Archive for the ‘Consciousness’ Category

Grand Challenges of Neuroscience: Day 4

Saturday, July 7th, 2007

After a bit of a hiatus, I'm back with the last three installments of "Grand Challenges in Neuroscience". picture-1.png

Topic 4: Time

Cognitive Science programs typically require students to take courses in Linguistics (as well as in the philiosphy of language).  Besides the obvious application of studying how the mind creates and uses language, an important reason for taking these courses is to realize the effects of using words to describe the mental, cognitive states of the mind.

In fact — after having taken courses on language and thought, it seems that it would be an interesting coincidence if the words in any particular language did map directly onto mental states or brain areas.  (As an example, consider that the amygdala is popularly referred to as the "fear center".) 

It seems more likely that mental states are translated on the fly into language, which only approximates their true nature.  In this respect, I think it's important to realize that time may be composed of several distinct subcomponents, or time may play very different roles in distinct cognitive processes.

Time. As much as it is important to have an objective measure of time, it is equally important to have an understanding of our subjective experience of time.  A number of experimental results have confirmed what has been known to humanity for some time: Time flies while you're having fun, but a watched pot never boils.   
Time perception strongly relates cognition, attention and reward.  The NSF committee proposed that understanding time is going to be integrative, involving brain regions whose function is still not understood at a "systems" level, such as the cerebellum, basal ganglia, and association cortex.  

Experiments?

The NSF committee calls for the develpoment of new paradigms for the study of time.  I agree that this is critical.  To me, one of the most important issues is the dissociation of reward from time (e.g., "time flies when your having fun"):  most tasks involving time perception in both human and non-human primates involved rewarding the participants. 

In order to get a clearer read on the neurobiology of time perception and action, we need to observe neural representations that are not colored by the anticipation of reward.

-PL 

Brain image from http://www.cs.princeton.edu/gfx/proj/sugcon/models/
Clock image from http://elginwatches.org/technical/watch_diagram.html

Grand Challenges of Neuroscience: Day 3

Sunday, May 13th, 2007

Topic 3: Spatial Knowledgeskaggs96figure3.png

Animal studies have shown that the hippocampus contains special cells called "place cells".  These place cells are interesting because their activity seems to indicate not what the animal sees, but rather where the animal is in space as it runs around in a box or in a maze. (See the four cells in the image to the right.)

Further, when the animal goes to sleep, those cells tend to reactivate in the same order they did during wakefulness.  This apparent retracing of the paths during sleep has been termed "hippocampal replay".

More recently, studies in humans — who have deep microelectrodes implanted to help detect the origin of epileptic seizures — have shown place-responsive cells.  Place cells in these studies were found not only in the human hippocampus but also in nearby brain regions.

The computation which converts sequences of visual and other cues into a sense of "place" is a very interesting one that has not yet been fully explained.  However, there do exist neural network models of the hippocampus that, when presented with sequences, exhibit place-cell like activity in some neurons.

The notion of place cell might also extend beyond physical space.  It has been speculated that computations occur to convert sequences events and situations into a distinct sense of "now".  And indeed, damage to the hippocampus has been found not only to impair spatial memory but also "episodic" memory, the psychological term for memory for distinct events.

Experiments? 

How can we understand the ways in which we understand space? Understanding spatial knowledge seems more tangible than understanding the previous two topics in this series. It seems that researchers are already using some of the most effective methods to tackle the problem.

First, the use of microelectrodes throughout the brain while human participants play virtual taxi games and perform problem solving tasks promises insight into this question.  Second, computational modeling of regions (e.g., the hippocampus) containing place cells should help us understand their properties and how they emerge.  Finally, continued animal research and possibly manipulation of place cells in animals to influence decision making (e.g., in a T-maze task) may provide an understanding of how spatial knowledge is used on-line. 

-PL 

History’s Top Brain Computation Insights: Day 15

Monday, April 16th, 2007

A split brain patient verbally reports the left-brain information, while reporting with his hand for right-brain information 15) Consciousness depends on cortical communication; the cortical hemispheres are functionally specialized (Sperry & Gazzaniga – 1969)

It is quite difficult to localize the epileptic origin in some seizure patients. Rather than removing the gray matter of origin, neurosurgeons sometimes remove white matter to restrict the seizure to one part of the brain.

One particularly invasive procedure (the callosotomy) restricts the seizure to one half of cortex by removing the connections between the two halves. This is normally very effective in reducing the intensity of epileptic events. However, Sperry & Gazzaniga found that it comes at a price.

They found that presenting a word to the right visual field of a patient without a corpus callosum allowed only the patient's left hemisphere to become aware of that word (and vice versa). When only the side opposite the one which was presented the word was allowed to respond, it had no idea what word had been presented.

The two hemispheres of cortex could not communicate, and thus two independent consciousnesses emerged.

Sperry & Gazzaniga also found that the left hemisphere, and not the right, could typically respond linguistically. This suggested that language is largely localized in the left hemisphere. (See the above figure for illustration.)

The functional distinction between the left and right hemispheres is supported by many lesion studies. Generally, the left hemisphere is specialized for language and abstract reasoning, while the right hemisphere is specialized for spatial, body, emotional, and environment awareness. The boundary between these specializations has been trivialized in the popular media; it is actually quite complex and, in healthy brains, quite subtle.

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ composed of functional column units whose spatial dedication determines representational resolution, is composed of many specialized regions involved in perception (e.g., touch: parietal lobe, vision: occipital lobe), action (e.g., frontal lobe), and memory (e.g., temporal lobe), which depend on inter-regional communication for functional integration.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries.]

-MC

Predicting Intentions: Implications for Free Will

Thursday, March 8th, 2007

News about a neuroimaging group's attempts to predict intentions hit the wire a few days ago. The major theme was how mindreading might be used for unethical purposes.

What about its more profound implications?

If your intentions can be predicted before you've even made a conscious decision, then your will must be determined by brain processes beyond your control. There cannot be complete freedom of will if I can predict your decisions before you do!

Dr. Haynes, the researcher behind this work, spoke at Carnegie-Mellon University last October. He explained that he could use functional MRI to determine what participants were going to decide several seconds before that decision was consciously made. This was a free choice task, in which the instruction was to press a button whenever the participant wanted. In a separate experiment the group could predict if a participant was going to add or subtract two numbers.

In a way, this is not very surprising. In order to make a conscious decision we must be motivated by either external or internal factors. Otherwise our decisions would just be random, or boringly consistent. Decisions in a free choice task are likely driven by a motivation to move (a basic instinct likely originating in the globus pallidus) and to keep responses spaced within a certain time window.

Would we have a coherent will if it couldn't be predicted by brain activity? It seems unlikely, since the conscious will must use information from some source in order to make a reasoned decision. Otherwise we would be incoherent, random beings with no reasoning behind our actions.

In other words, we must be fated to some extent in order to make informed and motivated decisions.

-MC 

Pinker on ‘The Mystery of Consciousness’

Saturday, January 27th, 2007

Illustration for TIME by Istvan OroszTime magazine has just published an intriguing article on the neural basis of consciousness. The article was written by Steven Pinker, a cognitive scientist known for his controversial views on language and cognition.

Here are several excerpts from the article…

On the brain being the basis for consciousness:

Scientists have exorcised the ghost from the machine not because they are mechanistic killjoys but because they have amassed evidence that every aspect of consciousness can be tied to the brain. Using functional MRI, cognitive neuroscientists can almost read people's thoughts from the blood flow in their brains. 

On a new basis for morality emerging from neuroscience: 

…the biology of consciousness offers a sounder basis for morality than the unprovable dogma of an immortal soul… once we realize that our own consciousness is a product of our brains and that other people have brains like ours, a denial of other people's sentience becomes ludicrous.

On the evolutionary basis of self deceit:

Evolutionary biologist Robert Trivers has noted that people have a motive to sell themselves as beneficent, rational, competent agents. The best propagandist is the one who believes his own lies, ensuring that he can't leak his deceit through nervous twitches or self-contradictions. So the brain might have been shaped to keep compromising data away from the conscious processes that govern our interaction with other people.

 -MC