Final answer:
Decreases in synaptic efficacy, known as long-term depression (LTD), can occur in various brain states which include waking, NREM sleep, and REM sleep, and do not depend on the specific sequence of neuronal firing.
Step-by-step explanation:
Decreases in synaptic efficacy can occur irrespective of the ordering of firing of the presynaptic and postsynaptic neuron in the state of long-term depression (LTD). LTD is characterized by a prolonged decrease in synaptic coupling between these neurons and can happen in various brain states, including waking, NREM sleep, and REM sleep. This is different from long-term potentiation (LTP), which typically requires a specific sequence of neuronal firing to strengthen synaptic connections. Importantly, the process of LTD is crucial for learning and memory as it facilitates the weakening and pruning of less-used synaptic connections.