Pattern Detection
Humans tend to register the unfamiliar rather than the familiar, the unusual rather than the usual. Theory of Signal Detection assumes that there are two stages of information processing in the task of detection: First sensory evidence of presence or absence of signal or noise, secondly a decision whether evidence is strong enough based also on probability and positive outcomes. Psychophysics attempts to measure the relationship of physical stimulation and the psychological sensations it produces. Absolute human sense threshold in vision is defined as a candle in a clear dark night at distance of 30 miles. In touch the threshold is the wing of a fly falling on a cheek from 1 cm. Difference thresholds are defined as just noticeable difference (JND) or the smallest change can be detected in 1 of 2 trials. JND for weight is 2%, brightness 2%, and loudness 10%. Counterintelligence Manuals on observation techniques refer to psychological research which asserts that on average 85% of what is learned from a real life situation is through the visual sense and only 13% is learned through the sense of hearing while 2% is through the sense of touch, smell and taste. The human visual system is not able to detect movement of less than 1/10 of a second, which is the basis for a variety of illusions or tricks which involve a quick hand, the "sleight of hand". In the space domain, vision is better than audition, so typically vision dominates, but in the time domain, just as exemplified in speech perception, hearing resolution is better than vision. Hearing may be as much as 10 times better than vision in resolving events that happen very close together in time.
Scientists study how senses help the brain fill perception gaps to give meaning to events. Experiments suggest that the moment of perception, what the brain thinks is "now," may have happened as much as half a second ago. The light had to reach the eyes, and the sound had to reach the ears, then both signals get processed by the brain so that they look simultaneous. "But it's nothing but trickery of the brain, creating truth from illusions and it takes a lot of effort to make it come out right."
As to how this all happens one answer is that it's an illusion and things are not actually all that coherent.