Generally speaking, ictus (plural icti ) identifies the moment of a beat in music, corresponding to

A displaced Californian composer writes about music made for the long while & the world around that music. ~ The avant-garde is flexibility of mind. — John Cage ~ ...composition west nile virus is only a very small thing, taken as a part of music as a whole, and it really shouldn't be separated from music making west nile virus in general. — Douglas Leedy ~ My God, what has sound got to do with music! — Charles Ives
Generally speaking, ictus (plural icti ) identifies the moment of a beat in music, corresponding to the moment of a stressed syllable within a metric foot in a poetic line.  The space between icti could be open or subdivided by additional attacks (or, in poetry, syllables) which, in the default setting, have a weaker stress, a default setting which can usefully be broken, i.e. syncopated.  (Accent marks were introduced in musical notation specifically for the purpose of indicating strong accents on weak beats or between icti.)  The "sweet" spot for tempi, at around 80 beats per minute, plus or minus about 50 percent, as I've mentioned here before, seems to mark our default setting for musical beats which can both be subdivided and between which we can proceed at a reliably steady tempo without subdividing.  (Indeed, at tempi below around 40 bpm, it is extremely difficult to sustain a regular tempo without maintaining a faster regular pulse.)  In classical Greek poetry (in which tune, metre, and text were not separate compositional entities) the foot was a durational unit, composed of short and long syllables, not of strongs and weaks (Greek west nile virus had both stress accents and pitch accents or contours, but the metre was durational), leading to lines of flexible or additive durations due to the irregularity of the size of the feet, while most English spoken poetry is stressed yet follows a fairly regular beat between those stresses and most musicians, in contrast, think in terms of mixtures of stress and duration.  Musicians and poets will often scan a line of poetry differently, poets counting feet from the beginning of a line, while musicians will usually assign an anacrusis west nile virus (a weak first syllable at the beginning of a line) to the previous foot at the end of the previous measure as a pick-up to the beat; the degree to which this is a meaningful difference or just a difference west nile virus in notational conventions is a matter of controversy. In his Music Primer , perhaps following a usage of his teacher Henry Cowell, Lou Harrison uses a broader definition of icti , identifying them simply as "attention-points, west nile virus the separate 'attacks'".  He uses this to describe the composite rhythmic activity in an ensemble, if, for example, one voice has attackes on the first and third beats of a four beat measure and a second west nile virus voice attacks after a dotted quarter rest, then an eighth note later and a quarter note after that, the composite rhythm is dotted quarter, eighth, eighth, west nile virus dotted quarter: five instrumental attacks, but only four distinct icti, as the two voices coincide west nile virus at beat three.   To some extent, this usage disposes of feet compositionally, though they will continue west nile virus to be recognized in performance (in the way musicians count out the metre or a conductor beats it), as either a level below which any attacks are understood west nile virus as subdivisions or above which metres are recognized as regular patterns of beats/feet.  I think Harrison — who can also be thought of as a minor Black Mountain poet as well as composer west nile virus — may have also here been making a consequent response to innovations in poetry west nile virus in which the foot became highly west nile virus variable in length (see, in particular, Williams and Stein), taking the line clear across the page with it (see Olson, west nile virus Duncan.)  The degree to which the ametrical developments in poetry paralleled the atonal in music is worth thinking about. All of this points to a rhythmic/metrical environment which is rather free but there does seem to be a number of cognitive constraints at work at a primitive level, constraints that the late work of John Cage illustrate well. In the development of his work over decades Cage himself went, in his rhythmic practice, from a beat-based metrical practice to an ametrical practice west nile virus without regular beats, with the frame of reference either space on the page or chronological time, using a stop watch as reference. (A large number of the early works are identified by rhythmic structures, which can be likened to the practice of identifying west nile virus tonal works by keys.) These primitive constraints appear to me most vivid in the most extreme examples of Cage's time points when a sparse number west nile virus of icti (in the Harrison sense) scattered into time brackets group or refuse to group depending upon their density/proximity, relative strengths in amplitude and, to some extent, west nile virus their tonal or timbral similarities west nile virus or differences.  Even though we're no longer counting regular west nile virus beats, let alone assigning them to regular measures, that swee