Gamma-Rhythmic Gain Modulation

Cognition requires the dynamic modulation of effective connectivity, i.e., the modulation of the postsynaptic neuronal response to a given input. If postsynaptic neurons are rhythmically active, this might entail rhythmic gain modulation, such that inputs synchronized to phases of high gain benefit from enhanced effective connectivity. We show that visually induced gamma-band activity in awake macaque area V4 rhythmically modulates responses to unpredictable stimulus events. This modulation exceeded a simple additive superposition of a constant response onto ongoing gamma-rhythmic firing, demonstrating the modulation of multiplicative gain. Gamma phases leading to strongest neuronal responses also led to shortest behavioral reaction times, suggesting functional relevance of the effect. Furthermore, we find that constant optogenetic stimulation of anesthetized cat area 21a produces gamma-band activity entailing a similar gain modulation. As the gamma rhythm in area 21a did not spread backward to area 17, this suggests that postsynaptic gamma is sufficient for gain modulation

A new unifying account of the roles of neuronal entrainment

Rhythms are a fundamental and defining feature of neuronal activity in animals including humans. This rhythmic brain activity interacts in complex ways with rhythms in the internal and external environment through the phenomenon of ‘neuronal entrainment’, which is attracting increasing attention due to its suggested role in a multitude of sensory and cognitive processes. Some senses, such as touch and vision, sample the environment rhythmically, while others, like audition, are faced with mostly rhythmic inputs. Entrainment couples rhythmic brain activity to external and internal rhythmic events, serving fine-grained routing and modulation of external and internal signals across multiple spatial and temporal hierarchies. This interaction between a brain and its environment can be experimentally investigated and even modified by rhythmic sensory stimuli or invasive and non-invasive neuromodulation techniques. We provide a comprehensive overview of the topic and propose a theoretical framework of how neuronal entrainment dynamically structures information from incoming neuronal, bodily and environmental sources. We discuss the different types of neuronal entrainment, the conceptual advances in the field, and converging evidence for general principles

Assessing Neuronal Synchrony and Brain Function Through Local Field Potential and Spike Analysis

Studies of neuronal network oscillations and rhythmic neuronal synchronization have led to a number of important insights in recent years, giving us a better understanding of the temporal organization of neuronal activity related to essential brain functions like sensory processing and cognition. Important principles and theories have emerged from these findings, including the communication through coherence hypothesis, which proposes that synchronous oscillations render neuronal communication effective, selective, and precise. The implications of such a theory may be universal for brain function, as the determinants of neuronal communication inextricably shape the neuronal representation of information in the brain. However, the study of communication through coherence is still relatively young. Since its articulation in 2005, the theory has predominantly been applied to assess cortical function and its communication with downstream targets in different sensory and behavioral conditions. The results herein are intended to bolster this hypothesis and explore new ways in which oscillations coordinate neuronal communication in distributed regions. This includes the development of new analytic tools for interpreting electrophysiological patterns, inspired by phase synchronization and spike train analysis. These tools aim to offer fast results with clear statistical and physiological interpretation

State-dependent modulation of cortico-spinal networks

Beta-band rhythm (13-30 Hz) is a dominant oscillatory activity in the sensorimotor system. Numerous studies reported on links between motor performance and the cortical and cortico-spinal beta rhythm. However, these studies report divergent beta-band frequencies and are, additionally, based on differently performed motor-tasks (e.g., motor imagination, muscle contraction, reach, grasp, and attention). This diversity blurs the role of beta in the sensorimotor system. It consequently challenges the development of beta-band activity-dependent stimulation protocols in the sensorimotor system. In this vein, we studied the functional role of beta-band cortico-cortical and cortico-spinal networks during a motor learning task. We studied how the contribution of cortical and spinal beta changes in the course of learning, and how this modulation is affected by afferent feedback to the sensorimotor system. We furthermore researched the relationship to motor performance. Consider that we made our study in the absence of any residual movement to allow our findings to be translated into rehabilitation programs for severely affected stroke patients. This thesis, at first, investigates evoked responses after transcranial magnetic stimulation (TMS). This revealed two different beta-band networks, i.e., in the low and high beta-band reflecting cortical and cortico-spinal activity. We, then, used a broader frequency range in the beta-band to trigger passive opening of the hand (peripheral feedback) or cortical stimulation (cortical feedback). While a unilateral hemispheric increase in cortico-spinal synchronization was observed in the group with peripheral feedback, a bilateral hemispheric increase in cortico-cortical and cortico-spinal synchronization was observed for the group with cortical feedback. An improvement in motor performance was found in the peripheral group only. Additionally, an enhancement in the directed cortico-spinal synchronization from cortex to periphery was observed for the peripheral group. Similar neurophysiological and behavioral changes were observed for stroke patients receiving peripheral feedback. The results 6 suggest two different mechanisms for beta-band activity-dependent protocols depending on the feedback modality. While the peripheral feedback appears to increase the synchronization among neural groups, cortical stimulation appears to recruit dormant neurons and to extend the involved motor network. These findings may provide insights regarding the mechanism behind novel activity-dependent protocols. It also highlights the importance of afferent feedback for motor restoration in beta-band activity-dependent rehabilitation programs

Dynamic coupling between whisking, barrel cortex, and hippocampus during texture discrimination: A role for slow rhythms

Increasing amounts of work have demonstrated that brain rhythms might constitute clocking mechanisms against which to coordinate sequences of neural firing; such rhythms may be essential to the coding operations performed by the local networks. The sequence of operations underlying a tactile discrimination task in rats requires the animal to integrate two streams of information, those coming from the environment and, from reference memory the rules that dictate the correct response. The current study is a follow up on the work which has described the hippocampal representation of the tactile guided task. We have used a well-established texture discrimination task, in which rats have to associate two stimuli with two different reward locations. We placed microelectrodes in primary somatosensory cortex and the CA1 region of hippocampus to perform recordings of spiking activity and local field potentials when the animal touched the discriminandum as well as when he was in a resting state. We also performed recording on an arena in which the animal moved freely and did not perform any task. Earlier work has demonstrated that tactile signals reach the hippocampus during texture discrimination, presumably through the somatosensory cortex. We predicted that neurons in the primary somatosensory cortex (S1) are entrained to the oscillatory theta rhythm that permeates the hippocampus. Our expectation is that such coherence could serve to increase the reliability of synaptic transmission, linking the acquisition of new sensory information with associative processes. We addressed the following issues: Is the timing of action potentials in S1 modulated by the ongoing hippocampal theta rhythm? If so, is the occurrence of this modulation aligned in time to the period in which the hippocampus acquires tactile signals? We also predicted that the 10-Hz whisking that characterizes the acquisition of texture information would be more strongly phase locked to theta rhythm than the whisking in the air that is not accompanied by any explicit tactile task. We speculate that such phase locking could be a means to synchronize sensory and hippocampal processing. The notion that the coordination between brain areas might be related to the rhythmic of sensorimotor cycles is particularly appealing. We have found that the firing of 18% of barrel cells was significantly modulated by hippocampal theta during the half-second period of active tactile discrimination. Importantly, we found that during periods of rest interleaved in the session, neurons significantly decreased the degree of phase-locking with respect to touch. We hypothesize that areas involved with motivational processes as basal ganglia could gate the entrainment during task related epochs. S1 neurons were classified as those excited by contact with the discriminandum, and those not excited by contact. The firing of both sorts of neurons was modulated by CA1 theta rhythm during exploration of the texture. However the theta phase to which they fired preferentially was opposite; contact-responsive neurons tended to fire in the upward phases of the cycle whereas contact non-responsive neurons tended to fire in the downward phase of the cycle suggesting that theta rhythm might have the function of temporally separating sensory cortical neurons according to their functional properties and the information they carry. By clustering touch-sensitive neurons to a certain time window and separating them from \u2018non-informative\u2019 neurons, theta rhythm could increase the efficiency not only of information tranfer to hippocampus but also the efficiency of information encoding/decoding. We also found phase and amplitude relationships between whisking and hippocampal theta during the goal-directed tactile task; the relationships disappear when the animal moves along an open arena, still actively whisking but not engaged in the texture discrimination task. We were able to show, for the first time to our knowledge, that CA1 theta rhythm can exert a behavioral state-dependent modulatory effect on sensory cortex. S1 neuron firing and whisking activity are entrained to hippocampal theta rhythm when the animal collects meaningful tactile information from the environment

Fractals in the Nervous System: conceptual Implications for Theoretical Neuroscience

This essay is presented with two principal objectives in mind: first, to document the prevalence of fractals at all levels of the nervous system, giving credence to the notion of their functional relevance; and second, to draw attention to the as yet still unresolved issues of the detailed relationships among power law scaling, self-similarity, and self-organized criticality. As regards criticality, I will document that it has become a pivotal reference point in Neurodynamics. Furthermore, I will emphasize the not yet fully appreciated significance of allometric control processes. For dynamic fractals, I will assemble reasons for attributing to them the capacity to adapt task execution to contextual changes across a range of scales. The final Section consists of general reflections on the implications of the reviewed data, and identifies what appear to be issues of fundamental importance for future research in the rapidly evolving topic of this review

On the role of neuronal oscillations in auditory cortical processing

Although it has been over 100 years since William James stated that everyone knows what attention is , its underlying neural mechanisms are still being debated today. The goal of this research was to describe the physiological mechanisms of auditory attention using direct electrophysiological recordings in macaque primary auditory cortex (A1). A major focus of my research was on the role ongoing neuronal oscillations play in attentional modulation of auditory responses in A1. For all studies, laminar profiles of synaptic activity, (indexed by current source density analysis) and concomitant firing patterns in local neurons (multiunit activity) were acquired simultaneously via linear array multielectrodes positioned in A1. The initial study of this dissertation examined the contribution of ongoing oscillatory activity to excitatory and inhibitory responses in A1 in passive (no task) conditions. Next, the function of ongoing oscillations in modulating the frequency tuning of A1 during an intermodal selective attention oddball task was investigated. The last study was aimed at establishing whether there is a hemispheric asymmetry in the way neuronal oscillations are utilized by attention, corresponding to that noted in humans. The results of the first study indicate that in passive conditions, ongoing oscillations reset by stimulus related inputs modulate both excitatory and inhibitory components of local neuronal ensemble responses in A1. The second set of experiments demonstrates that this mechanism is utilized by attention to modulate and sharpen frequency tuning. Finally, we show that as in humans, there appears to be a specialization of left A1 for temporal processing, as signified by greater temporal precision of neuronal oscillatory alignment. Taken together these results underline the importance of neuronal oscillations in perceptual processes, and the validity of the macaque monkey as a model of human auditory processing

Music in the brain

Music is ubiquitous across human cultures — as a source of affective and pleasurable experience, moving us both physically and emotionally — and learning to play music shapes both brain structure and brain function. Music processing in the brain — namely, the perception of melody, harmony and rhythm — has traditionally been studied as an auditory phenomenon using passive listening paradigms. However, when listening to music, we actively generate predictions about what is likely to happen next. This enactive aspect has led to a more comprehensive understanding of music processing involving brain structures implicated in action, emotion and learning. Here we review the cognitive neuroscience literature of music perception. We show that music perception, action, emotion and learning all rest on the human brain’s fundamental capacity for prediction — as formulated by the predictive coding of music model. This Review elucidates how this formulation of music perception and expertise in individuals can be extended to account for the dynamics and underlying brain mechanisms of collective music making. This in turn has important implications for human creativity as evinced by music improvisation. These recent advances shed new light on what makes music meaningful from a neuroscientific perspective

Music in Research and Rehabilitation of Disorders of Consciousness: Psychological and Neurophysiological Foundations

According to a prevailing view, the visual system works by dissecting stimuli into primitives, whereas the auditory system processes simple and complex stimuli with their corresponding features in parallel. This makes musical stimulation particularly suitable for patients with disorders of consciousness (DoC), because the processing pathways related to complex stimulus features can be preserved even when those related to simple features are no longer available. An additional factor speaking in favor of musical stimulation in DoC is the low efficiency of visual stimulation due to prevalent maladies of vision or gaze fixation in DoC patients. Hearing disorders, in contrast, are much less frequent in DoC, which allows us to use auditory stimulation at various levels of complexity. The current paper overviews empirical data concerning the four main domains of brain functioning in DoC patients that musical stimulation can address: perception (e.g., pitch, timbre, and harmony), cognition (e.g., musical syntax and meaning), emotions, and motor functions. Music can approach basic levels of patients’ self-consciousness, which may even exist when all higher-level cognitions are lost, whereas music induced emotions and rhythmic stimulation can affect the dopaminergic reward-system and activity in the motor system respectively, thus serving as a starting point for rehabilitation