BowersDamianDavis09PR.comment-on-BotvinickPlaut06

Psychological Review © 2009 American Psychological Association2009, Vol. 116, No. 4, 986–997 0033-295X/09/$12.00 DOI: 10.1037/a0017097COMMENTSA Fundamental Limitation of the Conjunctive Codes Learned in PDPModels of Cognition: Comment on Botvinick and Plaut (2006)Jeffrey S. Bowers and Markus F. Damian Colin J. DavisUniversity of Bristol Royal Holloway, University of LondonA central claim shared by most recent models of short-term memory (STM) is that item knowledge iscoded independently from order in long-term memory (LTM; e.g., the letter A is coded by the samerepresentational unit whether it occurs at the start or end of a sequence). Serial order is computed bydynamically binding these item codes to a separate representation of order. By contrast, Botvinick andPlaut (2006) developed a parallel distributed processing (PDP) model of STM that codes for item-orderinformation conjunctively, such that the same letter in different positions is coded differently in LTM.Their model supports a wide range of memory phenomena, and critically, STM is better for lists thatinclude high, as opposed to low, sequential dependencies (e.g., bigram effects). Models with context-independent item representations do not currently account for sequential effects. However, we show thattheir PDP model is too sensitive to these effects. A modified version of the model does better but stillfails in important respects. The successes and failures can be attributed to a fundamental ...