The process of perception is very much affected by attention, a phenomenon that involves filtering of incoming stimuli.
Human beings do not pay attention to everything in their environments; nor do they attend to all the stimuli impinging on their sense organs. Rather than becoming overwhelmed by the enormous complexity of the physical world, we attend to some stimuli and do not notice others. William James recognized the importance of attention very early: “A thing may be present to a man a hundred times, but if he persistently fails to notice it, it cannot be said to enter his experience.”
Human beings have a great deal of control over which stimuli they attend to and which they do not. The process of attention is a selective one. At a cocktail party, a person may pay strict attention to a conversation with the boss while “tuning out” nearby chatter. Yet even though the person’s attention is given to a certain category of incoming information he or she can still monitor other events, such as the arrival of new refreshments at the bar, the entrance of new guests, or the music on the stereo. The person’s attention can also shift very suddenly because of this monitoring process. For example, if another guest mentions the person’s name, the person’s attention is likely to shift from the current conversation to the dialogue in which his or her name was mentioned.
Studies of attention have found that people have a limited ability to process incoming information. If they are not attending to it, they usually cannot recall very much of it. In an experimental technique called shadowing, subjects listen to one set of stimuli through one sensory channel and another set of stimuli through a different channel. They try to repeat aloud, or shadow, the messages coming in through one of the two channels. Typically, they recall very little or nothing of the message coming in through the other channel, even if the same stimuli are repeated over and over to the unattended sensory channel. For example, subjects who shadow a set of words entering in one ear, while another set is presented dichotically to the other ear, usually cannot recall anything from the unattended list of words.
It might seem as though the person became functionally deaf in one ear. But apparently the words are heard; they just do not remain very long in the person’s memory. If the experimenter interrupts the shadowing task and quickly asks the subject to recall as many words as possible from the unattended channel, the person can often remember the last five or six words.
The fact that the person can choose what to attend to, what to perceive, and how to filter incoming sensory information, leads to the inevitable conclusion that perception is not at all mechanical.