Human perception pulses in and out four times per second. Perception doesn't flicker on and off, but every 250 milliseconds it cycles between periods of maximum focus and periods of a broader situational awareness.
A team of researchers from Princeton University and the University of California-Berkeley who studied monkeys and humans have discovered that attention pulses in and out four times per second. Brain rhythms have been known for almost a century, since electroencephalograms better known as EEGs were invented in 1924.
According to one of the researchers, they can now link brain rhythms for the first time to behavior, on a moment-to-moment basis. This pulsing attention must present an evolutionary advantage, the researchers suggest, perhaps because focusing too intently on one subject could allow a threat to catch us by surprise. This seems to be an elegant way to allocate brain resources -- to sample the environment and not have any lapses.