Memory consists of three main systems: sensory storage, short-term storage, and long-term storage. In 1968, psychologists Richard Atkinson and Richard Shiffrin proposed an information processing model of memory referred to as the modal model. According to this model, information passes from sensory memory to short-term memory to long-term memory. Information can be lost at each stage of processing unless strategies are used to maintain it. Sensory memory is the storage of information from the senses that lasts a few seconds or less before passing that information to short-term memory. There are two main types of sensory memory: iconic memory and echoic memory. Iconic memory is a fast-decaying store of visual information that lasts less than a second. Echoic memory is a fast-decaying store of auditory information that lasts about three to four seconds. In one study, participants had to report whether two tones were the same or different. They were highly accurate when the tones were played back-to-back. But their accuracy declined as the time between tones increased. After a three-second interval, participants were essentially guessing.Active attention to sensory memories allows people to work with that information in short-term memory. Short-term memory is a system for storing information for more than a few seconds but less than a minute. It can store about five to nine units of nonsensory information (such as letters, words, sentences, and numbers) for 5 to 20 seconds. Maintaining information in short-term memory requires active rehearsal, such as mentally reciting a phone number. Without rehearsal, information in short-term memory will be quickly forgotten through the natural decay of that information over time. It can also be easily displaced by additional information, such as a second phone number. Long-term memory is the system that stores facts, experiences, and skills for periods ranging from a few minutes to a lifetime. Reviewing and rehearsing information in short-term memory can move it into long-term memory. The best rehearsal strategies are elaborative strategies that focus on the meaning of the information to be remembered and connections to other information. Information from long-term memory can be retrieved and moved into short-term memory. Although not all information in long-term memory is preserved forever, it may be stored but inaccessible due to problems retrieving the information. It can also be forgotten due to interference from earlier or later learning.