Визуалды жад жүйесінің құрауышы: Икондық жад

Component of the visual memory system
Iconic memory is the visual sensory memory register pertaining to the visual domain and a fast decaying store of visual information. It is a component of the visual memory system which also includes visual short term memory (VSTM) and long term memory (LTM). Iconic memory is described as a very brief (<1 second), pre categorical, high capacity memory store. Natural observation of the light trail produced by glowing ember at the end of a quickly moving stick sparked the interest of researchers in the 1700s and 1800s. They became the first to begin empirical studies on this phenomenon 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 directs tests of visual persistence. Although it has been debated throughout history, current understanding of iconic memory makes a clear distinction between visual and informational persistence which are tested differently and have fundamentally different properties. Informational persistence which is the basis behind iconic memory is thought to be the key contributor to visual short term memory as the precategorical sensory store.

The two main components of iconic memory are visible persistence and informational persistence. The first is a relatively brief (150 ms) pre categorical visual representation of the physical image created by the sensory system. This would be the "snapshot" of what the individual is looking at and perceiving. The second component is a longer lasting memory store which represents a coded version of the visual image into post categorical information. This would be the "raw data" that is taken in and processed by the brain. A third component may also be considered which is neural persistence: the physical activity and recordings of the visual system. Neural persistence is generally represented by neuroscientific techniques such as EEG and fMRI.

Visible persistence is the phenomenal impression that a visual image remains present after its physical offset. This can be considered a by product of neural persistence. Visible persistence is more sensitive to the physical parameters of the stimulus than informational persistence which is reflected in its two key properties. :
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. The duration of visible persistence is inversely related to stimulus luminance. When the luminance, or brightness of a stimulus is increased, the duration of visible persistence decreases. Due to the involvement of the neural system, visible persistence is highly dependent on the physiology of the photoreceptors and activation of different 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. Different techniques have been used to attempt to identify the duration of visible persistence. The Duration of Stimulus Technique is one in which a probe stimulus (auditory "click") is presented simultaneously with the onset, and on a separate trial, with the offset of a visual display. The difference represents the duration of the visible store which was found to be approximately 100 200 ms. In the first paradigm, an image is presented discontinuously with blank periods in between presentations. If the duration is short enough, the participant will perceive a continuous image. Similarly, the Moving Slit Technique is also based on the participant observing a continuous image. Only instead of flashing the entire stimulus on and off, only a very narrow portion or "slit" of the image is displayed. When the slit is oscillated at the correct speed, a complete image is viewed.

Underlying visible persistence is neural persistence of the visual sensory pathway. A prolonged visual representation begins with activation of photoreceptors in the retina. Although activation in both rods and cones has been found to persist beyond the physical offset of a stimulus, the rod system persists longer than cones. Other cells involved in a sustained visible image include M and P retinal ganglion cells. M cells (transient cells), are active only during stimulus onset and stimulus offset. P cells (sustained cells), show continuous activity during stimulus onset, duration, 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.

Information persistence represents the information about a stimulus that persists after its physical offset. It is visual in nature, but not visible. Sperling's experiments were a test of informational persistence. The non visual components represented by informational persistence include the abstract characteristics of the image, as well as its spatial location. Due to the nature of informational persistence, unlike visible persistence, it is immune to masking effects.

Iconic memory provides a smooth stream of visual information to the brain which can be extracted over an extended period of time by VSTM for consolidation into more stable forms. One of iconic memory's key roles is involved with change detection of our visual environment which assists in the perception of motion.

Iconic memory enables integrating visual information along a continuous stream of images, for example when watching a movie. In the primary visual cortex new stimuli do not erase information about previous stimuli. Instead the responses to the most recent stimulus contain about equal amounts of information about both this and the preceding stimulus. As such change blindness can be defined as being a slight lapse in iconic memory. When scenes are presented without an ISI, the change is easily detectable. It is thought that the detailed memory store of the scene in iconic memory is erased by each ISI, which renders the memory inaccessible. This reduces the ability to make comparisons between successive scenes. These rapid eye movements occur in approximately 30 ms and each fixation lasts for approximately 300 ms. Research suggests however, that memory for information between saccades is largely dependent on VSTM and not iconic memory. Instead of contributing to trans saccadic memory, information stored in iconic memory is thought to actually be erased during saccades. A similar phenomenon occurs during eye blinks whereby both automatic and intentional blinking disrupts the information stored in iconic memory.

The development of iconic memory begins at birth and continues as development of the primary and secondary visual system occurs. By 6 months of age, infants' iconic memory capacity approaches adults'. By 5 years of age, children have developed the same unlimited capacity of iconic memory that adults possess. The duration of informational persistence however increases from approximately 200 ms at age 5, to an asymptotic level of 1000 ms as an adult (>11 years). A small decrease in visual persistence occurs with age. A decrease of approximately 20 ms has been observed when comparing individuals in their early 20s to those in their late 60s. Throughout one's lifetime, mild cognitive impairments (MCIs) may develop such as errors in episodic memory (autobiographical memory about people, places, and their contex), and working memory (the active processing component of STM) due to damage in hippocampal and association cortical areas. Episodic memories are autobiographical events that a person can discuss. Individuals with MCIs have been found to show decreased iconic memory capacity and duration. Iconic memory impairment in those with MCIs may be used as a predictor for the development of more severe deficits such as Alzheimer's disease and dementia later in life. Previous studies have shown that glucocorticoids have been closely linked to impact higher cognitive functioning. Glucocorticoid exposure causes severe memory retrieval impairment, explicitly advancing iconic memory decay. It reduces the active maintenance and storage of sensory information by altering transient neural responses during the initial stimulus processing stages. Elevated cortisol levels have also been associated with faster iconic memory decay and top down processing impairment, putting individuals at a higher risk of developing Dementia and AD. Iconic memory formation has been previously described as attention free and fleeting, however newer studies have shown that in fact it does require attention. IM is shown to decay at a faster rate if attention focus is not appropriately met to the attention load. This allows for the information that is being transported into working memory to be retained more precisely. Iconic memory decay has been found to occur at a rapid speed after the visual stimulus is no longer present. Without active retrieval, iconic memory averages to disappear within half a second. The theory of gradual decay in visual working memory claims that the accuracy at which the stimulus is remembered in iconic memory deteriorates over time. However, information stored in sensory memory is considered to facilitate exponential decay.

In 1960, George Sperling became the first to use a partial report paradigm to investigate the bipartite model of VSTM.

The effects of masking were identified by the use of a circle presented around a letter as the cue for recall. When the circle was presented before the visual stimulus onset or simultaneously with stimulus offset, recall matched that found when using a bar or tone. However, if a circle was used as a cue 100 ms after stimulus offset, there was decreased accuracy in recall. As the delay of circle presentation increased, accuracy once again improved. This phenomenon was an example of metacontrast masking. Masking was also observed when images such as random lines were presented immediately after stimulus offset.