Dissecting medial temporal lobe contributions to item and associative
Shaozheng Qina,b,d,⁎, Mark Rijpkemaa, Indira Tendolkarc, Carinne Piekemaa, Erno J. Hermansa,
Marek Bindere, Karl Magnus Peterssona, Jing Luod, Guillén Fernándeza,b
aDonders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, 6500 HB Nijmegen, The Netherlands
bDepartment of Neurology, Radboud University Nijmegen Medical Center, 6500 HB Nijmegen, The Netherlands
cDepartment of Psychiatry, Radboud University Nijmegen Medical Center, 6500 HB Nijmegen, The Netherlands
dKey Laboratory of Mental Health, Institute of Psychology, Chinese Academy of Sciences (CAS), 100101 Beijing, China
ePsychophysiology Unit, Jagiellonian University, Cracow, Poland
a b s t r a c t a r t i c l ei n f o
Received 10 October 2008
Revised 15 January 2009
Accepted 23 February 2009
Available online 9 March 2009
A fundamental and intensively discussed question is whether medial temporal lobe (MTL) processes that
lead to non-associative item memories differ in their anatomical substrate from processes underlying
associative memory formation. Using event-related functional magnetic resonance imaging, we implemen-
ted a novel design to dissociate brain activity related to item and associative memory formation not only by
subsequent memory performance and anatomy but also in time, because the two constituents of each pair to
be memorized were presented sequentially with an intra-pair delay of several seconds. Furthermore, the
design enabled us to reduce potential differences in memory strength between item and associative memory
by increasing task difficulty in the item recognition memory test. Confidence ratings for correct item
recognition for both constituents did not differ between trials in which only item memory was correct and
trials in which item and associative memory were correct. Specific subsequent memory analyses for item and
associative memory formation revealed brain activity that appears selectively related to item memory
formation in the posterior inferior temporal, posterior parahippocampal, and perirhinal cortices. In contrast,
hippocampal and inferior prefrontal activity predicted successful retrieval of newly formed inter-item
associations. Our findings therefore suggest that different MTL subregions indeed play distinct roles in the
formation of item memory and inter-item associative memory as expected by several dual process models of
the MTL memory system.
© 2009 Elsevier Inc. All rights reserved.
The ability to remember single items and to bind different
aspects of an experience into a coherent episode are key features of
episodic memory (Tulving, 1983, 2002). However, a fundamental
and debated question is whether neural processes leading to a
memory for individual items (i.e. non-associative or item memory)
differ from processes underlying memory formation for associations
among disparate elements of an experience (i.e. associative or
One line of research suggests that distinct medial temporal lobe
(MTL) subregions with different architecture and connectivity like the
hippocampus and the parahippocampal gyrus, which is covered by
entorhinal, perirhinal, and parahippocampal cortices, support distinct
mnemonic operations along the dimension of non-associative item
and associative memory (Brown and Aggleton, 2001; Eichenbaum et
al., 1994, 2007; Yonelinas et al., 2002; Davachi, 2006). These two-
component models distinguish between the hippocampal contribu-
tions critically important for associative memories and the perirhinal
contributions sufficient for item memory. More recently, Eichenbaum
et al. (2007) extended their model into a three-component model in
which the parahippocampal cortex contributes to associative memory
bycoding contextual informationwhile the perirhinal cortex supports
single item memory and the hippocampus supports associative
memory by binding item(s) with relevant contextual information.
Several lines of evidence support such a division of labor especially
during memory retrieval, because item recognition memory is
correlated with activation reductions in the perirhinal cortex while
associative retrieval or recollection is correlated with a hippocampal
activity increase (Weis et al., 2004; Gonsalves et al., 2005; Henson,
2005; Montaldi et al., 2006; Tendolkar et al., 2008; for reviews see
Diana et al., 2007; Eichenbaum et al., 2007).
Contrary to this divided labor account, another line of research
suggests that MTL subregions work together in a cooperative and
complementary way and thus differences found between hippocampal
NeuroImage 46 (2009) 874–881
⁎ Corresponding author. Centre for Cognitive Neuroimaging, Donders Institute for
Brain, Cognition and Behaviour, Radboud University Nijmegen, P.O. Box 9101, 6500 HB
Nijmegen, The Netherlands. Fax: +31 24 36 10989.
E-mail address: email@example.com (S. Qin).
1053-8119/$ – see front matter © 2009 Elsevier Inc. All rights reserved.
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/ynimg
and parahippocampal contributions are rather related to memory
strength instead of item and associative memories (Squire et al., 2007).
In such a unitary MTL model, the hippocampus at the top of the
hierarchy might be associated with stronger memories than the
parahippocampal region, which is subordinate and thus associated
with weaker memories. Several lines of evidence support this memory
strength scenario, especially during memory formation (Rutishauser et
al., 2006; Tendolkar et al., 2007), because the parahippocampal region
in particular the perirhinal cortex may support weaker memories for
both items and associations. For example, activity in the hippocampus
and the perirhinal cortex correlates with subsequent memory strength
(Shrager et al., 2008), and the perirhinal cortex appears also involved
in associative memory formation (Jackson and Schacter, 2004; Kirwan
and Stark, 2004; Uncapher et al., 2006; Staresina and Davachi, 2006,
Taken together, there are conflicting findings regarding the
question whether hippocampal and parahippocampal contributions
to memory formation are distinct between processes for item and
associative memories or not. To tackle this issue, we developed a new
design for our event-related fMRI study of memory formation
mitigating two central problems. Firstly, to get the best possible
separation of brain activity related to item and associative memory
formation, we aimed at separating these processes not only
behaviorally and anatomically, but also temporally. Therefore, we
presented items sequentially with an intra-pair delay of several
seconds and participants could not form task relevant associations
when encoding the first event, because the second one was not yet
known to them. Secondly, differences in memory strength between
the item and the associative memory formation conditions may have
been reduced. To do so, we increased the difficulty of the item
recognition memory test by pairing each item to be memorized with
an itemof the samesemantic categoryas a lure in the itemrecognition
memory test. Furthermore, encoding trails (inter-item associations)
whose constituents were recognized with similar levels of confidence
(i.e. memory strength) in the item recognition memory test, were
further sorted into the unsuccessful (i.e. item memory) and the
successful associative memory formation conditions, according to
memory performance of the final associative recognition memory
test. If we find similar item- and association-related subsequent
memory effects in the hippocampus and the perirhinal cortex, our
data would be in line with a unitary MTL model. However, if we find
different subsequent memory effects despite similar memory con-
fidence, our data would rather support a process dissociation in line
with the two- or three-component models of the MTL mentioned
Materials and methods
Twenty-four young healthy university students (mean age=23.8±
3.5 years; 8 males) without any neurological or psychiatric history and
normal/corrected-to-normal vision participated in this experiment.
Participants gavewritteninformed consentaccordingtothelocalethics
committee and the declaration of Helsinki. We excluded behavioral and
fMRI data from five participants: Four participants showed poor
memory performance (mean correct rate b0.54 in either the item or
the associative recognition memory test) and one participant caused
excessive movement artifacts (larger than 6 mm).
We selected 824 color photographs of different objects depicted on
a black background and covering a wide range of semantic categories
from a commercially available database (Hemera Photo-Objects;
http://www.hemera.com/). This stimulus set contained two exem-
plars of each object category (e.g. two hammers, two dogs etc.). One
exemplar of each category was used as a study item and the other one
as the lure for item recognition memory test. The assignment to the
study and the lure set was balanced across participants. For each set,
we created 200 pairs of semantically unrelated objects for the study
phase, againbalancedacross participants,and sixadditionalpairsfora
pre-scan training session. In a separate pilot study, all pairs were
screened bythree additionalparticipantstoavoidsemanticallyrelated
objects being paired together. The 34 pairs in which at least one
subject indicated initially a semantic relationship were modified so
that no semantic relationship was detected in a re-test
The experiment consisted of a paired-associate learning phase and
two subsequent memory tests. During the study phase, fMRI scans
were acquired while participants memorized 200 pairs of unrelated
objects. Each trial started with the first object (‘Event-1’) presented at
the center of the screen for 2 s, followed by a variable delay period
the second object (‘Event-2’) presented at the center of the screen for
2 s (see Fig. 1). Trials were separated by an inter-trial-interval (ITI)
ranging from 6 to 10 s (in steps of 1 s; average 8 s). Participants were
explicitly instructed to memorize objects and object-pairs for two
subsequent memory tests.To makesurethateachpair wasperceivedas
Fig.1. Experimental design. Each trial consisted of two unrelated objects presented sequentially during the paired-associate learning task inside the scanner. The first object (Event-
1) was presented for 2 s, followed byavariable delay period (Delay:6–10 s), and the second object (Event-2)was presentedagain for 2 s. About 3 h afterthis study phase, participants
performed item and associative recognition memory tests outside the scanner (see Task procedure for more details).
S. Qin et al. / NeuroImage 46 (2009) 874–881
such within a continuous stream of sequentially presented objects, we
implemented two features: (1) The first object of each pair was
presented together with an open bracket on its left hand side and the
second one with a closed bracket on its right hand side; (2) a fixation
grouped dots were presented during the intra-pair delay period. Thirty
null events of 6 s duration and showing a central fixation cross were
randomly inserted between trials. The whole study phase was divided
into two runs. Prior to scanning, participants performed additional six
trials in order to familiarize with task procedures.
About 3 h after the study phase, participants were asked to perform
two memory tests outside the scanner. First, an item recognition
memory test was given with 400 previously studied objects (all items
as lures. Participants were asked to judge whether each object was an
scale (1 = absolutely sure it was a new object; 2 = somewhat sure it
an old object; 5 = somewhat sure it was an old object; 6 = absolutely
sure it was an old object). This task was self-paced with a minimal
presentation time of 2 s and a maximal one of 6 s. Objects presented as
thefirsteventduringthestudyphasethatreceiveda6 or5 ratinginthe
item recognition memory test served as cue-objects for a subsequent
associative recognition memory test. In thistest, two-alternative forced
choice paradigm (2-AFC) was used. That is, each cue-object was shown
together with two studied objects presented as a second event during
the study phase (see Fig. 1). One of the two objects was previously
object and served as a lure. This approach may bear a potential risk of
reducing the power to detect associative memory formation, because
appears more likely that most decisions are made on the basis of two
actually present items instead of one absent and one present item and
study session as well as in the single item recognition memory test to
ensure that the decision about the correct association was not based
counterbalanced tothe rightand left side (see Fig.1). This task wasalso
8 s. Participants were asked to indicate by an appropriate button press
which object had been associated with the cue-object during the study
associative recognition memory test was limited to 100 trials, because
half of the second event objects served as lures. However, all
participants had more than 100 item hits related to the first event
with a ‘5’ or ‘6’ rating in the item recognition memory test and thus we
selected randomly 100 objects for the associative memory test out of
these hit trials. To exclude trials potentially contaminated by guessing,
participants were instructed to give an unsure response if they did not
remember which association was the correct one.
fMRI data acquisition
During MRI scanning, whole brain T2⁎-weighted EPI-BOLD fMRI
data were acquired with a Siemens Sonata 1.5 T MR-scanner using an
ascending slice acquisition sequence (35 axial slices, volume
TR=2.75 s, TE=40 ms, 90° flip-angle, slice-matrix size=64×64,
a T1-weighted MP-RAGE sequence (volume TR=2250 ms,
TE=3.93 ms, 15° flip-angle, 176 sagittal slices, slice-matrix
size=256×256, slice thickness=1 mm, field of view=256 mm).
fMRI data analysis
Image pre-processing and statistical analysis was performed using
SPM5 (www.fil.ion.ucl.ac.uk/spm). The first five volumes of each
participant's EPI-datawere discarded toallow for T1equilibration.The
functional EPI-BOLD contrast images were realigned and the mean of
functional images was coregistered to the structural MR image using
mutual information optimization. Subsequently, functional images
were slice-time corrected, spatially normalized, resampled to create
3 mm isotropic voxels and transformed into a common stereotactic
space, as defined by the SPM5 MNI T1 template, as well as spatially
filtered by convolving the functional images with an isotropic 3D
Gaussian kernel (8 mm FWHM).
Encoding trials were sorted into several categories (see Table 1)
based on subsequent memory performance. Transient neural activity
associated with successful formation of associative and item memory
is at issue. Therefore, we created separate regressors to assess the
activity of the three temporal components in each trial (Event-1,
Delay, Event-2)as a functionof subsequent memory performance, and
convolved these with the canonical hemodynamic response function.
The onset and offsetof the delay period vector were spaced apart from
the first and the second event vectors by 2 s to increase the
orthogonality of the regressors (Murray and Ranganath, 2007;
Hannula and Ranganath, 2008). This approach resulted in an averaged
co-linearity estimate of all pairs of regressors of less than 0.32.
In relation to the item recognition memory test, encoding trials in
which both the first event and the second event received a ‘1’, ‘2’ or ‘3’
the two events received a ‘4’ rating were regarded as ‘unsure’, and
encoding trials in which the first event received a ‘5’ or ‘6’ rating were
regarded as ‘Event-1 hits’ (see Behavioral results). Trials in which the
first event was remembered were subdivided into the following cate-
gories depending on associative recognition memory test: (1) Single
incorrectly associated with the second event; (2) Associations remem-
bered or successful associative memory formation (‘association’): Both
events and their association were correctly recognized; (3) Trials in
Averaged number of trials on three conditions of interest.
Mean (SEM)34.47 (1.85)21.26 (1.64)40.21 (3.15)
Note. ‘Forgotten’: Neither the first event nor the second event was later remembered in
the item recognition memory test; ‘Item’: Both events or items were recognized, but
incorrectly associated with the second event; ‘Association’: Both events and their
association were correctly recognized.
Fig. 2. Behavioral performance in item recognition memory test. Distributions of mean hit
and false alarm rates: Mean (SEM) proportions of responses are shown on the y-axis and
confidence ratings (‘1’: absolutely new; ‘6’: absolutely old) are depicted on the x-axis.
S. Qin et al. / NeuroImage 46 (2009) 874–881
whichtheassociationwas‘remembered’ correctly,but the secondevent
was not recognized were put into a separate bin, because theyare likely
contaminated by guessing and different levels of memory strength for
the two constituents of an association. Also, all remaining trials were
included into a condition ‘of no interest’. In addition, the realignment
parameters were included to account for movement-related variability,
and the low-level null events were modeled separately. The data was
analyzed using the general linear model and statistical parametric
intensity normalization, and serial correlations correction using auto-
regressive AR(1) model.
The relevant contrast parameter images generated at the single
subject level were submitted tothe second-level group analysis. Atthe
group level, the relevant contrasts between conditions were com-
puted using a two (onsets: Event-1, Event-2) by three (Memory:
Forgotten, Item, Association) analysis of variance (ANOVA). In the
whole brain analysis, results from the random effects analyses were
initially thresholded at pb0.001 (uncorrected), and the cluster size
statistics were used as the test statistic. Unless otherwise specified,
only clusters significant at pb0.05 (corrected for multiple non-
independent comparisons; Worsley et al.,1996) are reported together
with the MNI coordinates of their local maximum. Given our clear
hypothesis regarding the MTL, across both hemispheres two separate
anatomical regions of interest (ROI) were created one for the
hippocampus and the other for the PHG. For this purpose, individual,
hand-drawn anatomical ROIs (i.e. on the basis of a standardized
protocol; Pruessner et al., 2000, 2002) were normalized and averaged
and then used as search volume for the small volume correction
(SVC). Furthermore, to characterize the response patterns in brain
regions involved in item and associative memory formation, the beta
values separated for each conditionwere extracted from these regions
using MarsBar (http://marsbar.sourceforge.net/; Brett et al., 2002).
The distribution of averaged response proportions in the item
recognition memory test (mean±SEM, collapsed across the first and
Brain activations derived from subsequent memory analyses.
Brain region BACluster
Event-1: Item vs. forgotten
Posterior inferior temporal cortex
Posterior fusiform gyrus
Posterior parahippocampal cortex
Event-2: Association vs. item
Inferior prefrontal cortex
Note. A threshold pb0.001 (uncorrected) was used in the whole brain volume search
and only clusters with 6 or more significant voxels are reported.
Event-1, time-locked to the onset of the first event; Event-2, time-locked to the onset of
the first event; L, left; R, right; BA, Brodmann area; MNI, MNI coordinates (SPM5).
aCluster level pb0.01 the whole brain volume corrected.
bCluster level pb0.05 small volume correction (SVC) corrected for the MTL ROIs.
cVoxel level pb0.001 the whole brain volume uncorrected.
Fig. 3. Brain regions involved in item memory formation. Activation clusters (pb0.001 uncorrected) superimposed on averaged (n=19) high-resolution T1-weighted images with
coronal orientation, (A) Activation in the right posterior inferior temporal cortex; (B) Activation in the right posterior parahippocampal cortex; (C) Activations in the bilateral
perirhinal cortex. Bars in the graphs represent the parameter estimates (arbitrary units) of the three conditions of interests. The data for these graphs were only extracted for
illustrative purposes and not for testing effects statistically. Notes: R, right.
S. Qin et al. / NeuroImage 46 (2009) 874–881
second event) are shown in Fig. 2. A two (stimulus type: old vs. new)
by six (confidence rating: 6-point scale) repeated-measure ANOVA
revealed an interaction between confidence rating and stimulus type
(F(5, 90)= 59.92, pb0.001). Further paired-sample t tests showed that
the proportion of ‘6’ and ‘5’ ratings was significantly higher for old than
new items (minimum t(18)=4.07, pb0.001). The proportion of ‘4’
ratings did not differ between old and new items (t(18)=−0.69,
p=0.49). The proportion of ‘1’, ‘2’ and ‘3’ ratings for old items was
lower than for new ones (minimum t(18)=−4.48, pb0.001). These
results demonstrate successful discrimination between studied and
unstudied items at all confidence levels except level 4. Associations
were clearly retrieved above chance level (mean proportion of hits:
0.67; SEM: 0.08; t(18)=8.63; pb0.001), indicating successful memory
formation for inter-item associations. Moreover, performance level for
associative memory formation (mean of discriminability d′=0.68;
SEM: 0.07) and only item memory formation (mean of discriminability
d′=0.86; SEM: 0.12) appears similar (t(18)=−1.60,p=0.13), indicating
a similar level of discriminability. Furthermore, when focusing on ‘5’ and
‘6’ confidence ratings, as done for the fMRI analysis, we observed no
difference in the proportion of ‘6’ ratings in the ’item’ and the
‘association’ bins (mean proportion±SEM: 0.86±0.04 vs. 0.90±0.04
respectively; t(18)=1.33, p=0.20), indicating again similar confidence
levels in the two conditions of interest. However, the statistical
comparison of d′ might be problematic, because we used two different
memory tests to measure item and associative memory. Thus, the
estimates of d′ may have different variances.
To reveal brain activity related to item memory formation, we
analyzed the data time-locked to the onset of the first event and
contrasted trials that were related to a subsequent success in
recognizing the item but failure to retrieve the association (i.e.
‘item’) with trials related to subsequent failure to remember both
the item and the association (i.e. ‘forgotten’). We found activated
areas in the right fusiform gyrus potentially covered by parahip-
pocampal cortex (local maximum at [33, −42, −21]; cluster
pb0.05 SVC), in the right posterior inferior temporal cortex (local
maximum [48, −66, −9]; cluster pb0.05 corrected), and in the
anterior parahippocampal gyrus bilaterally, presumably covered by
perirhinal cortex (local maxima at [−39, −18, −21], [42, −15,
−27]; with a less stringent threshold of pb0.001 uncorrected) (see
Table 2; Fig. 3).
To identify brain regions related to successful associative
memory formation, we analyzed the data time-locked to the onset
of the second event and contrasted trials related to subsequent
associative retrieval success (i.e. ‘association’) with trials related to
subsequent success to recognize the item only (i.e. ‘item’). In the
MTL, we revealed an activation in the left hippocampus (local
maximum at [−33, −24, −21]; pb0.05 SVC). When masking this
result with the hippocampal ROI, based on a normalized and
averaged volume of individual hippocampal masks, 11 out of 15
voxels remained, confirming that this cluster clearly lies within the
hippocampus. Outside the MTL, we found an activation linked to
successful associative memory formation in the left inferior
prefrontal cortex (local maxima at [−48, 30, −6]), but with a less
stringent threshold of pb0.001, uncorrected (see Table 2; Fig. 4).
Given potential differences in brain activity between trials with
confidence ratings of 5 and 6, we conducted an additional analysis
focusing on trials with a confidence rating of 6. Although the
number of trials appears insufficient for the ‘item’ bin, we still
identified very similar left hippocampal activation, when contrasting
Fig. 4. Brain regions involved in associative memory formation. Selected activation clusters (pb0.001 uncorrected) superimposed on averaged (n=19) high-resolution T1-weighted
images with coronal and sagittal orientations: (A) Activation in the left hippocampus; (B) Activation in the left ventral inferior prefrontal cortex. Bars in the graphs represent parameter
S. Qin et al. / NeuroImage 46 (2009) 874–881
associative with item memory formation, just at a less stringent
threshold (pb0.005 uncorrected).
Implementing a new task design in which processes underlying
the formation of item memory and associative memory were
separated in time and differences in memory strength between the
two types of memory formation were potentially reduced, we found
clear evidence for differences between hippocampal and parahippo-
campal contributions. While the hippocampal activity at encoding
was associated only with subsequent associative memory retrieval,
activity in the parahippocampal region predicted subsequent single
item recognition. This dissociation is in line with several previous
studies suggesting a two-component model of the MTL during
encoding (Davachi et al., 2003; Ranganath et al., 2003; Chua et al.,
2007; for reviews see Eichenbaum et al., 1994; Davachi, 2006). Given
that subsequent confidence was similar for the two constituents in
item and associative memories, differences in memory strength
cannot readily explain the dissociation found. Thus, our data appear
not supportive for a unitary MTL model proposed by Squire et al.
(2007) in which different subcomponents act together in a comple-
mentary way while supporting both item and associative memory
Our findings do indeed suggest that the MTL subregions play
distinct roles in itemand associative memory formation. The posterior
inferior temporal cortex (including posterior fusiform gyrus) may
initially contribute to the perceptual analysis of individual items (or
objects) during encoding (Owen et al., 1996; Wagner et al., 1998;
Brewer et al., 1998; Kao et al., 2005; Summerfield et al., 2006).
Thereafter, a neocortical processing stream converges in the para-
hippocampal region where perceptual representations are unitized
and thus can be transformed into meaningful representations for
further usage (such as object identities and semantic meanings to
facilitate associative memory formation) in the rhinal cortex (Nobre
and McCarthy, 1995; Fernández et al., 2001; Levy et al., 2005),
indicating that it is important for item memory formation (Davachi et
al., 2003; Ranganath et al., 2003; Kao et al., 2005; Chua et al., 2007).
This processing stream enables a-contextual encoding(suchas object-
related prototypical context) (Bar, 2004) and rapid familiarity
detection when an item is already known by the very same process
(Fernández and Tendolkar, 2006). Such a gradient accounts for the
inferior temporal to perirhinal processing is consistent with some
hierarchical models, which are mainly derived from studies in non-
human primates, patients with intracranial event-related potential
recordings as well as from neuroimaging studies in humans,
suggesting that neocortical input of visual object features is
sequentially processed in a posterior to anterior stream in order to
transform the representational information from a perceptual level
into a semantic one (Mishkin et al., 1983; Tanaka et al., 1991;
Fernándezet al.,2001; Vinckieret al.,2007). Whenthe secondevent is
perceived, relevant representational information converges further
into the hippocampus, allowing representations of the first and the
second event to be successfully bound together into a newassociation
(Qin et al., 2007; Cheng et al., 2008; for review see Eichenbaum,
2004). The hippocampus is thought to be an optimal site to process
these converging streams from the perirhinal cortex and the
parahippocampal cortex that convey detailed information about
item features and integrative information about the relevant context
(Diana et al., 2007; Eichenbaum et al., 2007).
However, our data does not entirely exclude a single-process
model for the MTL memory system which posits that recognition is
based on a uni-dimensional value of memory strength. Despite
similar performance levels, our contrast between item and associa-
tive memory might still be confounded by differences in memory
strength. It may appear conceivable that a larger number of features
were encoded for trials in which associative memory retrieval was
successful as compared to trials in which only single items were later
recognized. However, a monotonic increase in memory strength in
subsequently forgotten trials over subsequent item memory trials to
subsequent associative memory trials appears not in line with the
data extracted from those activation areas. Only if one assumes a
non-linear relationship between memory strength and the BOLD
signal as proposed previously (Squire et al., 2007), our data could
also be interpreted as being in line with a unitary MTL model.
However, several studies have shown linear functions between
neural activity and associated BOLD responses in a wide range of
regions, but non-linear components occur at the extremes of the
function and these non-linearities appear to be different in different
brain structures (Boynton et al. 1996; Soltysik et al. 2004). Although
there is no good reason to assume that different non-linear neural-
BOLD functions occur within the MTL during normal memory tasks,
there is to the best of our knowledge no study confirming the
linearity of the neural-BOLD coupling in the MTL. Hence, we cannot
exclude this alternative interpretation currently, but it appears not
The findings we obtained might be specifically depending on
which kind of associations were formed and tested here. There is
initial evidence that intra- and inter-item associations are supported
by different MTL subregions (Mayes et al., 2004). The formation of
intra-item associations appears to be related to processing in the
parahippocampal region (Staresina and Davachi, 2006; Tendolkar et
al., 2007), while inter-item associations are formed in the hippo-
campus (Mayes et al., 2004). The associations implemented here
were created by arbitrary pairings of semantically (and percep-
tually) unrelated objects and thus, they can be clearly regarded as
inter-item associations. Hence, our conclusion about the specific
role of the hippocampus in associative memory formation is most
likely limited to the formation of inter-item associations. Several
neuroimaging studies have found consistently that the hippocam-
pus was involved in storing new inter-item associations (such as
word-pairs, face-name pairs) (Henke et al., 1999; Sperling et al.,
2003; Kirwan and Stark, 2004; Prince et al., 2005; Chua et al., 2007;
Qin et al., 2007). This interp\retation is consistent with the idea
that the hippocampus is a general binding mechanism that
associates separate items together (that might not be integrated
and unified by the neocortex) to form a new association
(Eichenbaum et al., 1994; Mayes et al., 2004).
An alternative interpretation of our hippocampal finding is related
to the temporal delay between the two items to-be-associated. It is
well accepted that the hippocampus is critically engaged in
bridging discontinuities across time by its role in forming ‘context
units’ and ‘glueing’ together of sequential items (Wallenstein et al.,
1998; Eichenbaum, 2004). A recent neuroimaging study confirmed
that the human hippocampus is indeed involved in sequence
disambiguation by which overlapping sequences are kept separate
and remembered correctly (Kumaran and Maguire, 2006). How-
ever, the presentation order was irrelevant for our task and only
two items were paired together and thus our task can hardly be
regarded as a sequence learning task. Nevertheless, the hippocam-
pus was also found to be involved in ‘discontinuity association’ by
bridging a delay between the constituents of word-pairs in
anticipation of a semantic relatedness judgment (Luo and Niki,
2005), but this finding was not related to memory formation.
Therefore, our present findings add to the aforementioned findings
and suggest that the hippocampus is strongly involved in forming
new inter-item associations between items that occur separated in
In addition to the hippocampus, the inferior prefrontal cortex
was activated when associative memory formation was successful.
The inferior prefrontal cortex may interact with the hippocampus
and thus facilitate the formation of new associations by its role in
S. Qin et al. / NeuroImage 46 (2009) 874–881
recovering semantic information (Wagner et al., 2001) and in
enabling semantic elaborative processes (Sperling et al., 2003;
Jackson and Schacter, 2004; Prince et al., 2005; Ranganath and
Blumenfeld, 2007). Nevertheless, this is not mutually exclusive from
the view that the inferior prefrontal cortex involves generally in
episodic memory formation including memories for items and
associations as shown by numerous functional imaging studies (see
reviews Fernández and Tendolkar, 2001; Wagner et al., 2001; Paller
and Wagner, 2002; Simons and Spiers, 2003; Ranganath and
While this single study cannot exclude all alternative explana-
tions for our findings like differences in strategic processing or
differences in test formats for item and associative recognition
memory our findings are most likely in line with dual process
models of the MTL in which the parahippocampal region has a
particular role in non-associative item memory and the hippocam-
pus in memory for newly formed inter-item associations. This
conclusion could be further strengthened by studies that confirm a
linear neural-BOLD function in the MTL and by studies that control
memory strength strictly quantitatively across item- and associative
We thank Dr. Atsuko Takashima and Dr. Jianhui Wu for helpful
discussions. We also thank all participants for their participation in
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