User:DavidFieni/sandbox

The Neuroscience of Rhythm refers to the various forms of rhythm generated by the brain. This could refer to something as simple as clapping your hands to a beat or something as complicated as the circadian rhythm. Rhythm is defined as a "movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions." . Neurons in the human brain are capable of producing electrical signals by firing in specific patterns. While not much is known about the cognitive side of pattern generation contribute to various rhythms, there are several models, both computational and animal, that attempt to explain this phenomena. Furthermore, a great deal more is known about central pattern generation. Collectively these unique oscillations not only allow humans to enjoy music or walk but they govern the very processes that keep us alive.

Models
Many computational models have attempted to quantify the process of how various rhythms are created by humans. Most of these are a computational attempt to use knowledge from signal processing or computer science in order to explain how a beat could be perceived and generated. This is an extremely effective method to discover the underlying mechanisms of rhythmic processing.

Avian Song Learning
One of the most useful model of pattern recognition and generation is found in birds. Many species of young birds posses the ability to recognize, learn, and generate a new song upon reaching adulthood. Two very famous computational neuroscientist Kenji Doya and Terrence J. Sejnowski created a model of this using the zebra finch as a target organism. The zebra finch is perhaps one of most easily understood examples of this among birds. The young zebra finch is exposed to a "tutor song" from the adult, during a critical period. This is defined as the time of life that learning can take place, in other words when the brain has the most plasticity. After this period, the bird is able to produce an adult song, which is said to be crystallized at this point. Doya and Sejnowski evaluated three possible ways that this learning could happen. The first was an immediate, one shot perfection of the tutor song. While this is a very attractive model for certain species such as humans, it was unlikely that this took place is birds because the learning process takes a long time. The second scheme was that of error learning. This refers to signal generated by the avian brain that corresponds to the error between the tutor song and newly generated template song. This is a very reasonable explanation, so reasonable if fact that it is the adopted theory of more current neuroscientist, such as Dr. Sam Sober of Emory University. Unfortunately Doya and Sejnowski found too many issues with most of the models based on this at the time, which led the to stray away from this explanation. They settled on the third scheme which uses reinforcement learning to explain the process. Reinforcement learning consists of a "critic" in the brain capable of evaluating the difference between the tutor and the template song. Assuming the two are closer than the last trial, this "critic" then sends a signal activating NMDA receptors on the articulator of the song. In the case of the zebra finch, this articulator is the robust nucleus of archistriatum or RA. The NMDA receptors allow the RA to be more likely to produce this template of the tutor song, thus leading to learning of the correct song.

Autonomic Rhythms
The autonomic nervous system is responsible for many of the regulatory processes that sustain human life. A great deal of these are dependent upon a certain rhythm, such as sleep, heart rate, and breathing.

Circadian Rhythm
Circadian literally translates to "about a day" in Latin. This refers to humans 24 hour cycle of sleep and wakefulness. This cycle is driven by light. The human body must photoentrain itself in order to make this happen. Although the rods, are responsible for sensing light they are not what sets the biological clock. The photosensitive retinal ganglion cells contain a pigment called melanopsin. This photopigment is depolerized in the presence of light, unlike the rods and cones which are hyperpolerized. Melanopsin encodes the day-night cycle to the suprachiasmatic nucleus (SCN) via the retinohhypothalamic tract. The SCN evokes a response from the spinal chord. These preganglionic neurons in the spinal chord modulate the superior cervical ganglia, which synapses on the pineal gland. The pineal gland synthesizes the neurohormone melatonin from tryptophan. Melatonin is secreted into the bloodstream where it effects neural activity by interacting with melatonin receptors on the SCN. The SCN then is able to influence the sleep wake cycle, acting as the "apex of a hierarchy" that governs physiological timing functions.