ABSTRACT
How information is encoded in the brain is a fundamental question that has been asked repeatedly in the course of decades of brain research. We should start by enquiring what is meant by a neural code. In general terms a code is a representation of an event existing in a given domain by means of a signal of a different quality. A simple example: a message in words may be encoded in a series of digits. In this case the essence of the code is an algorithm that converts letters into numbers. In the nervous system we know how the force of a muscle contraction is encoded in the frequency of ring of motorneurons, or how the intensity of a sensory stimulus is encoded in the pattern of ring of specic neurons along the sensory pathways. These are examples of rather simple neurophysiological phenomena. The situation is more complex as one goes from simple motor or sensory encoding toward the question of how cognitive functions are encoded in nervous activity. There exist some intriguing examples of how complex stimuli, like a face of an individual, may be encoded in the ring pattern of some rather specic neurons (Quian Quiroga et al. 2005, 2009). In a general perspective, experimental evidence supports the concept that encoding of complex cognitive processes takes place by means of spatiotemporal patterns of neural activity of populations of neurons, that is, by the joint ring patterns of the latter, as discussed below. Those neuronal ring patterns are reected in local eld potentials (LFPs) and local magnetic elds according to fundamental biophysical rules. The activity of neurons, however, is not limited to a display of series of action potentials but comprises also other changes of membrane currents caused by synaptic activities and intrinsic phenomena, in the form of membrane depolarizations and hyperpolarizations. Thus, LFPs reect combinations of the activities of neuronal populations that are manifested as the latter encode, transmit, and process information in the brain. LFPs are the basic building blocks of EEG/MEG
22.1 Introduction ......................................................................................................................... 431 22.2 Formation of Dynamical Neuronal Assemblies: Coactivation and the Emergence
of Oscillations .................................................................................................................... 432 22.3 Relationships between LFPs/EEG/MEG Signals and Cognitive Processes:
Enabling/Disabling, Modulating, and Encoding ................................................................ 433 22.4 Oscillatory Coding: Role of Frequency and Power of LFPs/EEG/MEG Signals .............. 435 22.5 Oscillatory Coding: Role of Phase of LFPs/EEG/MEG Signals ........................................ 435 22.6 Multiple Combinations of EEG/MEG Signals Associated with Cognitive Processes ....... 437 22.7 Slow EEG Signals Enabling/Modulating the Encoding of Memories ............................... 438 22.8 EEG/MEG Signals Modulating Attention and Encoding of Information .......................... 439 22.9 Encoding of Information in the Gamma Frequency Range ............................................... 441 22.10 Interactions between Brain Oscillations Associated with Cognitive Processes ..................442 22.11 Perspectives and Concluding Remarks ............................................................................... 443 References ......................................................................................................................................444
