Tulving's Distinction
Endel Tulving made a fundamental distinction in the way that memories are encoded: he classifies a memory as semantic if it is a piece of knowledge without a context (a blue shirt) and episodic if the knowledge is tied to a very specific context of a particular incident (the blue shirt a subject was wearing on the day his first child was born).
Semantic memory is highly relevant on the conceptual and functional levels: to know what a blue shirt is does not require ever having worn one, and to distinguish a blue shirt from a white one does not require the subject to recall an occasion on which each color of shirt was worn. The knowledge exists in the nature of an atom, not tied to a molecule, which can be picked out of the memory storage to be used, alone or in conjunction with other atoms, quite readily.
Episodic memory does not have such relevance. Subjects who recall events from their past quite often recall a plethora of details that have no significant relevance - the color of shirt he was wearing on the occasion of the birth of his child is an extraneous detail that has nothing to do with the event in a functional sense. He might also recall what he ate for breakfast that morning, the color of the nurse's scrubs, a song that was playing on radio when he got the news, the color of the carpet in the waiting room, and a myriad of other details that are related only by proximity to the event.
However, that's not to say that episodic memory is frivolous, as it is the basis for most procedures: when we seek to perform a task that we have performed in the past, we draw upon an aggregated memory of past experiences to guide us in our present behavior - such that among the inconsequential details are other bits of data that are quite important (such as remembering that, when jump-starting a car, the red cable attaches to the positive terminal and the black to the negative, and never vice-versa). That is to say that without episodic memory, a person would not benefit from learning, but would have to think his way through every problem as if it were the first time he had ever encountered it.
Of particular importance is the fact that episodic memory is much stronger and easier to recall than semantic memory. Asked to recall what color of shirt he was wearing on the fifth of March, 1972, it is likely even a person of exceptional intelligence would be completely unable to answer, even if he concentrated on the matter for some time. But if he is mindful (or reminded) of some significant event that occurred on that date, he is far more likely to remember the detail in the context of that event, even decades later.
The event itself doesn't necessarily have to be highly significant, though my examples might suggest so. If you ask a colleague what they had for lunch a week ago, they would be unlikely to be able to answer, but seed their memory with an episode, such as a discussion you had on that day, and they can recall the detail in the context of the day's events: "We met to discuss that at eleven o'clock, then I went back to my desk to check my email, then I went to the cafeteria and had a bowl of tomato soup and a chicken salad sandwich." This will come out along with many other completely irrelevant details about the events of the day - but it will far more likely be uncovered by an episodic rather than a semantic cogitation.
Memory and Experience Design
The relevance of Tulving's distinction to user experience, particularly to consistency and relevancy, is that consistency touches upon (weaker) semantic memory rather than (stronger) episodic memory. That is to say the ability to recognize a warning message requires the user to have learned the appearance of such a message as a semantic unit, rather than their ability to call to mind a previous episode in which such a message was encountered.
This is particularly important to experience design in that our first impulse is to reduce friction by crafting the elements of experience to be familiar to the user, similar in a semantic sense to the elements that they recall: a warning message is different to an error message, and different to an informational message, not merely because the color of the text is different (semantic) but because the user sees that specific treatment each time an event of a given kind occurs.
Consider also that the experience we are providing to the user in the present will also serve as a memory that will provide this familiarity in future interactions. Given that the user will discern from the present context of what is before him that the text is meant as a warning to be heeded, its various qualities will first be experienced in the context of a task (episodic) and, with repetition, will be atomized for semantic consideration.
And of particular importance is the experience of the present, as separated from past and future, because the ability to discern from context without relying upon memory of either kind facilitates the cognitive task by not requiring the user to access memory of either kind.
Memory as Facility
Ultimately, memory is a facility that can be leveraged when necessary to complete a task, but unless the task is a mnemonic test, memory itself it is not the purpose of a task. In terms of customer experience, the minimization of effort, physical or mental, is a method of facilitation - and to rely upon memory is to increase the difficulty of the task, a practice that should be approached with discretion and, where possible, avoided.
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