Investigating Dissociations Betweend Perceptual Categorization and Explicit Memory

Investigating D issociations Betw een Perceptual C ategorization and Explicit M em ory M arci A . Flanery ( m arci.flanery@ vanderbilt.edu ) Thom as J. Palm eri ( thom as.j.palm eri@ vanderbilt.edu ) Brooke L. Schaper ( brooke.l.schaper@ vanderbilt.edu ) D epartm ent of Psychology; V anderbilt U niversity N ashville, TN 37240 U SA A bstract A re dissociations betw een categorization and explicit m em ory in tests of am nesics and norm als evidence for m ultiple m em ory system s? O r could these dissociations be artifacts arising from m ethodologies used in som e ex- perim ents? W e report a series of studies exploring this issue. U sing norm als in various states of sim ulated am - nesia w e show that categorization at test is w ell above chance even in the absence of prior exposure to category m em bers. W e also show that subjects perform w ell w hen tested w ith item s that conflict w ith categories they had studied earlier. W e argue that subjects in som e para- digm s can extract inform ation about categories from the test rather than rely on m em ory for studied category m em bers. In further studies, w e generalize these findings to other stim uli and other category structures that have been used in tests of am nesics and norm als. Introduction D o categorization and explicit m em ory rely on inde- pendent neural m em ory system s? Evidence for m ultiple system s com es from dissociations between categoriza- tion and explicit m em ory in studies of norm als and am - nesics. A m nesics are reported to categorize at levels com parable to norm als but are significantly worse at explicit m em ory. Such dissociations seem to im ply that separate system s m ay exist and pose clear problem s for theories that assum e a single underlying m em ory sys- tem , such as well-known exem plar m odels. The evidence is clear that am nesics have im paired explicit or declarative m em ory. The focus of this paper is on whether data from studies testing am nesics clearly provide evidence for intact abilities to learn new per- ceptual categories. O ur goal is to exam ine whether som e categorization perform ance can be explained in the absence of positing a separate im plicit system for category learning that is spared in am nesia. O ur ap- proach has been to utilize the sam e paradigm s and m ethodologies found in the am nesia literature to study norm al subjects under conditions that sim ulate aspects of am nesia. To create “am nesia” in norm als, we used a variety of techniques such as elim inating the study ses- sion altogether, introducing delays between study and test, and surreptitiously switching the test stim uli to those from an unstudied category. W e follow the am ne- sia literature in testing these effects using a variety of stim uli, including distortions of dot patterns, object-like stim uli with discrete features, and sim ple form s placed in categories separated by quadratic boundaries. In this paper, we review som e behavioral evidence for dissociations from studies of am nesics and norm als. For each case, we present data from studies we conducted that provide a possible alternative explanation for intact categorization by am nesics. D ue to space constraints, we will only present our results in sum m arized form without detailed description of the m ethods or statistical analyses. A fter sum m arizing our initial work along these lines reported by Palm eri and Flanery (1999), we describe several new experim ents that expanded upon these initial results in several im portant ways. Learning C ategories of D ot Patterns A classic m ethodology for studying categorization and recognition has been the Posner and K eele (1968) dot pattern paradigm . To create a pattern, a sm all num ber of dots are random ly scattered on a grid. To create a cate- gory, a pattern is random ly generated and designated the prototype. Category m em bers are generated by ran- dom ly distorting the prototype by varying degrees. K nowlton and Squire (1993) used a variant of this paradigm to test am nesics on categorization and recog- nition. For categorization, subjects were exposed to 40 high distortions. Subjects were tested on judging m em - bers and nonm em bers of that category. For categoriza- tion, m em bers were 4 repetitions of the prototype, 20 low distortions, and 20 high distortions. N onm em bers were 40 random ly generated patterns. For recognition, subjects were exposed to five random patterns eight tim es each. In the recognition test, they were asked to discrim inate between the five old patterns and five new patterns. N o corrective feedback was provided in either condition. K nowlton and Squire (1993) reported a dis- sociation between categorization and recognition when com paring am nesics and norm als. A s shown in Figure 1, am nesics categorized as well as norm als but were significantly im paired at recognition m em ory. This dissociation seem ed to provide evidence for separate system s. H owever, N osofsky and Zaki (1998) showed that a single-system m odel could account for a