Final answer:
The question involves synaptic efficacy in neuroscience, which is affected by synaptic plasticity involving processes such as LTP and LTD. These processes can be influenced by the presence of neuromodulators or signaling molecules acting as a 'third party'.
Step-by-step explanation:
The question addresses the concept of synaptic efficacy, which refers to the strength or potency of a synaptic transmission. In neuroscience, synaptic efficacy can be altered through a process known as synaptic plasticity. This involves changes in the synapse strength due to increased or decreased activity. Co-activity, or the simultaneous activation of pre- and postsynaptic neurons, can lead to long-term potentiation (LTP) or long-term depression (LTD), which are long-lasting increases and decreases in synaptic strength, respectively.
LTP and LTD are thought to be cellular mechanisms underlying learning and memory. The presence or absence of a third party, mentioned in the question, refers to neuromodulators or signaling molecules that can modulate synaptic plasticity. For example, neurotransmitters like dopamine, serotonin, or even ions such as calcium can act as a 'third party' to influence whether synaptic potentiation or depression occurs.
As a result, synaptic plasticity is not just a binary interaction between two neurons but a complex process that can be modulated by other factors, like neuromodulators, which impact whether and how synaptic changes take place following neuronal co-activity.