Monday, September 12, 2005

The “brain chip”

Dr. K. Rohiniprasad

The brain is being invaded by the all-pervasive silicon chip at last. American scientists have cultured colonies of rat nerve cells on silicon semiconductor substrate. Rat hippocampus cells connect with electrodes on the silicon to form a "bionic chip". It duplicates activity patterns of sections of the hippocampus. These chips are composite, containing numerous separate components, each of which acts like a single cell. It is hoped that hybrid brain-electronic systems of this type may soon help us understand the process by which brains perform complex functions, including specific pattern recognition. Scientists hope to implant similar chips into living brains as both experimental instruments and eventually as prostheses. Once the initial trials are complete, the researchers can begin tests on monkeys where they prevent parts of the hippocampus from working and by-pass it with the chip. Such systems may also form part of a new generation of chemical detectors. Others plan to use the wired colonies of cells in the study of the effects of the hormone oestrogen and allied substances on the brain.

The technique of culturing on silicon was developed over nearly two years. To make the chip scientists first dissociated about 80,000 of the cells of hippocampus from their intricate network of interconnections with each other and placed them as individual cells on a specially prepared silicon test bed fitted with a matrix of electrodes. In this process the cells affix themselves to the test bed and grow, sending out processes and reforming synaptic contacts with each other. They can live on the chips for months. Scientists have learned to make and use masks made by traditional photo-resist processes to "sculpt" the colonies into desired shapes by the proper use of the substrate coating to which the cells adhere. By this method colonies have been made on electronic contacts, encouraging some connections and preventing others. Thus the neurons can be made to grow only in certain directions and the electrode matrix the neurons grow on can serve as input device for the cells. It is possible to monitor their activity so that the spontaneous patterns of synaptic activity that occur with the neurons talking to each other can be observed. It is possible to try and find the algorithms that define their activity. The researchers can also stimulate the nerve cell colonies at any desired point, and then monitor the cellular chemical activity that takes place at the synapses of hippocampal neurons. This can be manipulated at the individual neuron level by administration of minute, precisely-measured quantities of substances to enhance or inhibit any given neurotransmitters. The "bionic chip" can eventually do information processing on its own.

Some day, by actually implanting and interfacing a chip with living brain cells, scientists can get living cells to interact and talk to such chips. Such an implant might restore function lost by disease or injury. If someone with the prosthesis regains the ability to store new memories, then it is safe to assume that the brain chip works. Sceptics warn about major difficulties in taking out the existing hippocampus and implanting such devices since it could cause damage. Like heart transplants people may find the technology hard to accept at first. Right now the chip is "miles away" from being used in human brains.