Speaking of a person essentially means speaking of the story of a human being. When summoning a personal picture of a client, of a friend or even of ourselves we can’t prescind from referring to memories. Memories are, from an evolutionary point of view, an essential function for human beings because they allow us to  retrieve consolidated solutions to constant everyday problems. Furthermore mermory has been studied by both clinical psychology, privileged subject of Sigmund Freud and his followers, and neurosciences in which, thanks to the work of Erich R. Kandel (nobel prize for medicine in 2000), it has been possible to hypothesize how the formation of new memories is connected to physical modifications of synapses (links between neurons).
Also thanks to these prolific research fields, the general concept of “memory” has often been divided in sub-components, reformulated and perfected. It is important to remember a fundamental neuropsychological distinction (often proved valid) between declarative memory, accessible and of easy verbalization (“what did I have for lunch today?”) and procedural memory, observable in behaviours and hard to verbalize (let’s just think to all the actions we must do to drive a car).
In a recent review [1] results of different studies (including many studies conducted by the same team of researchers) were compared to explore the cerebral modifications caused by meditation and hypnosis. Authors differentiate these techniques reporting that, while in hypnotic state, a strong suggestibility can be observed and this can lead to the retrieval of suppressed memories. This process is used in hypnotic therapy to allow clients to retrieve a certain experience, hard to retrieve otherwise, along with all its perceptual and emotional components. Data collected by the authors showed how hypnosis, speaking of brainwaves, is characterized by alpha frequencies (typical of relaxed waking states) especially on the temporal and central lobes. Furthermore, hypnotic activity produces a theta activation (typical of dreams and imaginative activities) that is diffused to both hemispheres. Using PET (positron emission tomography) authors have also studied the process of learning, using a highly imaginative word recall test. Thanks to this brain imaging technique, it has been possible to  identify which areas where active during hypnosis, showing a pronounced bilateral activity of occipital an prefrontal cortices compared to meditation.
Another well studied aspect concerns intrusive memories. Such phenomena are typical of several disorders such as the post traumatic stress disorder, obsessive-compulsive disorder, depression and phobias. These memories have characteristics in common such as non-intentionality of their retrieval and the significant disturbance that they cause. Several studies have demonstrated that hypnosis can be a useful model to study such kind of intrusion. in particular, in a recent work [2] the ability of hypnosis to elicit intrusive memories was tested. Experimenters asked to a group of hypnotizable subjects to describe an unpleasant event of their life. During hypnosis a group of subjects (post-hypnotic amnesia group)received the suggestion to remembter the unpleasant event after a certain stimuli (for example a word or a sound). Experimenters asded To another group (i9ntentional recall group) to spontaneously recall the event right after the end of the induction. After the hypnotic phase subjects completed a cognitive task in which the stimulus associated to the event was presented. Results demonstrated that the post-hypnotic amnesia group recalled the event in an intrusive, unintentional way and with a high degree of unpleasantness compared to the control group. Furthermore, the cognitive task resulted more difficult because of the interference of intrusive memories.
Hypnosis can therefore hold two roles in the wideness of the research aboud memory: on the one hand it can be used in clinical practice to recover and strengthen important memories, on the other hand it can be an instrument to explain complex phenomena that can be observed when studying relationships between mind and brain and that are hard to understand in the framework of neurobiological sciences.

[1] Halsband U., Mueller S., Hinterberger T., Stickner S. (2009). Plasticity changes in the brain in hypnosis and meditation. Contemporary Hypnosis, 26(4): 194–215.
[2] Hill Z., Hung L., Bryant R.A. (2010). A hypnotic paradigm for studying intrusive memories. J. Behav. Ther. & Exp. Psychiat. 41: 433-437.