RAPID COMMUNICATION

Domain-Specific Impairment of Source Memory Following a RightPosterior Medial Temporal Lobe Lesion

Jan Peters,1,2* Benno Koch,3 Michael Schwarz,3 and Irene Daum1,2

ABSTRACT: This single case analysis of memory performance in apatient with an ischemic lesion affecting posterior but not anterior rightmedial temporal lobe (MTL) indicates that source memory can be dis-rupted in a domain-specific manner. The patient showed normal recog-nition memory for gray-scale photos of objects (visual condition) andspoken words (auditory condition). While memory for visual source(texture/color of the background against which pictures appeared) waswithin the normal range, auditory source memory (male/female speakervoice) was at chance level, a performance pattern significantly differentfrom the control group. This dissociation is consistent with recent fMRIevidence of anterior/posterior MTL dissociations depending upon thenature of source information (visual texture/color vs. auditory speakervoice). The findings are in good agreement with the view of dissociablememory processing by the perirhinal cortex (anterior MTL) and parahip-pocampal cortex (posterior MTL), depending upon the neocortical inputthat these regions receive. VVC 2007 Wiley-Liss, Inc.

KEY WORDS: source memory; recollection; parahippocampal gyrus;hippocampus; auditory

INTRODUCTION

Damage to the medial temporal lobe (MTL) is known to result insevere memory deficits (Squire et al., 2004). Anatomically, MTL forms ahierarchical network (Lavenex and Amaral, 2000), with perirhinal andparahippocampal cortices constituting the input fields of MTL. Perirhinalcortex (anterior MTL) and parahippocampal cortex (posterior MTL) havedistinct patterns of reciprocal connections with the neocortex (Suzuki andAmaral, 1994; Lavenex et al., 2002): while perirhinal cortex is predomi-nantly connected to inferior-temporal regions of the ventral visual process-ing stream, parahippocampal cortex is strongly connected to regions ofthe dorsal visual stream, but also to superior temporal auditory association

cortices. In functional terms, these distinct patterns ofneocortical connections may serve as the substrates ofdistinct memory processes (Buffalo et al., 2006; Dava-chi, 2006). In monkeys, lesions to perirhinal corteximpair performance in delayed nonmatching to sampletasks, consistent with the anatomical position of peri-rhinal cortex in the ventral visual processing stream(Nemanic et al., 2004). Lesions to parahippocampalcortex, on the other hand, disrupt delayed nonmatch-ing to location tasks, consistent with the involvementof the parahippocampal cortex in the dorsal visualstream (Alvarado and Bachevalier, 2005). In the presentstudy, we employed a memory task using source infor-mation thought to be relayed to perirhinal cortex(background texture/color) and parahippocampal cor-tex (speaker voice).

In functional neuroimaging studies, posterior para-hippocampal cortex has been implicated in the proc-essing of spatial contextual information (Cansinoet al., 2002; Davachi et al., 2003; Kahn et al., 2004)and in spatial scene perception (Epstein and Kanw-isher, 1998). Perirhinal cortex, on the other hand, hasbeen reported to be involved in object and face per-ception (Murray and Richmond, 2001; Lee et al.,2005) and familiarity- and item-based recognition(Henson et al., 2003; Ranganath et al., 2004). Thesefindings are consistent with the idea of a close associa-tion between the functional role of anterior and poste-rior parahippocampal regions and their respectiveinterconnected neocortical regions.

In a recent event-related fMRI study, memory forvisual and auditory items (pictures of objects and spo-ken words) and their context (background againstwhich objects appeared and speaker voice) was foundto be clearly related to different MTL subregions(Peters et al., 2007a). Activity in posterior parahippo-campal regions was related to successful retrieval ofauditory but not visual source information, while peri-rhinal cortex showed the reverse pattern. This findingis consistent with anatomical reports of strong projec-tions between auditory association cortices and para-hippocampal cortex and projections between inferiortemporal cortices and perirhinal cortex (Suzuki andAmaral, 1994). In the present single case analysis, the

1 International Graduate School of Neuroscience, Ruhr-University ofBochum, Germany; 2Department of Neuropsychology, Institute of Cog-nitive Neuroscience, Ruhr-University of Bochum, Germany; 3Depart-ment of Neurology, Klinikum Dortmund, GermanyGrant sponsor: International Graduate School of Neuroscience, Ruhr-University of Bochum.*Correspondence to: Jan Peters, Department of Neuropsychology, Insti-tute of Cognitive Neuroscience, Ruhr-University of Bochum, Uni-versitatsstrabe 150, D-44780 Bochum, Germany.E-mail: jan.peters1@gmail.comAccepted for publication 16 March 2007DOI 10.1002/hipo.20297Published online 2 May 2007 in Wiley InterScience (www.interscience.wiley.com).

HIPPOCAMPUS 17:505–509 (2007)

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same source memory task was administered to a patient withan ischemic lesion limited to the right posterior parahippocam-pal gyrus, posterior hippocampus, and fusiform gyrus. Giventhe location of the lesion, the patient was expected to showimpaired auditory source memory in the presence of intact vis-ual texture/color source memory.

A deficit in auditory source memory rather than both sourceand item memory was expected, as the task involved incidentalencoding of source (see Detailed Methods), thus requiring par-ticipants to recollect perceptual rather than verbally recoded in-formation during retrieval. Given the previous finding of intactverbal recognition memory in the patient (Peters et al., 2007b),we expected that intact semantic and possibly phonologicalfunctions in the patient may suffice to support auditory itemmemory, whereas memory for speaker voice would be severelyreduced due to a disruption of posterior MTL connectionswith auditory association cortices.

The patient is a 39-yr-old right-handed woman who wasdiagnosed with an infarction of the right posterior cerebral ar-tery 4 months prior to participation. Structural magnetic reso-nance imaging (MRI) indicates that damage is limited to poste-rior hippocampus, parahippocampal gyrus, and fusiform gyrus(Peters et al., 2007b). A recent transversal scan illustrating theapproximate extent of her lesion in the anterior–posteriordimension is depicted in Figure 1, and neuropsychological datacan be found in the Detailed Methods section.

To keep the results of the single case study strictly compara-ble to the fMRI study, the identical source memory task wasadministered (see Detailed Methods). In short, the investigationconsisted of an encoding and a retrieval session separated by a10-min interval. During encoding, words spoken by a male orfemale speaker (auditory trials) and gray-scale photos of objectspresented against a background of lawn or clouds (visual trials)were presented and item-based encoding was emphasized. Dur-ing retrieval, old and new words and pictures were presented ina contextually neutral format: all words were spoken in a‘‘robot’’ voice and pictures were presented against a gray back-ground. Participants made old/new judgements and, followingold judgements, were prompted to indicate the speaker voice(male/female) or the background texture (lawn/clouds) of theoriginally presented item in a forced choice format.

Data from the auditory and the visual condition were ana-lyzed separately. Proportions of hits with and without correctsource judgements, misses, correct rejections, and false alarmswere determined (Table 1). To assess item memory, correctedrecognition scores [Pr 5 hitrate 2 false alarm rate] (Snodgrassand Corwin, 1988) were obtained, and source memory per-formance was assessed by comparing the percentage of hitsaccompanied by correct source judgements to the chance-levelperformance of 50% (Table 1). To compare the patient’s per-formance to the control group, we used (i) control-referencedz-scores and (ii) Crawford and Howell’s (1998) modified t testand the revised standardized difference test RSDT (Crawfordand Garthwaite, 2005) to test for a dissociation of auditory andvisual source memory in the patient (see Detailed Methods).

Item memory in terms of Pr (Table 1) was significantly abovechance in the control group, both for auditory (t(1,9) 5 7.135,

FIGURE 1. Transversal T2-weighted MR scan of the patient.Damage includes right posterior hippocampus, parahippocampalgyrus, and parts of the fusiform gyrus, while anterior MTL struc-tures (perirhinal/entorhinal cortex and anterior hippocampus) areunaffected.

TABLE 1.

Behavioral Measures for the Control Group (n 5 10) and the Patient, Separately for Auditory and Visual Trials

Old items New items Measures for statistical analysis

Hits with

source

Hits without

source Misses

Correct

rejections

False

alarms

Corrected

recognition (Pr)

Hits source

correct (%)

Auditory trials

Controls 0.41 (0.13) 0.17 (0.08) 0.42 (0.16) 0.81 (0.11) 0.19 (0.11) 0.39 (0.17) 70.23 (10.59)

Patient 0.42 0.47 0.11 0.43 0.57 0.31 47.19

Visual trials

Controls 0.46 (0.15) 0.27 (0.07) 0.25 (0.15) 0.95 (0.04) 0.05 (0.04) 0.69 (0.18) 61.74 (9.60)

Patient 0.63 0.25 0.12 0.85 0.15 0.73 71.59

Reponses to old and new items are reported as proportions of trials. All measures for the control group are reported as mean (SD).

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P < 0.001) and visual trials (t(1,9) 5 11.925, P < 0.001). Visualitem memory was significantly better than auditory item mem-ory (t(1,9) 5 27.282, P < 0.001). A reliable positive correlationbetween auditory and visual item memory performance wasobserved in the control group (r 5 0.732, P 5 0.016). In linewith previously reported recognition memory data (Peters et al.,2007b), item memory performance of the patient was unre-markable. Z-score analysis yielded scores of 20.47 for auditoryand 0.18 for visual item memory (see Fig. 2). Crawford andHowells test did not yield a deviation from the control group initem memory (auditory condition: t(1,9) 5 20.449, P > 0.3,visual condition: t(1,9) 5 0.170, P > 0.3). The RSDT indicatedthat the difference between auditory and visual item memoryobserved in the patient (0.31 vs. 0.73) was not significantly differ-ent from the performance pattern in the control group (t(1,9) 50.746, P> 0.3). The relatively poor item memory in the auditorycompared with the visual condition may be linked to the fact thatvisual trials benefit more from dual-coding than auditory trials(Paivio, 1991), thus supporting visual memory performance, andall photos in this study were easily verbalized.

Source memory in terms of the percentage of Hits with cor-rect source judgements (Table 1) significantly exceeded thechance level of 50% in the control group (auditory trials: t(1,9) 56.039, P < 0.001, visual trials: t(1,9) 5 3.867, P 5 0.004) andperformance was better in the auditory condition (t(1,9) 52.610, P 5 0.028). The correlation between the measure inboth modalities was positive but nonsignificant (r 5 0.485,P 5 0.155, see Fig. 3). In terms of z-scores (see Fig. 2), audi-tory source memory was significantly impaired in the patient(z 5 22.176, P 5 0.015) whereas visual source memory was

fully intact (z 5 1.026, P > 0.1). The clear discrepancy in per-formance between both modalities in the patient is illustratedin Figure 3. Crawford and Howells test also yielded intact vis-ual source memory (t(1,9) 5 0.978, P > 0.1) and significantlyimpaired auditory source memory (t(1,9) 5 22.074, P 50.034). The RSDT indicated that the performance differencebetween auditory and visual source memory was significantlylarger than the performance discrepancy observed in the controlgroup (t(1,9) 5 2.701, P 5 0.024, two-tailed), fulfilling the for-mal Crawford and Garthwaite (2005) criteria for a classicalneuropsychological dissociation, i.e., auditory source memory issignificantly reduced, visual source memory is intact, and theRSDT yields a significant discrepancy between tasks.

We additionally analyzed the performance difference betweenauditory and visual source memory by computing absolute dif-ferences in source accuracy scores. The controls mean absolutedifference was 10.87 points (standard deviation (SD) 5 7.39,range 0.83–24.59), whereas the patient showed an absolute dif-ference of 24.24 (z 5 1.81). The patient’s absolute differencescore was substantially greater (>8 points) than all but a singlecontrol subject’s score.

Could the observed impairment in the patient be attributedto an auditory perceptual rather than mnemonic deficit? Toensure that the source information was adequately processedduring encoding, participants were prompted to indicate thespeaker voice and the background texture of each presenteditem (see Detailed Methods). The patient correctly identifiedthe speaker voice in 100% of the auditory encoding trials andcorrectly identified the background texture in 100% of the vis-ual encoding trials. Critically, this shows that the observedimpairment is not merely the result of a perceptual impairmentin voice recognition, but clearly represents a mnemonic deficit.

FIGURE 2. Z-scores of the patient referenced to the controlgroup (n 5 10) for all experimental measures (see Table 1). Z-scores above/below the dotted lines (z 5 61.64) indicate a signifi-cant deviation from the control group (P < 0.05).

FIGURE 3. Auditory source memory performance in thepatient (grey dot) and controls (black dots) plotted against visualsource memory performance (i.e., percentages of Hits with correctsource judgements). The dotted lines indicate chance level per-formance (50%). The solid black line depicts a linear fit of thecontrol data (r 5 .485, P 5 0.155).

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The finding of slightly reduced auditory item memory in thepatient raises the possibility of a potential joint auditory itemand source memory deficit. The source memory task may havesimply been more difficult, thus revealing a larger deficit in thepatient. Although it is doubtful if auditory item and sourcememory can be considered different tasks as required by thetest, the RSDT was applied to the data to exploratively assessthe discrepancy between auditory item and auditory sourcememory in the patient. This revealed a trend towards a dissocia-tion in the patient (t(1,9) 5 21.329, P 5 0.11). As the size ofthe deviation from controls differed substantially between audi-tory item and source memory in absolute terms (z 5 20.471compared with z 5 22.176), these findings argue against a gen-eral impairment on the auditory task. Rather, the patient’s defi-cit appears to be confined to auditory perceptual informationand not extend to semantic and phonological informationwhich is sufficient to support old/new judgements. Previousfindings of intact verbal recognition memory and intact recollec-tion of intentionally encoded source information are consistentwith this interpretation (Peters et al., 2007b). The data thussuggest that the patient’s impairment is either attributable to aselective retrieval deficit for auditory perceptual information orto a deficit in binding auditory features during encoding.

Taken together, the findings of this single case support theidea that distinct regions of MTL support different types ofsource memory, depending on the type of neocortical inputthat they receive. Regions in the inferior temporal gyrus whichform the main input to the perirhinal cortex are known to con-tain neurons responsive to color, patterns, and color–patternconjunctions (Tanaka et al., 1991). Given that we used texture/color source information in the visual condition of our task, itis likely that this type of source is predominantly relayed to theperirhinal cortex. Human voice information, on the otherhand, is known to be processed in regions of the auditory asso-ciation cortex in the superior temporal gyrus (Belin et al.,2000). As such regions project predominantly to the parahip-pocampal cortex in the primate (Suzuki and Amaral, 1994), itis likely that voice information is predominantly relayed to theparahippocampal cortex. In the present case, intact perirhinalstructures may be sufficient to support the encoding and/orreconstruction of texture/color information whereas the corre-sponding mechanisms for the auditory modality may be dis-rupted as a result of damage to right parahippcampal cortex.

The current findings from a single case analysis add further evi-dence to the view that distinct mnemonic processes can be linkedto distinct regions of MTL (Davachi, 2006; Eichenbaum, 2006).Furthermore, the present results support the idea that MTL-neo-cortical connectivity patterns may form the basis for functionaldissociations in MTLwith respect to memory processing.

DETAILED METHODS

Subjects

General intelligence in the patient was estimated using thesubtests ‘‘similarities’’ and ‘‘picture completion’’ from a short

German version (Dahl, 1972) of the Wechsler Adult Intelli-gence Scale (Wechsler, 1981). The patient’s IQ estimate was inthe normal range (IQ 5 106). Short term and working mem-ory as assessed by the digit span and block span tasks from theWechsler Memory Scale (Wechsler, 1987) were unremarkable(digit span forward 5 8, backward 5 4, block span forward 57, backward 5 7).

The patient’s WMS-R (Wechsler, 1987) verbal memory scoreis in the normal range (score 5 89), although somewhat lowerthan expected on the basis of her IQ. Estimates of recollectionand familiarity in a word list-discrimination procedure basedon previously described a task (Yonelinas, 1994; Zoppelt et al.,2003) are in the normal range (Peters et al., 2007b). Consistentwith the right-sided location of the lesion, delayed recall of theRey complex figure was impaired (score 5 7/36) while copywas intact (score 5 35/36).

A group of n 5 10 subjects matched to the patient on age(mean age 5 38.20, SD 5 15.54, patient age 5 39 yr, z-score5 0.14), education (years of education mean 5 12.33, SD 51.37, patient years of education 5 13 yr, z-score 5 0.48), andhandedness was recruited by advertisement and served as con-trol group for the patient on the source memory task. Informedwritten consent was obtained from all subjects prior to theirparticipation and they were reimbursed for participation andtravel expenses.

Task

A set of 230 stimuli was used. Visual items were greyscalephotographs of common objects, whereas auditory items werethe corresponding spoken words. Spoken words were recordedfrom a male and a female speaker in stereo at a sampling rateof 44.1 kHz. Eighty items were randomly selected to serve asvisual targets and 80 items were randomly selected to serve asauditory targets. During encoding, 40 randomly selected visualtargets were presented against a flat ‘‘lawn’’ texture and 40 werepresented against a flat ‘‘clouds’’ texture. Forty randomlyselected spoken words were presented in a female voice duringencoding and the remaining 40 words were presented in a malevoice. Auditory and visual trials were randomly intermixed. Par-ticipants were instructed to memorize the spoken words andthe pictures for a later memory test. Item-based encoding wasemphasized. To ensure that participants adequately processedsource information during encoding, they were prompted toindicate the background texture (lawn/clouds) or the speakervoice (male/female) after each item presentation.

During recognition, all items were presented in a contextu-ally neutral format: pictures were presented against a gray back-ground and words were spoken in a neutral ‘‘robot’’ voice. Theneutral voice was constructed by frequency modulation of therecordings of a second male speaker. Of the 70 items not pre-sented during encoding, 35 were randomly chosen to serve asauditory distractors and 35 served as visual distractors. Thus,during recognition, 80 old items and 35 new items were pre-sented in each modality. Again, auditory and visual trials wererandomly intermixed. Participants made old/new judgements

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(the prompt was displayed for 2,900 ms) and, following oldjudgements, were promted to indicate the background texture(visual trials) or the speaker voice (auditory trials) of the itemduring encoding in a forced-choice manner (this prompt wasdisplayed for 3,000 ms). Subjects were encouraged to guess ifthey were unsure about the source. During encoding and recog-nition, pictures were presented for 2,300 ms and the spokenduration of all words ranged between 800 and 1,100 ms.

Data Analysis

In single-case studies, the performance of a patient is gener-ally compared with a control group in terms of z-scores refer-enced to the control group. This approach provides poor con-trol over the Type I error rate if control samples are small (i.e.,<50), as is typically the case in experimental single-case studies(Crawford and Garthwaite, 2005). A modified t test (Crawfordand Howell, 1998) has been shown to provide adequate controlover the Type I error rate even when control samples sizes aremodest in size (Crawford and Garthwaite, 2005). Thus, bothz-scores and results from Crawford and Howell’s test arereported for item and source memory measures. To assesswhether there was a significant performance difference betweenauditory and visual source memory in the patient, the revisedstandardized difference test (RSDT) described by Crawford andGarthwaite (2005) was used. Unlike classical statistical tests oftest performance differences such as the Payne and Jonesmethod (Payne and Jones, 1957), the RSDT controls for theType I error rate in case of small control samples, as shown byMonte Carlo simulations (Crawford and Garthwaite, 2005). Inaddition, the method takes the correlation between test scoresin the control group into account.

Acknowledgment

We thank Benjamin Brummerheinrich for help with testingcontrol subjects.

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