Visual Processing by Hierarchical and Dynamic Multiplexing

The complexity of natural environments requires highly flexible mechanisms for adaptive processing. Furthermore, the processing of single and multiple stimuli is strongly context and goal dependent. The large repertoire of frequencies and waveforms of what is commonly referred to as neuronal oscillations could be an ideal candidate for implementing such flexibility in neural systems. Here we present a framework in which highly dynamic multiplexing and phase coding schemes underlie and structure the attention-guided processing of complex visual scenes. Importantly, we suggest that the dynamic fluctuations of excitability vary rapidly in terms of magnitude, frequency and wave-form over time, i.e. they are not necessarily sustained oscillations. We propose that different elements of a single object should be processed within a single cycle (or burst) of alpha band activity (7-14Hz). This allows for the formation of coherent object representations while simultaneously separating multiple objects across multiple alpha cycles. Each element (e.g. the eyes or ears of the object cat) would still be processed separately in time - expressed as different gamma band bursts (> 30Hz) - along the alpha phase. Since the processing capacity per alpha cycle is limited, an inverse relationship between object resolution and size of attentional spotlight ensures independence of the proposed mechanism from absolute object complexity. The specific frequency and wave-shape of the respective fluctuations involved would depend on the nature of the object that is processed and on cognitive demands. Additionally, we suggest that the processing of multiple objects would further be organized along the phase of flexible slower fluctuations (e.g. delta or theta). Alternatively, saccades could drive the reset of this slow fluctuations. Complex scene processing and exploration, involving covert attention and eye movements, would therefore be associated with multiple frequency changes, both in the alpha and lower frequency range. This framework therefore embraces the idea of a hierarchical organization of visual scene processing, independent of the temporal dynamics of the environment.