Two papers released in the journal Neuron discuss these gaps, which happen 4 times every second. Hence the object of our focus only gets these quick snapshots and we have to piece them together to create the appearance of continual attention. The reason our focused attention is diverted so frequently is due to evolutionary selection pressures to remain vigilant to dangers in the environment around us. Thus any environmental distraction will interrupt our focus with this frequency.
This reminds me of what I wrote in my review of Thompson’s book, chapter 2:
Consciousness appears to be in a continuous stream, yet it is in fact broken into discontinuous, discrete moments, each of which is conditioned on a variety of contextual factors. […] Thompson wonders if we can measure the gaps between these discrete, phasic moments. […] The referenced experiments showed that we tend to perceive a stimulus when there is a peak in a brain wave cycle, and not so when it hits a trough. That is, much like the longer waking and sleeping cycle, brain wave cycles that happen in milliseconds have a similar effect on perception. These experiments support the hypothesis of discrete, phasic moments. This holds true even for sustained attention of the meditative type. It’s true that such meditative focus increases our ability to sustain attention, yet it is not continuous and alternates in millisecond intervals consistent with brain wave function.