**4. Long term depression**

LTD is an activity dependent reduction in the efficacy of neuronal synapses. It can generally last for hours or longer. It brings about a long lasting decrease in synaptic strength. LTD can be defined as a long lasting decrease in the synaptic response of neurons to stimulation of their afferents following a long patterned stimulus (Collingrigde et al., 2010). LTD is generally considered as a reversal of LTP as it is understood that if synapses continue to increase in strength, eventually they would reach some level of maximum efficacy which might cause saturation and then they may be unable to encode new information. This would result in neurons coming to a stage of complete inactivity or over activity. LTD is also considered to be the initial step in synaptic elimination (Bastrikova et al., 2008; Beckner et al., 2008) as it is known that those synapses which lose their efficacy are eliminated.
