On the modularity of recognition memory for object form and spatial location: a topographic ERP analysis

\

Pergamon

PII] S9917Ð2821"86#99017Ð9

Neuropsycholo`ia\ Vol[ 25\ No[ 4\ pp[ 330Ð359\ 0887 Þ 0887 Elsevier Science Ltd[ All rights reserved Printed in Great Britain 9917Ð2821:87 ,08[99¦9[99

On the modularity of recognition memory for object form and spatial location] a topographic ERP analysis AXEL MECKLINGER Max!Planck!Institute of Cognitive Neuroscience\ Leipzig\ Germany "Received 08 May 0886^ accepted 01 Au`ust 0886#

Abstract*Event!related potentials from 50 scalp sites were used to examine the brain processes subserving recognition memory for object forms and spatial locations[ Subjects memorized line drawings of highly familiar objects and their spatial locations within a two!dimensional matrix[ Prior to the test phases a cue indicated whether object!based or spatially!based recognition judgements were required[ Recognition judgements were faster and more accurate for spatially!based than for object!based judgements[ A variety of topographical di}erences in the ERP waveforms as a function of recognition task emerged] First\ when the cue indicated that object!based judgements were required\ negative slow wave activity extending for several hundred ms with a maximum at frontal recording sites was obtained[ Conversely when spatially!based judgements were required\ slow wave activity developed over parieto! occipital areas[ Second\ early portions of the old:new e}ects evoked by the test items "i[e[ 299Ð599 ms after stimulus onset# showed a similar anterior!posterior dissociation as a function of recognition task[ Third\ for object!based\ but not for spatially!based\ judgements\ late old:new e}ects "i[e[ 699Ð0599 ms# were found with a clear maximum at right frontal recordings[ The results are consistent with the view that functionally and anatomically di}erent brain systems are involved in recognition memory for object form and spatial location[ They further suggest that the retrieval of object forms involves conceptual semantic integration processes[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Key Words] Old:new e}ects^ ERPs^ N399^ visual memory^ memory retrieval[

behavioral\ neuropsychological and neuroimaging stud! ies have been conducted[ They suggest that functional processes of rather di}erent types and complexities in the visual!spatial domain can be functionally and ana! tomically subdivided along these two lines[ For example\ behavioral studies employing the selective interference approach suggest that spatial processing and object pro! cessing draw from separate pools of working memory resources[ Tresch et al[ ð44Ł reported that working mem! ory for object forms was selectively impaired by a con! currently performed object interference task "color discrimination#\ whereas spatial working memory was selectively impaired by a spatial interference task "move! ment discrimination#[ Similar results have been reported by Logie ð11Ł\ Baddeley and Lieberman ð1Ł and Logie and Marchetti ð13Ł^ see ð12Ł for an overview#[ The notion of separable working memory systems for object forms and spatial locations was also con_rmed by recent neu! roimaging studies ð20\ 49Ł[ A second line of evidence for di}erent processing mod! ules for {{what|| and {{where|| information is provided by neuropsychological studies] Patients with temporo!

Introduction Recent research provides converging evidence for the notion that spatial and object information is processed by functionally and anatomically di}erent systems in the primate|s brain[ In 0871\ Ungerleider and Mishkin ð47Ł reported results from experimental lesion studies with monkeys\ which indicated that a ventral pathway\ that connects the visual cortex with inferior temporal lobe regions is engaged in the processing of object properties of a stimulus such as object form\ contour or color\ whereas a dorsal pathway running from the primary vis! ual cortex to posterior parietal regions mediates the pro! cessing of spatial stimulus features\ such as absolute or relative spatial location[ Since this initial report of a {{what|| and a {{where|| system in the primate|s visual system a large variety of  Address for correspondence] Dr[ Axel Mecklinger\ Max! Planck!Institute of Cognitive Neuroscience\ Inselstra)e 11Ð15\ 93092 Leipzig\ Germany^ tel[ ¦¦230!8839!003^ fax ¦¦230! 8839!002^ e!mail] mecklingÝcns[mpg[de

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occipital lesions have large impairments in tasks requiring the imaging of object features but have no di.culties in spatial tasks ð4Ł[ Conversely\ patients with damage to the posterior parietal region show the reverse de_cit ð26Ł[ The separability of object and spatial processing functions\ even for tasks involving long!term memory rep! resentation of visual!spatial information\ was con_rmed by a recent PET study ð20Ł[ In this study the regional cerebral blood ~ow was examined while subjects had to recognize either object or spatial features of previously learned pictures[ Increased blood ~ow during the retrieval of object features from long!term memory was found in the right inferior temporal region\ whereas the retrieval of spatial features led to increased blood ~ow in the right posterior parietal lobe[ Both memory tasks were associated with increased blood ~ow in dorso!lateral pre! frontal regions[ Despite the growing evidence for the separability of object and spatial processing functions in the human brain\ several aspects of this dissociation require further speci_cation[ For example\ recent studies suggest that the ventral stream can be further subdivided into subsystems specialized for the processing of perceptual features of complex visual stimuli\ e[g[ faces or word!like letter strings\ as well as for processing of a word|s semantic context ð22Ł[ Similarly\ it remains to be speci_ed which processing functions for visual!spatial information can be separated along these two lines and which can not[ It is conceivable that the initial encoding of object and spatial features is subserved by di}erent neuronal struc! tures\ whereas the same neuronal structures are involved in higher level storage and processing functions for object and spatial information[

The ERP Approach In this study we examined the separability of object and spatial processing components involved in recognition memory by means of a topographical analysis of event! related potentials "ERPs#[ ERPs are small voltage oscil! lations measured at the scalp which are time!locked to the processing of external events[ They involve a sequence of de~ections "i[e[ components# which mark the passage of information through the brain ð00Ł[ Di}erences in tim! ing and scalp topography of particular ERP components can be used to make inferences about the timing and the spatial con_guration of brain activity involved in cognitive processes[ If di}erent scalp topographies are found for an ERP component elicited in two di}erent experimental tasks\ it can be assumed that these two experimental tasks activated di}erent patterns of neu! ronal activity and re~ect di}erent functional processes ð04\ 32Ł[ For example\ the scalp topography of negative slow wave activity in the ERPs has been reported to be speci_c to the kind of information held in working memory ð10\

18\ 30Ł[ Employing a delayed!matching!to sample task\ Ruchkin et al[ ð30Ł obtained negative slow wave activity over left frontal brain areas when phonological infor! mation "i[e[ pronounceable nonwords# is retained in working memory[ Conversely parietally focused negative slow waves were found to be associated with the retention of visuo!spatial information "spatial patterns#[ This pat! tern of results indicates that holding verbal and visuo! spatial information in working memory can be func! tionally dissociated\ and that it is associated with di}er! ential neuronal activity[ Using a similar task\ Mecklinger and Pfeifer ð18Ł found di}erent slow wave patterns when either object forms or spatial patterns had to be main! tained in working memory[ Retaining the shape of geo! metrical objects over a period of several seconds was associated with pronounced negative slow wave activity over right temporal and bilateral frontal areas\ whereas\ during the retention of two!dimensional spatial con! _gurations\ negative slow wave activity was most pro! nounced over bilateral occipital and parietal areas[ ERPs have also been used as direct tests of the pro! cessing components involved in recognition memory using verbal ð02\ 34Ł or pictural stimuli ð2\ 5Ł "for an overview\ see ð33Ł#[ A consistent _nding of these studies is that correctly recognized {old| "i[e[ previously studied# stimuli evoke more positive going waveforms than cor! rectly recognized {new| "i[e[ not previously studied# stim! uli[ This e}ect begins around 199Ð299 ms post stimulus and persists for several hundred milliseconds[ For verbal stimulus materials these old:new di}erences have been found to be larger over the left hemisphere ð34Ł[ The old:new e}ects are not observed by old stimuli that are erroneously classi_ed as new ð21Ł\ and they are inde! pendent of the response requirements for old and new responses[ It is generally assumed that old:new e}ects occur whenever a judgement that an item is old is accompanied by the retrieval of memory information formed by its initial presentation ð25\ 35Ł\ i[e[ recollection[ However\ several dissociations between recognition memory performance and ERP old:new e}ects have been reported\ which indicate that recognition memory impairments are not necessarily associated with reduced old:new e}ects ð28\ 36Ł[ Some evidence on the neuronal mechanisms contributing to the ERP old:new e}ects was provided by recent ERP studies with temporal lobectomy patients ð37\ 40Ł[ Smith and Halgren ð40Ł examined rec! ognition memory for words in patients with left of right anterior temporal lobectomy and in normal controls[ Reliable old:new e}ects in the ERPs were found for the right!sided patients and for normal controls\ but not for the patients with left!sided lesions[ Conversely\ Rugg et al[ ð37Ł found that old:new e}ects during a continuous recognition memory task for words were signi_cantly smaller in both\ left and right anterior temporal lobec! tomy patients as compared to normal controls[ These _ndings suggest that scalp!recorded ERP old:new e}ects may serve as an index of the memory processes subserved by the medio!basal temporal lobes ð43Ł[

A[ Mecklinger:Recognition memory for object forms and spatial locations

The Present Study

Methods

In the current study\ a recently developed study!test paradigm ð15\ 16Ł was employed[ Subjects are required to memorize composite stimuli containing object and spatial information[ It was only prior to the test phase that they were informed whether they had to make recognition judgements based on the object or spatial features of the stimuli previously studied[ This paradigm enables us to examine processes underlying object!based and spatially! based recognition judgements using the same study phases and the same test stimuli for both types of infor! mation[ These tasks require the encoding and storage of both object and spatial stimulus features during the study phase[ Upon presentation of the task cue\ indicating that either recognition judgements for object or spatial fea! tures will be required\ the subjects need to selectively rehearse either the one or the other type of information\ and upon presentation of the test stimuli\ retrieve either the one or the other information from memory[ The ERP analysis focused on the cue intervals and test intervals in both recognition tasks[ In a recent study using this procedure the topographic distribution of the old:new e}ects in the test phases of both tasks was found to be task speci_c] Object!based judgements evoked frontally maximal old:new e}ects\ and spatially!based recognition judgements evoked parietally focused e}ects in a time period spanning from 299 to 599 ms after the onset of the test items ðcf 16Ł[ In this latter study\ geometrical objects "circle\ square\ diamond etc[#\ which were pre! sented at the positions of a 2×3 spatial matrix\ were used as stimulus materials[ The goal of the present study was to examine whether similar topographical dissociations of the old:new e}ects are obtained when standardized line drawings typically employed in studies on picture processing ð42Ł are used as stimulus materials[ If di}erent neuronal systems are involved in accessing and retrieving familiar objects and their spatial locations in memory\ we would expect topo! graphical and temporal dissociations in the old:new e}ects evoked in the two recognition tasks[ Moreover\ to the extent to which rehearsal of objects and spatial locations is mediated by di}erential cortical structures\ we expect topographically!distinct slow wave activity in the cue intervals of both tasks[ In order to allow a detailed examination of the scalp distribution of the ERP e}ects a high density electrode montage was employed[ In most cases\ especially for late ERP components\ a large num! ber of neuronal sources contribute to the scalp recorded components\ only some of which are a}ected by an exper! imental factor ð19Ł[ In these situations di}erence wave! forms obtained by subtracting the waveforms evoked in two experimental conditions can be used to dissociate the brain regions involved in cognitive processing in more detail ð04Ł[ In the current study we used scalp potential topographic maps of di}erence waveforms to analyze the temporal and topographical distribution of ERP e}ects in the cue interval and the test phases of both memory tasks[

Subjects

332

Twenty!two volunteers "09 male# between 08 and 14 years of age "mean] 10[5 years# participated in this experiment[ All subjects were students at the University of Leipzig\ were right handed and had normal or corrected!to!normal vision[ They reported to be in good health and were paid 01 DM:h for their participation[ None of the subjects had any prior experience with the task[

Stimuli All stimuli were presented on a VGA monitor under the control of a 375 computer[ They consisted of line drawings of 01 familiar and simple objects drawn from a standardized set of 159 line drawings ð42Ł[ The objects were] glasses\ pipe\ key\ hammer\ table\ scissors\ sock\ book\ lamp\ envelope\ cup\ and hat[ The mean familiarity and complexity scores were 3[95 and 1[91\ respectively[ All objects were presented in one of 01 equally spaced squares of a 3×2 grid "sidelines 16[4 cm and 10[4 cm# colored blue against a light grey background[

Procedure The subjects were seated comfortably in an acoustically and electrically shielded dimly lit chamber in front of the 06ý moni! tor[ They sat at a distance of about 89 cm from the screen and held a small response box on their lap[ Each subject performed one session including 69 study!test blocks[ The temporal sequence of a block is displayed in Fig[ 0[ The subject started each block by a button press[ Next\ four objects were presented sequentially at random positions within the 3×2 spatial matrix[ The subjects were instructed to mem! orize both the four objects and their respective spatial locations in the matrix[ 0999 ms after the end of the study phase the words {{bitte warten|| "please wait# were presented for 2 s in the center of the screen[ To interfere with verbal rehearsal\ subjects were instructed to repeat aloud the phonologically di.cult word {{Pfau|| "Engl[ peacock# 2 times[ Similar articulatory sup! pression tasks have been shown to prevent subjects from ver! bally recoding visual stimuli ðcf 0Ł[ After this interference task\ one of two cues was presented for 0999 ms\ indicating that in the upcoming test phase either object!based or spatially!based recognition memory judgements will be required[ The cues con! sisted of the three letters OBJ in the object task and POS in the spatial task[ Two seconds after the onset of the cue\ the _rst test stimulus was presented[ Each test phase was comprised of eight consecutive recognition trials[ In the spatial task the sub! jects were required to indicate on each trial whether or not one of the positions from the study list\ irrespective of object identity was presented[ In the object task the subject indicated whether or not one of the study list objects were presented irrespective of its spatial position[ In both tasks the same stimuli were presented and subjects pressed one button to respond old and another button to respond new with either the left! or right! hand index _nger\ respectively[ Old and new responses were equiprobable within each test phase[ After each response\ feed!

 Snodgrass and Vanderwart ð42Ł used 4!point scales for both ratings[ For the familiarity rating 0 and 4 indicated very unfam! iliar and very familiar\ respectively whereas for the complexity rating 0 indicated very simple and 4 very complex line drawings[

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A[ Mecklinger:Recognition memory for object forms and spatial locations

Fig[ 0[ Schematic illustration of the time course of stimulus presentation within an experimental block[ S0 to S3 indicate the four study stimuli[ T0 to T7 the eight test stimuli[

back was provided "correct\ false\ no response#[ In the object recognition test phases\ two of the four old "new# objects were presented at positions which also occurred in the study phase^ the other two old "new# objects were presented at unstudied positions[ In order to minimize the processing of irrelevant stimulus features "object features in the spatial task and spatial features in the object task# old objects in the test phases were never presented at exactly the same spatial positions as during study ðcf 16Ł[ The same constraints were applied to the spatial recognition test phases[ Note that although no exact rep! lications of the study list items were presented in the test phases\ objects "positions# in the test phases could nevertheless be com! bined with one of the three other positions "objects# of the study list[ The order of tasks was randomized across the 69 study!test blocks and both tasks were presented with the same probability "i[e[ 49)#[ The four study list items and the eight test items within each block were presented with the restriction that spatial positions in the left and right half as well as in the upper and lower half of the grid were equiprobable[ The assignment of response hand to response button was counterbalanced across subjects[ The subjects were instructed to memorize both the objects and their respective spatial positions during study\ and to preserve a visual image of the objects and their positions[ They were also informed that\ in 49) of the blocks\ they will have to recognize spatial positions\ and in the other 49) of the blocks they will have to recognize object forms[ They were told to respond as fast and accurately as possible[ Six practice blocks were given to the subjects at the beginning of the session[ Includ! ing electrode application and removal\ each session lasted about 1[4 h[

ERP recording The EEG activity was recorded with tin electrodes mounted in an elastic cap "Electrocap International# from 50 scalp sites of the extended 09Ð19 system[ The spatial layout of the scalp electrodes is displayed in Fig[ 1[ Electrode labeling is based on the standard nomenclature described in ð38Ł[ The ground electrode was positioned 09) of the distance between the two preoccular points right to Cz[ The vertical Electro!oculogram "EOG# was recorded from elec! trodes located above and below the right eye[ The horizontal EOG was recorded from electrodes positioned at the outer canthus of each eye[ Electrode impedance was kept below 4 KOhms[ The right mastoid was actively recorded as an additional channel[ All scalp electrodes were referenced to the left mastoid and were o/ine re!referenced to linked mastoids[ The EEG and EOG were recorded continuously with a band pass from DC to 69 Hz and were AÐD converted with 05 bit resolution at a sampling rat of 149 Hz[

Data analysis Behavioral data[ Reaction time was de_ned as the interval between the appearance of the recognition items and the sub! jects| keypress[ All of the reaction time averages were composed of correct responses only[ ERP data[ ERPs time!locked to the onset of the task cues and the test items were computed for each subject at all rec! ording sites with epochs extending from 199 ms before stimulus onset until 1999 ms thereafter[ The nature of processing in the cue intervals was assumed to be in the domain of rehearsal\ whereas the processing in the test phases was assumed to be more in the domain of retrieval[ The ERPs evoked by the test stimuli were selectively averaged for old and new responses and only trials containing correct responses were entered in these subject averages[ The average voltages in the 199 ms preceding the items were examined for systematic di}erences as a function of experimental task[ Because no systematic e}ects were found\ this epoch served as a baseline\ i[e[ its mean value was subtracted from each data point in the waveforms[ Prior to averaging\ each epoch was scanned for EOG and other artifacts[ Whenever the SD in a 199 ms time interval exceeded 49 mV\ the epoch was rejected[ After artifact rejection there were no signi_cant di}er! ences in the horizontal or vertical EOG as a function of task or response type in none of the time windows used for the quanti_cation of the ERP data[ ERPs to the cues were quant! i_ed as mean amplitudes in consecutive time windows of 149 ms between 649 and 1999 ms post stimulus[ The time intervals used for quanti_cation of the ERP responses in the test phase will be reported in the result section[ Because some of the components were not clearly visible as peaks at all electrode sites\ mean amplitude measures were considered more reliable for component scoring than peak measures[ In order to avoid a loss of statistical power that is implicated when repeated! measure ANOVAs are used to quantify multi!channel and multi!time window data ð8\ 24Ł\ electrode sites were pooled to nine topographical regions[ The following regions were de_ned] left frontal "F8\ F6\ F4\ AF6#^ medial frontal "AF2\ AFZ\ AF3\ F2\ Fz\ F3#^ right frontal "F09\ F7\ F5\ AF7#^ left temporal "FT8\ FT6\ T8\ T6\ CP4\ TP6#^ medial temporal "FC2\ FCZ\ FC3\ C2\ Cz\ C3#^ right temporal "FT09\ FT7\ T7\ T09\ CP5\ TP7#\ left parieto!occipital "P6\ P4\ PO6\ P8#^ medial parieto! occipital "Pz\ PO2^ POZ^ PO3^ O0^ OZ\ O1# and right parieto! occipital "P7\ P5\ PO7\ P09#[ According to Homan et al[ ð03Ł\ who established a correspondence between electrode site and underlying cerebral structure using radiographic techniques\ the medial frontal region is approximately over the middle frontal gyrus "Brodmann area "BA# 35#[ The left and right frontal regions are approximately over the inferior frontal gyri "BA 34#[ The left and right temporal regions cover approxi! mately the middle and superior temporal gyri "BA 10 and 11#\

A[ Mecklinger:Recognition memory for object forms and spatial locations

334

Fig[ 1[ Layout of the locations of the 50 scalp electrodes used for ERP recording[ The shaded areas mark the 5 regions of interest used for statistical analyses[

whereas the medial temporal region is approximately over the precentral gyrus "BA 3#[ Finally the left and right parieto! occipital regions cover approximately the posterior part of the middle temporal gyri and the anterior occipital sulcus "BA 08\ 26#\ whereas the medial parieto!occipital region is approxi! mately over the gyri occipitales and the superior parietal lobe "BA 06\ 6#[ The location of these nine regions on the scalp is indicated by the shaded areas in Fig[ 1[ The ERP measures in the cue interval were subjected to repeated!measure ANOVAs with factors anterior!posterior dimension "ant!pos dimension# "2 levels#\ lateral dimension "2 levels# and task "1 levels#\ whereas the ERPs evoked by the test stimuli were quanti_ed in ANOVAs with the factors ant!pos dimension\ lateral dimension and response type "old vs new# separately for the two recognition tasks[ In case of signi_cant interactions involving the task or response type factors\ one! way ANOVAs with the factor response type or task were per! formed to examine the e}ects of the experimental manipulations for the ant!pos dimensions\ lateral dimensions and:or the topo! graphical regions[ When interactions were accompanied by main e}ects of response type or task\ measures of treatment magnitude "V1 ð08Ł# for the single e}ects of task or response type will also be reported in order to elucidate the interactions[ All e}ects with two or more degrees of freedom in the numerator were adjusted for violations of sphericity which are inherent in repeated!measure analyses according to the formula of Green! house and Geisser ð09Ł[ In order to avoid reporting large amounts of statistical results not relevant for the issues under investigation "e[g[ lateral dimension×ant!pos dimension inter!

actions#\ only main e}ects or interactions including the task and response type factors will be reported[ Scalp potential topo! graphic maps of selected ERP results were generated using a two!dimensional spherical spline interpolation ð27Ł and a radial projection from Cz\ which respects the length of the median arcs[

Results Behavioral data Mean reaction times and accuracy data for old and new responses in the object and spatial task are displayed in Table 0[ Subjects responded faster and more accurately in the spatial than in the object task[ Old responses were also faster than new responses\ with this di}erence being larger for spatially!based than for object!based judg! ments[ Accuracy in both tasks was higher for old responses\ with the di}erence between old and new responses being larger in the object than in the spatial task[ This pattern of results was con_rmed by statistical analyses] Repeated!measure ANOVAs with factors task and response type revealed main e}ects of task "reaction times] F"0\08#  033[0\ P ³ 9[9990^ accuracy] F"0\08# 

335

A[ Mecklinger:Recognition memory for object forms and spatial locations

Table 0[ Performance results "reaction times and accuracy# for old and new responses in the object and the spatial memory task[ The standard error of the mean is presented in parentheses Memory task

Reaction time "ms#

Accuracy ")#

Object memory old new

587 "29# 641 "29#

77[5 "0[3# 81[5 "0[9#

Spatial memory old new

422 "16# 514 "16#

83[9 "0[9# 84[5 "9[7#

06[4\ P ³ 9[9994# and response type "reaction times^ F"0\08#  099[58\ P ³ 9[9990^ accuracy] F"0\08#  7[38\ P ³ 9[997#[ Moreover interactions between task and response type were obtained for reaction times\ F"0\08#  10[4\ P ³ 9[9991\ and for accuracy\ F"0\08#  4[38\ P ³ 9[92[ In order to examine the extent to which subjects exclus! ively processed object and spatial features in the two recognition tasks\ trials in which the irrelevant infor! mation in the test phase "spatial locations in the object task and objects in the spatial task# was part of the study list were compared with those trials in which new irrel! evant features were presented[ Reaction times and accu! racy for old objects were 580 ms and 77[7) when these objects were presented at locations also included in the study phases\ whereas for old objects presented at pre! viously unstudied positions\ reaction times and accuracy were 694 ms and 77[3)[ The corresponding di}erences for old responses to spatial locations with previously studied and unstudied objects were 423 ms "83[0)# and 422 ms "83)#\ respectively[ None of these di}erences were signi_cant "F ³ 0#\ indicating that subjects exclus! ively processed the stimulus features relevant for the rec! ognition judgments in both tasks[

Event related potentials Cue!interval[ Figure 2 displays the ERP waveforms at the three midline electrodes "Fz\ Cz and Pz# and at lateral frontal\ temporal and parieto!occipital recording sites evoked by the cue indicating either the object or the spatial memory task[ The cues evoked pronounced par! ietal maximal P299 components peaking at about 499 ms in both tasks[ Di}erences between the cues indicating either the object or the spatial task emerged at about 699 ms[ These di}erences took the form of a fronto!centrally more nega! tive right lateralized slow wave for the object cue as compared to the spatial cue[ It extended until about 0799 ms after cue onset[ Over the parieto!occipital region at about 0649 ms a di}erent pattern emerged[ In this time interval more pronounced negative slow wave activity was evoked by the spatial cue at left parieto!occipital

recordings "e[g[ PO6#[ The topographical distributions of the ERPs evoked by the object and the spatial cue are further illustrated in Fig[ 3 which illustrates the topo! graphic maps calculated for the di}erence between the object and the spatial task cue at 0099 ms and 0799 ms after cue onset[ These observations were con_rmed by a series of stat! istical analyses[ The results of the three!way repeated! measure ANOVAs performed for the mean amplitudes in consecutive time windows of 149 ms duration are dis! played in Table 1[ Main e}ects of task were obtained in the four time intervals from 649 to 0649 ms\ and the three!way interactions were signi_cant in the 0999Ð0149 ms and the 0149Ð0499 ms interval[ These latter inter! actions re~ect the fact that the task di}erences were sub! stantially larger at the medial frontal and the medial and right temporal regions "V1 × 9[20# than at the other regions "V1 ³ 9[11#[ Additionally\ interactions of task and ant!pos dimension were obtained in the four intervals from 0999 to 1999 ms[ For the time intervals from 0999 to 0649 ms these interactions indicate that the object cue evoked more negative going slow waves than the spatial cue at frontal and temporal regions\ P ³ 9[90\ but not at the parieto!occipital regions\ P × 04[ In the 0649Ð1999 ms time interval negative slow wave activity at the frontal regions was larger after the object cue than the spatial cue\ P ³ 9[94[ At the parieto!occipital regions in this time interval an interaction task×lateral dimension was obtained\ F"1\27#  6[20\ P ³ 9[995\ indicating that the spatial cue evoked more negative going slow wave activity than the object cue at the medial and right\ P ³ 9[94\ but not at the left parieto!occipital region[ Old:new effects for object and spatial information[ The ERP waveforms evoked by old and new items in the test phases of the object and spatial task are presented in Figs 4 and 5[ Early parieto!occipital N099 and frontal P199 components were obtained at 053 ms and 193 ms\ respec! tively\ for both tasks and response types[ Starting around 299 ms the waveforms evoked by old stimuli were more positive going than for new stimuli[ These old:new e}ects were observable in both tasks with a broad temporal and topographical distribution[ In the object task\ early old:new e}ects "i[e[ in the 299Ð 599 ms time range# were most pronounced at fronto! central recordings[ They resulted from an enhanced front! ally focused negativity to new objects that peaked at 339 ms "at the Fz electrode#[ For old objects this negativity was almost absent and only discernible as a small dip in the rising slope of the P299[ The P299 was more pro! nounced and peaked earlier "413 ms# for old objects as compared to new objects\ for which the maximum at the Pz electrode was reached at 593 ms[ Starting around 699 ms and extending until 0599 ms post stimulus onset late old:new di}erences in the object task were apparent at right frontal and to a smaller extent at right temporal recording sites whereas no such e}ects were observable at the posterior recordings[ These e}ects arose from a right frontally focused positivity evoked by old objects[

A[ Mecklinger:Recognition memory for object forms and spatial locations 336

Fig[ 2[ ERP waveforms averaged across subjects elicited by the cues indicating that in the upcoming test phase either object!based or spatially!based recognition judgements will be required[ In this and the following _gures the waveforms are plotted for nine electrodes representing the left\ medial and right frontal "F6\ Fz\ F7#\ temporal "T6\ Cz\ T7# and parieto! occipital "PO6\ Pz\ PO7# regions[ The vertical lines indicate cue onset and the vertical EOG is plotted in the upper right corner[

337

A[ Mecklinger:Recognition memory for object forms and spatial locations

Fig[ 3[ Topographic maps of the di}erences between the ERP responses to the object cue and the spatial cue at 0099 and 0799 ms after cue onset[ The distance between the isopotential lines is 9[7 mV[ Bright areas indicate positive di}erences between the two conditions and dark areas indicate negative di}erences[ Electrode positions are marked by small circles[

Table 1[ ANOVA results for the _ve consecutive time windows in the cue interval F!values

Task Task×lat Dim[ Task×a!p Dim[ Task×lat[ Dim×a!p Dim[

df

649Ð0999 ms

0999Ð0149 ms

0149Ð0499 ms

0[08 1[27 1[27 3[65

09[94 9[42 1[28% 0[28

09[03 0[34 5[24 2[34$

09[13 0[14 8[05 3[29$

0499Ð0649 ms 5[03$ 0[86 5[16 0[72

0649Ð1999 ms 9[24 9[81 00[99 0[31

Note] lat Dim[  lateral dimension^ a!p Dim[  anterior!posterior dimension[   P ³ 9[90^ $  P ³ 9[94^ %  P ³ 9[09[

The scalp topographies of the old:new di}erences in the object task at two time points\ representative of early and late e}ects "399 ms and 0199 ms respectively# are displayed in Fig[ 6[ In the spatial task both responses evoked larger P299 components than in the object task and the cor! responding old:new di}erences were most pronounced at parieto!occipital recordings[ Examination of the wave! forms| morphology indicates that in contrast to the object task\ the early spatial old:new e}ects are restricted to a smaller time interval "i[e[ 299Ð499 ms# and mainly result from a larger and earlier rising P299 component for old as compared to new responses[ Mean P299 latencies "as measured at Pz# in this task were 399 ms and 413 ms for

old and new responses\ respectively[ Late old:new e}ects during spatially!based judgments were present between 699 ms and 0499 ms with a widespread and slightly right lateralized distribution[ The scalp topographies of the early and late old:new e}ects in the spatial task are further illustrated in the topographic maps displayed in Fig[ 7[ Based on the temporal extension of the old:new e}ects\ three time windows were selected for quanti_cation[ In the object task the time windows were 299Ð599 ms\ 699Ð 0099 ms and 0099Ð0599 ms[ In the following these time periods will be referred as the early\ middle and late time intervals[ Due to the shorter duration of the early old:new di}erences for spatially!based judgements\ the early time

A[ Mecklinger:Recognition memory for object forms and spatial locations 338

Fig[ 4[ ERP waveforms averaged across subjects elicited by old and new objects[ The vertical lines indicate the onset of the objects which were presented for 0999 ms[ For details see legend of Fig[ 2[

349 A[ Mecklinger:Recognition memory for object forms and spatial locations

Fig[ 5[ ERP waveforms averaged across subjects elicited by old and new spatial positions[ For details see legend of Fig[ 2[

A[ Mecklinger:Recognition memory for object forms and spatial locations

340

Fig[ 6[ Topographic maps of the old:new di}erences in the object recognition task at two time points\ representative for the early "399 ms# and late time interval "0199 ms#[ The distance between two isopotential lines in 9[4 mV and bright areas indicate large old:new e}ects[

interval was set to 299Ð499 ms in the spatial task[ The results of the three!way ANOVAs for both tasks are displayed in Table 2[ In the object task in the early time interval a main e}ect of response type and the interactions response type×lateral dimension and response type×lateral dimension×ant!pos dimension were obtained[ The latter interactions re~ect the fact that the old:new e}ects were larger in magnitude at the medial and right frontal regions "V1  9[28 and 9[34\ respectively#\ and the medial and right temporal regions "V1  9[40 and 9[23#\ as com! pared to the left frontal and temporal regions\ "V1  9[95 and 9[13#[ Moreover\ the old:new e}ects were of the same magnitude at the three parieto!occipital regions[ In the middle and late time intervals in the object task the inter! actions response type×lateral dimension and response type×ant!pos dimension were signi_cant[ Both inter! actions were embedded in the triple interaction response type×lateral dimension×ant!pos dimension[ Post hoc tests indicated that in these two late time intervals the old:new di}erences were signi_cant at the right and medial frontal regions and the medial temporal region\ P ³ 9[90\ but not at the other regions\ P ³ 9[06[

For the spatial task in the early time interval a main e}ect of response type and the interactions response type×lateral dimension and response type×ant!pos dimension were obtained[ The latter interaction resulted from the fact that early old:new e}ects in the spatial task increased in magnitude from the frontal "V1  9[98#\ over the temporal "V1  9[46# to the parieto!occipital regions\ "V1  9[64#[ They were also larger at the medial "V1  9[48# and the right hemisphere regions "V1  9[46# than at the left hemisphere regions "V1  9[24#[ In the middle and late time intervals main e}ects of response type and interactions response type×lateral dimension were found[ In both time intervals the old:new di}erences were signi_cant at the medial and right hemisphere regions\ P ³ 9[990\ but not at the left hemisphere regions\ P × 9[04[ Sin`le trial analyses[ As indicated by Figs 4 and 5\ the reduced ERPs for new responses in the P299 time interval were accompanied by delayed P299 amplitudes in both tasks[ The di}erence in P299 latency between old and new responses\ as measured at the Pz electrode\ amounted to 79 ms for object!based judgments and 013 ms for spatially!based judgments[ Based on this pattern of

341

A[ Mecklinger:Recognition memory for object forms and spatial locations

Fig[ 7[ Topographic maps of the old:new di}erences in the spatial recognition task[ The maps were calculated for the same time points shown in Fig[ 6 "i[e[ 399 ms and 0199 ms#[ For details\ see legend of Fig[ 6[

Table 2[ ANOVA results for the three time intervals in the test phases of the object and the spatial task F!values Object task

Response type Response type×lat Dim[ Response type×a!p Dim[ Response type×lat Dim[×a!p Dim[

df

299Ð599 ms

0[08 1[27 1[27 3[65

06[87 5[38 9[69 00[00

Spatial task

699Ð0099 ms 2[86% 5[56 8[61 7[32

0099Ð0599 ms

df

299Ð499 ms

699Ð0099 ms

0099Ð0599 ms

2[34% 3[14$ 7[92 7[46

0[08 1[27 1[27 3[65

18[46 01[89 6[38 9[70

10[67 04[94 0[07% 9[89

8[86 6[90 1[63% 0[59

Note] lat Dim[  lateral dimension^ a!p Dim[  anterior!posterior dimension[   P ³ 9[90^ $  P ³ 9[94^ %  P ³ 9[09[

results\ it is conceivable that new responses were associ! ated with more latency variability of P299\ leading to reduced amplitudes in the averaged waveforms for each single subject[ In order to examine the extent to which old:new e}ects in the P299 time interval were confounded with latency jitter\ single trial quanti_cation for P299 amplitude was conducted for old and new responses in both tasks[ This analysis was restricted to the three mid!

line electrodes Fz\ Cz\ and Pz and the single trial P299 measures were examined in repeated!measure ANOVAs with factors response type "1 levels# and electrode "2  In this analysis P299 was de_ned as the maximum positive voltage between 299 and 799 ms post!stimulus relative to the prestimulus baseline in the low pass _ltered "2[4 Hz cut!o} frequency# single trials at the Fz\ Cz\ and Pz recording[

A[ Mecklinger:Recognition memory for object forms and spatial locations

levels#[ The mean values averaged across subjects for both response types in both tasks are presented in Table 3[ As apparent from the table and from statistical analy! ses\ old responses evoked larger P299 amplitudes than new responses in the object task\ F"0\08#  8[47\ P ³ 9[995\ and the spatial task\ F"0\08#  3[49\ P ³ 9[93[ Post hoc tests also revealed that in the object task\ the old:new e}ects were signi_cant at the frontal recording\ P ³ 9[990\ but not at the parietal recording\ P  9[09\ whereas the opposite topographical pattern was obtained in the spatial task "Fz] P  9[10^ Pz] P ³ 9[90#[ Thus\ the single trial analysis replicates the topographical dis! sociations of the old:new e}ects evoked by object!based and spatially!based judgments[ Topo`raphical pro_le analysis[ Object!based and spa! tially!based recognition judgments evoked topo! graphically and temporally di}erent old:new e}ects[ During object recognition\ the old:new di}erences were largest at fronto!central recording sites in the early P299 time interval\ and displayed a right frontal maximum from 699 and 0599 ms post stimulus[ In contrast\ during spatial recognition judgments\ old:new e}ects in all three time periods were largest at parieto!occipital recordings and were lateralized to the right hemisphere over tem! poral and parieto!occipital recording sites in the middle time interval\ i[e[ between 699 and 0099 ms[ These topo! graphical dissociations might suggest that di}erent neu! ronal sources contribute to the old:new e}ects in both tasks\ and in the early and late time intervals[ However\ any inferences on intracranial sources\ drawn from di}erential scalp to togrpahies of ERPs in di}erent con! ditions\ presuppose that the topographical di}erences between conditions are not confounded with di}erences in absolute amplitude between the contrasted conditions ð6\ 04Ł[ We therefore performed a topographic pro_le analysis by scaling the data so that the RMS amplitudes of the old:new e}ects in the early and late time intervals were the same in both recognition tasks ð14Ł[ Since any di}erences in the magnitude of the old:new e}ects are removed after this normalization\ interactions among the task factor and the region factor "incorporating the nine

Table 3[ P299 amplitudes "in mV# for the old and new responses in the object and the spatial recognition task at the frontal\ central and parietal midline electrodes\ i[e[ Fz\ Cz\ Pz[ The standard error of the mean is presented in parentheses P299 amplitude "mV# Memory task

Fz

Cz

Pz

Object memory old new

00[1 "9[7# 8[6 "9[6#

01[8 "0[9# 00[3 "9[7#

01[8 "0[1# 01[9 "0[9#

Spatial memory old new

01[4 "9[7# 01[9 "0[9#

05[0 "0[0# 04[2 "0[1#

04[7 "0[2# 03[6 "0[2#

342

topographical regions of the initial ANOVAs# can unam! biguously be taken as evidence for di}erent locations or orientations of the neuronal generators of the old:new e}ects in both tasks[ Given the similarity of the ANOVA results for the middle and late time window in both tasks\ this topographical pro_le analysis was restricted to the early and late e}ects in both tasks[ The ANOVAs with factors task "object vs spatial# and topographical region "8 levels# performed for the normalized old:new di}er! ences revealed reliable interactions in the early\ F"7\041#  2[43\ P ³ 9[91\ and the late time interval\ F"7\041#  6[42\ P ³ 9[90\ suggesting that the early and late old:new e}ects in both tasks arose from di}erent neuronal structures[ Moreover\ an ANOVA contrasting the topographic pro_les of early and late old:new e}ects within the object and the spatial tasks gave rise to a signi_cant time window×region interaction for the object task\ F"7\041#  4[74\ P ³ 9[990\ but not for the spatial task\ F ³ 0\ indicating that the neuronal sources of the object old:new e}ects but not the spatial old:new e}ects changed over time[ Effects of task dif_culty[ The two recognition tasks not only di}ered in the type of information to be retrieved but also in task di.culty[ As is apparent from Table 0\ in the object task subjects took about 049 ms longer to respond and made 4) more errors than in the spatial task[ From this it follows that\ rather then re~ecting dissociations in the retrieval of object form and spatial location\ the topographical distributions of the old:new e}ects in both tasks could also be related to task di.culty[ For example\ it is conceivable that recognition judgments for di.cult to retrieve information are correlated with anteriorly focused old:new e}ects\ whereas less di.cult recognition judgments are correlated with posteriorly focused old:new e}ects[ To examine this issue\ a median split of the 19 subjects was performed based on the per! formance di}erence between both tasks[ For this analysis\ corrected recognition scores "CR#\ i[e[ the hit rates minus the false alarm rates ð41Ł were used as a measure of recognition performance[ In the following the group showing larger between!task di}erences in CR will be referred to as the large di}erence group\ whereas the group with small di}erences will be referred to as the small di}erence group[ If the topographic dissociation of the old:new e}ects in both tasks re~ects the activation of di}erent brain systems for object form and location retrieval\ this dissociation should be present in both groups of subjects[ Conversely\ if the topographic e}ects arose from di}erences in task di.culty\ they should be absent for the subjects with small di}erences in task per! formance[ For the large di}erence group the corrected recognition scores were 9[65 and 9[80 in the object and spatial task\ respectively[ The corresponding values for the small di}erence group were 9[75 "object task# and 9[76 "spatial task#[ As revealed by a t!test for depended samples this di}erence was signi_cant for the large\ P ³ 9[9991\ but not for the small di}erence group\ P × 9[07[

343

A[ Mecklinger:Recognition memory for object forms and spatial locations

The ERPs at the three midline electrodes for both groups of subjects in the object and spatial recognition tasks are displayed in Fig[ 8[ The corresponding scalp topographies of the early and late old:new e}ects in both tasks are illustrated in Fig[ 09[ As is apparent from the _gures\ the ERP e}ects of both groups resemble those obtained in the initial analysis[ For both groups the early old:new e}ects had a fronto!central maximum in the object task and were most pronounced at the parieto! occipital recordings in the spatial task[ For the late time interval the e}ects yielded a right frontal maximum in the object task and were slightly right lateralized in the spatial task[ Notably\ these e}ects were highly similar for both groups[ These observations could be con_rmed by statistical analyses[ The between!subject factor group "large vs small di}erence group# was added to the initial ANOVAs performed for each task and time interval[ None of the analyses revealed signi_cant e}ects of group or interactions involving the group factor\ P × 9[07[ Moreover\ ANOVAs performed separately for each of the groups gave rise to signi_cant three!way interactions response type×ant!pos dimension×lateral dimension in all three time intervals in the object task[ For the spatial task\ interactions between response type and lateral dimension were obtained in all three intervals[ These results did not di}er in any signi_cant respect between the two groups[ The absence of any di}erences in the ERP old:new e}ects between the two groups of subjects showing large and small di}erences in task performance suggests that di}erential task di.culties are not adequate to account for the topographic di}erences of the object!based and spatially!based old:new e}ects[ These results rather sug! gest that the topographic di}erences in the old:new e}ects result from the requirement to retrieve di}erent kinds of information from memory[

Discussion Performance measures In this study we examined the processing components underlying recognition memory for object forms and spa! tial locations[ The analysis of performance measures indi! cates that subjects responded faster and more accurately when recognition judgments were based on spatial locations than on object forms[ It is conceivable that the relative advantage of spatial judgments arose from the fact that the objects shared more overlapping features and were thus less discernible than the twelve grid locations relevant for the spatial recognition judgments[ Thus\ the performance di}erences to some extent could be attributed to longer encoding times rather than more di.cult retrieval operations in the object task as com! pared to the spatial task[ In support of the latter notion\ the between!task di}erences in response speed for old responses were considerably larger than the correspond!

ing di}erences in performance accuracy[ It is further note! worthy that the subjects could selectively attend to spatial information during the spatially!based judgments and to object information during object!based judgments[ This conclusion can be derived from the observation that per! formance was not better when old objects were presented at previously studied positions than when they were pre! sented at previously unstudied positions[ The same results were obtained for locations _lled with previously studied: unstudied objects[ This pattern of results supports the view that processing of irrelevant information plays a negligible role in the two recognition tasks and that the two tasks were exclusively handled as either spatial or object tasks[ With these considerations in mind\ we will not turn to the discussion of the ERP data[

Event!related potentials Cue!interval[ The ERP results provide several lines of evidence for the view that di}erent processing com! ponents underlie recognition memory for object forms and spatial locations[ Starting around 649 ms after pres! entation of the task cues\ the ERPs were more negative going over frontal and central recording sites when the cue indicated that the object task will have to be per! formed as compared to the cue indicating the spatial recognition task[ This ERP di}erence lasted about 0999 ms with largest between!task di}erences at mid! and right!frontal recordings[ The reversed pattern\ i[e[ more negative going waveforms evoked by the spatial cue\ emerged in the last 149 ms of the cue interval at the parieto!occipital regions[ From the data at hand it cannot unambiguously be decided whether the observed fronto! central and parieto!occipital slow wave di}erences arose from more negative going activity in one task or more positive going activity in the other[ However\ several lines of evidence support the view that the di}erential slow wave activity results from modulations of fronto!cen! trally distributed negative slow waves evoked by the object cue\ and parieto!occipital negative slow waves evoked by the spatial cue[ A variety of previous studies indicate that negative slow wave activity with a duration of several hundred milliseconds had task speci_c top! ographies and can be related to retention:rehearsal oper! ations in memory tasks ð17\ 39\ 45Ł[ For example\ Uhl et al[ ð45Ł report bilateral frontal negative slow waves in a non!verbal associate learning tasks when unfamiliar faces had to be retained in working memory[ In tasks requiring the maintenance of spatial information in memory for several seconds\ negative slow wave activity at parieto! occipital recordings was found to increase with the amount of spatial information retained in memory ð18\ 30Ł\ or when spatial con_gurations ð17Ł or location sequences ð31Ł had to be retained in memory[ Based on these topographical and functional similarities between the slow wave pattern evoked by the task cues in the present study\ and those reported in previous studies\

A[ Mecklinger:Recognition memory for object forms and spatial locations 344

Fig[ 8[ ERP waveforms for old and new objects and spatial locations for the large "left panel# and small di}erence group "right panel#[ The waveforms are displayed at the three midline electrodes Fz\ Cz\ Pz[

345 A[ Mecklinger:Recognition memory for object forms and spatial locations Fig[ 09[ Topographic maps of the old:new di}erences in the object and spatial recognition task for the large "left panel# and small di}erence group "right panel#[ The maps were calculated for the same time points shown in Fig[ 6 "i[e[ 399 ms and 0199 ms#[ For details\ see legend of Fig[ 6[

A[ Mecklinger:Recognition memory for object forms and spatial locations

it can be assumed that the fronto!central and parieto! occipital slow wave patterns indeed re~ect modulations of negative slow waves related to the rehearsal of those stimulus features required for the upcoming recognition judgments[ The di}erential scalp topography of the slow wave pattern in the cue interval also replicates results in a previous study using this paradigm ð16Ł[ In the latter study the magnitude of parieto!occipital negative slow waves was found to vary with the amount of spatial processing required in the cue interval[ However\ in con! trast to this latter study in which parieto!occipital nega! tive slow wave activity emerged as early as 649 ms after cue onset\ the present parieto!occipital slow wave pattern is only present in the last 149 ms prior to the test phase[ An explanation for this temporal discrepancy could be that in the present study the rehearsal demands were larger for the less discriminable object features as com! pared to the rehearsal of spatial locations[ It is con! ceivable that this processing aspect was accompanied by larger and also more wide spread negative slow wave activity in the object cue interval which masked parieto! occipital distributed slow wave activity evoked by the spatial task cue[ In support of this view\ it is in the last 199Ð299 ms of the cue interval when the magnitude of the slow wave di}erence related to object rehearsal is reduced and slow wave activity related to spatial rehearsal emerges "cf Fig[ 2#[ Test phase*early old:new effects[ The earliest sign of di}erential processing of old and new items in both tasks emanated at 299 ms after the onset of the test stimuli[ In the spatial task\ early old:new e}ects were restricted to bilateral temporal and parieto!occipital regions\ with a clear maximum at parieto!occipital recordings[ The cor! responding early e}ects during object!based recognition judgments were topographically more widespread with a maximum over frontal and temporal regions[ These early old:new e}ects were superimposed on P299 components with similar frontal to parietal increasing scalp topo! graphies in both tasks\ but with larger amplitudes during spatially!based recognition judgments[ Moreover\ the old:new e}ects in the object task to some extent arose from a frontally focused negativity\ that was more pro! nounced with new than with old objects[ In showing a pronounced topographical dissociation on the anterior! posterior dimension\ the present results replicate those obtained in the Mecklinger and Meinshausen ð16Ł study in which geometrical objects rather than line drawings of familiar objects were used as stimulus materials[ Notably\ the anterior!posterior dissociation of the old:new e}ects in both tasks was preserved when the analyses were restricted to a group of subjects for which performance in both tasks was on the comparable level\ suggesting that di}erential task di.culty cannot account for the present pattern of ERP results[ It has repeatedly been argued that old:new di}erences to some extent can be attributed to di}erences in target detection with old items yielding larger {targetness| and

346

thus evoking larger P2999 components than new items ð07\ 21Ł[ However\ the current topographical dissociation of the old:new e}ects in both tasks provides strong evi! dence against the notion of {unspeci_c target detection| processes[ A more plausible account of the early old:new e}ects can be derived from the functional properties of the N399 component[ It has been proposed that the amplitude of the N399 is correlated with the ease with which an item can be integrated into a preceding context ð02\ 29Ł[ Given that an item is represented by a code that represents the item within a particular domain of processing\ be it semantic\ lexical or phonological\ it can be subject to contextual integration ð35Ł[ Consistent with this argu! ment\ enhanced N399 components have also been found for meaningful non!verbal stimuli that are incongruent with a preceding context[ This e}ect was obtained in di}erent paradigms\ for example\ picture matching ð3Ł\ object decision ð01Ł\ and in a continuous recognition memory task for pictures ð5Ł that bears similarities to the present paradigm[ In contrast to the posteriorly dis! tributed N399 evoked by linguistic stimuli\ the nega! tivities in response to non!linguistic symbolic stimuli in the above mentioned studies displayed a more frontal scalp distribution ð3\ 5\ 7\ 01Ł[ Given these apparent func! tional and topographical similarities\ the frontal maximal N399 evoked by new objects can be assumed to be associ! ated with the integration of new objects into a conceptual! semantic context generated by the study list objects[ Notably\ evidence for an enhanced frontally focused negativity to new objects has also been reported by Mec! klinger and Meinshausen ð16Ł who employed geometrical objects\ rather than line drawings of highly familiar objects as stimulus materials[ The morphology of the waveforms for old objects suggest that\ in addition to attenuated N399s old objects also give rise to more pro! nounced P299 activity\ and that both phenomena par! tially overlap in the 299Ð599 ms time range[ The early old:new e}ects evoked during spatially! based recognition judgments were topographically di}erent from those obtained for object!based judgments[ Examination of the waveforms| morphology indicates that spatially!based old:new e}ects did not result from modulations of the N399 component\ but rather re~ect the fact that the P299 was more pronounced for old as compared to new responses\ with this di}erence being largest over parieto!occipital regions[ In support of this view\ the parietal maximal scalp topography of the spatial old:new e}ects concurred with the topographic dis! tribution of the P299[ Parietally focused modulations of the P299 during recognition judgments have been reported by Johnson et al[ ð05\ 06Ł[ The authors found that the P299 at parietal\ but not at frontal recordings\ increased with the number of correctly classi_ed old and new words[ They further report a positive correlation between the magnitude of this parietal P299 and the hit rates during recognition judgments[ Based on these _ndings it is assumed that the parietal P299 is associated

347

A[ Mecklinger:Recognition memory for object forms and spatial locations

with the accessibility of memory representations during recognition judgments[ In light of the higher hit rates "i[e[ correct old responses# for spatially!based as compared to object!based judgments\ it is conceivable that the par! ietally focused P299 components during spatially!based judgments in the present study re~ects the higher strength or discriminability of spatial information in memory[ In summary\ a reasonable but still tentative interpret! ation of the anterior!posterior dissociation of the early old:new e}ects is that retrieval of object forms involves conceptual semantic integration processes\ re~ected by an anteriorly focused N399 to new objects that gives rise to anteriorly focused old:new di}erences[ Conversely\ retrieval of spatial locations presumably relies on more readily accessible structural representations and is re~ected by more posteriorly focused old:new e}ects that arose from modulation of parietal P299 components[ Since inferences from the scalp topography of ERP com! ponents to activated brain structures are in most cases problematic\ only very tentative assumptions about the brain structures generating the old:new e}ects can be made[ Based on the observation that old:new e}ects are attenuated after lesions to the medial temporal lobes ð37\ 40Ł\ and given the proposed functional characteristics of the old:new e}ects during recognition judgments\ it is conceivable that neuronal activity in the medial temporal lobes contributes to these e}ects[ Notably\ based on the results from studies employing intracranial ERP record! ings\ it was argued recently that anterior regions of the medial temporal cortex are engaged in processing con! textually constrained semantic information from pictures and words[ Posterior parts of the medial temporal cortex did not respond to semantic manipulations\ but rather were found to be more involved in lower!order perceptual processing of visual stimuli ð22\ 23Ł[ Further exper! imentation will be required to examine whether or not the anterior and posterior distributed early old:new e}ects at the scalp re~ect processes in di}erent regions of the medial temporal lobes[ Test phase] late old:new effects[ In the later\ i[e[ 699Ð 0599 ms time intervals\ old:new e}ects were still present in both tasks[ In showing a parieto!occipital maximum\ these late old:new e}ects for spatially!based judgments were highly similar in scalp topography to those observed in the early P299 time interval[ Accordingly\ the topo! graphic pro_le analysis did not reveal any reliable topo! graphical or localization di}erences between the early and late spatial old:new e}ects[ In contrast\ in the object task the late old:new e}ects were restricted to the right frontal recordings and\ as revealed by the pro_le analysis\ were generated in di}erent neuronal structures than the early object!based old:new e}ects[ Late right frontal old: new e}ects in the object task were even obtained for a group of subjects for which object and spatial task performance was on a comparable level[ This indicates that the presence of these late e}ects in the object but not the spatial task cannot solely be related to the higher di.culty of the object task[ The right frontally focused

old:new e}ects in the late time period resemble results of a previous experiment in which recognition memory processes for object forms and object locations were con! trasted using a similar paradigm ð16Ł[ The late onset and long temporal extension of these processes argue against a close causal relationship of the e}ects with the response and most likely indicate a contribution of post!decision evaluation processes ð5Ł[ Evidence for an involvement of right frontal cortex in memory retrieval was provided recently by Wilding and Rugg ð48Ł[ The authors found a late right frontal positive slow wave between 0099 and 0399 ms evoked by correctly classi_ed words[ Since it was larger for those words for which also correct source judgments "the voice in which the words were spoken during study# were made\ the authors conclude that this late slow wave re~ects post retrieval processes like the recollection of information from the study episode[ In light of the apparent topo! graphical and temporal similarities of the late ERP pat! terns found by ð48Ł and in the present study\ it is reasonable to assume that the present e}ects re~ect pro! cesses that operate on the products of the retrieval process\ like recovering the spatial or temporal position of the object forms during study[ The absence of any right frontally!distributed old:new e}ects for spatially! based judgments might suggest that post decision control process are contingent upon retrieval processes operating on semantically represented information[ In conclusion\ the current results provide several lines of evidence for the view that the processing components underlying recognition memory for objects and spatial information can be functionally dissociated and are mediated by di}erent neuronal structures[ The parieto! occipital distribution of the slow wave pattern related to the rehearsal of spatial locations in the cue interval sug! gests that parieto!occipital cortical areas closely related to those which are initially involved in perception and identi_cation of spatial information ð46\ 47Ł are also acti! vated when spatial information is retained in working memory[ The view that percept!like\ structural repre! sentation formats underly spatial memory processes are tentatively supported by the more posterior located old: new di}erences during spatially!based recognition judg! ments[ In contrast\ the frontally distributed old:new e}ects for object forms might indicate that\ for object forms\ conceptual!semantic integration processes are a necessary constituent of the retrieval process[ This pat! tern of results\ as well as the selective occurrence of late and right frontally distributed old:new e}ects for object! based recognition judgments\ suggests that retrieval of object information involves a more distributed network of neuronal structures than the corresponding network mediating recognition of spatial locations[

Acknowled`ements*Parts of these results have been presented at the 3th Annual Meeting of the Cognitive Neuroscience Society in Boston in March 0886[ I wish to thank Heike Bothel

A[ Mecklinger:Recognition memory for object forms and spatial locations for data collection as well as Erdmut Pfeifer for his valuable support in software production[ The helpful comments of A[ D[ Friederici and P[ Gorrell on earlier versions of this manu! script are greatly acknowledged[

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