Online research in philosophy

Single-unit data from the cortex of monkeys performing working-memory tasks support the main point of the target article. Those data, however, also indicate that the activation of long-term memory is essential to the processing of all cognitive functions. The activation of cortical long-term memory networks is a key neural mechanism in attention (working memory is a form thereof), perception, memory acquisition and retrieval, intelligence, and language.

The notion that the working-memory system is not to be located in the prefrontal cortex, but rather constituted by the interplay between temporal and frontal areas, is of some attraction. However, at least for the domain of sentence comprehension, this perspective is promoted on the basis of sparse data. For this domain, the authors not only missed out on the chance to systematically integrate event-related brain potential (ERP) and neuroimaging data when interpreting their own findings on semantic aspects of working memory, but also neglected syntactic aspects of working memory and computation altogether.

Working-memory retention as activated long-term memory fails to capture orchestrated processing and storage, the hallmark of the concept of working memory. The event-related potential (ERP) data are compatible with working memory as a mental workspace that holds and manipulates information on line, which is distinct from long-term memory, and deals with the products of activated traces from stored knowledge.

Different hypotheses about the mechanisms underlying working memory lead to different predictions about working memory capacity when information is distributed across the two hemispheres. We present preliminary data suggesting that memory scanning time (a parameter often associated with working memory capacity) varies depending on how information is subdivided across hemispheres. The data are consistent with a distributed model of working memory.

Caplan & Waters propose a dedicated linguistic working memory to handle “interpretive” language comprehension, but there are data suggesting that more general working memory capacity can predict syntactic comprehension difficulty, and their claims depend on the existence of a principled distinction between “interpretive” and “post-interpretive” processes, which seems unlikely. Other conceptions of the source of individual differences also deserve consideration, as more flexible explanations of the phenomena.

The target article differentiates a new, syntactic component in verbal working memory. We suggest that several more components could be differentiated to make a model of working memory complete. Next, syntax is not always separable from the subject's verbal memory capacity as measured by standard working memory tasks. Finally, interference between different processes cannot be taken as evidence for the processes sharing the same resources. Interference might be a result of active mutual inhibition.

The metric devised by Halford, Wilson & Phillips may have considerable potential in distinguishing between the working memory demands of different tasks but may be less effective in distinguishing working memory capacity between individuals. Despite the strengths of the metric, determining whether an effect is caused by relational complexity or by differential levels of expertise is currently problematic.

Caplan & Waters's neuropsychological evidence for two types of verbal working memory rests entirely on a very restricted definition of “syntactic complexity,” one in terms of word order. This opens the possibility that the dissociation they observe relates to the differential use of word-order strategies rather than to the structure of verbal working memory.

A promising approach to more refined models consistent with the Caplan & Waters hypothesis is based on similarity-based interference, a general principle that applies across working memory domains. This may explain both the fine details of syntactic working memory phenomena and the gross fractionation for which Caplan & Waters have found evidence. Detailed models of syntactic processing that embody similarity-based interference fare well cross-linguistically.

In challenging current conceptions of the role of working memory in sentence processing, Caplan & Waters consider studies comparing young and older adults on sentence processing. This commentary raises two challenges to Caplan & Waters's conclusions: first, working memory tasks appear to be age invariant. Second, the production of complex syntactic constructions appears not to be age invariant.