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One of the most intriguing problems in reading science is how the reader’s knowledge of orthographic units is used in skilled reading (Grainger, this volume). The long‐standing answer is that readers come to recognize a word as a whole unit rather than a string of letters. J.M. Cattell’s famous experiments (1886; reviewed in Huey 1908) were intended to demonstrate this. After viewing a briefly exposed string of letters, Cattell attempted to report all the letters in the string. When the letters spelled a word, he could report more letters than when he viewed a random letter string.

In fact, Cattell’s experiments could not distinguish perception of the whole word from memory for some of its letters. Remembering enough letters would prompt retrieval of a word that contains them, making the report of the letter string a mix of perception, memory, and a bias to respond with words. Nevertheless, Cattell’s explanation (and Huey’s) stood unchallenged until the independent publications of experiments by Reicher (1969) and Wheeler (1970).

Reicher (1969) and Wheeler (1970) controlled for response bias by asking participants which of two letters had been briefly presented (and masked) in a particular position. For example, given the string lake, probing whether k or t had appeared in the third position would not produce a word bias, because either letter completes a word. The publication of these experiments stimulated a generation of research on the “word superiority effect,” eventually leading to a modified conclusion: Letters within nonword pronounceable strings (pseudowords) are also perceived better than random strings of letters. Letters in real words are perceived a little better than letters in these pseudowords, but the largest difference seems to concern the internal structure of the letter string, its word‐like orthography and phonology.

McClelland and Rumelhart (1981) explained both the word superiority effect and the pseudoword superiority effect in a new approach, a model that connected three hierarchical levels – words, letters, and letter segments – with bi‐directional activation between adjacent levels of the hierarchy. Activation spreading from letters up to words accumulates recognition evidence for specific words; and activation from a word down to the letter level accumulates evidence for the letters in that word. Thus, letters are perceived better in pseudowords than letter strings because they receive feedback from words that contain these letters (e.g., the k in loke receives feedback from lake and like). Similarly, bi‐directional activation causes k to be better perceived in lake than loke, producing word superiority effect.

This approach became a model for how to conceptualize “interaction” in a precise way. The explicit representation of letters and words in a lexical memory system later gave way to Parallel Distributed Processing (PDP) models that learned connections rather than having them built in (Plaut et al., 1996; Seidenberg & McClelland, 1989; Seidenberg et al., this volume). However, the principles of the original interactive model with “localized” lexical representations were retained in other models of alphabetic reading (e.g., Grainger & Jacobs, 1996). Many computational models have been developed since these earlier models, which were restricted by small lexicons and limited generality across word reading tasks (Norris, 2013). These problems, and the focus on alphabetic writing, continue to challenge the generality of reading models.