**4.1 Types of LTD**

LTD can be either homosynaptic or heterosynaptic. Homosynaptic LTD is induced by a conditioning input. It is input specific. It is restricted to the individual synapse which is activated by a low frequency stimulus (LFS) i.e., it happens in the same synapse that receives the induction. It is associative and it correlates with postsynaptic activation of the neuron by an active presynaptic neuron. Homosynaptic LTD is in turn of two types. LTD which follows an LTP is often known as depotentiation. If LTD is observed from base line conditions, with low frequency stimulus, then it is *de novo* LTD. Heterosynaptic LTD refers to depression at synapses neighboring the activated ones but are not directly activated themselves (Abraham et al., 2007). Heterosynaptic LTD occurs at synapses that are not potentiated. It occurs consequent to a non-conditioning input in association with either LTP or LTD. LTD relies on both pre and postsynaptic expression mechanisms although the maintenance mechanism is not fully understood (Bliss and Cooke, 2011).
