ABSTRACT
Frequency. As has already been discussed, some researchers have questioned the findings of category-specific deficits in AD on the basis that pertinent variables (such as colour or familiarity) of the stimuli were not adequately controlled for. Even if researchers have been able to respond to most of these objections, the dispute highlights the fact that the category of an object is not the only or even the principal determinant of semantic loss in AD. Rather, the factor that appears overwhelmingly to influence loss of knowledge is something related to the frequency with which an object is encountered, in that rarer objects are invariably “lost” before more commonly encountered objects. In experimental situations researchers have tried to take this factor into account by equating stimuli on available measures, such as printed word frequency, subjective familiarity ratings, prototypicality, and age of acquisition. Although these measures are highly correlated, each of these measures has its own merits and weaknesses depending on the experiment being carried out. For instance, prototypicality is an important factor to control for in an object classification task but it is problematic to equate stimuli on this basis in naming tasks because common objects can have two values of prototypicality. An example of an object with two values is the knife that has a high typicality value as a weapon and a low value as a tool. Furthermore, whole classes of objects might be more familiar than others, so that prototypicality has little meaning. For example, “apple” and “gun” are both equally typical for their categories (fruit and weapon) and yet “apple” is rated as much more familiar than “gun” (Snodgrass & Vanderwort, 1980). Word frequency would need to be taken into account as a factor in tasks using printed words because it can influence the ease with which a word can be read, although Snodgrass and Yuditsky (1996) found that it did not correlate with picture naming accuracy on the Snodgrass and Vanderwart set of pictures, and that it correlated only minimally with RT.
