Reaction time as a measure of inter- and intraobject visual similarity: Letters of the alphabet

Interobject visual similarity is often measured by the time that subjects require to say that two objects are not identical, with long RTs indicating high similarity. In Experiment 1, using a complete set of uppercase alphabetic stimuli, we show: (1) RTs correlate strongly with direct ratings of visual similarity. (2) Stimuli that are similar as measured by RT or by direct judgment have the same profile of similarity to all other stimuli in the set as measured by correlational analysis. (3) The order in which the stimuli are compared has only a small effect on measured similarity. All of these results indicate desirable properties of any measure of similarity.Intraobject similarity is a concept pertinent to the relation of an object to itself. In Experiments 1 and 2 we show: (1) All letters are not equally similar to themselves, since RT for the “same” response is different for different letters. (2) The relation between RT and intraobject similarity is opposite to that for interobject similarity, with short RTs indicating high intraobject similarity. (3) Even though intraobject similarities differ, each letter is more similar to itself than to any other letter in the set. We discuss the implications of these results in terms of assumed proximity constraints underlying similarity data.