ABSTRACT
The brain is a highly interconnected network of neurones, in which the activity in any neurone is necessarily related to the combined activity in the neurones that are afferent to it. Due to the widespread presence of reciprocal connections between brain areas, reentrant activity through chains of neurones is likely to occur. Certain pathways through the network may be favoured by inhomogeneity in the number or efficacy of synaptic interactions between the neural elements as a consequence of developmental and/or learning processes. In cell assemblies interconnected in this way, some ordered sequences of intervals within spike trains of individual neurones, and across spike trains recorded from different neurones, will recur. Such recurring, ordered, and precise (in the order of few ms) interspike interval relationships are referred to as “spatiotemporal patterns” of discharges. This term encompasses both their precision in time, and the fact that they can occur across different neurones, even recorded from separate electrodes. This chapter introduces the fundamental assumptions and algorithms that lead to the detection of complex patterns of neural discharges and introduces a way of interpreting this activity within the framework of non-linear dynamics. Empirical results of experimental and simulation studies are provided in different sections of the chapter. KEYWORDS: Spatiotemporal firing patterns; Neural dynamics; Brain theory; Time code; Frequency code; Multi-unit recordings; Non-linear dynamics; Sensorimotor association
1. BRIEF HISTORICAL INTRODUCTION
In 1753-1755 the physiologist Albrecht von Haller published in Göttingen an historical essay, the “Dissertation on the Irritable and Sensitive Parts of Animals” (original title: De partium corporis humani sensibilius et irritabilus). This work was based on numerous experiments of vivisection, and on stimulation of organs using the new knowledge
offered to physiology by physics, chemistry and natural history. With a rudimentary technique of stimulation, Von Haller classified the parts as irritable, sensible or elastic and noted that the reactions varied between different parts of the brain. The historical importance of the work by Von Haller is not so much related to the results obtained, but rather in systematically developing a transdisciplinary approach to brain research using the new technologies of his time. In 1791 the Italian physician Luigi Galvani started the publication of a remarkable series of studies that demonstrated muscle twitch in a frog by touching its nerves with electrostatically charged metal, and later using two dissimilar uncharged metals. These observations led Galvani to postulate that the circulation of a particular body fluid, that exists naturally in the nerves in a state of disequilibrium, provided the stimuli for the muscle fibres to contract. In addition, normal muscular contraction without a source of electrostatic electricity was, in Galvani’s view, evidence for the existence of an additional and “natural” form of electricity, that he called “animal electricity”. This latter statement set the scene for the famous Galvani-Volta controversy. Alessandro Volta, a friend of Galvani and an Italian physicist who, in 1775, invented the electrophorus-a device to generate static electricity-gave an opposed and experimentally valid explanation of Galvani’s experiment. The electricity did not come from the animal tissue but was generated by the contact of different metals, brass and iron, in a moist environment. The interest in this controversy resides in the confrontation of two basic, irreducible interpretations of the same observation, derived from each scientist’s different background: Galvani saw the frog phenomenon as the work of biological organs, Volta as that of a physical apparatus. The outcome of the controversy was exceptional. On the one hand the challenge of Volta’s opinion led Galvani to perform a new series of experiments that demonstrated muscular contraction by touching the exposed muscle of one frog with the nerve of another frog, thus showing for the first time the existence of bioelectric forces. On the other hand, Volta focused his research efforts upon the study of electric fluids between dissimilar metals, and in 1800 he presented the first electric battery, providing future researchers with a stable source of electricity not dependent on electrostatic forces.
