 and
500 ms, did not improve when considering correlations among simultaneously
recorded units, and generalized to very different images. The feasibility of
decoding sensory information from human extracellular recordings has implications
for the development of brain-machine interfaces.
Publication Types: Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S.
PMID: 17671106 [PubMed – indexed for MEDLINE]
3: J Cogn Neurosci. 2007 Mar;19(3):479-92.
Local field potentials and spikes in the human medial temporal lobe are selective
to image category.
Kraskov A, Quiroga RQ, Reddy L, Fried I, Koch C.
California Institute of Technology, California, USA.
Local field potentials (LFPs) reflect the averaged dendrosomatic activity of
synaptic signals of large neuronal populations. In this study, we investigate the
selectivity of LFPs and single neuron activity to semantic categories of visual
stimuli in the medial temporal lobe of nine neurosurgical patients implanted with
intracranial depth electrodes for clinical reasons. Strong selectivity to the
category of presented images was found for the amplitude of LFPs in 8% of
implanted microelectrodes and for the firing rates of single and multiunits in
14% of microelectrodes. There was little overlap between the LFP- and
spike-selective microelectrodes. Separate analysis of the power and phase of LFPs
revealed that the mean phase was category-selective around the theta frequency
range and that the power of the LFPs was category-selective for high frequencies
around the gamma rhythm. Of the 36 microelectrodes with amplitude-selective LFPs,
30 were found in the hippocampus. Finally, it was possible to readout information
about the category of stimuli presented to the patients with both spikes and
LFPs. Combining spiking and LFP activity enhanced the decoding accuracy in
comparison with the accuracy obtained with each signal alone, especially for
short time intervals.
Publication Types: Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S.
PMID: 17335396 [PubMed – indexed for MEDLINE]
4: Curr Biol. 2006 Oct 24;16(20):2066-72.
A single-neuron correlate of change detection and change blindness in the human
medial temporal lobe.
Reddy L, Quiroga RQ, Wilken P, Koch C, Fried I.
Computation and Neural Systems, California Institute of Technology, Pasadena,
California 91125, USA. lreddy@klab.caltech.edu
Observers are often unaware of changes in their visual environment when attention
is not focused at the location of the change . Because of its rather intriguing
nature, this phenomenon, known as change blindness, has been extensively studied
with psychophysics as well as with fMRI . However, whether change blindness can
be tracked in the activity of single cells is not clear. To explore the neural
correlates of change detection and change blindness, we recorded from single
neurons in the human medial temporal lobe (MTL) during a change-detection
paradigm. The preferred pictures of the visually responsive units elicited
significantly higher firing rates on the attended trials when subjects correctly
identified a change (change detection) compared to the unattended trials when
they missed it (change blindness). On correct trials, the firing activity of
individual units allowed us to predict the occurrence of a change, on a
trial-by-trial basis, with 67% accuracy. In contrast, this prediction was at
chance for incorrect, unattended trials. The firing rates of visually selective
MTL cells thus constitute a neural correlate of change detection.
Publication Types: Comparative Study Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S.
PMID: 17055988 [PubMed – indexed for MEDLINE]
5: Neuron. 2006 Feb 2;49(3):433-45.
Object selectivity of local field potentials and spikes in the macaque inferior
temporal cortex.
Kreiman G, Hung CP, Kraskov A, Quiroga RQ, Poggio T, DiCarlo JJ.
McGovern Institute for Brain Research, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA. kreiman@mit.edu
Local field potentials (LFPs) arise largely from dendritic activity over large
brain regions and thus provide a measure of the input to and local processing
within an area. We characterized LFPs and their relationship to spikes (multi and
single unit) in monkey inferior temporal cortex (IT). LFP responses in IT to
complex objects showed strong selectivity at 44% of the sites and tolerance to
retinal position and size. The LFP preferences were poorly predicted by the spike
preferences at the same site but were better explained by averaging spikes within
approximately 3 mm. A comparison of separate sites suggests that selectivity is
similar on a scale of approximately 800 microm for spikes and approximately 5 mm
for LFPs. These observations imply that inputs to IT neurons convey selectivity
for complex shapes and that such input may have an underlying organization
spanning several millimeters.
Publication Types: Comparative Study Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S.
PMID: 16446146 [PubMed – indexed for MEDLINE]
6: Nature. 2005 Jun 23;435(7045):1102-7.
Comment in: Nature. 2005 Jun 23;435(7045):1036-7.
Invariant visual representation by single neurons in the human brain.
Quiroga RQ, Reddy L, Kreiman G, Koch C, Fried I.
Computation and Neural Systems, California Institute of Technology, Pasadena,
California 91125, USA.
It takes a fraction of a second to recognize a person or an object even when seen
under strikingly different conditions. How such a robust, high-level
representation is achieved by neurons in the human brain is still unclear. In
monkeys, neurons in the upper stages of the ventral visual pathway respond to
complex images such as faces and objects and show some degree of invariance to
metric properties such as the stimulus size, position and viewing angle. We have
previously shown that neurons in the human medial temporal lobe (MTL) fire
selectively to images of faces, animals, objects or scenes. Here we report on a
remarkable subset of MTL neurons that are selectively activated by strikingly
different pictures of given individuals, landmarks or objects and in some cases
even by letter strings with their names. These results suggest an invariant,
sparse and explicit code, which might be important in the transformation of
complex visual percepts into long-term and more abstract memories.
Publication Types: Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov’t Research Support, U.S. Gov’t, Non-P.H.S. Research Support, U.S. Gov’t, P.H.S.
PMID: 15973409 [PubMed – indexed for MEDLINE]
Dear Alfredo,
clearly the reactional firing of the neurons in the first study has been related to recognition. Consciousness is not about recognition though, it is about cognition!
Recognition of a significant image may mean many things. A reaction may be due to an emotion that is also triggered by recognizing something.
So I think it is premature to assume this finding could be related to consciousness.
A similar objection must be brought in towards the second research. When a test person realizes a change, that is a major realization. Of course that kind of mental realization must have a neural correlate in firings of neurons.
I think the findings could be very important though.
So thank you for bringing them up here.
Dear Hans:
Please note that in the older papers the authors made a relation of neuron firing with memory and recognition only, but in the new one (to appear in PNAS) they made the connection with consciousness. Why? Because they used a new kind of task (similar to Del Cul et al.; pleasee see the notice “The 300 ms Wave”) with a variation in the duration of presentation of stimuli. When the duration is not sufficient for couscious reportability the stimulus is considered to be sub-threshold. Some cells fire only when reportability occurs. Therefore, the activation of these cells is in one-to-one relation with the (conscious) reportability of the stimulus. We can conclude that such an activation is necessary for perceptual consciousness of the stimulus.
I agree with you that they (apparently) did not prove that it is sufficient. My opinion is that neuron firing is just a step in a chain of events that begin with the formation of (stimulus-evoked) Local Field Potentials and continues with oscillatory synchrony of several brain areas. This is the assumption of Global Workspace theories (see my notice on Bernard Baars and the Del Cul et al. paper; incidentally, I will review this paper for N.Network group “Neuroscience”, to appear next month).
Thanks for the comment!
Best,
Alfredo
The paper appeared here
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Alfredo