Re: your mail (Al Bregman )

Subject: Re: your mail
From:    Al Bregman  <bregman(at)HEBB.PSYCH.MCGILL.CA>
Date:    Mon, 12 Oct 1998 13:03:53 -0400

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 DIFFERENCES EXCEED THE SENSORY CAPACITY TO REGISTER DIFFERENCES (e.g., in 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. Al ---------------------------------------------------------------------- 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(at) Lab Web Page: ---------------------------------------------------------------------- Email to AUDITORY should now be sent to AUDITORY(at) LISTSERV commands should be sent to listserv(at) Information is available on the WEB at

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