Brain cells perform balancing act

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Medical News Today 02 Feb 2005

Brain cells perform balancing act, MIT

Researchers at MIT's Picower Center for Learning and Memory have uncovered an

important new way that the brain performs complex functions such as pattern

recognition. The study will appear in the Feb. 1 issue of Nature Neuroscience.

The work, led by Mriganka Sur, Sherman Fairchild Professor of Neuroscience and

head of the Department of Brain and Cognitive Sciences at MIT, has implications

for understanding the cellular mechanisms underlying many higher level

functions, including consciousness.

Within the visual cortex, brain cells work together in localized circuits on

tasks such as pattern recognition. At a molecular level, this involves matching

the correct positive, or excitatory wires, with the correct negative, or

inhibitory wires. An exquisite balance in the interplay between plus and minus

inputs on individual neurons is essential to stabilize and shape circuits of

thousands of cells.

Earlier work has shown that brain cells contain many individual processing

modules that each collect a set number of excitatory and inhibitory inputs. When

the two types of inputs are correctly connected together, powerful processing

can occur at each module. What's more, the modules have their own built-in

intelligence that allows them to accommodate defects in the wiring or electrical

storms in the circuitry. If any of the connections break, new ones automatically

form to replace the old ones. If the positive, excitatory connections are

overloading, new negative, inhibitory connections quickly form to balance out

the signaling, immediately restoring the capacity to transmit information.

" We describe a key principle by which neuronal networks in the brain compute new

properties from simple inputs. We use a set of sophisticated techniques,

including optical imaging, patch clamp recording from neurons in the intact

brain, high-resolution tracing of connections and computational modeling, to

show that networks in the visual cortex exquisitely balance plus and minus

inputs as a critical element of computing a new property such as orientation

selectivity or sensitivity to an edge of light, " Sur said. " Indeed, neurons and

networks cannot function appropriately without such a balance. "

PATTERNS OF ACTIVITY

The primary visual cortex of monkeys and cats contain regions where neurons are

tuned to the vertical, horizontal and diagonal lines that give shape to images

we see. These regions are dotted with " pinwheel centers " around which all

orientations are represented. Areas far from the pinwheel centers contain

neurons that are tuned to a specific line orientation, not all of them at once.

Visual stimulation evokes different patterns of synaptic inputs at the pinwheel

centers and the surrounding areas.

Yet in all regions, neurons are finely tuned to line orientation and edges. In

this study, Sur and colleagues look at how processing networks in the brain

transform the inputs they receive through visual stimuli to create outputs that

can be used for perception and action.

Their results suggest that a key principle is at work through which higher

functions are accomplished. This principle is a simple rule of spatial

integration-a single mechanism that is able to balance plus and minus inputs to

allow neurons to respond quickly and accurately no matter what their location in

the cortex.

It's almost as if neurons in less-than-prime real estate compensate for their

location by altering the way they respond to inputs. They accomplish this

through a fine-tuning mechanism that adjusts to the type of input they receive

at their particular location. Thus, the inhibitory tuning balances excitation

and yields sharp spike tuning at all locations.

" These ideas form the beginning of an important new way to understand how the

brain creates new functions, " Sur said. " All higher functions of the brain,

particularly complex functions such as pattern recognition or even

consciousness, likely use such principles as a basic building block. "

In addition to Sur, authors include MIT postdoctoral fellows Marino,

Schummers and C. Lyon, who collaborated with a group led by Klaus

Obermayer at Berlin University of Technology.

This work was supported by the National Institutes of Health.

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