Sensory memory is the first stage of memory processing, holding large amounts of sensory information for very brief periods. It serves as a buffer that maintains a relatively complete record of sensory input just long enough for relevant information to be selected for further processing. Without sensory memory, the perceptual world would be experienced as a series of disconnected snapshots rather than a continuous flow.
Iconic Memory
George Sperling's (1960) landmark partial-report experiment demonstrated the existence of a brief, high-capacity visual store — iconic memory. When participants viewed an array of 12 letters for 50 ms, they could report only about 4-5 letters (the whole-report limit). But when a tone cue indicated which row to report, presented immediately after the display, participants could report 3-4 of the 4 letters in any cued row — implying that about 9-10 of the 12 letters were initially available. This rich representation decayed within approximately 250-500 ms.
Partial report (immediate cue): ~75% of any row
Estimated capacity: ~9-10 items
Duration: decays to whole-report level within ~300 ms
Echoic Memory
The auditory equivalent of iconic memory is echoic memory, which stores auditory information for a somewhat longer period — approximately 2-4 seconds. Darwin, Turvey, and Crowder (1972) demonstrated echoic memory using a partial-report paradigm adapted for audition, and Efron's (1970) work on auditory persistence showed that brief sounds continue to be "heard" after their offset. The longer duration of echoic compared to iconic memory reflects the inherently temporal nature of auditory information — speech and music unfold over time and require longer integration windows.
Functional Role
Sensory memory serves several critical functions. It provides temporal integration, bridging gaps between successive fixations (in vision) or successive segments of ongoing speech (in audition). It provides a raw data buffer from which attention can select relevant information for further processing. And it maintains a record of the recent sensory past that can be sampled retroactively — explaining why you can "hear" something someone said a moment ago that you were not initially attending to.
In the influential Atkinson-Shiffrin (1968) modal model, sensory memory (the sensory register) is the first of three stores: sensory register, short-term store, and long-term store. Information flows from sensory memory to short-term memory through the process of attention and pattern recognition. Only information attended to and transferred to short-term memory survives beyond the brief sensory trace.
Neural Basis
Iconic memory appears to involve both retinal persistence and cortical processes in early visual areas. ERP studies show that the visual cortex maintains activity for several hundred milliseconds after stimulus offset, corresponding to the duration of the iconic trace. Echoic memory likely involves sustained activity in auditory cortex, with the mismatch negativity (MMN) component providing evidence that the auditory system maintains sensory representations against which incoming sounds are compared.