  The brain has the phenomenal ability to reorganize itself by forming new
connections among neurons and by pruning others. The so-called neural or brain
plasticity facilitates the modification of brain structure and function over
different time scales. Plasticity might occur due to external stimuli received
from the environment, during recovery from brain injury, or due to
modifications within the body and brain itself. In this paper, we study the
combined effect of short-term (STP) and spike-timing-dependent plasticities
(STDP) on the synaptic strength of excitatory coupled Hodgkin-Huxley neurons
and show that plasticity can facilitate the formation of modular neural
networks with complex topologies that resemble those of networks with
preferential attachment properties. In particular, we use an STDP rule that
alters the synaptic coupling intensity based on time intervals between spikes
of postsynaptic and presynaptic neurons. Previous works have shown that STDP
may induce the appearance of directed connections from high to low-frequency
spiking neurons. On the other hand, STP is attributed to the release of
neurotransmitters in the synaptic cleft of neurons that alter its synaptic
efficiency. Our results suggest that the combined effect of STP and STDP with
high recovery time facilitates the formation of connections among neurons with
similar spike frequencies only, a kind of preferential attachment. We then
pursue this further and show that, when starting with all-to-all neural
configurations, depending on the STP recovery time and distribution of neural
frequencies, modular neural networks can emerge as a direct result of the
combined effect of STP and STDP.
