AKRI

Thinking : Memory, physical memory.

Analysis

The construction and operation of human memory is the target of considerable research. The focus is to understand more clearly, the construction and operation at levels ranging from the most subtle changes that constitute the storage of a memory to the general organisation of memory and the complexity of conscious and unconscious recall.

Research also continues into the construction and operation of machine memory. However, the focus here is to improve its performance and develop new construction methods and techniques. Over recent years, the speed of machine memory has increased dramatically and the size (and cost) has similarly reduced.

Debate

One reoccurring theme in debates in this area is whether machines can ever be as intelligent as humans. This component of that debate would concentrate on memory, a more manageable sub section. The debate could be broken down further.

Will the capacity of machine memory ever exceed that of human memory (in an individual system rather than the whole of machine memory in the world)? If we can accept a yes answer to this, although this is by no means a certainty, Other questions need to be considered.

Will machine memory ever be as serviceable as human memory given the ability of human memory to cope with a great variety of problems?

Will machine memory ever be as efficient as human memory, more storage space may simply mean that nothing can be found?

Will machine memory ever be able to select the things it stores, ignoring pointless memories (indeed, do humans do this)?

Human Memory Construction

Overview

Human memory exists in the brain. This is an organ, which works through an electro-chemical process. There are about 1011 active brain cells or neurons in a normal human brain. About 1010 of these are in the cerebral cortex. It is believed that memories are stored at the junctions between neurons, the Synapses. As learning takes place the number and specialisation of chemical receptors at the receptor site changes to reflect how the neuron will respond in future to a similar stimulus.

Where does memory exist in the brain?

People have postulated that ultimately, at the end of the memory chain, there are neurons that respond to specific items. For instance, there could be a granny cell, which fires when you see or think about your granny. This is not likely to be true. It is probable that memories are distributed within the various sub-sections of the brain. Memories for complex occasions may be distributed throughout many sections involving vision, sound, touch etc. Unlike computer memory, if one cell fails, it does not lead to any particular memory failure.

It is however known that the Hippocampus is crucial for laying down long term memories. Damage to this area of the brain may result in a person being completely unable to remember new things yet able to remember clearly, events from before the damage.

Memories may be composed of elements from any or all of the sensory modalities and may therefore be truly diverse as far as storage is concerned. However, it is now known that certain structures in the brain have considerable responsibility for the storage and retrieval of long term memories. The hippocampus , part of the forebrain, is crucial for long term memory. Damage to this area of the brain can result in serious amnesia for past events and also considerable difficulty in creating new memories. It is not clear whether damage is affecting storage or retrieval since both would produce the same effect. Frontal lobe damage has been shown to affect the performance of the central executive part of working memory.

The cerebral cortex and the hippocampus are relatively recent parts of the brain. Declarative memory the hippocampus and some related nearby areas of the cortex are thought to be involved in declarative memory, which itself may be a relatively recent phenomenon. The basal ganglion seems to be responsible for procedural knowledge. The cerebellum may play a key role in classical conditioned response acquisition.

Certain neurotransmitters have been found to be closely related to memory events and hormonal effects are also known to increase glucose in the brain leading to improved event memory at certain times.

Brain Structures

Diagram : Brain Structures

Human Memory Operation

Operation

The mechanism of memory is believed to take place at the synapse, the junction between neuron cells. Both the actual synaptic connection and the way the synapse responds to stimulus, represent the elementary units of memory. Synaptic junctions form as the brain is developing but it is also possible that new junctions form later in life.

Synaptic Operation

When a signal, an action potential, travels down the axon of a neuron, it may reach many synaptic junctions with other cells. At these junctions, chemical transmitters such as acetycholine, adrenaline, noradrenalin, serotonin etc are released and travel the very short distance across the synaptic junction. Receptor sites on the dendritic spine of the next cell can each accept a molecule of transmitter. If enough receptor sites are activated, the dendrite will signal the cell body of the next neuron. Receptor sites may cause an excitatory (more likely to signal) or an inhibitory (less likely to signal) action. It is the sum of these effects that will eventually cause a response in the next neuron.

Memories are laid down when permanent changes are made to the effect of the release of chemical transmitters. Postsynaptic sites may become more sensitised to the effect of the chemicals or additional receptor sites may be grown.

Graphic : Synaptic Operation

Neurological Research

The great advance in the study of neurons was helped by the development of a staining technique that allowed individual cells to be viewed under a microscope. As the technology of microscopy has developed, it has been possible to investigate the activity of individual neuron cells. Initially, the large cells, such as those found in the giant squid provided information about how neurons transmitted information (the action potential). Later, the study of simple learning in uncomplicated animals such as Aplysia, a marine snail, has lead to the discovery that synaptic changes are taking place during learning.

It has also been shown that the Hippocampus, a region in the brain, stores long term memories for weeks before transferring them to other parts of the brain. The hippocampus is essential for long term memory in humans.

In 1973, Timothy Bliss and Terjelomo, in Oslo, first demonstrated that neurons in the Hippocampus have a remarkable plasticity, the kind that would be required for learning. They found that a brief high frequency train of action potentials in one of the neural pathways of the hippocampus produced an increase in synaptic strength in that pathway. The effect can last from hours to weeks.

Machine Memory Construction

There are several forms of machine memory. Each type is dependent on the technology used to implement it. Most machine memory, of the computer form, is digital. The basic storage unit is therefore a single bit (an on or off state). The technology used to implement machine memory offers different ways of storing bits (on or off states). A typical modern personal computer would contain examples of each of the three construction methods described below.

Fixed Magnetic Discs

Photo : Hard
              Drive

Fixed Magnetic dics would be the main bulk storage media. These hold information for long periods even when there is no power to the machine. This is similar to the way music cassette tapes hold information. When the computer starts up, it recieves its initialisation information from the magnetic fixed disc, after following some instructions from a special type of silicone based storage chip. Magnetic discs can easily be written to as well as read.

Silicon Based Memory

Silicon Chips

Silicon based memory constructions are digital circuits on silicon chips. It would be the main working memory in the computer. Information contained within it would change as different software is used. The information is all lost (usually) when the computer is switched off.

Optical Memory

Graphic : CD-ROM

Optical memory constructions are encoded and read by LASER. They are employed as bulk carriers of digital information and most computers have some method of reading (and possibly writing to) optical compact discs. It is very common for new software to be delivered on an optical disc because these can contain large amounts of digital code and are very cheap to make.

Machine Memory Operation

The operation of the main three types of computer storage is technology dependent. Magnetic storage devices work by magnetising ferrous material in one of two orientations. This information can be read back as the magnetic material is passed across a read head (coil of wire). Movement between the material and write/read heads is necessary for this process.

Silicon storage devices are based on the storage of an electrical charge in either a bi-stable device (a circuit with two stable states) or a capacitive junction, usually a field effect transistor.

Optical devices work by burning tiny pits or holes in a special material to reveal a reflective coating underneath. Information is read back by shining a tiny light on the track and reading the reflection which changes depending on whether a pit is reached or not. Again, movement between the material and the read/write head is required for this process to work.

Sources

"Your Memory: A Users Guide"

by Alan Baddeley: Peguin 1993 provides an excellent treatment for human memory, particularly operation.

Scientific American special issues on "The Brain": 1979 and "Mind and Brain": 1992, provide a good treatment of the physical construction of human memory

References to machine memory can be found in computer architecture and digital electronic texts.