Iconic Memory

iconic memory

Iconic memory is a fast-decaying storage of visual information and the visual sensory memory register belonging to the visual domain. It’s part of the visual memory system, together with visual short-term memory (VSTM) and long-term memory (LTM).

It supports VSTM by delivering a cohesive representation of our full visual sense for a very short time. Iconic memory aids in accounting for phenomena such as change blindness and experience continuity during saccades. It is no longer seen as a single thing but rather as a collection of at least two different components, visible persistence and informational persistence.

Visible Persistence

The phenomenal sensation that a visual picture remains after its physical offset is referred to as visible persistence. This is a byproduct of neuronal persistence.

Visible persistence is more sensitive to the physical aspects of the stimulus than informational persistence, as seen by its two essential qualities:

  1. The duration of visible persistence is inversely related to stimulus duration. This means that the longer the physical stimulus is presented for, the faster the visual image decays in memory.
  2. The duration of visual persistence is inversely linked to the luminance of the stimulus. The duration of visual persistence reduces as the luminance, or brightness, of a stimulus increases. Because of the neural system’s involvement, visible persistence is highly dependent on photoreceptor physiology and the activation of various cell types in the visual cortex. This visible representation is subject to masking effects whereby the presentation of interfering stimulus during, or immediately after stimulus offset interferes with one’s ability to remember the stimulus.

Underlying visible persistence is the neural persistence of the visual sensory pathway. A prolonged visual representation begins with activation of photoreceptors in the retina.

Although both rod and cone activation has been observed to remain beyond the physical offset of a stimulus, the rod system endures longer than the cone system. M and P retinal ganglion cells are also engaged in a prolonged visual picture. M cells (transient cells) are only active at stimulus onset and offset.

Cortical persistence of the visual image has been found in the primary visual cortex (V1) in the occipital lobe, which is responsible for processing visual information.

Informational Persistence

Informational persistence refers to knowledge about a stimulus that survives after its physical offset. It is visual in nature, yet not visible. Sperling’s tests were a test of informational persistence.

The length of the stimulus is the most important factor in determining the duration of informational persistence. As the length of the stimulus rises, so does the duration of the visual code.

Non-visual components represented by informational persistence include the image’s abstract features as well as its geographical position. Unlike visible persistence, informational persistence is immune to masking effects due to its nature.

The properties of this component of iconic memory suggest that it plays a critical function in representing a post-categorical memory store for which VSTM can access information for consolidation.

Although there is less research on the brain representation of informational persistence than on visual persistence, new electrophysiological techniques are revealing cortical areas implicated. Informational persistence, as opposed to apparent persistence, is assumed to rely on higher-level visual areas beyond the visual cortex.

Iconic memory’s role in change detection has been related to activation in the middle occipital gyrus (MOG). MOG activation was found to persist for approximately 2000ms suggesting a possibility that iconic memory has a longer duration than what was currently thought. Iconic memory is also influenced by genetics and proteins produced in the brain.

Brain-derived neurotrophic factor (BDNF) is a part of the neurotrophin family of nerve growth factors. Individuals with mutations to the BDNF gene which codes for BDNF have been shown to have shortened, less stable informational persistence.

Function of Iconic Memory

Iconic memory sends a steady stream of visual information to the brain, which the visual short-term memory can extract over time and consolidate into more stable forms. One of the primary responsibilities of iconic memory is to identify changes in our visual environment, which aids in the perception of motion.

Iconic memory allows for the integration of visual information along a continuous stream of visuals, such as when viewing a movie. New stimuli do not delete knowledge about prior stimuli in the primary visual cortex. Instead, responses to the most recent stimuli contain roughly equal quantities of information regarding both that stimulus and the one before it.

This one-back memory may be the main substrate for both the integration processes in iconic memory and masking effects. The particular outcome depends on whether the two subsequent component images (i.e., the “icons”) are meaningful only when isolated (masking) or only when superimposed (integration).

Saccadic Eye Movement

It has been proposed that iconic memory plays a role in maintaining experience continuity during saccadic eye movements. These quick eye movements last about 30 milliseconds, and each fixation lasts about 300 milliseconds.

However, research suggests that memory for information between saccades is mostly based on VSTM rather than iconic memory. Information stored in iconic memory is assumed to be lost during saccades rather than contributing to trans-saccadic memory. A similar process occurs during eye blinks, in which both automatic and deliberate blinking disturbs the information stored in iconic memory.

Change Blindness

The brief representation in iconic memory is thought to play a key role in the ability to detect change in a visual scene. The phenomenon of change blindness has provided insight into the nature of the iconic memory store and its role in vision. Change blindness refers to an inability to detect differences in two successive scenes separated by a very brief blank interval, or interstimulus interval (ISI).

The difference is immediately discernible when scenes are presented without an ISI. Each ISI is considered to obliterate the detailed memory store of the scene in iconic memory, rendering the memory unavailable. This limits the capacity to make comparisons between scenes.

Discovery Timeline

Many people throughout history have documented the presence of a prolonged physiological image of a thing after its physical offset. Aristotle claimed that afterimages were engaged in the dream experience in one of the earliest written accounts of the phenomenon. He described how afterimages appeared when staring at the sun or a bright light and then closing one’s eyes.

Researchers in the 1700s and 1800s were intrigued by the natural observation of the light trail generated by a flaming ember at the end of a rapidly moving stick. They were the first to do empirical research on this phenomena, which came to be known as visual persistence.

The role of visible persistence in memory received a lot of attention in the 1900s since it was thought to be a pre-categorical representation of visual information in visual short-term memory (VSTM). George Sperling began his renowned partial-report studies in 1960 to prove the existence of visual sensory memory and some of its properties such as capacity and endurance.

It was not until 1967 that Ulric Neisser termed this quickly decaying memory store iconic memory. Approximately 20 years after Sperling’s original experiments, two separate components of visual sensory memory began to emerge: visual persistence and informational persistence. Sperling’s experiments mainly tested the information pertaining to a stimulus, whereas others such as Coltheart performed direct tests of visual persistence.

Di Lollo presented a two-state model of visual sensory memory in 1978. Despite historical dispute, contemporary knowledge of iconic memory distinguishes between visual and informational persistence, which are assessed differently and have fundamentally different qualities. As the precategorical sensory store, informational persistence, which underpins iconic memory, is regarded to be the primary contributor to visual short-term memory.

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