Fig 2. Hebbian learning with homeostatic MPDP on inhibitory synapses.

A conductance based integrate-and-fire neuron is repeatedly presented with a fixed input pattern of activity in presynaptic inhibitory or excitatory neuron populations (top row—blue dots are excitatory, red dots are inhibitory spike times). The number of input neurons is N_i = 142 for the inhibitory population and N_e = 571 for the excitatory population. Second row shows the membrane potential before learning. The upper red line is the threshold for potentation of inhibitory synapses ${\vartheta }_P^I$, lower red line is resting potential and threshold for depression ${\vartheta }_D^I$. The third row shows the voltage as before with added teacher input by an additional population of excitatory neurons; this input induces a spike at t = 100ms. The fourth row shows the voltage after 100 learning steps with MPDP on inhibitory synapses only, with teacher input, the next row shows the recall without teacher. The spike is almost at the same position in the recall case. The last row shows the voltage after 1000 recall trials during which the inhibitory synapses were allowed to change under the MPDP rule. Despite this, the output spike is still close to the desired time, which shows that the output is approximately stable.