Archive for the ‘Perception’ 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 26

Friday, April 27th, 2007

The fusiform face area and the extrastriate body area 26) Some complex object categories, such as faces, have dedicated areas of cortex for processing them, but are also represented in a distributed fashion (Kanwisher – 1997, Haxby – 2001)

Early in her career Nancy Kanwisher used functional MRI (fMRI) to seek modules for perceptual and semantic processing. She was fortunate enough to discover what she termed the fusiform face area; an area of extrastriate cortex specialized for face perception.

This finding was immediately controversial. It was soon shown that other object categories also activate this area. Being the adept scientist that she is, Kanwisher showed that the area was nonetheless more active for faces than any other major object category.

Then came a slew of arguments purporting that the face area was in fact an 'expertise area'. This hypothesis states that any visual category with sufficient expertise should activate the fusiform face area.

This argument is based on findings in cognitive psychology showing that many aspects of face perception once thought to be unique are in fact due to expertise (Diamond et al., 1986). Thus, a car can show many of the same perceptual effects as faces for a car expert. The jury is still out on this issue, but it appears that there is in fact a small area in the right fusiform gyrus dedicated to face perception (see Kanwisher's evidence).

James Haxby entered the fray in 2001, showing that even after taking out the face area from his fMRI data he could predict the presence of faces based on distributed and overlapping activity patterns across visual cortex. Thus it was shown that face perception, like visual perception of other kinds of objects, is distributed across visual cortex.

Once again, Kanwisher stepped up to the plate. (more…)