Subjects are exposed to a stimulus, such as a sound or image, so briefly that they never become consciously aware of it and cannot recall it later. Yet tests show that the stimulus has been imprinted on the brain at some level. In one set of experiments, subjects are shown a list of words too briefly for it to be stored in short-term memory. Later the subjects are asked to play a game similar to the television game Wheel of Fortune . Subjects who have previously been exposed to a list of words containing octopus are much more likely to guess correctly, even though they cannot explicitly recall whether the list included octopus.
PET scans monitor short-lived radioactive isotopes of oxygen that have been injected into the blood.
High levels of the isotope indicate increased blood flow and thus increased neural activity. MRI dispenses with the need for an injection of a radioactive substance. A powerful electromagnetic pulse causes certain atoms to align in a particular direction, like iron filings arranged around a magnet. When the magnetic field is relaxed, the atoms emit radiation at characteristic frequencies. Those regions of the brain that are most active are assumed to be crucial to the activity.