Memory

In its most general sense, a memory is the trace of some past event by which that past event can subsequently be reconstrued. In normal use, we think of memory as information that is encoded in our brains in a way that enables it to be stored quietly away, and then subsequently retrieved when needed. This notion has been generalised to the idea of digital electronic information stores as memory, following the perhaps unsafe analogy between animal brains and digital computers. Metaphorically, we sometimes talk of memory in physical objects, such as when a deformable object holds an imprint of a previous form.

This article is about memory as information stored in our brains for subsequent deliberate retrieval. As we experience new things every day of our lives, our brains change in small but sometimes important ways. Nothing in our brains is fixed, neurons are dying steadily, but some new ones are being born throughout our lives. Each of the nerve scells in our brains smakes many thousands sof connections, called synapses, with other nerve cells - and these are constantly in flux, the number and strength of these connections changes according to our experience. Thus experience changes our brains in these and many other ways, and in a sense, all of these changes are part of learning - in that they are the mechanisms by which past experience influences future behaviour. In a sense all learning involves 'memory', but memory as we use the word is not just about learning. In natural usage, a memory is a detailed reconstruction in the mind of some past event. We believe that ultimately memories must be encoded in the brain by changes in the patterns or strengths of connectivity between neurons. We also know that some parts of the brain like the hippocampus are especially important for remembering certain types of things. However to pretend that neuroscientists have an adequate understanding of how even the simplest memory of an event is encoded in the brain would be quite wrong. There is an enormous "gap" in our understanding; we know a great deal about the fundamental mechanisms by which nerve cells operate, but we don't know how these allow us to store and retrieve memories as we understand them.

Neurological Basis of Memory
The neuropsychologist and theoretical neuroscientist Donald Hebb (1904 - 1985) was the first to distinguish between short-term memory and long-term memory. When the brain receives a sensory input, for example, visual and auditory stimuli, a sensory memory retains an exact copy of what is seen or heard, but this memory lasts for at most only a few seconds. What is retained longer than this depends on selective attention - things that we "notice" may be stored in short-term memory for up to a few minutes. This memory is thought to depend on electrical activity in neuronal circuits, and is very easily destroyed by interruption or interference. Short term memory includes iconic memory, to hold visual images; acoustic memory, to hold sounds; and working memory, an active process to keep a memory until it is put to use.

Memories stored for longer than this are stored in long-term memory. Hebb's main contribution to neuroscience was his theory that the basis of long-term memory was a form of synaptic plasticity - a long term alteration in the strength of connections between neurons now thought to involve a phenomenon called "long-term potentiation" (LTP). This is relatively permanent storage, and it requires the synthesis of new proteins.

Whether information is stored in long-term memory depends on its 'importance'; for any animal, memories associated with stress or trauma are potentially important for the adaptive value that such memories have for future avoidance behaviour, and the hormones that are released during stress are thought to have an important role in determining what memories are preserved. In humans, acute traumatic stress is associated with acute secretion of epinephrine and norepinephrine (adrenaline and noradrenaline) from the adrenal medulla and more prolonged secretion of cortisol from the adrenal cortex. Acute increases in these hormones are thought to facilitate memory while chronic stress associated with prolonged hypersecretion of cortisol may have the opposite effect. The limbic system, including the hippocampus and amygdala in particular, is critically involved in memory storage and retrieval as well as giving emotional significance to sensory inputs.The hippocampus is important for explicit memory, and for memory consolidation; it is also very sensitive to stress and has a role in recording the emotions of a stressful event. The hippocampus receives input from many different parts of the neocortex and sends its output out to different parts of the brain. The amygdala is thought to assign emotional values to sensory inputs which are then elaborated upon by the neocortex and imbued with personal meaning; thus patients with amygdalar damage are no more likely to remember emotionally charged words than nonemotionally charged ones. The amygdala may also integrate internal representations of the external world in memory image form associating emotional experiences with these memories. The septo-hippocampal system is thought to record memory in temporal and spatial dimensions, and plays an important role in storing and categorizing incoming stimuli in memory.