Psychology 403.
Laboratory in Cognitive Psychology
Experiment 7.
Fall, 2003
Signal Detection
Overview
Participants are presented a pair of lines and asked to decide whether or not they differ in
length. On half of the trials the lines will differ and on half they will not. On the trials on which they
differ, half will differ by a small amount and half will differ by a larger amount. Over two blocks of
trials the participants will receive differing pay-offs for hits and false alarms. The experiment is
designed to test the major features of signal detection theory; that sensitivity (d') and criterion (ß)
are independent parameters of performance, each influenced differently by the variables in the
experiment.
According to signal detection theory, detection of stimuli in many circumstances can be
conceptualized as a noise-limited task. On trials on which the critical aspect of the stimulus is not
present, internal (and sometimes external) sources of noise will produce activity that is
indistinguishable from the kind of activity that is evoked by the presentation of a signal. This
activity is, in the simplest version of the theory, thought to be normally distributed. The effect of a
signal is to add a constant amount to that internal activity. Thus it is possible to conceive of two
distributions of activity, one resulting from noise alone and one from signal plus noise, with
differing means but both having the same standard deviation. According to signal detection
theory, sensitivity is indexed by the degree of separation of the means of these distributions. It is
characterized by d', the difference between the means measured in standard deviations. Given
the overlap between the distributions of noise and signal-plus-noise activity, the observer must
establish a criterion or response rule to determine how to respond to ambiguous internal signals
that might result from noise or from signal-plus-noise. The location of the criterion is influenced by
different factors than those that influence sensitivity. In the present experiment two variables will
be manipulated, signal strength and pay-offs (rewards for hits and penalties for false alarms).
Within each of two blocks, both signal strengths will be presented along with trials in which no
signal is presented. Between blocks the pay-offs will be manipulated. The experiment is
designed to determine whether signal strength and pay-off manipulations affect measures of
sensitivity and the location of the criterion in the manner predicted by signal detection theory.
These predictions are to be spelled out in the lab reports based on the extensive lectures and
outline distributed in class (Zacks, 2003).
The signal in this experiment will be a difference in the length of a pair of lines. On nonsignal trials the lines will be the same lengths. In the weaker signal condition, the difference will
be small, and in the stronger signal condition the difference will be larger.
Method
A participant will initiate a trial by pressing the space bar on the computer. A plus sign
serving as a fixation target will appear for 500 milliseconds and then disappear followed by a pair
of horizontal lines randomly located in one of 2 locations on the screen. The lines will both lie
along an imaginary, horizontal line, and the ends nearest each other will be separated by a fixed
amount (approximately 0.1 inches). One of the lines will be one of three lengths, 20, 21, or 22
units long (where 20 units=approximately 1 inch), and the other will either be the same length (nosignal trials), one unit longer (weak signal) or two units longer (stronger signal). When they differ
in length, the longer line will be randomly located on the left or the right side of the pair. The pair
of lines may appear in one of two locations, centered approximately 1.5 inches to the left or right
of the fixation point. Participants will press the N key on the keyboard to indicate "no", there is no
difference in the line lengths, or the Y key to indicate "yes", there is a difference in line length.
The lines will remain present for 1 second. Participants must respond during the second that the
lines are present or within 2 seconds of the termination of the lines. They will receive feedback
after every trial telling them whether or not their response was correct.
In one block, participants will receive 5 points for every hit and will lose 1 point for every
false alarm. In the other block, participants will receive 1 point for every hit and lose five points for
every false alarm. Participants will be motivated to earn as many points as possible by the
promise of a reward provided by the instructor. The order of presentation of the two different payoff conditions will be randomly determined for each participant. Before the experimental blocks,
participants will receive practice with the same stimuli (but with no points determined by their
responses). Participants will be required to attain an overall accuracy of 70% in a practice block
before proceeding to the experimental blocks. If the participant fails to achieve 70% correct the
block will be repeated. The data will consist of false alarm rates for the non-signal condition and
hit rates for the weaker and stronger conditions for each of the two blocks (run with the same
stimuli but different pay-offs).
REFERENCES
Zacks, J. (2003) Signal Detection Theory. A lecture outline prepared for the students of
Psychology 403, Laboratory Course in Cognitive Psychology.