Brain inspired experiments-Short term memory transition to long term memory

Nowadays the information storage in brain is understood as the communication between neurons through synapses and learning process is established by modification of synaptic weight or plasticity mchanism categorized as short-term memory (STM) and long-term memory (LTM). What researchers calls as short-term memory is small amount of information which brain holds for a short period of time (not more than 30 s). On the other hand, long-term memory doesn’t mean an everlasting retention time in the brain, thus anyhting longer than 30 s.

memories transform from short-term memory into long-term memory through repeated stimulation. In addition retention loss in a nanoscale memristor device bears striking resemblance to memory loss in biological systems following a stretched-exponential function (SEF):

where φ(t) is the relaxation function, τ is the characteristic relaxation time, I0 is the prefactor, and β is the stretch index ranging between 0 and 1. The stretchedexponential behavior originates from the wide distribution of activation energies and the associated wide range of relaxation times among different relaxation processes in a disordered system.

Electrical tests for a synapse device

1. Paired-Pulse Facilitation (PPF) and posttetanic potentiation (PTP):

For a certain amount of stimulations (such as 10 pulses) when the interval time between pulses is relatively short, at the read voltage, the memristor conductance is incremented. See the example below:

Dependence of the transition efficiency on stimulation rate. (a) Current through the memristor recorded after each stimulation pulse, at different pulse interval conditions.

(b) Current increase (ΔI) after every stimulus plotted against pulse number for different pulse interval conditions. Each measurement was repeated five times. Solid marks and error bars represent the mean and standard deviation (SD), respectively.

(c) Extracted I2
- I1 and I10 - I1, representing PPF and PTP, versus the interval of the stimulation pulses (in milliseconds). Inset shows the voltage profile used for this measurement. The lines are simply guides to the eyes.

Ref. of figure

2. STM to LTM transition experiment:

Not only the amplitude of the memristor conductance is improved upon repeated stimulation the retention time also improves significantly with stimulation. To verify this effect, we applied stimuli of identical voltage pulses with fixed height, width, and
pulse-to-pulse interval. Here different numbers of stimulations (for example N = 5 to N = 40 in steps of 5) are applied to the same memristor device starting from the same initial state, and the retention time at the small voltage (V = 0.1 V or < 0.5 V) will be recroded right afterthe last stimuli in each stimulation series.

Characteristic relaxation time (τ) then is obtained through the fitting in panel a (figure above) and the prefactor (I0) plotted with respect to the number of stimulations (N).

Ref. of figure

As can be seen the STM can only be sustained by constantly rehearsing the same stimulus, while LTM, despite the presence of natural forgetting (I mean the decay of retention time for each N), can be maintained for a much longer period of time without follow-up stimuli. 

We note that in human brain, forgetfulness is not always a disadvantage since it releases memory storage for more important or more frequently accessed pieces of information and is thought to be necessary for individuals to adapt to new environments.

Still more to write in this lesson