There are many different hypotheses about how human memory works that often overlap with each other. In recent years research has clarified key aspects of sensory memory, one of the oldest concepts in the field of scientific psychology applied to this basic process.

In this article we will define the characteristics of the three main types of sensory memory that have been described so far: the iconic, the echoic and the haptic memory, which work with visual, sound and tactile stimuli, respectively.

What is sensory memory?

The sensory memory allows us to retain information obtained through the senses for a short period ; subsequently, these signals will be discarded or transmitted to other memory stores of longer duration, the working memory and the long-term memory, through which it will be possible to operate on the immediate stimuli.

The concept of “sensory memory” was coined by Ulric Gustav Neisser in 1967. His model was based on basic research and defined sensory memory as a short term record , of unlimited and precautionary capacity, i.e. prior to cognitive information processing and therefore beyond conscious control.

Previously, in 1958, Donald Eric Broadbent had proposed the existence of a perceptual system through which all sensory stimuli would pass before reaching short-term memory and being filtered for the conscious processing of the most relevant items.

In his original formulation Neisser considered that there are two types of sensory memory : the iconic, which processes visual information, and the echoic, based on auditory and verbal stimuli. Subsequently, solid evidence has been found in favour of the existence of haptic memory, related to touch and proprioception.

Types of sensory memory

Although it is considered that there are probably short-term mnemonic stores for all the senses, those that have been studied in greater depth are iconic, echoic and haptic memory .

Iconic Memory

The most investigated type of sensory memory is the iconic one, which records visual information. The most relevant contributions to this phenomenon were made by George Sperling in the 1950s and 1960s, but subsequently authors such as Neisser, Sakkit and Breitmeyer have updated the concept of iconic memory.

Through his pioneering studies with tachistoscope, Sperling concluded that people have the ability to simultaneously retain 4 or 5 items after staring for an instant at a broad stimulating array. Other researchers found that iconic memory persists for about 250 milliseconds.

In this case , the visual track that we keep in our short-term memory is called an “icon”. There is currently a debate as to whether this icon is located in the central or the peripheral nervous system; in any case, the conception that the iconic memory is fundamentally a laboratory artefact with no ecological validity predominates.

This phenomenon is most probably related to the persistence of neuronal stimulation in the photoreceptors located in the retina, i.e. the cones and rods. This system could have the function of allowing the processing of visual stimuli by the perceptive system.

2. Eco-Memory

Similar to the iconic one, the echo memory has been defined as a precategorical, short term record with a very high capacity. It differs from the iconic one in that it processes audio rather than visual information.

Echoic memory retains auditory stimuli for at least 100 milliseconds , allowing us to discriminate and recognize sounds of all kinds, including those that make up speech, which can be held for up to 2 seconds; thus, echoic memory is fundamental to understanding language.

It is understood that this type of memory records auditory information in sequence, thus focusing on its temporal properties. In part, the time the echo print is retained depends on stimulus properties such as complexity, intensity and tone.

A remarkable phenomenon in relation to echoic memory is the recency effect, which is specific to this type of memory. It consists of the fact that we remember better the last stimulus (or item) that we have processed than others that have been presented immediately before.

Echoic memory has been related to the hippocampus and to different areas of the cerebral cortex: the premotor, the prefrontal ventrolateral posterior left and the parietal posterior left. Injuries in these regions cause deficits in the perception of visual stimuli and in the speed of reaction to them.

3. Haptic memory

This concept is used to designate a mnemonic store that works with tactile information, and therefore with sensations such as pain, heat, itching, tingling , pressure or vibration.

The haptic memory has a capacity of 4 or 5 items, like the iconic one, although the print is kept for longer, about 8 seconds in this case. This type of sensory memory allows us to examine objects by touch and interact with them, for example to pick them up or move them properly.

It is believed that there are two subsystems that make up haptic memory. On the one hand we find the skin system, which detects the stimulation of the skin, and on the other the proprioceptive or kinesthetic system , related to muscles, tendons and joints. A distinction should be made between proprioception and interception, which involves the internal organs.

Haptic memory has been defined more recently than iconic and echoic memory, so the scientific evidence available on this type of sensory memory is more limited than that available on the other two described above.

The haptic memory depends on the somatosensory cortex , especially on regions located in the upper parietal lobe, which store tactile information. The prefrontal cortex, which is fundamental for planning movement, also seems to be involved in this function.