Rhythm generally means a movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions. In the performance arts, rhythm is the timing of events on a scale, of musical sounds and silences that occur over time, of the steps of a dance, or the meter of spoken language. In some performing arts, such as hip hop music, the delivery of the lyrics is one of the most important elements of the style. Rhythm may also refer to visual presentation, as timed movement through space, in recent years, rhythm and meter have become an important area of research among music scholars. Recent work in these areas includes books by Maury Yeston, Fred Lerdahl and Ray Jackendoff, Jonathan Kramer, Christopher Hasty, Godfried Toussaint, William Rothstein, and Joel Lester. In Thinking and Destiny, Harold W. Percival defined rhythm as the character and meaning of thought expressed through the measure or movement in sound or form, or by written signs or words. In his television series How Music Works, Howard Goodall presents theories that human rhythm recalls the regularity with which we walk, other research suggests that it does not relate to the heartbeat directly, but rather the speed of emotional affect, which also influences heartbeat. Yet other researchers suggest that certain features of human music are widespread. The perception and abstraction of rhythmic measure is the foundation of human instinctive musical participation, Joseph Jordania recently suggested that the sense of rhythm was developed in the early stages of hominid evolution by the forces of natural selection. Plenty of animals walk rhythmically and hear the sounds of the heartbeat in the womb, some types of parrots can know rhythm. There is not a report of an animal being trained to tap, peck. For this reason, the fast-transient sounds of percussion instruments lend themselves to the definition of rhythm, Musical cultures that rely upon such instruments may develop multi-layered polyrhythm and simultaneous rhythms in more than one time signature, called polymeter. Such are the cross-rhythms of Sub-Saharan Africa and the interlocking kotekan rhythms of the gamelan, for information on rhythm in Indian music see Tala. For other Asian approaches to rhythm see Rhythm in Persian music, Rhythm in Arabian music and Usul—Rhythm in Turkish music and this consists of a series of identical yet distinct periodic short-duration stimuli perceived as points in time. It is currently most often designated as a crotchet or quarter note in western notation, faster levels are division levels, and slower levels are multiple levels. Rhythms of recurrence arise from the interaction of two levels of motion, the faster providing the pulse and the slower organizing the beats into repetitive groups. Once a metric hierarchy has been established, we, as listeners, a durational pattern that synchronises with a pulse or pulses on the underlying metric level may be called a rhythmic unit. A rhythmic gesture is any pattern that, in contrast to the rhythmic unit
Percussion instruments have clearly defined sounds that aid the creation and perception of complex rhythms
A Griot performs at Diffa, Niger, West Africa. The Griot is playing a Ngoni or Xalam.