![]() ![]() The magnitude of both theta and gamma oscillations during encoding also appears to predict the efficacy of subsequent recall and theta can both modulate gamma amplitude and the firing of single neurons. Some studies report that theta phase rather than amplitude is correlated with cognitive processes, the so-called phase reset model, while others place more importance on coupling between theta amplitude and gamma frequency. There is still debate as to whether functionally important changes in theta or gamma involve amplitude or phase parameters, or both. Altered coupling has been reported both in the context of human cognitive and perceptual tasks in the cortex and in the rat hippocampus during item-context association learning, although how this might act to modulate neuronal activity has yet to be established. It may also provide a process of temporal segmentation that can maintain multiple working memory items. Ĭoupling between gamma amplitude and theta phase (theta-nested gamma) has been reported in both cortex and hippocampus and provides an effective combination for neuronal populations to communicate and integrate information during visual processing and learning. Modulation of oscillatory synchronization can also increase synaptic gain at postsynaptic target sites thereby potentiating responses to learned stimuli. Human electroencephalographic (EEG) recordings show event-related gamma activity indicating gamma as a signature of cortical networks underlying object representations. ![]() Fast frequency gamma oscillations (30-70 Hz) can provide tighter control and coordination than lower frequency ones and are hypothesised to be responsible for higher cognitive functions such as perceptual binding of visual features. These findings may reflect the patterns of synaptic plasticity and maintenance of the memory for a stimulus. In hippocampus the phase of theta functions as the clock signal for timing of pyramidal neurons and long-term potentiation (theta peaks) and depotentiation (theta troughs). Low frequency theta oscillations (4-8 Hz) have been observed to increase in terms of power during working memory tasks and in power and phase-locked discharge of single neurons in a visual memory task. The functions of both low and high frequency oscillations in the brain are the subject of considerable speculation. In this way learning can produce potentiation in neural networks simply through altering the temporal pattern of their inputs. A network model which can reproduce these changes suggests that a key function of such learning-evoked alterations in theta and theta-nested gamma activity may be increased temporal desynchronization in neuronal firing leading to optimal timing of inputs to downstream neural networks potentiating their responses. Conclusionsįace discrimination learning produces significant increases in both theta amplitude and the strength of theta-gamma coupling in the inferotemporal cortex which are correlated with behavioral performance. This desynchronization effect was confirmed in IT neuronal activity following learning and its magnitude was correlated with discrimination performance. The model showed that these changes could potentiate the firing of downstream neurons by a temporal desynchronization of excitatory neuron output without increasing the firing frequencies of the latter. By increasing N-methyl-D-aspartate receptor sensitivity in the model similar changes were produced as in inferotemporal cortex after learning. The neural network model developed showed that a combination of fast and slow inhibitory interneurons could generate theta-nested gamma. Neuronal activity was phase-locked with theta but learning had no effect on firing rates or the magnitude or latencies of visual evoked potentials during stimuli. ![]() Actual discrimination performance was significantly correlated with theta and theta-gamma coupling changes. The strength of this coupling was also increased following learning and this was not simply a consequence of increased theta amplitude. Over 75% of electrodes showed significant coupling between theta phase and gamma amplitude (theta-nested gamma). Resultsįollowing learning the amplitude of theta (4-8 Hz), but not gamma (30-70 Hz) oscillations was increased, as was the ratio of theta to gamma. A neural network model has been developed to simulate and aid functional interpretation of learning-evoked changes. Local field potential and multi-unit neuronal activity recordings were made from 64-electrode arrays in the inferotemporal cortex of conscious sheep during and after visual discrimination learning of face or object pairs. How oscillatory brain rhythms alone, or in combination, influence cortical information processing to support learning has yet to be fully established.
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