A localised group of neurons firing synchronously at 30-100 hz is referred to as a local field potential gamma oscillation. These are important for spike-timing-dependent plasticity to occur. Synchronized activity of 10–30 ms in the gamma frequency create a narrow time window for the coincident activation of pre-synaptic and post-synaptic cell used for STDP (for more details read here). Slower oscillations do not provide a narrow enough window and faster oscillations, having more than one cycle in the STDP window, cause the post-synaptic cell to receive inputs both before and after having generated a spike.
However, STDP occurs if pre-synaptic and post-synaptic action potentials are correlated. Notably this occurs even if two cells with equally weak inputs correlate, which is not the kind of result that is useful to learning as we wish to learn strong coincidences. Gamma synchronization is not necessarily time-locked to a stimulus. Due to these two reasons long term potentiation (strengthening) of synapses induced by synchronized gamma activity alone does not attain the specificity of memory encoding, but an additional mechanism is required.
The hippocampus is considered to play a major role in memory. Learning-dependent synchronization of hippocampal theta activity is associated with large event-related potentials with frequency in the theta (4-8 hz) and delta (0-4 hz) range that appear to result from phase the reset of theta activity occurring at a fixed interval after presentation of a stimulus. Theta reset determines the theta phase at which a given stimulus affects a cell. Theta band learning is non-Hebbian and only involves pre-synaptic and not post-synaptic spikes. If the stimuli arrive during the peak of the theta oscillation long term potentiation (strengthening of synapses) occurs, inputs arriving at a trough of the theta cycle induce long term depression (weakening of synapses). Axmacher et al note that a combination between theta and gamma learning dynamics my provide the required specificity for memory learning:
‘Whereas gamma-dependent plasticity alone may not distinguish between correlated weak and strong inputs and occurs not necessarily time-locked to a given stimulus, plasticity during theta reset has these features. Theta-dependent plasticity alone, on the other hand, is too coarse to encode stimulus features with a high temporal resolution: at least Hebbian LTP requires precise spike timing. Moreover, sequence encoding (sequences of items as well as spatial paths) has been suggested to depend on action potentials during subsequent theta phases, with gamma periods binding each item.’