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Re: your mail

Dear List,

I don't agree that discrimination implies underlying set size of 2, and
that this is the only difference between identification and
discrimination.  The critical difference is that in discrimination, the
items to be distinguished are physically present on the trial.

Here's an illustration:

Create a hundred figures resembling chinese characters but built out of
short, straight, connected lines, such that the only thing that
distinguishes them is the number, orientation, and position of the
component lines.  Confusing?  You bet!

In a discrimination experiment, on any one trial, randomly select two of
these figures, and present them side by side for as long as the subject
wishes.  Presumably the subject will score very high AS LONG AS THE
size of angle).  Even though the size of the underlying set of objects is
100, the two to be discriminated are always physically present.  From this
we see that set size is irrelevant in discrimination.  One need never have
formed a mental representation of the stimuli before the trial, if
inspection time is unlimited.

Now teach the subject names for the 100 stimuli.  On each trial, present
one at random, ask for its name, then give feedback about correctness.
Treat the number of trials to a criterion (say four correct responses in a
row for stimulus n) as a measure of the subject's ability to perform an
identification.  This will be an incredibly difficult task, even though
the underlying set size is the same as in the discrimination task, and
even though the subject could discriminate any member of the set from any
other member in the discrimination task.

The critical difference is that in discrimination, the to-be-discriminated
items are all physically present (there need not only be two).  The
psychological mechanisms for noticing differences between them involve
very-short-term memory as you glance back and forth from one to the other,
but this is not the same as the long-term memory required to remember
their names.

Of course when you speed up the responses in a discrimination task or you
present the stimulus very quickly, the role of long-term memory increases
and set size becomes important.

So every discrimination task taps (1) the limits of sensory capacity to
register differences, (2) very-short-term memory (3) long term-memory
representations.  By changing the properties of the task, you can tap
these processes in various combinations.

I think to understand what's going on it is important to consider the
underlying mental processes and not just the formal properties of the
measurement task.


Albert S. Bregman,  Professor,  Dept of Psychology,  McGill University
1205  Docteur Penfield Avenue,   Montreal,  Quebec,  Canada   H3A 1B1.
Phone: +1 514-398-6103 Fax: -4896  Email: bregman@hebb.psych.mcgill.ca
Lab Web Page: http://www.psych.mcgill.ca/labs/auditory/laboratory.html

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