Studies show that we can only process a portion of the information we receive on a daily basis. Both of the selective attention and inattentional blindness theories draw conclusions about the amount of verbal and non-verbal information we can process respectively. As we are limited in space, below is an explanation of both theories to help get you started.
A model for the theory of selective attention was first developed by Donald Broadbent. Simply put, the theory states that it's impossible for individuals to process information from every single stimulus in our environment, so we use selective attention to choose what stimuli we feel are important. By stimuli, we simply mean anything that prompts us to action, such as thinking, feeling, or doing. Broadbent discovered that we can really only pay attention to one channel of stimulus at a time, so we refer to Broadbent's theory as the single channel model. Broadbent tested his single channel theory by having participants listen to two things being spoken in each ear at the same time. In his test, the ears served as the stimulus channel. He found that individuals will filter and only focus on one thing being said in one ear. The information that is focused on will be retained, while the other information will be mostly lost, though some people may be able to repeat back bits of what was said due to the fact that our short-term memories are able to retain it; however, only the long-term memory will retain the information that was focused on. He also found that the meaning of what is being said is not a part of the filtering system. Instead, we only filter information based on physical characteristics, such as which ear the information is being spoken into and what the person's voice sounds like (McLeod, "Selective Attention").
Similarly, inattentional blindness also has to do with filtering stimulus but specifically stimulus received through the visual channel. Arien Mack and Irvin Rock first developed the theory of inattentional blindness in 1992 ("Inattentional Blindness"). Simply put, the theory asserts that when a person is overwhelmed by stimuli, then the person will develop a "temporary blindness effect," meaning that the person will miss seeing certain objects or stimuli ("Inattentional Blindness"). The reason why people develop temporary blindness is because, as they process information, they create mental pictures; therefore, too many images will prevent the ability to process new information in terms of creating mental pictures (Changing Minds, "Inattentional Blindness"). It has especially been noted that even low levels of alcohol will create temporary blindness. A classic study was performed in which participants were blindly served either alcoholic or non-alcoholic beverages. The participants were then asked to watch a "25-second video clip of six people playing with a ball" and asked to count the number of times the players passed the ball. During the middle of the game, someone dressed in a gorilla suit ran through the game "beating its chest." The gorilla remained onscreen for one-third of the video's length. The participants who were served alcohol proved to be "twice as likely to miss seeing the gorilla" because they were too focused on processing the information of the ball passes and the alcohol impeded their ability to process new stimuli, thereby impeding their ability to create new mental images (Changing Minds).