It’s all about adaptation: MMN as a by-product of synaptic plasticity and temporal binding
Leibnitz Institute for Neurobiology, Magdeburg, Germany
I consider the possibility that the MMN, rather than reflecting a dedicated change-detection mechanism, results from the general ability of the auditory cortex to represent temporally evolving complex sounds. Such an ability requires a memory system which binds information over time so that incoming sounds are represented in the context of preceding events. The neural mechanism producing this type of binding has remained elusive. Recent computational modelling results suggest that short-term synaptic plasticity - expressed as a variety of adaptation phenomena - offers just such a binding mechanism. In computational simulations of auditory cortex, plasticity/adaptation is necessary for core, belt, and parabelt columns to respond selectively to speech sounds and to other spectro-temporally complex stimuli. The simulations also demonstrate that the structure and short-term plasticity of auditory cortex are sufficient for MMN responses to arise in a variety of stimulation conditions, without the need to invoke separate change-detection mechanisms and MMN generators. Thus, in the context of explaining responses in the oddball paradigm, adaptation has two intertwined roles: First, it leads to the suppression of the N1 response to the standard. Second, it provides - perhaps counterintuitively - the mechanism whereby cortex differentiates between the standard and the deviant. Thus, it is possible that while adaptation modulates the N1m response and therefore underlies the MMN, its real functional significance is its contribution to temporal binding and, hence, to the ability of cortical cells to respond selectively to complex stimuli found in the natural environment.