Subliminal Perception
Subliminal perception refers to processing of environmental stimuli without conscious awareness. In other words, people are not aware that they are sensing the stimulus. Historically speaking, there is little evidence for subliminal perception as demonstrated in mainstream media. A famous example of the use of subliminal perception in the 1950s claimed that briefly flashing the message "drink Coca-Cola" to a theater audience resulted in increased cola sales. However, the study was eventually shown to be a hoax. Systematic review of other attempts at subliminal perception, such as tapes people listen to while sleeping, has shown no effects of subliminal perception on behavior.
However, brains are able to process some stimuli that appear too quickly for humans or other animals to notice consciously. There is some evidence that subliminal stimulation can affect behavior. Priming is a nonconscious memory effect in which exposure to a cue influences the activation of ideas or the recall of words and objects. Studies using subliminal cues show that priming can affect behavior. For example, a research participant seated in front of a screen may be presented with a picture of a mouse for such a short duration that they cannot consciously perceive it. Then, in a word-completion task following that exposure, they are more likely to fill in "_ou_e" as "mouse" than as "house."
Extrasensory Perception
By definition, extrasensory perception is outside of human awareness. However, this is not the same as subliminal perception. Instead, extrasensory perception (ESP) is defined as the perception of information without the use of the five major senses or the aid of previous experience. It refers to the ability to read minds or predict the future.
Many people believe in the possibility of ESP or other paranormal phenomena. One reason for this might be illusory correlation. There is a human tendency, or bias, to remember striking coincidences but forget more ordinary experiences. Thus there is a perception of a relationship between two coincidental events that, in reality, is not there. For example, a person who dreams about a car accident and then reads about a car accident the next day may believe they had predicted the crash. That person is unlikely to notice all of the bad dreams that weren't followed by accidents. They are also unlikely to notice all of the bad events they failed to predict. Bad dreams and car accidents are both relatively common—sometimes they will occur together due to mere chance.
Additionally, people tend to underestimate the likelihood of coincidences. For example, in a room full of 60 people, if someone claiming to have ESP stated, "I know there are two people here with the same birthday," those people might be very surprised to find out that claim is true. When asked, most people would say the room would need to hold hundreds if not thousands of people for this to be true, when, actually, in a room full of 60 people, there is a 99% chance that two people will have the same birthday. Although the odds are fairly high, people tend to estimate the probability as quite low and may be more likely to believe the predictor's claim of having ESP.
Many studies have attempted to detect the presence of ESP. Well-designed studies that prevent participants from cheating or accidentally getting access to information show that people do not have the ability to read minds.