How did Sperling (1960) provide evidence that iconic memory has large capacity and very short duration (mention partial and full recall techniques)

How did Sperling (1960) provide evidence that iconic memory has large capacity and very short duration (mention partial and full recall techniques)

Sperling wondered the quantity of information the people were able to take in from briefly presented stimuli. Thus, he carried out an experiment in which three rows of 4 letters each were shown for 50 milliseconds. He used two different methods:
- In the first one the participants were asked to recall all the letters they could remember from the 12 shown (full recall technique). The results indicated that an average of 4.5 out of 12 could were remembered from the participants.
- In the second method, after each presentation of the 12 letters a tone was presented. There were 3 possible cues, each one linked to one of the three rows in such a way that, when presenting the determinate tone after the whole group of letters, the participants would know which row they were asked to recall. In this partial recall technique it turned out that an average of 3.3 out of 4 letters (82%) were successfully remembered, no matter which row is asked to recall.

Thus, he concluded that people may be able to see around the 82% of the whole display but they cannot report all of them because while they are recalling the first ones, they are forgetting the rest.

Sperling was interested in both the capacity and duration of the iconic memory. He conducted a study in which he very briefly (50milicesconds) presented his participants with a set of letters arranged in 3 rows and 4 columns. He later asked his participants to recall all the letters they remember (full recall). They on average remembered 4.5 out of the 12 letters.
Sperling wasn't sure how to interpret his findings so he devised a second experiment where he used partial recall. As before, he flashed his participants with a set of letters, only this time after the letters were shown there was a sound. The sound (either low, medium or high) indicated which row the participant should focus their attention on and recall. The average recall was 3.3 out of 4. It's important to remember that the sound cue was presented after the letters were shown, so the participants could only direct their attention to the iconic memory not the actual stimulus.
The findings from both parts of the experiment allowed Sperling to conclude that the participants remembered about 82% of the letters which means that the capacity of the iconic memory is large. However, they could not remember all the letters because they decayed rapidly and were irretrievable from memory, proving that the duration of iconic memory is very short.