Iconic Memory and Visual Stimuli

People remember things in different ways. One of these ways is known as iconic memory, which involves the memory of visual stimuli. The word iconic refers to an icon, which is a pictorial representation or image. Iconic memory is how the brain remembers an image you have seen in the world around you.

For example, look at an object in the room you are in now, and then close your eyes and visualize that object. The image you "see" in your mind is your iconic memory of that visual stimuli. Iconic memory is part of the visual memory system, which includes long-term memory and visual short-term memory.

This article discusses what iconic memory is, how it works, and how it was first discovered. It also explores important phenomena that influence the persistence of visual stimuli.

History of Iconic Memory

In 1960, George Sperling performed experiments designed to demonstrate the existence of visual sensory memory. He was also interested in exploring the capacity and duration of this type of memory.

In Sperling's experiments, he showed a series of letters on a mirror tachistoscope to participants. These letters were only visible for a fraction of a second, but the subjects were able to recognize at least some of the letters. However, few were able to identify more than four or five letters.

In additional experiments, Sperling provided clues to help prompt memories of the letters. Letters were presented in rows, and the participants were asked to recall only the top, middle, or bottom rows.

The participants were able to remember the prompted letters relatively easily, suggesting it is the limitations of this type of visual memory that prevents us from recalling all of the letters. We see and register them, Sperling believed, but the memories simply fade too quickly to be recalled.

In 1967, psychologist Ulric Neisser labeled this form of quickly fading visual memory as iconic memory.

Examples of Iconic Memory

It can be helpful to consider a few examples of iconic memory and how it can aid you in your daily life. Consider some of the following scenarios:

• You glance over at a friend's phone as she is scrolling through her Facebook newsfeed. You spot something as she quickly thumbs past it, but you can close your eyes and visualize an image of the item very briefly.
• You wake up at night to get a drink of water and turn the kitchen light on. Almost instantly, the bulb burns out and leaves you in darkness, but you can briefly envision what the room looked like from the glimpse you were able to get.
• You are driving home one night when a deer bounds across the road in front of you. You can immediately visualize an image of the deer bolting across the road illuminated by your headlights.

Types of Visual Persistence

Iconic memory involves the persistence of visual information. There are three different types of visual persistence when it comes to iconic memory:

• Neural persistence: This type of persistence involves the continuation of neural activity even after the visual stimulus is no longer present. 
• Visible persistence: This form of persistence involves continuing to see an image after it is no longer present. An example of this would be briefly continuing to see the brightness of a flashlight after it has been turned off.
• Informational persistence: This relates to the information that is still available once a stimulus is no longer visible. For example, after an object is no longer visible, you may still be able to see the space around its previous location.

Research has also found two important effects influence iconic memory for visual stimuli. 

• Inverse duration effect: The longer a stimulus lasts, the shorter its persistence after it is absent.
• Inverse intensity effect: The more intense a visual stimulus is, the briefer its persistence once it disappears.

However, it is important to note that these phenomena do not apply to afterimages. Afterimages are produced when a stimulus is so intense that the retinal impression causes the continued activation of the visual system.

Impact of Iconic Memory

Iconic memory is believed to play a role in change blindness or the failure to detect changes in a visual scene. In experiments, researchers have shown that people struggle to detect differences in two visual scenes when they are interrupted by a brief interval. Introducing a brief interruption erases iconic memory, making it much more difficult to make comparisons.

One study found that individual differences in change blindness were related to both the strength and stability of the visual image as well as visual ability, including the strength of iconic memory.

One study found considerable variability in the duration of iconic memory. For some participants, iconic memory lasted up to 240ms, while for others, it lasted no more than 120ms. The researchers suggested that this may indicate that iconic memory has different layers linked to specific levels of visual hierarchy.