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Memory is the ability to store and retrieve information as and when required. While it is widely believed that memory can retain information for days, months, or even years, there is one type of memory that holds information for only a few milliseconds to a few seconds. This type of memory is called sensory memory.
We have five sense organs, and each of these sends sensory information to different parts of the brain. For example, the eyes send information to the primary visual cortex through long neural pathways. Similarly, the ears, nose, tongue, and skin also send sensory information for further processing.
When a neuron fires an action potential, it takes about 1–2 milliseconds to reach its peak depolarization, and then it takes another few milliseconds to return to its resting state. As a result, one neuron cannot fire continuously to send a signal. Therefore, our brain needs some kind of storage that can hold the information for a few milliseconds. These storage units are modality-specific, meaning that we have five sensory memories, one for each sense organ. Each sensory memory works differently from the others and does not interfere with their functioning. Sensory memory is closely connected to perception, and most research in this field has been done on visual and auditory sensory memory.
Iconic Memory
Iconic memory is a sensory memory storage system for visual material that holds information for up to about one second. It is named “Icon” given by Neisser in 1967. An example of Iconic memory is when the trail of a sparkler or a movie at 24 frames per second produces a persistence of vision. The fading of the overhead projector light in a classroom is also an example of Iconic memory. A classical study on Iconic storage was conducted by George Sperling in 1960. In his experiments, Sperling showed participants a grid of letters for a brief period and then asked them to recall as many letters as they could. In the “whole report” condition, participants were asked to recall all the letters they saw. In the “partial report” condition, participants were cued to recall only a specific row of letters. Sperling found that participants could recall about four to five letters out of twelve in the whole report condition, but up to three out of four letters in the partial report condition. The delay in cueing the tone resulted in the same outcome as the whole report. Sperling inferred that the Icon holds information that is not yet categorized.
Averback and Coriell (1961) showed that the Icon can be “erased” if another stimulus is presented after the initial stimulus, such as a circle after the letter matrix. Participants were then asked to report the letter where the circle was placed but were unable to do so. Other cues, such as color or brightness, can be used, but phonological sound is not possible.
Echoic Memory
Echoic memory is a sensory memory for auditory material. It is named “Echo” by Neisser in 1967. In the “four-eared” listening task conducted by Moray, Bates, and Barnett in 1965, four channels were created using stereophonic mixing. Each channel had a string of random letters. Whole and partial report using four lights showed that Echo also stores information briefly. Darwin et al. found that it can be cued by category, unlike Icon. Echoic memory might have a large capacity of up to 20 seconds. The suffix effect occurs when the recall cue (such as a beep) is also in the auditory mode, hindering the recall of the previously given auditory stimulus. This is a sort of “auditory mask.”
Sensory memories are modality-specific, meaning that visual sensory memory stores visual information, while auditory sensory memory stores auditory information. Sensory memory has a large capacity, but the length of time information is stored