Memory is the system that allows past experience to influence present behavior, and researchers have identified multiple stages and types of memory that work together. The process begins with encoding, the conversion of information into a form that can be stored. Encoding can take visual (iconic), acoustic (echoic), or semantic, meaning-based, forms. Once encoded, information may enter short-term memory, which holds a limited amount of information, roughly seven items plus or minus two, for about twenty to thirty seconds. Through further processing, some of this material is transferred to long-term memory, which has a virtually unlimited capacity and can store information indefinitely.
A more nuanced view is offered by Baddeley's model of working memory, which describes an active system for temporarily holding and manipulating information. This system includes the phonological loop for verbal information, the visuospatial sketchpad for spatial and visual material, the central executive for coordinating attention, and the episodic buffer for integrating information across domains. Long-term memory itself is divided into explicit (declarative) memory, which involves conscious recall and includes semantic memory for facts and episodic memory for events, and implicit (nondeclarative) memory, which operates unconsciously and includes procedural memory for skills and conditioned responses.
Several well-established phenomena illustrate both the strengths and vulnerabilities of memory. The serial position effect shows that people tend to remember items at the beginning of a list, through the primacy effect, and at the end of a list, through the recency effect, better than items in the middle. When the brain is injured, distinctive patterns of memory loss can occur. Retrograde amnesia is the inability to recall memories formed before a brain injury, while anterograde amnesia is the inability to form new memories after the injury; the case of patient H.M., who underwent bilateral hippocampal removal, is the classic demonstration of anterograde amnesia. Memory can also be distorted after the fact. The misinformation effect occurs when post-event information alters a person's recollection of the original event, a phenomenon extensively studied by Elizabeth Loftus, who showed how leading questions can change eyewitness testimony.
Memory is also subject to predictable patterns of decay. Hermann Ebbinghaus's forgetting curve shows that retention declines rapidly at first and then levels off, with the steepest losses occurring within the first hour of learning. Counteracting this decay is the principle of retrieval practice, sometimes called the testing effect: actively recalling information strengthens memory far more effectively than simply re-reading material, making retrieval practice one of the most powerful study strategies available.