Iconic Memory and Visual Stimuli

There are many different types of memories. One type is known as iconic memory, which involves the memory of visual stimuli. Iconic memory is how the brain remembers an image we've seen in the world around us.

Here we dive a bit deeper into iconic memory, including talking more about what it is, how it works, and how it was first discovered. We also explore important phenomena that influence the persistence of visual stimuli when creating this memory type.

What Is Iconic Memory?

The word 'iconic' refers to an icon, and an icon is a pictorial representation or image. So, iconic memory is the storage for visual memory that allows us to visualize an image after the physical stimulus is no longer present.

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 stimulus.

Iconic memory is part of the visual memory system, which includes long-term memory and visual short-term memory. It is a type of sensory memory that lasts just milliseconds before fading.

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 memory type.

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

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 row.

The participants were able to remember the prompted letters relatively easily, suggesting it is the limitations of this type of visual memory that prevent 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. Interestingly, Neisser is also known as the father of cognitive psychology.

Examples of Iconic Memory

It can be helpful to consider a few examples of iconic memory and how it exists in daily life. Consider some of these 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.

Iconic Memory and 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 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 found three important effects that 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.
• Inverse proximity effect: The greater the proximity between dots in a matrix, the shorter its persistence.

Impact of Iconic Memory

Iconic memory is believed to play a role in change blindness. This refers to 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 added 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.