Abstract
There is considerable interest in the role of coupling between theta and gamma oscillations in the brain in the context of learning and memory. Here we have used a neural network model which is capable of producing coupling of theta phase to gamma amplitude firstly to explore its ability to reproduce reported learning changes and secondly to memory-span and phase coding effects. The spiking neural network incorporates two kinetically different GABAA receptor-mediated currents to generate both theta and gamma rhythms and we have found that by selective alteration of both NMDA receptors and GABAA,slow receptors it can reproduce learning-related changes in the strength of coupling between theta and gamma either with or without coincident changes in theta amplitude. When the model was used to explore the relationship between theta and gamma oscillations, working memory capacity and phase coding it showed that the potential storage capacity of short term memories, in terms of nested gamma-subcycles, coincides with the maximal theta power. Increasing theta power is also related to the precision of theta phase which functions as a potential timing clock for neuronal firing in the cortex or hippocampus.
Highlights
The roles of different brain oscillatory rhythms, either alone or in combination, in controlling learning and memory functions have been the subject of intensive investigation and speculation
Biophysical models for generating hippocampal theta and gamma rhythms have already been provided by Kopell et al in [23], where it was claimed that theta nested-gamma activity is due to the h-current in the oriens-lacunosum molecular cell in the hippocampus
GGAserepresents the connection from inhibitory slow (INs) neuron to EX neuron mediated by GABAA receptors, gNMee represents the recurrent connection among EX neurons mediated by NMDA receptors, etc
Summary
The roles of different brain oscillatory rhythms, either alone or in combination, in controlling learning and memory functions have been the subject of intensive investigation and speculation. Local field potential (LFP) recordings in the hippocampus have shown that low frequency theta oscillations (4–8 Hz) are important in carrying information about memory processes [1,2] and function to decreasing reaction times in decision making tasks [3]. EEG and MEG as well as LFP recordings have revealed that synchronous firing of a group of neurons in visual processing is associated with binding problem in which gamma synchronization can combine features in a visual scene to form a coherent percept [7,8]. Modulation of oscillatory synchronisation can lead to the increase in synaptic gain at postsynaptic target sites thereby potentiating responses to learned stimuli [8,9]
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