In 1958 the English psychologist Donald Broadbent proposed the existence of a theoretical filter device located in our brain between the sensory register of incoming information and the short-term memory storage.
The psychologist theorised that humans process information with limited capacity and select them to be processed early; due to this limited capacity a selective filter is needed for the information processing. In his experimentations he made use of the dichotic listening test, a psychological test used to study selective attention within the auditory system; during the experiment the participants had been told to wear headphones in which a different auditory stimuli were presented to each ear at the same time. The participants were then told to attend and remember the information coming into one ear and neglect the information presented to the other one; this test showed the ability of the participants to recall information to the attended channel and the inability to recall the different stimuli in the unattended channel. In the second part of the test, one set of three digits was sent into one ear and another three digits’ set was sent into the other; they were then told to recall all numbers and set them in the order they wanted to; the result showed that the participants would recall the numbers in a ear-by-ear order, instead of any other order: for example, if 782 were presented to one ear and 980 to the other, the recall would be 782980. The function of this filter so would be the prevention of the overloading of the limited capacity mechanism, which is the short-term memory.
This theory is strictly linked to the famous cocktail party effect which shows how humans are able to focus the attention towards the auditory stimuli they find most interesting.
Another interesting theory was developed by a graduate student of Broadbent, Anne Treisman who proposed the Attenuation Theory. This hypothesis suggested that the theoretical filter would also attenuate the stimuli presented to the unattended channel, but if the stimuli pass a sort of threshold it would pass through and be perceived by the brain; the threshold in question would be determined by words’ meaning: important words (such as a name) would have a low threshold and the participant would easily gain awareness, while unimportant words (such as chair) would have an higher threshold to prevent the participant from gaining awareness inappropriately.