Signal Detection Theory

Signal Detection Theory

Arose from WWII Research on the man-machine interface. No assumptions about thresholds. Instead they assumed that one’s ability to detect a stimulus will vary from moment to moment. They also assumed that things such as attitudes, motivation, and stimulus quality all play a role.

Capacity of Sense Organs

How much can you see in a single glance? (you seen general scene but not details) Can we measure it? In fact, it’s really easy. Pennies experiment. Take handful of pennies and throw on floor behind you. Turn around real quick and glance. People report seeing only about 4 or 5 pennies even though they know there are more pennies than that on the floor. A fraction from all the things in the environment. You are not perceiving everything even though you are aware there are more things in the environment.

Psychologists used T-Scopes to study this phenomenon in a more controlled way: subjects were shown an array of letter and then asked to report as many letters as they could.


Flash of letters on white screen in rectangle – average number of letters seen is 4.3 (span of apprehension)

You saw twelve letters but only recalled 4.

What’s going on?

Sperling (1960)

did dissertation on this – You do see more than 4 or 5 letters but by the time you report 4 or 5, you’ve forgotten the rest. Even in visual perception, there is a very important memory component. How could this idea be tested?

Very elegant and logical experiment

His experiment:

Instead of using a whole-report procedure where you recall as many as you can. He did a variation. He used a partial-report procedure: subjects were asked to report just one row of the letter array. They were cued by a tone played immediately after the letters disappeared. The tone determined what row they needed to report. High tone would be top row. Medium tone was middle row and low tone was bottom row. When you do it this way instead of the whole report method, people on average report 3 of the 4 letter from the cued row. You didn’t know til after the letters had gone away. This implies they could have reported 3 letters from any row. That tells us that immediately after seeing the letters, subjects were aware of nine letters. Nine is very different from 4.3 in whole report studies.

What happens if we increase the delay between showing the letters and the tone?

If a delay occurs before the cuing tone is played, subjects do worse. After letters are shown, a delay of 150 msec (between 1 and 2 tenths of a second) before tone is played results in peoples reporting only about 2.4 letters. This delay showed a drop from 3 to 2.4. Implying they have access to 2.4 letters on each row tells us that people had 7.2 letters at their disposal. It doesn’t matter what row you ask them to recall. The results are the same.

Subjects who had a delay of 500 msec between letters and tone reported only about 2 letters. This implies that they had only 6 letters available in their memory.

Time matters crucially.

Sperling concluded that when you see something, it’s held in a very short-term sensory memory system. He referred to it as the icon.

Typically an icon lasts for about 250 msec.

It’s assumed the process acts in a similar way for other sensory organs.

In auditory memory system, it’s called an echo.

What does erasing sensory memory so quickly do?


Sensory memory retains what we need and hear long enough for us to process it. Cognitive system needs time to make sense out of what you just saw. It then fades away to be replaced by new sensory information. This is sometimes called the persistence of vision. It is responsible for things like illusory motion in motion pictures. Each picture is a static and ran at the right speed to give sense of motion.

Memory is crucial in first stages of sensory perception


Even low level processes, like our perception of the environment are affected by higher-level cognitive processes like memory. Allows for continuity of experience.

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