How does a lack of selective attention influence our perception?

Please take a look at the YouTube video below. Follow the instructions:
• Selective Attention Test

Were you able to see the gorilla? 

This phenomenon you just observed in the video is known as inattentional blindness - the failure to notice a stimulus that isn't attended, even in plain sight - provides evidence that attention is required for perception. This momentary unawareness is probably caused by the plethora of visual stimuli that demand attention.

There are a few requirements that an event or situation must meet to classify as an incident of inattentional blindness:
1. The observer should not recognize a visual event or item.
2. The entire optical stimulus must be visible.
3. It should be simple to identify if the observer consciously notices the signal.
4. The stimulus must be unexpected, and the inability to notice it is due to a breakdown in attention, not flaws in the visual picture or the optic stimulation.

Daniel Simons and Christopher Chabris conducted arguably the most well-known investigation on inattentional blindness, popularly referred to as the "Invisible Gorilla Test" (video).  Many spectators were preoccupied with the difficult task of tracking throws and passes; thus, despite the gorilla's evident prominence, they had missed it. In the experiment, the participants completely blocked out other visual information because they were preoccupied with counting how many times the white team passed the ball. Therefore, they missed detecting the gorilla. About 46% of the participants failed to notice the gorilla during their first trial.