Sensory memory, which is sometimes called a fleeting memory, gets consolidated as short-term memory, only if we choose to remember the event. This memory is replaced constantly. For instance, while watching a motion picture, one doesn't observe the gaps between frames, as each frame is held in the sensory register until the arrival of the next frame.
The human memory system involves the encoding, storage, and retrieval of information. The model for human memory suggested by Atkinson and Shiffrin (1968) includes three components called sensory memory, short-term memory, and long-term memory. The term 'sensory memory' refers to the first stage of memory that holds on to the incoming sensory/perceptual information. Before the information enters the short-term memory store, the data is acquired by our senses. The senses of sight and hearing hold on to the sensory input for a very brief duration.
In fact, the information is retained for the shortest duration in the sensory memory. From the sensory information, only the inputs that you decide to pay attention to, move on to the short-term/working memory. The sensory memory associated with the sense of sight is referred to as the iconic memory, whereas the memory associated with the sense of hearing is referred to as the echoic memory.
In the first stage of memory, an exact copy of the information gathered through the senses is stored for a very short duration. Our senses are working constantly, which is why we focus on a limited amount of information that we consider as relevant. The sensory memory has a large capacity. It is believed that it includes systems that are associated with each sense. In the encoding process, a substantial amount of information is gathered through the sense of sight and hearing.
The sensory memory comprises:
Iconic memory (Visual sensory input from the eyes)
Echoic memory (Auditory sensory input)
Haptic memory (Tactile sensory input)
Gustatory memory (Input related to the sense of taste)
Olfactory memory (Input related to the sense of smell)
Sensory memory, short-term memory, and long-term memory differ from each other, when it comes to the duration, capacity, and function. Sensory memory has a large capacity for information, but it has a brief duration. The short-term memory and long-term memory have limited and unlimited capacity respectively. While the duration of sensory memory ranges up to 4 seconds, the duration of the short-term memory is about 20-30 seconds. The information can be stored in long-term memory indefinitely.
An important function of sensory memory is to briefly store sensory impressions or the exact copies of the sensory inputs (visual, auditory, gustatory, olfactory, and haptic), as a buffer memory till it is transferred into the short-term memory. The element of attention is extremely important in case of sensory memory. Out of all the sensory inputs, only the information that we pay attention to gets transferred to the short-term memory. The rest of the information is discarded or forgotten.
The sensory information that is received vanishes in a few seconds, which in turn prevents us from getting overwhelmed by multiple sensory stimuli. Moreover, just within the brief duration for which it lasts, one can decide the parts of sensory information that should be retained and transferred to short-term memory. Since the mental images are constantly being replaced with the new sensory inputs in the sensory registers, continuity is maintained in the way we perceive our surroundings.
Iconic Memory vs. Echoic Memory
The terms 'iconic memory' and 'echoic memory' were coined by Ulric Neisser in 1967. Iconic memory is the sensory memory for vision, whereas echoic memory is the memory for audition (hearing). The term 'iconic' is derived from the word 'icon', and refers to a fleeting mental image that is formed after receiving the sensory input from the eyes. Lightning flash or sparkler trails are examples of iconic memory. Even when we blink, the visual inputs are held in the iconic memory, which is why there's no realization of the fact that our eyes were completely closed.
On the other hand, the term 'echoic' is derived from the word 'echo', and refers to the continuation of the sound or the sensory input from the auditory system. For instance, if you are engrossed in reading a book when someone says something to you, you might ask the person to repeat what he/she said. However, you might be immediately able to remember what that person had said, as the words would have been briefly stored in the echoic memory.
The major difference between iconic memory and echoic memory is regarding the duration and capacity. Echoic memory lasts up to 3-4 seconds in comparison to the iconic memory, which lasts up to one second. However, iconic memory preserves 8-9 items, in comparison to 4-5 items in case of echoic memory.
George Sperling's Experiment
It was in the 1960s that the existence of iconic memory was proven with the help of a series of experiments conducted by an American psychologist named George Sperling. In the experiment, the subjects were asked to recall a set of 9 letters (three letters each in three rows). The letters were projected on to a screen for a brief period (one-twentieth of a second) with the help of a tachistoscope.
It was observed that most participants were able to recall four or five letters. According to Sperling, the participants were unable to recall all the letters even when all of them had been registered, as the memory had faded too quickly.
In another experiment, a set of twelve letters were projected on to the screen, but a tone was sounded right after the letters were flashed. The subjects were given the instructions to recite only the letters in the top, middle, and bottom row on hearing the high, medium, and low tone respectively. It was observed that the participants had no difficulty in recalling the letters in the row indicated by the tone. On an average, most subjects were able to recall 3 out of 4 letters in the row. This proved that all the twelve letters had been registered in the visual sensory store.
Experiment by Guttman and Julesz (1963)
In 1963, Guttman and Julesz asked the participants to put on headphones and listen to the repeated segments of white noise (noise produced by a stimulus that contains all the audible frequencies of vibration) produced with the help of a computer. It was ensured that a repeating pattern of white noise was put together without any gap between repetitions. It was observed that the participants were unable to recognize the fact that the sound was being repeated, if the repeating segment of the noise lasted longer than a few seconds. All they could hear was a continuous sound with no pattern. On the other hand, the subjects could realize that they were hearing a repeated sound, when the segment of white noise lasted less than two seconds. Thus, the exact pattern of sound was preserved in the auditory sensory store for a brief duration. In a study conducted by Lu, Williamson, and Kaufman (1992), a neuroimaging technique called magnetoencephalography was used for mapping brain activity. This study revealed activity in the auditory cortex 2-5 seconds after the sound stimulus.
On a concluding note, the iconic memory, echoic memory, and other components of sensory store decay rapidly, but the sensory memory acts as a buffer and is an essential step in the storage of information in short-term memory. Major problems can arise, if the relevant information doesn't get transferred from the sensory memory to the short-term memory.