Sensory memory is our ability to retain impressions of sensory information after the original stimulus has ceased.
One of the earliest investigations into this phenomenon was in 1740 by Johann Andreas Segner (1704 - 1777) the German physicist and mathematician. In an elegant experiment Segner attached a glowing coal to a cartwheel and rotated the wheel at increasing speed until an unbroken circle of light was perceived by the observer. He calculated that the glowing coal needed to make a complete circle in under 100ms to achieve this effect.
Sperling ran a more systematic study of this effect in 1960. His experiment involved briefly presenting a grid of three rows of four letters for 50ms to volunteers. In one condition he asked participants to report as many letters as they could remember after the letters had been presented. In another he indicated, after the presentation of letters, which row of letters (first, second or third) he would like participants to try and recall. Sperling found that whilst participants could only report an average of four letters in the first condition, they usually sucessfully recalled the four letters of the chosen row in the second condition, even though they were only told which to report after the whole letter grid was presented. This suggests that for a brief period of time the whole grid was accessible to the participants as sensory memory.
Sensory memory is still considered to operate within this approximate time frame (under 1 second and certainly no more than 2) and so can be seen to be very short lived. It is also characterised by being outside of consciousness control (i.e. it happens automatically and unbidden). Despite retaining information for a very short period if time, it is not to be confused with short term memory (which typically lasts 10-15 seconds without rehearsal of the remembered material) and is so named to distinguish it from long term memory which can store information for as long as a lifetime.
Sensory memory is also accountable for the eyes' field of vision. In reality it is quite limited, but the sensory memory retains images that have already been seen. Thus creating an extended field of vision.