Memory

Everything that comes in passes first through that small account, and its balance doesn’t stay. For something to stay, it has to be transferred to another account, and the one who authorises the transfer is the hippocampushippocampusThe structure that turns recent experience into lasting memory. Everything your interface asks the person to remember tomorrow passes through it today.See it in Memory →See in the glossary →, a curved structure in the temporal lobe that got its name because it looks like a seahorse. It doesn’t store the memory. It decides what deserves to be stored and distributes it across the cortex, little by little, over hours and nights of sleep, in a process called consolidation. That’s why the person remembers where the button they used last week was and doesn’t remember the code that came by SMS ten minutes ago. One was transferred. The other never left working memoryworking memoryThe mental space where the person holds what they are using right now. It holds about four things, and they fade in seconds if they are not used.See it in Memory →See in the glossary →.

Before any information reaches consciousness, it passes through an ultra-fast buffer, and in vision it’s called iconic memoryiconic memoryThe trace an image leaves for about 250 milliseconds after it leaves the screen. It is what allows comparing two things in the blink of an eye, and what is lost when an element disappears too fast.See it in Memory →See in the glossary →: an image stays alive for about 250 milliseconds, in the measurement SperlingGeorge SperlingMeasured visual sensory memory and showed that it keeps almost everything for less than a second.One reference in this work:1960The information available in brief visual presentationsSee on Wikipedia ↗See in the bibliography → made in 1960 by projecting a grid of letters for an instant and asking the person to report a randomly chosen row after the screen went blank. It’s a quarter of a second, and it’s what allows you to compare two things on the screen without looking twice, because the second look still reaches the trace of the first. In sound the window is much wider, something like three seconds, and that’s why you can ask “sorry?” and understand the sentence before it’s repeated. This brief retention costs a lot of energy, and evolutionarily it wouldn’t make sense to prolong it, because we would keep irrelevant details getting in the way of immediate action.

The small account every interface draws on has a technical name, working memoryworking memoryThe mental space where the person holds what they are using right now. It holds about four things, and they fade in seconds if they are not used.See it in Memory →See in the glossary →, and it has an address. It depends on frontoparietal circuitsfrontoparietal circuitsThe pair that supports working memory: the prefrontal cortex holds the information active while the parietal lobe keeps track of where things are.See it in Memory →See in the glossary →, in which regions of the prefrontal cortexprefrontal cortexThe region behind the forehead, where the person decides, plans and sustains attention. It keeps one thing in focus at a time and inhibits the rest: it is where the bottleneck of conscious decision usually tightens.See it in Memory →See in the glossary → keep the neuronal patterns active while the parietal lobe integrates the spatial and sensory context. On average, we hold about four elements at the same time, in the number CowanCowanRedid Miller’s count with a better method and arrived at about four items, not seven. It is the number this site uses.One reference in this work:2001The magical number 4 in short-term memorySee in the bibliography → established in 2001 after reviewing the span tasks with verbal rehearsal blocked. There’s no point showing five complex menus at once, because working memory simply doesn’t accommodate them. And even within those few seconds, the representations are subject to synaptic noise, and distractions and competing details can erase them before use.

Part of that memory is about where things are, and it’s the oldest of all. Linked to the hippocampushippocampusThe structure that turns recent experience into lasting memory. Everything your interface asks the person to remember tomorrow passes through it today.See it in Memory →See in the glossary →, spatial memory allows us to navigate physical and digital environments: place cells in the hippocampus and grid cells in the entorhinal cortexplace and grid cellsThe internal GPS, in the hippocampus and its surroundings. It evolved to remember paths to water and food, and it is the same one that remembers where that button was on the previous screen.See it in Memory →See in the glossary → create an internal map of space, used both to remember where we clicked in an interface and to go back to the previous page. Keeping navigation patterns consistent, with the menu in the same place on every screen, exploits that ancient resource. And “ancient” here is stronger than it seems. Memory and navigation run on the same neuronal algorithms, and the hypothesis BuzsákiBuzsákiDescribed how the theta rhythm of the hippocampus sustains memory and navigation, the same machine for remembering and for finding one’s way in space.One reference in this work:2013Memory, navigation and theta rhythm in the hippocampal-entorhinal systemSee in the bibliography → and Moser defended in 2013 is that the mechanisms for remembering evolved from the mechanisms for finding one’s way in physical space. The brain didn’t gain a new part to deal with menus and the way back; it reuses the one that served to find the way to water.

When a piece of information is repeated or deemed relevant, the trace leaves the hippocampushippocampusThe structure that turns recent experience into lasting memory. Everything your interface asks the person to remember tomorrow passes through it today.See it in Memory →See in the glossary → and settles in areas of the cortex, and from then on memory divides by type of content. Episodic and semantic memory spread across the neocortex and, over time, stop depending on the hippocampus and the medial temporal lobemedial temporal cortexThe inner part of the temporal lobe, hippocampus and surroundings, through which fact and episode pass to become lasting memory. Its role is temporary: over time the memory ends up stored in the cortex and stops depending on this region.See it in Memory →See in the glossary →; procedural memory, tied to motor skills such as typing and gesturing on screens, consolidates in the basal gangliabasal gangliaTogether with the cerebellum, they store what the body learned to do without thinking: typing, swiping, reaching an icon. It is the memory that survives distraction.See it in Memory →See in the glossary → and the cerebellumcerebellumThe structure low at the back of the skull that automates movement and sequence. The more someone uses your product, the more the operation migrates here, and the costlier it gets to change what has already become automatic.See it in Memory →See in the glossary →; prospective memory, remembering to do something in the future, involves planning networks in the prefrontal cortexprefrontal cortexThe region behind the forehead, where the person decides, plans and sustains attention. It keeps one thing in focus at a time and inhibits the rest: it is where the bottleneck of conscious decision usually tightens.See it in Memory →See in the glossary →.

How much of that survives has a number, and it’s an old one. The forgetting curve EbbinghausEbbinghausMeasured his own memory for years, with nonsense syllables, and drew the forgetting curve and the serial position effect.One reference in this work:1885Über das GedächtnisSee in the bibliography → measured in 1885, memorising nonsense syllables, still holds: without reinforcement, whether review, repetition or emotional context, we lose up to 70% of new content within 24 hours, and MurreMurreReplicated Ebbinghaus’s forgetting curve more than a century later, and it held.One reference in this work:2015Replication and analysis of Ebbinghaus’ forgetting curveSee in the bibliography → and Dros replicated the curve in 2015 with the same result. Without repetition and without emotional charge, the trace weakens before consolidation is finished, and it’s the same mechanism that appears in the chapter on emotion.