November 08, 2008

Scientists Simulate Learning In Amoeba Using Memristor

It is surprising how small insects get energy from a wide range of food (not merely petrol or diesel), crawl, fly, reproduce and do so many maneuvers. Now it has been seen that amoeba, a unicellular organism, can learn and memorize too. We are far from creating devices of such versatility, let alone making them as compact as they are.

Amoebae can move, and they do this by changing the physical state they are made of: sol-gel state. The interior of amoebae contains endoplasm, which is in sol state; while the surrounding ectoplasm remains in gel state. The ectoplasm, being in gel state, is more viscous than it’s inside counterpart. When the organism moves, its contractile elements made of actin myofilaments contract, pulling the inside of the amoeba. This causes tension in the endoplasm, creating a change in the sol-gel state. If you squeezed a sponge ball that had been dipped in water, you would notice that water would spurt out from the pores of the sponge. Likewise, the increased tension inside, will create channels through the more viscous ectoplasm, courtesy some parts of ectoplasm (gel state) giving away (to sol state).

We know that reptiles hibernate in winter, when the humidity and temperature is low (we too are no exception). Amoebae too, slow their locomotion in response to these conditions. There are inherent oscillations within the amoeba (alternate sol gel transformation, changes in ionic flux etc) which are continuously adjusted with external signals like temperature and humidity. We, complex multicellular organisms, too have our own master oscillator (circadian clock) in the suprachiasmatic nucleus, which also continuously adjusts by lights falling on the retina.

Yoshiki Kuramoto of Kyoto University and colleagues subjected Physarum polycephalum, an amoeba, to three regularly-spaced dips in temperature and humidity, and found that its locomotive activity decreased. Thereafter, they noticed that a single dip was sufficient to elicit this response. It seems they adjusted their oscillations to the external cue and developed a conditioning later. The study implied that the amoeba anticipated that other such dips might be forthcoming, from the memory it learned. Such response did not occur when the temperature and humidity changes were irregular.

Memory in this case occurs due to the persistence of the channels etched by the organism in the ectoplasm. But this ‘memory’ did not persist for long, if we continued giving them a single dip instead of a regular triplet. This plasticity (change due to reorganization as a function of a stimulus) in amoeba has now been simulated with the aid of electronics by Massimiliano Di Ventra et al.

They used a capacitor, a resistor, an inductor in series and connected a ‘memristor’ in parallel with the capacitor. Memristors (for memory resistors)array of memristors are devices which consist of two layers of titanium dioxide (often present in medicine coatings and chewing gums). When current is applied to one layer, the resistance of the other changes. Leon Chua, of the University of California at Berkeley, predicted it long time ago; and now R. Stanley Williams and colleagues at Hewlett Packard have developed it. It can store memory like DRAM, but unlike DRAM it doesn’t forget when a current is no longer flowing. They hold promise as energy efficient chip for computers and we can also expect faster ‘booting’ of computers, since memory will already have been stored there. The adjoining figure shows ‘memristors’ in a row, as seen by atomic force microscopy (AFM).

Now when a current, fluctuating (AC) in a non periodical manner or a stable DC, was made to pass through the circuit, the memristor went to a low resistance state, virtually short circuiting and dampening the oscillation. However, with a regularly fluctuating current, whose frequency matched the resonant frequency of the circuit, the memristor went into a high resistance state, strengthening the oscillation. What connects electronics to amoeba is the memory that both the circuit retain. The memory of memristor, called memristance, is due to atomic rearrangement in the device. The high resistance state lingers for quite some time, so that next time one single pulse was necessary to put it into oscillation. This phenomenon is quite akin to the protozoal response.

It seems that those days are certainly not far when we will just need to jack-up a USB device in our head to boost up our memory.

Last modified: Nov 13, 2008
Reference: Tetsu Saigusa, Yoshiki Kuramoto (2008). Amoebae Anticipate Periodic Events Physical Review Letters, 100 (1) DOI: 10.1103/PhysRevLett.100.018101

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