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EXPERIMENT 2
Concurrent articulatory suppression
In order to assess whether subvocalization plays a differentrole in the LP and IP tasks, the
interference manipulation in Experiment 2 was the presence or absence of articulatory
suppression. Concurrent repetition of an irrelevant vocalization has long been known to
impair verbal short-term memory(Murray, 1967), and its interaction with the word length
effect (Baddeley, Thomson, & Buchanan, 1975) has been interpreted as its prevention of
subvocal rehearsal. Furthermore, changing-state articulation (e.g., “one–two–three–four–
one . . .”) produces a greater impairment than repetitive articulation (e.g., “the–the–the–
the–the . . .”; Macken & Jones,1995). This effectofchangingstate parallelsthatfound with
irrelevantsound (Jones & Macken,1995). Ifthe LPtaskdiffers fromthe IP taskin requiring
subvocalization to maintain and retrieve information about serial order, it should be more
sensitive to disruption from articulatorysuppression.
Method
Participants
The16participantsreplyingtoadvertsintheUCLPsychologyDepartmentconsistedof 7menand
9women (meanage of26.5 years),andthey were paidforparticipating.
Materials and procedure
Thesamematerialswere usedasthoseinExperiment1,withparticipantsattemptingoneprobetask
persession,consistingoftwoblocks,onewithsuppressionandonewithout.Giventhe tiringnature of
articulatorysuppression,thetwosessionswereheldonseparatedays.Thisalsoreducedthelikelihoodof
participantsusingthe same strategies(e.g.,serialrehearsal) forthetwoprobe tasks.The orderof probe
tasksandthe orderofsuppressionconditions were counterbalancedacrossparticipants.
Inthe suppressioncondition,participantswere instructedtorepeatthe sequence“one–two–three–
four”asquicklyaspossible tothemselves,loudlyenoughtoheartheirownvoice.Theywere instructed
tocontinue suppressionthroughout the block oftrials,thoughif theywanted a rest,they couldpause
betweenthe self-pacedtrials.
Results
Suppression caused considerableimpairmenton both tasks, butthe impairmentwas greater
ontheLPtask(M = 0.17,SD = 0.11)thanontheIPtask(M= 0.09,SD = 0.08;upper panel
of Figure 6). A 2 (suppression) × 2 (probe task) × 2 (session order) ANOVA showed a
significant interaction between suppression and probe task, F(1,14) = 9.84, p < .01.
However,therewasalsoasignificanteffectofsession order,F(1, 14) = 6.29,p < .05,andthe
three-wayinteraction approachedsignificance,F(1, 14) = 4.48,p = .05.There was ageneral
improvementin performancein thesecondsession,andthe interaction betweensuppression
and probe task was more noticeable in the second session than in the first. It seems that
combining tasks was difficult and that a certain amount of practice may increase selective
interferenceeffects.
SHORT-TERM MEMORY INTERFERENCE
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As in Experiment 1, one might argue that the Interference × Probe Task interaction
reflectsaceilingeffect,giventhatcontrolperformanceintheIPtaskisover 90%correctand
significantlygreaterthanthatin the LPtask,t(16)= 4.11,p < .001.Performanceon thetwo
taskswasbettermatchedbycomparingseven-itemlistsintheIPtaskwithfive-itemlistsinthe
LP task (lower panel of Figure 6), such that control performance on the tasks no longer
differed significantly, t(16) = 1.31, p = .21. Both tasks still showed a significant effect of
suppression, t(16) > 4.48, p < .001, and the interaction between probe taskand suppression
approached significance in a two-wayANOVA, F(1, 15)= 4.00, p = .06. Given that a one-
tailed criterion is more appropriate here, we conclude that the corresponding interaction in
themain analysis was notdue to aceilingeffect.
1318
HENSON ETAL.
Figure6. Overallperformance(upperpanel)withandwithoutarticulatory suppression(Experiment2)andwhen
controlperformancein IPandLPtasks equated(lowerpanel). SeeFigure2 formoredetails.
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Phonological similarity and probe type effects
The proportions of correct responses to confusable and nonconfusable positive and
negative probes (as designated in Experiment 1) are shown in Figure 7. In the IP task, the
effect of probe confusability was greater in the control condition than in the suppression
condition. Indeed, a 2 (suppression condition) × 2 (probe type) × 2 (probe confusability)
ANOVA showed a significant interaction between suppression and probe confusability,
F(1,15) = 6.94, p < .05. Pairwise tests showed a significant effect of confusability in the
control, t(16) = 4.74, p < .001,butnotin thesuppressioncondition, t(16) = 0.33(collapsing
acrossprobetypein bothcases).Thesedataareconsistentwith theproposalthatarticulatory
suppression preventsphonologicalrecoding(Baddeley,1986).Therewas alsoamaineffectof
probe type, F(1, 15)= 15.4, MSE = 0.07, p < .001, with negative probes producing higher
accuracy.
For the LP task, phonological similarity reduced correct responses to negative probes in
both control and suppression conditions. The was confirmed by a significant interaction
between probe confusability and probe type, F(1, 15)= 5.87, p < .05. Unlike the IP task,
however, suppression did not interact with probe confusability. Indeed, an effect of probe
confusability remained under suppression, t(16) = 4.63, p < .001 (collapsing across probe
type).
Serial position effects
Due to relatively small numbers of observations, data were collapsed across list length.
The upper panel of Figure 8 shows accuracy as a function of probe position and probe
task.Interpretationofserialpositioneffects iscompromisedbythefactthatPositions1–4are
collapsedoveralllistlengths,whereasPosition5isonlycollapsedoverlistlengths6and7,and
Position 6 is only from list length 7. However, interest concerns interactions with probe
position,whicharenotcompromised.A2(probetask)×2 (suppression condition)×6(probe
position) ANOVA revealed a significant interaction between probe task and probe position,
SHORT-TERM MEMORY INTERFERENCE
1319
Figure7. Accuracyas afunctionofprobetypeandprobeconfusabilitywithandwithoutarticulatorysuppression
(Experiment2).Cont=control; Supp=suppression.SeeFigure4 for moredetails.
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F(3.45, 51.78) =3.57,MSE = 0.14,p < .01,reflectinggreater recencyin theIPthan LPtask,
as in Experiment 1. No other interactions reached significance, however, suggesting that
suppression had auniformeffect over probe position.
Reactiontimesin thecontrolcondition oftheLPtaskincreased monotonicallywithprobe
position,as in Experiment1 (see lower panelofFigure8).However, this wasnottruein the
suppression condition, suggesting that suppression was effective in preventing serial
rehearsal. This was confirmed by a significant three-way interaction between probe task,
suppression, and probe position, F(3.05, 45.8)= 6.82, MSE= 0.01, p < .001, indicating a
greater interaction between suppression and probepositionintheLPtaskthanintheIPtask.
Indeed, suppression appeared to have littleeffect on the RTfunctions in theIP task.
1320
HENSON ETAL.
Figure 8. Accuracy and reaction times (RT) as a function of probe position with and without articulatory
suppression(Experiment2).Cont=control;Supp=suppression.Positionsmarkedwithanasterixarederivedfrom
fewerlists(seetext).SeeFigure5 formoredetails.
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Discussion
ThepresentexperimentsupportedthehypothesisthattheLPtaskinvolvesagreaterdegreeof
serialrehearsalthan doesthe IP task.Concurrentarticulatorysuppression,which is assumed
topreventrehearsal(e.g.,Baddeley,1986), had alargerdetrimentaleffectontheLPthan on
the IP task. Independent support for the assumption that suppression prevented subvocal
rehearsal was obtained from the finding that itflattened RT profiles as a function of probe
position in the LP task. It is interesting to note the sharp contrast between this latter effect
of suppression and the absence of a corresponding effect of irrelevant speech on RTs in
Experiment 1. We suggestthatwhile both manipulations degrade memory for serial order,
irrelevantspeech does notpreventparticipants fromusingsubvocalization torespond tolist
probes because,unlike suppression,itdoes notcapture the articulatory system.
Ifsuppressionprevents serialrehearsal,howcan peopleperformtheLPtaskabovechance
under suppression? One possibility is that sequential presentation of list items gives rise to
avisuospatially organized orthographic representation of the list, as well as a temporally
organizedphonologicalrepresentation(see,e.g.,Logie,DellaSala,Wynn,&Baddeley,2000).
Undernormalconditions,peoplemaycomparethevisuospatialrepresentationagainstthelist
probe,inparallel.Onlyifamismatchisdetected mighttheycomparethe phonologicalrepre-
sentation to the list probe via sequential, subvocal articulatory rehearsal. (This is another
potentialexplanation for why“yes” responses are faster than “no” responses for later probe
positions;seeDiscussiontoExperiment1.)Undersuppression,however,peoplemustrelyon
thevisuospatialrepresentation,reducingbutnotabolishingperformance.Theparallelnature
ofthevisuospatialcomparison would then explainthe flatter probepositioncurvesand faster
RTs (atleastfor later probepositions).
Suppression also exerted a significant detrimental effect on the IP task. This may be a
consequence ofarticulatory suppression preventing phonological recoding of the list items
and probe (Baddeley, 1986). This interpretation receives some support from the fact that
suppression removed the effectofphonological similarity in the IP task.In this case,correct
performanceoftheIPtaskundersuppressionmightbeattributabletoaformofvisualmemory
likethatdescribedabove.However,unliketheIPtask,theLPtaskcontinuedtoshowaneffect
of phonological similarity under suppression. This suggests that suppression may not have
prevented phonologicalrecoding(or serial rehearsal)completely—for example, participants
may have attempted to recode list items in between irrelevant articulations. Thus, inter-
pretationofdataobtainedwithsuppressioniscomplicatedbythefactthatitmayhavemultiple
interference effects. A more selective interference with a timing signal was attempted in
Experiment3,byintroducingtemporalgrouping intolist presentation.
EXPERIMENT 3
Temporal grouping
Temporalgroupingofasequencebytheinsertionofapauseeveryfew items iswellknownto
improveserialrecall(e.g.,Ryan,1969).Thistemporalgroupingeffectisindependentofword
length and phonological similarity (Hitch, Burgess, Towse, & Culpin, 1996). Temporal
grouping is also independent of articulatory suppression when the list items are presented
auditorily, butnot when presented visually(Hitch etal., 1996).Accordingtoseveralmodels
SHORT-TERM MEMORY INTERFERENCE
1321
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(Brownetal.,2000;Burgess&Hitch,1999;Henson&Burgess,1997;Hitchetal.,1996),such
grouping effects reflect a change in the nature of the timing signal underlying serial recall.
More specifically, grouping results in a differentiation of the timing signal into two com-
ponents:onetrackingthetimingofitems within groups and onetrackingthetimingofitems
(orgroups)withinlists.Ifmemoryforserialorderdependsonsuchatimingsignal,theLPtask
should showbetter performanceforgrouped than for ungrouped lists.Totheextentthatthe
IP task does not depend on memory for serial order, its performance should be relatively
insensitive to thetemporal rhythmoflistpresentation.
Method
Participants
The 18volunteersfromLancasterUniversity,6maleand12female,werepaidtoparticipateinthe
experiment.
Materials
Atotalof120listsof consonantswere constructedusing thestimulidescribedinExperiment1.Of
these,60containedsixitemsforpresentationassequencesoftwogroupsofthreeitems.Theremaining
60listswereassignedforungroupedpresentation.Ofthese,15listscontainedfiveitems,30containedsix
items,and15containedsevenitems.Theorderoflistlengthswithintheungroupedsetwasrandomized
inordertodiscouragetheuseofsubjectivegrouping.SelectionofpositiveandnegativeprobesfortheIP
task andLP taskwere as describedpreviously.
TimingofstimuliwasasthatinExperiment 1forthe ungroupedcondition.Forgroupedpresenta-
tiontherewasa750-mspausecorrespondingtothepresentationofanextra“blank”itembetweenItems
3and4.Participantswere toldto use the pausetogroupthe lettersinthreesinthe same waythat they
mightdofor atelephonenumber.
Procedure
PresentationofstimuliandinstructionswereasthoseinExperiment1exceptthattrialswerecontin-
uous rather than self-paced. All participants were given the ungrouped lists in the first of two test
sessions, with half performing the IP task first and half performing the LP task first. Participants
returnedfortheirsecondsessionsomehoursordayslater,whentheyweregiventhe groupedlists.The
orderofprobetaskswasagaincounterbalanced.Groupedlistswere not presentedinthe firstsessionin
ordertoavoidunwantedtransferofgroupingstrategiestotheungroupedlists.Whilethismeantthatany
improvement associated withgrouping may have reflectedgeneralpractice effects,we were primarily
interestedinthe interactionbetweengroupingandtask.Eachsessionlastedforabout50minutes.
Results
Overall performance
Tofacilitatecomparisons,datafromonlythesix-itemlistsin theungroupedcondition are
reported.Theproportions ofcorrectresponses for grouped and ungroupedlistpresentation
areshowninFigure9.Groupingimprovedperformanceby.03(SD = 0.08)intheIPtask,and
by.09(SD = 0.09)intheLPtask.A2(grouping)×2(probetask)ANOVAshowedsignificant
effectsofgrouping,F(1, 17) = 27.03,p < .001,andprobetask,F(1, 17) = 12.88,p < .05.The
1322
HENSON ETAL.
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interaction between grouping and probe task approached significance, F(1, 17) = 3.23,
p= .09,andwasreliablewhenconsideredasaone-tailedtestofthepredictedinteraction.Post
hoct-testsshowed asignificanteffectofgroupingon theLP task,t(18) = 4.04,p < .001, and
nottheIPtask,t(18) = 1.61,p = .13.However, interpretation ofthe differences betweenthe
probetasks is compromised bya possible ceiling effectin theIP task.
Serial position effects
Accuracyas afunction ofprobepositionforeachcondition isshown in theupperpanelof
Figure10.Mostnoticeableisthe greater accuracyforPosition 3thanPositions 2or 4in both
ungrouped and grouped conditions of the LP task. This pattern suggests that participants
were spontaneously subjectively grouping the ungrouped lists. A 2 (grouping) × 2(probe
task)× 5(probe position)ANOVA showed a significanteffectof grouping,F(1, 17) = 5.21,
MSE = 0.25, p < .05, a significant interaction between probe task and probe position,
F(3.13, 53.22) = 3.01,MSE = 0.54,p < .05, and an interaction between groupingand probe
task that again approached significance,F(1, 17) = 2.22,MSE = 0.57,p = .06.
As before, RTs in the LP task generally increased with probe position (lower panel of
Figure 10), butwith a marked deviation frommonotonicity on Position 3(like the accuracy
data). Grouping tended to reduce the overall gradient of RTs against probe position.
Groupinghad lessobviouseffectsonRTsintheIPtask.ThisgreatereffectofgroupingonLP
thanonIPprobepositioncurveswasconfirmedbyasignificantthree-wayinteractionbetween
grouping,probetask,andprobeposition,F(3.10,52.78)= 3.11,MSE = 0.15,p < .05.Two-
wayANOVAson the LPand IPtasks separatelyconfirmed an interaction between grouping
andprobeposition ontheLPtask,F(2.93,49.89) = 2.74,MSE = 0.26,p = .05,butnotinthe
IPtask, F(2.52, 42.77) = 1.14,MSE = 0.05,p = .34.
SHORT-TERM MEMORY INTERFERENCE
1323
Figure9. Overallperformancewithandwithouttemporalgroupingforsix-itemlists(Experiment2).SeeFigure2
for moredetails.
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Discussion
The present experiment provides support for the hypothesis that temporal grouping
primarily improves short-term memory for serial order. Although the interaction between
groupingandprobetaskonoverallperformancemayhavebeenconfoundedbyceilingeffects,
accuracy and RTs as a function of probe position confirm that grouping exerted a greater
influence on performanceofthe LP than on that ofthe IP task.Grouping increased perfor-
manceandloweredRTsonmostifnotallpositionsintheLPtask,particularlypositionsinthe
second group. However,grouping had no reliableeffects on probe position curves in the IP
task.
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Figure10. Accuracyandreactiontimes(RT)asafunctionofprobepositionwithandwithouttemporalgrouping
(Experiment3).G=grouped; U=ungrouped.SeeFigure5formoredetails.
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Both grouped and ungrouped probe position curves in the LP task showed improved
performanceon Position 3relativetoPosition 2.This “mini-recency”effectattheendofthe
groups is indicative of grouping strategies (Ryan, 1969). The fact that such evidence was
apparent even in the ungrouped condition suggests that participants were spontaneously
groupingin threes (themodal group size,Henson, 1996),despiteourprecautions tovarylist
lengthunpredictablyintheungrouped conditionandtoavoidcarry-overeffectsbytestingthe
ungroupedconditionfirst.Thepresenceofsubjectivegroupingin our ungrouped condition
wouldhaveweakenedtheinteractionbetweengroupingandtask.Thisisarecurringproblem
with (visual) grouping manipulations (Henson,1996).
Themini-recencyeffectonPosition 3intheLPtaskrepresentedfasterand moreaccurate
responding to negative list probes in which the paired transposition straddled a group
boundary. Such deviations of the grouped structure may be particularly easy to detect,
improving accuracy. However, it is difficult to explain why such responses are faster than
thoseon theprecedingpositionaccordingtoastrict,forward rehearsalstrategyfromthestart
of the list. One possibility is that participants have direct access to different groups and
sometimesinitiatedtheirrehearsalfromthestartofthesecond group.Inthiscase,theycould
immediatelynoticeanerroneous itematthestartofthesecond group when thetransposition
straddled thegroupboundary,thus producingshorter RTs on Position 3thanonPosition 2.
This strategywouldresultina shallowergradientacrossprobeposition for groupedthan for
ungroupedlists,consistentwiththeinteractionpattern between groupingand probeposition
in theLP task.
EXPERIMENT 4
Finger tapping
Experiment4examined the relativeinterference on IP and LP tasksofaconcurrenttapping
task,followingthesuggestionthatsuchrhythmicproductiontasksmightimpairtheencoding
of serial order in short-term memory bycompeting for a common timing signal (Burgess &
Hitch, 1999).Thissuggestion is consistentwiththeimagingstudyofHenson etal.(2000),in
which a dorsal premotor brain region was differentially activated as a function of serial
rehearsal and temporalgrouping in STM, given thatthe same brain region has been impli-
cated in rhythmic motor finger movements by imaging and neuropsychological studies
(Catalan etal.,1998;Halsbandetal., 1993).Ifthe same timingsignal is responsibleforserial
rehearsal and rhythmictapping,then theLP taskshouldshow agreater detrimental effectof
concurrenttappingthan theIP task. Furthermore, by increasingthetemporalcomplexityof
thetappingtask,fromsimple repetitivetappingtoamorecomplex“changing-state”rhythm
(Jones & Macken, 1995),we expected tosee an increasein the amountof interference.
Method
Participants
The18volunteersreplyingtoadvertsintheUCLPsychologyDepartmentconsistedof9menand9
women(meanageof 28.1years),andtheywerepaidforparticipating.
SHORT-TERM MEMORY INTERFERENCE
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Materials and procedure
Sixsetsof30listswerecreatedinthesamemannerasthatinExperiment1.TheIPandLPtaskswere
combinedwithone of three tappingconditions—notapping,regulartapping,andcomplextapping—
performedbyeachparticipantinsixseparateblocks.Thethreeblocksforeachprobetaskwerepresented
contiguously,withtheorderoftappingconditionscounterbalancedacrossparticipants.Halfthepartici-
pantsperformedtheLP blocks first,andhalf performedthe IP blocksfirst.
Inthe regularandcomplextappingtasks,participantspressedthespacebarwiththeirnondominant
handinsynchronywithacomputer-generatedtone(responsestotheprobe tasksbeingmadewiththeir
dominant hand). The tones hada durationof 200 msandpitchof 2 kHz.In the regulartapping task,
tones were produced at 320-ms intervals. In the complex tapping task, tones were produced in a
syncopatedrhythmliketheoneusedbySaito(1994).Therelativetimingoftonesanditemsintheprobe
tasksvariedduetotheirdifferentfrequencies,asshowninFigure11.Tonesbeganafewsecondsbefore
the firststimulusappearedandcontinueduninterruptedthroughoutthe block.
Baselinemeasuresofperformanceinbothtappingtaskswereobtainedforapproximately90 satthe
beginningandendofthe experiment.Participantswere given10practicetrialsateachprobe taskinthe
absenceofconcurrenttappingbeforereceivingtheexperimentalblocksinvolvingthattask.Threecovert
practice trials were added to the beginning of each block to ensure that participants had become
accustomed to combining the tapping and probe tasks. The only other procedural difference from
Experiment1wasthattrialswere notself-paced:Afterrecordinga response to theprobe task,the next
trialwas delayeduntil the beginning of the next two bar cycle of the tone sequence.The experiment
lastedapproximately1 hr 15 min.
Results
Overall performance
Concurrenttappingimpairedperformanceofboth IPand LPtasks (upperpanel,Figure
12), more so when tapping a complex than simple rhythm (impairment by regular and
complex tappingon IP task, M = 0.03,SD = 0.10and M = 0.11, SD= 0.09, respectively,
and on LP task, M = 0.07, SD = 0.07 and M = 0.16, SD = 0.09, respectively). This was
confirmed by a 2 (probe task) × 3 (tapping task) ANOVA showing significant effects of
probe task, F(1, 17) = 20.89, MSE = 0.01, p < .001, and tapping condition, F(1.97,
33.35)= 26.28, MSE = 0.01, p < .001. The interaction between probe task and tapping
approached significance, F(1.83, 31.07) = 3.07, MSE = 0.005, p < .06. A planned
comparison between performance with tapping, (complex+ regular)/2, versus notapping
revealed a significant interaction with probe task, F(1, 17) = 4.61, p < .05. However, the
difference between complex and regular tapping did not interact with the probe task
1326
HENSON ETAL.
Figure 11. Schematicofrelativetimingofprobetasks andregularandcomplextapping in Experiment4.
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