are sensory experiences that occur in the absence of real stimuli
Hallucination A hallucination is a perception in the absence of external stimulus that has qualities of real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. They are distinguishable from several related phenomena, such as dreaming, which does not involve wakefulness; pseudohallucination, which does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception; and imagery, which does not mimic real perception and is under voluntary control.[1] Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted stimulus (i.e., a real perception) is given some additional (and typically absurd) significance.
Sensory memory Echoic memory represents SM for the auditory sense of hearing. Auditory information travels as sound waves which are sensed by hair cells in the ears. Information is sent to and processed in the temporal lobe. The echoic sensory store holds information for 2–3 seconds to allow for proper processing. The first studies of echoic memory came shortly after Sperling investigated iconic memory using an adapted partial report paradigm.[13] Today, characteristics of echoic memory have been found mainly using a mismatch negativity (MMN) paradigm which utilizes EEG and MEG recordings.[14] MMN has been used to identify some of the key roles of echoic memory such as change detection and language acquisition. Change detection, or the ability to detect an unusual or possibly dangerous change in the environment independent of attention, is key to the survival of an organism.[14] One study focusing on echoic sensory changes suggested that when a sound is presented to a subject, it is enough to shape an echoic memory trace that can be compared to a physically different sound. Change-related cortical responses were detected in the superior temporal gyrus using EEG [15]. With regards to language, a characteristic of children who begin speaking late in development is reduced duration of echoic memory.[16] In short, "Echoic memory is a fast-decaying store of auditory information."[17] In the case of damage to or lesions developing on the frontal lobe, parietal lobe, or hippocampus, echoic memory will likely be shortened and/or have a slower reaction time.[18]