and temporal!lobe lesions - Semantic Scholar

\

Pergamon

PII] S9917Ð2821"87#99041Ð7

Neuropsycholoìa\ Vol[ 25\ No[ 5\ pp[ 388Ð493\ 0887 Þ 0887 Elsevier Science Ltd[ All rights reserved Printed in Great Britain 9917Ð2821:87 ,08[99¦9[99

Clustering and switching on verbal ~uency] the effects of focal frontal! and temporal!lobe lesions ANGELA K[ TROYER\' MORRIS MOSCOVITCH\$ GORDON WINOCUR\% MICHAEL P[ ALEXANDER& and DON STUSS Rotman Research Institute of Baycrest Geriatric Centre^ University of Toronto\ Ontario\ Canada^ $ Department of Psychology\ Baycrest Geriatric Centre^ Erindale College\ Mississauga\ Ontario\ Canada^ % Trent University\ Peterborough\ Ontario\ Canada^ and & Braintree Rehabilitation Hospital Network\ Braintree\ Boston University School of Medicine\ Memory Disorders Center\ Boston Veteran|s Administration Medical Center\ Massachusetts\ U[S[A[ "Received 3 September 0886^ accepted 09 October 0886#

Abstract*We examined the hypothesis that\ on verbal ~uency\ clustering "i[e[ generating words within subcategories# is related to temporal!lobe functioning\ whereas switching "i[e[ shifting between subcategories# is related to frontal!lobe functioning[ Tests of phonemic and semantic ~uency were administered to 42 patients with focal frontal!lobe lesions "FL#\ 12 patients with unilateral temporal!lobe lesions "TL# and 44 matched controls[ Performance by FL patients was consistent with our hypothesis] in comparison to controls\ patients with left!dorsolateral or superior!medial frontal lesions switched less frequently and produced normal cluster sizes on both phonemic and semantic ~uency[ Performance by TL patients was not consistent across ~uency tasks and provided partial support for our hypothesis[ On phonemic ~uency\ TL patients were unimpaired on both switching and clustering[ On semantic ~uency\ TL patients were impaired on switching in comparison to controls and left TL patients produced smaller clusters than right TL patients[ The best indices for discriminating the patient groups\ therefore\ were phonemic!~uency switching "impaired only with frontal lesions# and semantic!~uency clustering "impaired only with temporal!lobe lesions#[ Þ 0887 Elsevier Science Ltd[ All rights reserved Key Words] focal brain lesions^ temporal!lobe lesions^ frontal!lobe lesions^ semantic ~uency^ phonemic ~uency[

tal!lobe lesions "FL#\ in a majority of empirical studies\ impaired performance is obtained on phonemic ~uency\ both in comparison to control subjects ð0\ 3\ 4\ 06\ 10\ 11\ 12Ł and in comparison to patients with posterior cortical lesions ð2\ 05Ð07\ 11\ 12Ł[ This _nding\ however\ is not without contradiction[ In some studies\ phonemic ~uency performance by FL patients has been found to be similar to that of patients with posterior cortical lesions ð00\ 14\ 17Ł or to patients with di}use brain injury ð11Ł[ The patterns of performance by FL patients on semantic ~u! ency are also contradictory[ Although FL patients are not impaired on semantic ~uency in some studies ð2\ 3\ 00\ 19\ 17Ł\ impaired levels of performance have been obtained in others ð4\ 10\ 13\ 14Ł[ Thus\ it is clear that FL patients exhibit impairment on verbal ~uency tasks\ although the speci_c patterns of performance vary some! what between studies[ The cause of the ~uency impairment among FL pat! ients appears to be related to poor initiation and:or ~exi! bility of search and retrieval processes[ Consistent with this idea\ a patient with bilateral frontal!lobe lesions\ in comparison to age!matched controls\ generated fewer

Verbal ~uency tasks involve generating words according to speci_ed rules within 59 s intervals[ On phonemic ~uency\ participants are asked to generate words begin! ning with a speci_c letter\ such as f\ a and s ð0\ 1Ł[ On semantic ~uency\ participants are asked to generate words belonging to a semantic category\ such as animals ð19Ł[ As reviewed subsequently\ phonemic ~uency is often thought to be a test of frontal!lobe functioning and sem! antic ~uency a test of temporal!lobe functioning[ However\ there is emerging evidence that performance on these ~uency tasks\ as measured by the total number of words generated\ is not speci_c to lesions in any single brain region and additionally\ is sensitive to di}use brain damage[ Frontal!lobe dysfunction is generally thought to be associated with impaired phonemic ~uency and relatively intact semantic ~uency[ Among patients with focal fron! ' Address for correspondence] Angela Troyer\ Department of Psychology\ Baycrest Geriatric Centre\ 2459 Bathurst Street\ Toronto\ Ontario\ Canada M5A 1E0[ E!mail] troyerÝpsych[ toronto[edu[

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A[ K[ Troyer et al[:Verbal ~uency and focal brain lesions

words on a standard uncued version of semantic ~uency "i[e[ animals# but improved to the level of controls when semantic subcategory cues were provided "e[g[ farm ani! mals and jun`le animals# ð13Ł[ Additionally\ FL patients\ in comparison to patients with posterior lesions\ per! formed more poorly on a ~uency task requiring alter! nation between animals and s words\ despite no group di}erences when animals and s words were generated separately ð17Ł[ Both of these studies suggest a speci_c di.culty switching between categories or subcategories[ Additionally\ the ~uency impairment among FL patients does not appear to be due to a failure to use a strategy of generating words within subcategories[ On phonemic and semantic ~uency\ patients with frontal tumors and pat! ients with posterior tumors did not di}er in the pro! portion of words produced within clusters of three or more words\ in the size of the largest cluster\ or in the number of clusters produced ð01\ 02Ł[ Temporal!lobe dysfunction\ in contrast to frontal dys! function\ is generally thought to be associated with imp! aired semantic ~uency and relatively intact phonemic ~uency[ Presumably\ this impairment re~ects temporal! lobe related semantic!memory de_cits[ However\ ~uency studies of patients with focal temporal!lobe lesions "TL# have been somewhat contradictory[ TL patients per! formed better than FL patients on phonemic ~uency ð2\ 08\ 12Ł\ but were no more impaired than FL patients on semantic ~uency ð2\ 19Ł[ Patients with focal temporal! lobe atrophy\ or {{semantic dementia||\ were mildly impaired on phonemic ~uency and severely impaired on semantic ~uency ð8Ł[ Additionally\ patients who had undergone temporal lobectomy for the control of epilepsy were impaired on both semantic and phonemic ~uency ð03\ 04Ł[ Therefore\ the speci_c relationship between location of focal brain lesion and performance on pho! nemic and semantic ~uency is unclear[ The e}ect of lesion laterality\ regardless of location within the hemisphere\ on ~uency performance has been examined in a number of studies[ Generally\ patients with unilateral left!hemisphere lesions performed more poorly than patients with unilateral right hemisphere lesions on both phonemic and semantic ~uency ð09\ 03Ð06\ 08\ 19\ 11\ 12\ 17Ł[ Patients with bilateral lesions performed the worst ð0Ł[ In some studies\ patients with right!hemisphere lesions performed as well as non!lesion controls ð08\ 19Ł\ whereas in others\ patients with right hemisphere lesions were impaired ð03\ 04\ 12Ł[ Overall\ therefore\ the patient groups ranging from most impaired to least impaired\ respectively\ are those with bilateral lesions\ unilateral left!hemisphere lesions\ and unilateral right!hemisphere lesions[ As reviewed previously\ it is evident that the number of words generated on verbal ~uency tests is not necessarily sensitive to lesions in any one particular brain region[ We have argued ð15Ł that two important cognitive com! ponents of ~uency performance are clustering and swit! ching[ That is\ optimal ~uency performance involves producing clusters of semantically or phonemically

related words and once a subcategory is exhausted\ swit! ching to another[ We have demonstrated that switching was speci_cally decreased by divided attention ð15Ł and was consistently impaired in Parkinson|s disease ð16Ł\ providing preliminary evidence that switching is related to frontal!lobe functioning[ The purpose of the present study was to examine clustering and switching on ~uency tasks among patients with focal brain lesions[ We hypo! thesized that\ in comparison to controls\ switching would be impaired in patients with frontal!lobe lesions "regard! less of lesion laterality# and clustering would be impaired in patients with temporal!lobe lesions "especially left! temporal lesions#[ Our expectations regarding laterality were based on the nature of the ~uency components] switching is a cognitive!shifting task without obvious implications for lateralization\ whereas phonemic clus! tering is a language!related task and semantic clustering is a verbal semantic!memory task\ both of which may be more dependent on the left hemisphere[

Methods Participants Participants were patients with focal frontal!lobe or focal temporal!lobe lesions[ Because of group di}erences in demo! graphic characteristics "i[e[ age#\ each was matched with its own control group[ Demographic characteristics are presented in Table 0[ FL patients were grouped according to lesion site and ~uency performance via classi_cation and regression tree analyses\ as reported elsewhere ð14Ł[ All lesions were documented on CT or MRI scans[ Fourteen patients had lesions of the left dorsolateral and:or lenticular striate frontal regions "LDLF group#[ Eleven patients had lesions of the right dorsolateral and:or lenticular striate frontal regions "RDLF group#[ Seventeen patients had superior medial frontal lesions from either left\ right\ or bifron! tal damage "SMF group#^ right and left lesions were considered together because of previous evidence of lack of lateralized function in the superior medial frontal regions ð14Ł[ Eleven patients had inferior medial frontal lesions from either left\ right\ or bifrontal damage "IMF group#[ A small subgroup of patients "00)# with lesions in more than one frontal region were classi_ed according to the maximum lesion area[ The etiology of frontal pathology included cerebral vascular acci! dent "n 26#\ tumor "n 7#\ traumatic brain injury "n 7#\ and lobectomy "n 0#[ The mean time since lesion onset was at least 2 months for all patients[ Thirty!seven healthy control subjects were matched with the FL patients as closely as possible for demographic characteristics[ There were no group di}er! ences in age\ F"3\74# ³ 0\ or sex\ x1 "3\ n 89# 5[22\ P × 9[09[ However\ there were group di}erences in education\ F"3\74# 3[17\ P 9[992[ Among the 12 TL patients\ 8 had unilateral left!temporal! lobe lesions "LTL# and 03 had unilateral right!temporal!lobe lesions "RTL#[ The etiologies of the temporal!lobe lesions were lobectomy for intractable epileptic seizures "n 19# and cer! ebral vascular accident "n 2#[ Lesions were documented on CT or MRI scans or by surgical report[ Results of sodium amytal testing were available for 08 of the 19 patients with temporal lobectomy^ all were left!hemisphere language domi! nant[ Eighteen healthy control subjects were matched for demo! graphic characteristics with the TL patients[ There were no


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Table 0[ Demographic characteristics of patient groups and controls Frontal groups

n Age Education\ years Percentage female

Temporal groups

LDLF

RDLF

SMF

IMF

Ctl

LTL

RTL

Ctl

03 44[5 "8[6# 02[6 "2[1# 25)

00 44[3 "01[5# 00[5 "0[8# 53)

06 43[2 "00[7# 00[1 "2[6# 18)

00 41[4 "01[0# 00[6 "1[0# 16)

26 43[3 "03[3# 02[8 "1[2# 43)

8 26[3 "7[3# 01[5 "0[3# 45)

03 22[0 "01[8# 02[1 "1[5# 25)

07 25[1 "03[9# 02[3 "0[4# 45)

Note] LDLF left dorsolateral frontal\ RDLF right dorsolateral frontal\ SMF superior medial frontal\ IMF inferior medial frontal\ Ctl controls\ LTL left temporal lobe\ RTL right temporal lobe[ Standard deviations are shown in parentheses[

signi_cant di}erences between the LTL\ RTL and control groups in age\ F"1\27# ³ 0\ sex\ x1 "1\ n 30# 0[30\ P 9[49\ or education\ F"1\27# ³ 0[

hypotheses[ For the overall ANCOVAs\ ANOVAs\ and t tests\ an a level of 9[94 was used^ for the multiple post!hoc pairwise comparisons\ Bonferroni!corrected a!levels were used[

Procedures and scoring Tests of phonemic ~uency "i[e[ FAS test# ð0\ 1Ł and semantic ~uency "i[e[ animals# were administered on an individual basis[ Sixty seconds were allotted for each of the three phonemic trials and one semantic trial[ Three scores were obtained on each ~uency test] "a# total number of words generated\ excluding errors and repetitions^ "b# mean cluster size^ and "c# number of switches[ Detailed scoring rules for cluster size and switches have been provided elsewhere and produce scores with high interrater reliabilities "i[e[ r × 9[84# ð15Ł[ Brie~y\ on phonemic ~uency\ clusters consisted of successively generated words that began with the same _rst two letters "e[g[ arm and art#\ di}ered only by a vowel sound "e[g[ foot and fat#\ rhymed "e[g[ ~ight and fright#\ or were homonyms "e[g[ sail and sale#[ On semantic ~uency\ clusters consisted of successively generated words belonging to the same semantic subcategory\ such as farm ani! mals\ pets\ aquatic animals\ or insects[ Cluster size was counted beginning with the second word in each cluster and mean cluster size was calculated for the phonemic test and for the semantic test[ Switches were calculated as the number of transitions between clusters\ including single words\ for the phonemic and semantic tests[ The raw number of switches was utilized rather than correcting for the number of words generated "i[e[ a pro! portion score#\ because this index best represented the behav! iour of interest[ That is\ optimal ~uency performance\ as measured by the raw number of correct words generated\ requires the use of a large number of di}erent subcategories\ which is achieved by frequent switching[ Optimal ~uency per! formance would not be achieved with a high proportion swit! ching score but a low raw switching score "e[g[ 3 switches out of 4 words#[

Analyses To correct for group di}erences in education among the FL patient and control groups\ ~uency performance was analysed with analysis of covariance "ANCOVA#\ with education as the covariate[ Group di}erences between the TL patient and con! trol groups were conducted with ANOVA[ Post!hoc pairwise comparisons "Bonferroni|s test# were used to test whether each patient group di}ered from its respective control group[ Additionally\ we performed one!tailed t!tests on cluster size between the LTL and RTL groups\ based on our directional

Results Fluency data for the patient and control groups\ on both phonemic and semantic ~uency\ are presented in Table 1[

Frontal groups Switching performance by the FL patient and control groups was consistent with our hypotheses[ That is\ pho! nemic ~uency switching showed signi_cant group di}er! ences\ F"3\73# 05[90\ P ³ 9[990\ with impaired perform! ance by the LDLF group\ F"0\73# 49[82\ P ³ 9[990 and the SMF group\ F"0\73# 10[58\ P ³ 9[990\ in com! parison to controls[ Similarly\ semantic ~uency switching showed signi_cant group di}erences\ F"3\73# 3[61\ P 9[990\ with impaired performance by the LDLF group\ F"0\73# 02[48\ P ³ 9[990 and the SMF group\ F"0\73# 09[21\ P 9[991\ in comparison to controls[ Clustering performance by the FL groups was also consistent with our hypotheses[ There were no group di}erences in clustering on either phonemic ~uency\ F"3\73# 0[09\ P 9[253\ or semantic ~uency\ F"3\73# ³ 0[ The number of words generated on phonemic ~uency showed overall group di}erences\ F"3\73# 08[79\ P ³ 9[990\ with impaired performance by the LDLF group\ F"0\73# 56[07\ P ³ 9[990 and the SMF group\ F"0\73# 16[57\ P ³ 9[990[ The number of words gen! erated on semantic ~uency also showed overall group di}erences\ F"3\73# 01[97\ P ³ 9[990\ with impaired performance by the LDLF group\ F"0\73# 26[96\ P ³ 9[990\ the RDLF group\ F"0\73# 01[52\ P 9[990\ the SMF group\ F"0\73# 02[72\ P ³ 9[990 and the IMF group\ F"0\73# 7[43\ P 9[993[

491

A[ K[ Troyer et al[:Verbal ~uency and focal brain lesions Table 1[ Fluency data for patient groups and controls Frontal groups

Phonemic ~uency Switches Cluster size Words generated Semantic ~uency Switches Cluster size Words generated

Temporal groups

LDLF

RDLF

SMF

IMF

01[4 "5[8# 9[22 "9[11# 06[1 "8[9#

14[2 "6[8# 9[17 "9[07# 21[8 "09[8#

05[3 "6[9# 9[27 "9[12# 11[8 "7[9#

14[6 "8[6# 9[29 "9[11# 23[1 "00[4#

4[7 "2[1# 0[10 "9[67# 00[8 "3[9#

5[8 "1[2# 0[91 "9[49# 03[0 "1[6#

4[8 "1[1# 0[23 "9[66# 03[3 "4[0#

6[3 "1[4# 9[86 "9[21# 04[0 "1[4#

Ctl

LTL

RTL

Ctl

18[5 "7[9# 9[30 "9[07# 31[6 "00[3#

13[9 "09[0# 9[38 "9[39# 22[5 "09[3#

15[2 "8[9# 9[36 "9[05# 27[8 "09[7#

17[3 "7[9# 9[28 "9[18# 30[0 "00[5#

8[3 "2[5# 0[11 "9[38# 19[1 "3[7#

6[8 "0[7# 9[80 "9[43# 04[1 "2[2#

6[2 "1[5# 0[24 "9[47# 07[7 "4[4#

09[7 "2[8# 0[05$ "9[44# 11[4 "3[6#

Note] LDLF left dorsolateral frontal\ RDLF right dorsolateral frontal\ SMF superior medial frontal\ IMF inferior medial frontal\ Ctl controls\ LTL left temporal lobe\ RTL right temporal lobe[ Standard deviations are shown in parentheses[ P ³ 9[94\ P ³ 9[90\ P ³ 9[990 for overall analyses^ $ P ³ 9[94 for left vs right temporal groups[

Temporal groups Findings with the TL patients and controls were less clear\ but provide some support for our hypotheses[ On phonemic ~uency switching\ there were no overall group di}erences\ F"1\27# ³ 0[ On semantic ~uency switching\ there were overall group di}erences\ F"1\25# 4[91\ P 9[901\ with impaired performance by the RTL group in comparison to controls\ F"0\25# 7[37\ P 9[995[ On phonemic ~uency cluster size\ there were no overall group di}erences\ F"1\27# ³ 0\ and there was no di}er! ence between LTL and RTL patients\ t"10# 9[19\ P 9[735[ On semantic ~uency cluster size\ there were no overall group di}erences\ F"1\25# 0[55\ P 9[193^ however\ consistent with our a priori hypothesis\ LTL patients produced smaller clusters than RTL patients\ t"08# 0[70\ P 9[932[ For the number of words generated\ there were no group di}erences on phonemic ~uency\ F"1\27# 0[27\ P 9[152[ There were group di}erences on semantic ~uency\ F"1\13# 4[66\ P 9[998\ with impaired per! formance by the LTL group in comparison to controls\ F"0\25# 03[48\ P 9[990[

Discussion Our _ndings provide general support for the idea that switching on verbal ~uency is related to frontal!lobe func! tioning\ whereas clustering is related to temporal!lobe functioning[ The pattern of performance by the FL pat! ients was completely consistent with our hypothesis[ That is\ in comparison to controls\ FL patients as a group

switched less frequently and produced normal cluster sizes on both phonemic and semantic ~uency[ Per! formance by TL patients was not consistent across ~uency tasks and provided partial support for our hypothesis[ On phonemic ~uency\ TL patients were un! impaired on both switching and clustering[ On semantic ~uency\ TL patients were impaired on switching in com! parison to controls and LTL patients produced smaller clusters than RTL patients[ The best indices for dis! criminating the patient groups\ therefore\ were pho! nemic!~uency switching "impaired only with frontal lesions# and semantic!~uency clustering "impaired only with temporal!lobe lesions#[ This is consistent with pre! vious research ð16Ł in which these same indices dis! criminated between dementia groups[ Phonemic!~uency switching was impaired only in Parkinson|s dementia "in which frontal and subcortical regions are preferentially involved# and semantic!~uency clustering was impaired only in Alzheimer|s dementia "in which temporal and parietal regions are preferentially involved#[ We have obtained converging evidence\ therefore\ that switching is related to frontal!lobe functioning whereas clustering is related to temporal!lobe functioning[ The patients with frontal lesions were not homo! geneous in their ~uency performance\ but\ rather\ showed subgroup performance di}erences[ Speci_cally\ switching was impaired in the LDLF and SMF groups and unim! paired in the RDLF and IMF groups[ This is consistent with research indicating impaired initiation of behaviour and perseveration in patients with dorsolateral frontal lesions ð07Ł and superior!medial frontal lesions "i[e[ sup! plementary motor area and anterior cingulate gyrus# ð5\ 6Ł[ Presumably\ switching on verbal ~uency tasks would require initiation of numerous searches through semantic


and lexical memory\ and cognitive ~exibility to rapidly shift between subcategories[ Although switching was impaired on both ~uency tasks in our FL patients\ switching on semantic ~uency was also impaired in our TL patients[ It is possible that this unexpected switching de_cit resulted from damage to pathways connecting the anterior frontal and temporal regions subsequent to anterior temporal lobectomy[ Thus\ the switching impairment in our TL patients may be a result of additional frontal dysfunction[ On the other hand\ it is also possible that switching on semantic ~uency is mediated both by frontal and anteriorÐtemporal regions[ It is apparent that the frontal lobes are directly involved in switching\ either for initiating switches or for locating a new subcategory or both[ The frontal lobes may also require information indicating that the point of diminishing returns has been reached in searching a particular semantic cluster[ Perhaps it is this information that is not e.ciently transmitted in the semantic ~uency task in TL patients[ In contrast\ because phonemic pro! cessing is not as dependent on the anterior temporal lobe as semantic processing\ switching was normal in TL patients on this task[ If the frontal lobes are the repository of the switching mechanism itself\ frontal damage should lead to de_cits in switching on both tasks\ as seen in our FL patients[ Although semantic!~uency clustering was impaired in our LTL patients\ somewhat unexpectedly\ phonemic! ~uency clustering was not impaired in this group[ This is similar to previous _ndings that impaired clustering is not always consistent across ~uency tasks ð15\ 16Ł[ Rather\ it appears that di}erent cognitive processes are involved\ at least to some degree[ Semantic!~uency clustering may depend on access to and integrity of semantic stores\ whereas phonemic!~uency clustering may depend on fund of word knowledge and the ability to analyse the phonemic characteristics of words[ Thus\ despite the fact that clustering on phonemic and semantic ~uency both involve the ability to classify and group words with simi! lar characteristics\ additional cognitive processes vary between these tasks\ resulting in the possibility of incon! sistent performance across ~uency tasks[ Our frontal!lesion _ndings share some similarities with those of Laine ð01\ 02Ł\ who found that patients with frontal tumors produced normal cluster sizes on verbal ~uency in comparison to patients with posterior tumors[ They also found no group di}erences in the number of clusters of three or more words\ which was their measure of {{shifting||\ whereas our FL patients were impaired on switching when clusters of all sizes\ including single words\ were considered[ This suggests that switching alone\ regardless of the size of clusters between which the patient is switching\ may be the crucial ability that is sensitive to lesions of the frontal lobe[ The lateralizing e}ects we obtained for clustering and switching were generally consistent with our expec! tations[ We had no a priori reason to expect switching\ a cognitive!shifting task\ to be di}erentially a}ected by

492

right! versus left!hemisphere lesions[ Indeed\ we found that switching was signi_cantly impaired in patients with left!frontal\ superior!medial!frontal\ and right!temporal lesions[ On the other hand\ we expected that clustering words in phonemic or semantic subcategories\ which is a language!related task\ would be more greatly a}ected by left!temporal lesions[ This is indeed the pattern we obtained for semantic!~uency clustering\ although pho! nemic!~uency clustering was unimpaired in all patient groups[ The pattern of _ndings in Table 1 may appear to sug! gest that switching and number of words generated are highly dependent variables\ because group di}erences in the number of words generated were always associated with group di}erences in switching[ However\ additional research has indicated that a decrease in the number of words generated can be obtained without decreased switching but\ rather\ with decreased clustering "i[e[ among patients with Alzheimer|s disease# ð16Ł[ Impaired switching\ therefore\ is not necessary for obtaining a decreased number of words generated[ Consistent with the idea that phonemic ~uency is a test of frontal!lobe functioning\ the number of words generated on phonemic ~uency was impaired in the FL group but not the TL group in comparison to controls[ This is not only a test of frontal functioning\ however\ as impaired performance has also been obtained by patients with parietal!lobe lesions ð14Ł[ Number of words gen! erated on semantic ~uency\ on the other hand\ was imp! aired in both of our patient groups\ suggesting that performance on this test is not speci_c to temporal!lobe functioning[ This is consistent with a number of previous studies ð4\ 10\ 13Ł[ Speci_c e}ects\ therefore\ were obtained for the clustering and switching components\ but not for the total number of words generated[ The conclusions drawn in this study are limited by several factors\ including small TL patient sample sizes\ heterogeneity of lesion size and etiology\ and possible di}use brain dysfunction in addition to focal lesions among the patients with tumors or traumatic brain injury[ Additionally\ because of the early age at seizure onset in the temporal lobectomy groups\ some functional reorganization may have occurred[ Despite these sample characteristics\ which likely reduced the probability of _nding group di}erences\ we obtained signi_cant di}er! ences for most of our critical comparisons[ Acknowledèment*The present research was supported by a grant from the Medical Research Council of Canada to M[ Moscovitch and G[ Winocur and grants from the Ontario Mental Health Foundation and Medical Research Council of Canada to D[ Stuss[ A[ Troyer was supported by the Katz Post! doctoral Research Fellowship[

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C[ and Silveri\ M[ C[\ Neuropsychological correlates of localized cerebral lesions in non!aphasic brain! damaged patients[ Journal of Clinical Neuro! psycholo`y\ 0870\ 2\ 42Ð52[ Miller\ E[\ Verbal ~uency as a function of a measure of verbal intelligence and in relation to di}erent types of cerebral pathology[ British Journal of Clinical Psy! cholo`y\ 0873\ 12\ 42Ð46[ Milner\ B[\ E}ects of di}erent brain lesions on card sorting[ Archives of Neurolo`y\ 0852\ 8[ Milner\ B[\ Some e}ects of frontal lobectomy in man[ In The frontal `ranular cortex and behavior\ eds J[ M[ Warren and K[ Akert[ McGraw!Hill\ New York\ 0853[ Newcombe\ F[\ Missile wounds of the brain[ Oxford University Press\ London\ 0858[ Owen\ A[ M[\ Downes\ J[ J[\ Sahakian\ B[ J[\ Polkey\ C[ E[ and Robbins\ T[ W[\ Planning and spatial work! ing memory following frontal lobe lesions in man[ Neuropsycholoìa\ 0889\ 17\ 0910Ð0923[ Pendleton\ M[ G[\ Heaton\ R[ K[\ Lehman\ R[ A[ W[ and Hulihan\ D[\ Diagnostic utility of the Thurstone word ~uency test in neuropsychological evaluations[ Journal of Clinical Neuropsycholo`y\ 0871\ 3\ 296Ð 206[ Perret\ E[\ The left frontal lobe of man and the sup! pression of habitual responses in verbal categorical behaviour[ Neuropsycholoìa\ 0863\ 01\ 212Ð229[ Randolph\ C[\ Braun\ A[ R[\ Goldberg\ T[ E[ and Chase\ T[ N[\ Semantic ~uency in Alzheimer|s\ Par! kinson|s\ and Huntington|s disease] Dissociation of storage and retrieval failures[ Neuropsycholo`y\ 0882\ 6\ 71Ð77[ Stuss\ D[ T[\ Alexander\ M[ P[\ Hamer\ L[\ Palumbo\ C[\ Dempster\ R[\ Binns\ M[\ Levine\ B[ and Izukawa\ D[\ Brain mechanisms of verbal ~uency] The e}ects of focal anterior and posterior brain lesions on initiation\ production\ and strategic per! formance[ Journal of the International Neuro! psycholoìcal Society\ in press[ Troyer\ A[ K[\ Moscovitch\ M[ and Winocur\ G[\ Clustering and switching as two components of ver! bal ~uency] Evidence from younger and older healthy adults[ Neuropsycholo`y\ 0886\ 00\ 027Ð035[ Troyer\ A[ K[\ Moscovitch\ M[\ Winocur\ G[\ Leach\ L[ and Freedman\ M[\ Clustering and switching on verbal ~uency tests in Alzheimer|s and Parkinson|s disease[ Journal of the International Neuro! psycholoìcal Society\ 0887\ 3\ 026Ð032[ Vilkki\ J[ and Holst\ P[\ Speed and ~exibility on word ~uency tasks after focal brain lesions[ Neuro! psycholoìa\ 0883\ 21\ 0146Ð0151[