## 1 Introduction  
In this paper, we review the general features of the dissipative quantum model of brain and the role played by entanglement and quantum field theory (QFT) phase correlations in modeling brain functional activity. We start by recalling that in 1967, [PERSON] and [PERSON] proposed to study the brain as a many-body problem [1; 2]. In those years, QFT was achieving great successes in the study of condensed matter and elementary particle physics. The theoretical and experimental successful developments were indeed due in a substantial way to the structure of QFT, which is characterized by the existence of infinitely many unitarily inequivalent representations of the canonical commutation relations (CCR) [3; 4]. These representations describe physically different state spaces or phases of the system, and their existence allows the possibility of the spontaneous breakdown of symmetry, which in turn implies the dynamical formation of long-range correlations [5; 6; 7; 8]. In their paper [1], [PERSON] and [PERSON] were observing: \"First of all, at which level should the brain be studied and described? In other words, is it essential to know the behavior in time of any single neuron in order to understand the behavior of natural brains? Probably the answer is negative. The behavior of any single neuron should not be significant for the functioning of the whole brain, otherwise higher and higher degree of malfunctioning should be observed,... the activity of any single neuron is not significant, but rather the patterns of activity of clusters of them;... the existence of similar and almost simultaneous responses in several regions of the brain (a kind of long-range correlation) to a particular stimulation technique does not find any explanation in terms of activity of the single nerve cells: new non-classical mechanisms have to be looked for... it is strongly suggestive of a quantum model\". One of the motivations underlying the proposal by [PERSON] and [PERSON] was in the fact that although brain studies were improving and there were many progresses in molecular and cellular biology, the understanding of brain functional activity was still a fully-open problem. Neuroscientists were considering with great interest what was going on in other disciplines in the hope of finding possible help in their own field of study. For example, in the search for an explanation for the observed long-range correlations among neurons, [PERSON] [9; 10] suggested to apply to brain studies the concepts of the hologram and coherence developed in laser physics in those years [11]. The one posed by [PERSON] in the Nineteen Forties was in fact still an unsolved dilemma [12]: \"... Here is the dilemma. Nerve impulses are transmitted...form cell to cell through definite intercellular connections. Yet, all behavior seems to be determined by masses of excitation...within general fields of activity, without regard to particular nerve cells...What sort of nervous organization might be capable of responding to a pattern of excitation without limited specialized path of conduction? The problem is almost universal in the activity of the nervous system\". The concepts of \"field\" and \"masses of excitation\" in [PERSON]'s remarks were taken to be central ones in the successive years by [PERSON], who established a comprehensive quantitative treatment of brain activity in his celebrated book \"Mass action in the nervous system\" of 1975 [13]. Referring to his studies and to the state of neuroscience research in those years, he observed in 1991 [14] that \"My own group's studies, carried out over more than 30 years at the University of California at Berkeley, suggest that perception cannot be understood solely by examining properties of individual neurons, a microscopic approach that currently dominates neuroscience research. We have found that perception depends on the simultaneous, cooperative activity of millions of neurons spread throughout expanses of the cortex.