answer:It’s a “bandwidth” problem. The senses deliver way more information than the brain can process into perception. The scope of information being monitored can be wide, including a broad range of information (at the expense of resolution); or the scope can be narrow, freeing up bandwidth for resolution. If a complex visual field is being monitored the brain will prioritize how it allocates bandwidth, typically including tasks like identifying the objects in the field, patterns of movement and spacial relationships. These are low resolution tasks, but they convey information pertinent to basic functioning, so they get prioritized. When the visual input is filtered, there is less raw information to monitor and so bandwidth is freed for more detailed perceptual processing. This, by the way, is the mechanism behind how meditation quiets mental chatter. The chatter consumes a lot of bandwidth. By mobilizing the attention in a monitoring task like a high-resolution monitoring of the breath or a very broad monitoring of the whole sensory stream, the chatter is deprived of bandwidth and dies down.