Cognitive Sciences ePrint Archive
Alzheimer’s Disease and Herpes Simplex Encephalitis
Keith R. Laws1* and Giuseppe Sartori2
Abstract
& Most studies examining category specificity are single-case
studies of patients with living or nonliving deficits. Never-
theless, no explicit or agreed criteria exist for establishing
category-specific deficits in single cases regarding the type of
analyses, whether to compare with healthy controls, the
number of tasks, or the type of tasks. We examined two
groups of patients with neurological pathology frequently
accompanied by impaired semantic memory (19 patients with
Alzheimer’s disease and 15 with Herpes Simplex Encephalitis).
Category knowledge was examined using three tasks ( picture
naming, naming-to-description, and feature verification). Both
patient groups were compared with age- and education-
matched healthy controls. The profile in each patient was
examined for consistency across tasks and across different
analyses; however, both proved to be inconsistent. One
striking finding was the presence of paradoxical dissocia-
tions (i.e., patients who were impaired for living things on
one task and nonliving things on another task). The findings
have significant implications for how we determine cate-
gory effects and, more generally, for the methods used to
document double dissociations across individual cases in this
literature. &
INTRODUCTION
Although various tasks and statistical techniques have
been used to try to document category effects, a critical
and surprisingly unaddressed question concerns what
would be considered acceptable as evidence of a
‘‘category effect’’ (Laws, 2004, in press). Most category
effects ref lect an initial demonstration of poor picture
naming for one category and, in some cases, poor
picture naming is the only evidence presented (e.g., in
studies of Alzheimer’s patients, see Laws, in press). One
thing is certain, that is, that picture naming is implicitly
viewed as the main task for confirming the presence of a
category effect (and then that the additional tasks are
used to elaborate on that). This assumption ref lects the
fact that most models predict that naming is impaired if
semantic memory is impaired (and vice-versa). Never-
theless, evidence has existed for some time to show that
this is not necessarily the case. For example, Laws,
Evans, Hodges, and McCarthy (1995) described patient
SE who had impaired (associative, nonvisual) semantics
for animals but could name pictures of animals.
Recent studies have raised issues about the statistical
methods used to assess category effects with naming
data (Laws, in press; Laws, Gale, Leeson, & Crawford,
2005). In particular, the common practice of making
comparisons between the absolute number of living and
1Nottingham Trent University, UK, 2University of Padua, Italy
*Now at the University of Hertfordshire, UK
nonliving items that are named (verified, identified, and
so on) rather than comparing the patient data with
control data. This may be misleading about the presence
and even direction of category effects. Laws (in press)
noted several consequences of failing to evaluate patient
naming against the performance of matched normal
healthy controls. The most common analysis in over
two-thirds of all category-specific studies is the within-
patient comparison of the absolute scores (using x2:
Laws, in press); and this reveals quite different dissoci-
ations from those disclosed when a control comparison
is included. Indeed, Laws, Gale, et al. (2005) have
demonstrated how patients may show false negatives,
false positives, and even paradoxical dissociations (i.e.,
impaired for living on one task and for nonliving on
another task). Hence, the absolute living-nonliving
naming difference alone (be it exceptionally large or
nonexistent) provides an unreliable indicator for both
the presence and the direction of category effects.
This raises an important related methodological issue
concerning how we define a category effects through
a test profile. In fact, no a priori test profile exists
for documenting a category disorder (e.g., a category
difference for picture naming, attribute verification,
naming-to-description, f luency, drawing or any com-
bination of these). It also raises a critical issue con-
cerning whether such disorders are empirically and/or
theoretically driven. For example, we could make an
arbitrary empirical decision such that for a category
effect to be convincing, minimally requires a patient
D 2005 Massachusetts Institute of Technology
JournalofcognitiveNeuroscience 17:9, pp. 1453-1459