More than Spikes: Common Oscillatory Mechanisms for Content Specific Neural Representations during Perception and Memory This Review Comes from a Themed Issue on Brain Rhythms and Dynamic Coordination Sciencedirect Independent Contributions of Lfp Power and Phase to Neural Representation Content-spe

Although previous research into the mechanisms underlying sensory and episodic representations has primarily focused on changes in neural firing rate, more recent evidence suggests that neural oscillations also contribute to these representations. Here, we argue that multiplexed oscillatory power and phase contribute to neural representations at the mesoscopic scale, complementary to neuronal firing. Reviewing recent studies which used oscillatory activity to decipher content-specific neural representations, we identify oscillatory mechanisms common to both sensory and episodic memory representations and incorporate these into a model of episodic encoding and retrieval. This model advances the idea that oscillations provide a reference frame for phase-coded item representations during memory encoding and that shifts in oscillatory frequency and phase coordinate ensemble activity during memory retrieval. Introduction Reliable, flexible representations of the external world are crucial for encoding sensory perceptions into memories. Decades of research have documented changes in neural firing rate to varied and unique (''content specific'') environmental features. Beyond studies of single neuron firing rates, more recent work has investigated how the synchronized activity of many neurons, which results in oscillations of the local field potential (LFP) and the electroencephalogram (EEG), contributes to neural representation. Local and inter-areal synchronization supports dynamic coordination of incoming sensory information [1,2], inter-areal interactions [3,4], and synaptic plasticity [5,6], each of which are integral to both perception and memory. Thus, we argue that oscillations may be particularly relevant for encoding perceptual experiences into memory representations by coordinating spatially distributed and co-activated neuronal groups (i.e. cell assemblies). These coordinated representations thus constitute memory traces, or ''engrams'' [7,8]. Neural oscillations influence neural activity and human behavior We begin by addressing general concerns regarding the role of oscillations in the ''neural code'', which is dis-tinguishable in two senses. Coding in a ''strong'' sense implies that neural computation is causally driven by some configuration of spikes or extracellular signal, which implies that the brain is using this code to represent information. Coding in a ''weak'' sense is descriptive and refers to the decodability of some mental state by observing a brain state characterized by different metrics such as power, phase, and frequency. Although there is limited evidence for oscillatory coding via ephaptic effects in the ''strong'' sense, considerable evidence using pattern classification and information theoretic approaches indicates a role for oscillatory coding in the ''weak'' sense, as discussed below. The LFP is primarily a manifestation of aggregated synaptic activity and other transmembrane potentials …