The Connectional Anatomy of Visual Mental Imagery: Evidence From a Patient with Left Occipito-Temporal Damage

Abstract

Most of us can use their “mind’s eye” to mentally visualize things that are not in their direct line of sight, an ability known as visual mental imagery. Extensive left temporal damage can impair patients’ visual mental imagery experience, but the critical locus of lesion is unknown. Our recent meta-analysis of 27 fMRI studies of visual mental imagery highlighted a well-delimited region in the left lateral midfusiform gyrus, which was consistently activated during visual mental imagery, and which we called the Fusiform Imagery Node (FIN). Here we describe the connectional anatomy of FIN in neurotypical participants and in RDS, a right-handed patient with an extensive occipitotemporal stroke in the left hemisphere. The stroke provoked right homonymous hemianopia, alexia without agraphia, and color anomia. Despite these deficits, RDS had normal subjective experience of visual mental imagery and reasonably preserved behavioral performance on tests of visual mental imagery of object shape, object color, letters, faces, and spatial relationships. We found that the FIN was spared by the lesion. We then assessed the connectional anatomy of the FIN in the MNI space and in the patient’s native space, by visualizing the fibers of the inferior longitudinal fasciculus (ILF) and of the arcuate fasciculus (AF) passing through the FIN. In both spaces, the ILF connected the FIN with the anterior temporal lobe, and the AF linked it with frontal regions. Our evidence is consistent with the hypothesis that the FIN is a node of a brain network dedicated to voluntary visual mental imagery. The FIN could have a specific role in integrating high-level visual representations in domain-preferring regions in the ventral temporal lobe with elements of semantic knowledge stored in the anterior temporal lobe and in the language circuits.

Full text

The connectional anatomy of visual mental imagery:
evidence from a patient with left occipito-temporal
damage
Dounia Hajhajate1, Brigitte Kaufmann1, Jianghao Liu1, 2, Katarzyna Siuda-Krzywicka1, and Paolo
Bartolomeo1*
1. Sorbonne Université, Institut du Cerveau - Paris Brain Institute - ICM, Inserm, CNRS,
AP-HP, Hôpital de la Pitié-Salpêtrière, F-75013 Paris, France
2. Dassault Systèmes, France
*Corresponding author: paolo.bartolomeo@icm-institute.org ORCID 0000-0002-2640-6426
Keywords: Perception & imagery; patients; cerebrovascular; behavioral; lesion mapping; white
matter tractography.
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Abstract
Most of us can use their “mind’s eye” to mentally visualize things that are not in their direct line
of sight, an ability known as visual mental imagery. Extensive left temporal damage can impair
patients’ visual mental imagery experience, but the critical locus of lesion is unknown. Our
recent meta-analysis of 27 fMRI studies of visual mental imagery highlighted a well-delimited
region in the left lateral midfusiform gyrus, which was consistently activated during visual mental
imagery, and which we called the Fusiform Imagery Node (FIN). Here we describe the
connectional anatomy of FIN in neurotypical participants and in RDS, a right-handed patient
with an extensive occipitotemporal stroke in the left hemisphere. The stroke provoked right
homonymous hemianopia, alexia without agraphia, and color anomia. Despite these deficits,
RDS had normal subjective experience of visual mental imagery and reasonably preserved
behavioral performance on tests of visual mental imagery of object shape, object color, letters,
faces, and spatial relationships. We found that the FIN was spared by the lesion. We then
assessed the connectional anatomy of the FIN in the MNI space and in the patient’s native
space, by visualizing the fibers of the inferior longitudinal fasciculus (ILF) and of the arcuate
fasciculus (AF) passing through the FIN. In both spaces, the ILF connected the FIN with the
anterior temporal lobe, and the AF linked it with frontal regions. Our evidence is consistent with
the hypothesis that the FIN is a node of a brain network dedicated to voluntary visual mental
imagery. The FIN could have a specific role in integrating high-level visual representations in
domain-preferring regions in the ventral temporal lobe with elements of semantic knowledge
stored in the anterior temporal lobe and in the language circuits.
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Introduction
Visual mental imagery denotes our ability to use our “mind’s eye” to mentally visualize things
that are not in our direct line of sight. Brain-damaged patients with extensive left temporal
damage often have impaired visual mental imagery (Bartolomeo, 2002, 2008; Bartolomeo,
Hajhajate, Liu, & Spagna, 2020; Spagna, 2022), but the crucial lesion site in the temporal cortex
is unknown. Our recent meta-analysis of 27 fMRI studies of visual mental imagery (Spagna,
Hajhajate, Liu, & Bartolomeo, 2021) highlighted the importance of a well-delimited region within
the FG4 field (Lorenz et al., 2015) of the left midfusiform gyrus, that we labeled Fusiform
Imagery Node (FIN). This finding is consistent with the available literature on lesion
neuropsychology, neuroimaging, and direct cortical stimulation, which indicates the left inferior
temporal lobe as the region most commonly implicated in voluntary generated mental images
(Liu, Spagna, & Bartolomeo, 2021). The localization of the FIN in the left ventral temporal cortex
suggests a possible role as a bridge between domain-preferring visual regions (Mahon &
Caramazza, 2011) and amodal semantic networks (Fairhall & Caramazza, 2013; Lambon
Ralph, Jefferies, Patterson, & Rogers, 2017), perhaps including the language circuits (Bouhali et
al., 2014).
However, neuroimaging evidence such as that coming from the Spagna et al’s (2021)
meta-analysis is correlative, not causal. To establish a causal role of FIN in visual mental
imagery, the study of brain-damaged patients is mandatory. Here we describe patient RDS, a
58-year-old, right-handed patient who 7 years before testing had an extensive left
occipitotemporal stroke. The stroke provoked right homonymous hemianopia, alexia without
agraphia, and color anomia (Siuda-Krzywicka et al., 2019; Siuda-Krzywicka et al., 2020). These
deficits were stable at the time of the present testing. Despite his deficits, RDS had preserved
mental imagery introspection and behavioral performance. To estimate the importance of FIN
for visual mental imagery in this patient, we mapped it on the native space of his brain.
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Moreover, we assessed its connectional anatomy by mapping it and the lesion in the MNI and
native spaces; we then visualized the fibers of the two most important long-range white matter
tracts passing through the FIN, the inferior longitudinal fasciculus (ILF) and the arcuate
fasciculus (AF).
Methods
We used a French version (Santarpia et al., 2008) of the VVIQ questionnaire (Marks, 1973) to
assess RDS’s subjective vividness of visual mental imagery. In addition, the patient and a group
of 18 neurotypical participants, aged 22-48, performed a computerized version of the BIP -
Battérie Imagerie-Perception (Bourlon et al., 2009). The current version of the battery assesses
imagery of object shapes (Fig. 1A), object colors (Fig. 1B), faces (Fig. 1C), letters (Fig. 1D) and
spatial relationships on an imaginary map of France (Fig. 1E) (Bartolomeo, Bachoud-Lévi,
Azouvi, & Chokron, 2005; Bourlon, Oliviero, Wattiez, Pouget, & Bartolomeo, 2011; Bourlon,
Pradat-Diehl, Duret, Azouvi, & Bartolomeo, 2008). On each trial, participants hear a pair of
nouns followed by an adjective: e.g. “cherries.... strawberries... dark”. They are requested to
vividly imagine the items they hear. Immediately after, participants have to select the noun that
is best represented by the adjective: in the previous example, it is “cherries”, which are darker
than strawberries. Then they go on to indicate the overall vividness of their mental images on a
4-level Likert scale, pressing one of 4 buttons, where button 1 represents “no image at all” and
button 4 represents a “vivid and realistic image”. RDS performed a slightly more extended
version of the battery (18-20 items per imagery domain) than controls did (12 items per domain).
A perceptual control task (Fig. 3F) employed the same stimuli used for the imagery tasks,
except that stimuli were presented in an audio-visual format.
To compare RDS’s performance with controls’, we used the Singlims tool (Crawford &
Garthwaite, 2002). The program tests whether an individual's score is significantly different from
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a control sample and provides a point estimate of the abnormality of the scores. Here, 2-sided
p-values were considered as statistically significant at p < .05.
To assess the spatial relationships of the FIN with the patient’s lesion, we took the FIN
volume from our meta-analysis (Spagna et al., 2021) and overlapped it with the patient’s T1
images in the native space.
To identify the fibers that are connected to the FIN, the functional ROI derived from our
previous meta-analysis including 27 articles (Spagna et al., 2021) was used. We visualized the
fibers of two major white matter tracts passing through the FIN volume: the occipito-temporal
ILF (Catani, Jones, Donato, & ffytche, 2003), and the AF (Catani, Jones, & ffytche, 2005). Fiber
tracking was done on the group average template constructed from a total of 1065 subjects (Fig.
3A), that comes with DSIstudio (https://dsi-studio.labsolver.org/). A multishell diffusion scheme
was used, and the b-values were 990,1985 and 2980 s/mm2. The number of diffusion sampling
directions were 90, 90, and 90, respectively. The in-plane resolution was 1.25 mm. The slice
thickness was 1.25 mm. The diffusion weighted images were resampled at 2.0 mm isotropic.
The b-table was checked by an automatic quality control routine to ensure its accuracy
(Schilling et al., 2019). The diffusion data were reconstructed in the MNI space using q-space
diffeomorphic reconstruction (Yeh & Tseng, 2011) to obtain the spin distribution function (Yeh,
Wedeen, & Tseng, 2010). A diffusion sampling length ratio of 1.7 was used. The output
resolution of is 2 mm isotropic. The restricted diffusion was quantified using restricted diffusion
imaging (Yeh, Liu, Hitchens, & Wu, 2017). A deterministic fiber tracking algorithm (Yeh,
Verstynen, Wang, Fernández-Miranda, & Tseng, 2013) was used with augmented tracking
strategies (Yeh, 2020) to improve reproducibility.
To identify the fibers that are connected to the FIN, the functional ROI derived from a
meta-analysis including 27 articles was used (Spagna et al., 2021) (specification of FIN: MNI
coordinates -42, -54, 18). In order to grow this region into the white matter, a dilatation by 1 mm
was applied in DSIstudio.
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For fiber tracking, the anatomy prior of a tractography atlas (Yeh, Panesar et al. 2018)
was used to separately map “Inferior_Longitudinal_Fasciculus_L” and the
“Arcuate_Fasciculus_L” with a distance tolerance of 16 mm. A seeding region was placed at
“Inferior_Longitudinal_Fasciculus_L” and the FIN served as a ROI (Figure 3 in orange;
Supplementary Video 1). The same procedure was also used to identify fibers of the the
“Arcuate_Fasciculus_L” that are connected to the FIN (Figure 3 in blue; Supplementary Video
1). The anisotropy threshold, the angular threshold (between 15 to 90 degrees) and the step
size (from 0.5 voxel to 1.5 voxels) were randomly selected. Tracks with length shorter than
10mm or longer than 200 mm were discarded. For visualization purposes, a total of 500 tracts
were calculated. Topology-informed pruning (Yeh et al., 2019) was applied to the tractography
with 16 iterations to remove false connections.
To visualize the spatial relationships of the brain lesion with the FIN connections, the
patient’s structural MRI image was normalized into MNI space by using the normalization tool in
the Brain voyager software (https://www.brainvoyager.com/). The patient’s brain lesion was then
manually delineated by an experienced rater, by using the MRIcron software
(https://www.nitrc.org/projects/mricron/).
For fiber tracking in the patient’s native space (Figure 3B), a DTI diffusion scheme was
used, and a total of 60 diffusion sampling directions were acquired. The b-value was 1505
s/mm². The in-plane resolution was 2 mm. The slice thickness was 2 mm. The diffusion MRI
data were rotated to align with the AC-PC line. The tensor metrics were calculated using DWI
with b-value lower than 1750 s/mm². For fiber tracking the same procedure explained above
was used. To match the patient’s anatomy, the functional ROI of the FIN (Spagna et al., 2021)
was coregistered to the patient’s individual T1-weighted MRI using spm12 toolbox in Matlab.
The resulting neuroanatomical localization in the patient space was checked and corrected by
an experienced rater.
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To identify the cortical areas that connected by the fibers passing through the FIN, as
well as the the fibers that were damaged by the lesion, we used the connectivity matrix function
in DSI studio. For the HCP1065 template, several matrices representing the number of fibers
terminating within regions of the FreeSurferDKT_Cortical atlas were calculated. The connectivity
matrix was calculated by using the count of the connecting tracks restricted to the left
hemisphere. In creating the connectivity matrix the ROIs were specified as pass regions
(Ghulam-Jelani et al., 2021). In total, 4 connectivity matrices were generated; two fascicles (AF
or ILF) passing through two ROIs (FIN or Lesion).
Results
RDS reported vivid visual mental imagery overall (VVIQ score 77/80). For each domain of the
battery, the mean trial-by-trial vividness was 3.8/4 (object shape) 2.7/4 (object color), 3.7/4
(letters), 3.1/4 (faces), and 3.5/4 (map of France). His performance on the battery (Table 1)
revealed reasonably preserved abilities in all the tested imagery domains. No significant
difference emerged between RDS’s performance and controls’ (all⏐t⏐s < 0, all ps >.5). On the
perceptual control task, RDS’s performance was also at or near ceiling (range 95%-100%
correct).
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Table 1. Performance (number and percentage of correct responses) of RDS and controls on the visual mental imagery battery
Subject Object Shape Object Color Letters Faces Map of France
RDS 15/18 (83%) 16/20 (80%) 19/20 (95%) 14/20 (70%) 13/18 (72%)
Controls: mean 10.18/12 (85%)
9.19/12 (77%) 11.62/12 (97%)
10.13/12 (83%)
10.87/12 (91%)
Controls: S.D. 35.90% 24.47% 17.44% 36.40% 29.22%
Controls: range
75%-100% 58.33% - 100%
83.33%-100% 66.66%-100% 75%-100%
9/12-12/12 7/12-12/12 10/12-12/12 8/12-12/12 9/12-12/12
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T1-weighted MRI (Fig. 2) showed that RDS’s ischemic lesion encompassed the calcarine
sulcus, the lingual, fusiform, and parahippocampal gyri in the left hemisphere, as well as the
callosal splenium. In the fusiform gyrus, the lesion’s lateral border corresponded to the
midfusiform sulcus (see Fig. 2). Thus, the FG4 field, which lies lateral to the midfusiform sulcus
(Lorenz et al., 2015) and contains the FIN (Spagna et al., 2021), was entirely spared by the
lesion.
White matter tractography (Fig. 3, supplementary video) demonstrated intact fibers
passing through the FIN and belonging to two main systems: the ILF, linking the FIN to occipital
regions and to anterior temporal regions important for semantic knowledge (Lambon Ralph et
al., 2017), and the AF, connecting the FIN to perisylvian language circuits (Catani et al., 2005).
Most intact ILF fibers connected the regions labeled as left_fusiform and left_inferior_temporal
in the Desikan et al’s atlas (Desikan et al., 2006). Many ILF fibers leading to
left_lateral_occipital, left_lingual, left_pericalcarine, left_superior_temporal were instead
disconnected by the lesion. Concerning the AF, most intact fibers connected
left_inferior_temporal to left_pars_opercularis, left_caudal_middle_frontal, and
left_rostral_middle_frontal; fewer fibers connected left_inferior_temporal with
left_pars_triangularis and left_precentral. The lesion mainly disconnected fibers going from
left_inferior_temporal to left_caudal_middle_frontal.
Discussion
The occipito-temporal stroke suffered by RDS totally destroyed the portion of the left fusiform
gyrus situated medially to the midfusiform sulcus, but spared its lateral part, which contains the
FIN (Spagna et al., 2021) in cytoarchitectonic sector FG4 (Lorenz et al., 2015). Lesion
reconstruction and white matter tractography strongly suggest that the connectivity of the FIN
with the anterior temporal lobe and with language circuits was also spared by the lesion. RDS’s
intact introspection and reasonably preserved performance for visual mental imagery, together
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with the preservation of the FIN, is thus consistent with the hypothesis of a causal implication of
the FIN in this ability. Moreover, in this patient the FIN could not communicate with the left
primary visual cortex, which was totally destroyed by the lesion; as a consequence, our data
support the additional hypothesis that visual mental imagery engages the FIN in a top-down
fashion, perhaps through the ILF from the anterior temporal lobe important for semantic
processing (Lambon Ralph et al., 2017), and through the AF from prefrontal regions and
language circuits. The present results thus add to extensive neuropsychological evidence
(Bartolomeo et al., 2020; Liu et al., 2021; Spagna, 2022) against the dominant model of visual
mental imagery, which emphasizes the role of early visual cortex in this ability (Kosslyn et al.,
1999; Kosslyn, Thompson, Kim, & Alpert, 1995; Pearson, 2019). It is of note that, in the present
patient, splenial disconnection prevented any direct communication between the right, intact
visual cortex and the posterior part of the left hemisphere.
Our results nicely complement evidence coming from brain-damaged patients with
impaired visual mental imagery. For example, two patients found themselves unable to build
visual mental images after a closed head trauma (Moro, Berlucchi, Lerch, Tomaiuolo, & Aglioti,
2008). In both cases, the damage affected the left BA 37, and was likely to include the lateral
portion of the fusiform gyrus with the FIN. Traumatic brain injuries, like those suffered by Moro
et al.’s patients, typically provoke diffuse axonal injury, which is likely to disrupt white matter
connectivity in the large-scale brain networks supporting visual mental imagery (Bartolomeo,
2008; Mechelli, Price, Friston, & Ishai, 2004). Thus, FIN dysfunction leading to impaired visual
mental imagery should not be interpreted in a localist way, but as a source of perturbation of
large-scale brain networks (Bartolomeo, 2011). Another patient with left temporal damage had
impaired perception and imagery for orthographic material, but not for other domains
(Bartolomeo, Bachoud-Lévi, Chokron, & Degos, 2002). Damage or disconnection of domain-
preferring regions such as the visual word form area (Dehaene & Cohen, 2011) may account for
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such domain-selective deficits of visual mental imagery, as opposed to domain-general imagery
deficits which, in the present framework, may instead result from FIN dysfunction.
In apparent contradiction with the present evidence, a recent case report (Thorudottir et
al., 2020) described patient PL518, an architect who spontaneously complained to have
become unable to visualize items after a bilateral stroke in the territory of the posterior cerebral
artery. The lesion included the left medial fusiform gyrus, but spared its lateral portion. However,
the lesion did extend more laterally in the fusiform white matter than similar lesions in patients
without visual mental imagery deficits (see their Fig. 3A). Thus, disconnections within the
fusiform white matter might have contributed to FIN dysfunction and consequent visual mental
imagery impairment in PL518, perhaps in combination with the extensive accompanying lesions
in the right hemisphere, which might have deprived the left hemisphere of potential inter-
hemispheric compensation (Bartolomeo & Thiebaut de Schotten, 2016).
The present evidence suggests a crucial role for the FIN in visual mental imagery.
Specifically, the FIN might integrate, on the one side, elements of semantic knowledge stored in
the anterior temporal lobe (Lambon Ralph et al., 2017; Persichetti, Denning, Gotts, & Martin,
2021), and distributed linguistic representations (Popham et al., 2021), with, on the other side,
high-level visual representations in domain-preferring regions in the ventral temporal cortex
(Mahon & Caramazza, 2011). Following the lead of Heinrich Lissauer’s seminal ideas
(Bartolomeo, 2021; Lissauer, 1890; Lissauer & Jackson, 1988), we propose that dissociations in
performance between perceptual and imagery abilities may emerge when the FIN or other high-
level visual regions in the ventral temporal cortex are deafferented from perceptual input
processed in more posterior regions. Such posterior disconnections would result in impaired
perception with preserved imagery (Bartolomeo et al., 1998). The present evidence suggests
that, in these cases, visual mental imagery would be supported by the ILF and AF connections
to the FIN.
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A more direct test of this model would have required fMRI evidence of spared FIN
activity during visual mental imagery in our patient; unfortunately, however, when we attempted
such an experiment RDS had a panic reaction in the MRI machine, and subsequently declined
to participate in any further neuroimaging exams. As a consequence, the localization of the FIN
in the patient’s brain must be considered as a (likely) approximation, because it derives from our
meta-analysis of fMRI studies (Spagna et al., 2021). Finally, we note that although single patient
studies can achieve a level of detail unattainable in group studies, without extensive replication
one cannot always exclude the influence of idiosyncratic variations of the mind/brain
(Bartolomeo, Seidel Malkinson, & de Vito, 2017). This seems, however, unlikely for RDS,
because there are reasons to consider his premorbid neurocognitive profile as representative of
the general population (Siuda-Krzywicka et al., 2019). Accumulating evidence from in-depth
studies of other patients, with brain lesions inducing or not deficits of visual mental imagery, will
be important to confirm or refute the present model.
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Figure legends
Fig. 1. Examples of trials of the visual mental imagery battery performed by patient RDS. In
different trials, participants are invited to decide about (A) the overall shape of animals or
objects (round or long); (B) which fruit or vegetable is darker or lighter in color; (C) the general
facial shape of celebrities (round or oval); (D) which handwritten word contains at least one
ascender (t, l, d), or a descender (j, p, y); (E) which of 2 auditorily presented cities is right or left
or Paris in an imaginary map of France. The perceptual task (F) is similar to the imagery tasks,
except that stimuli are presented in an audio-visual format.
Fig. 2. T1-weighted MRI showing RDS’s lesion (yellow) in native space, as well as the FIN
location (orange).
Fig. 3. Connectional anatomy of the FIN (red), and its spatial relationship with a reconstruction
of the patient’s lesion (green). Fibers passing through the FIN are visualized from two major
tracts: the ILF (orange), which connects the FIN to occipital and temporal areas, and the AF
(blue). Panel A shows the fibertracking on the HCP1065.2mm template, Panel B shows the
fibertracking on the patient’s individual DTI, both with automated fiber tracking that come with
DSIstudio (Version 2021.12.03 by Yeh; http://dsi-studio.labsolver.org/). The supplementary
video shows a 3D visualization of the ILF fibers and the AF fibers passing through the FIN, and
their spatial relationship with the patient’s brain lesion.
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Fig. 1
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Fig. 2
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Fig. 3
See also the supplementary video at the following link: https://bit.ly/3JrFYse
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Statements and declarations
Funding
This work was supported by Agence Nationale de la Recherche through ANR-16-CE37-0005
and ANR-10-IAIHU-06 to PB, by the École des Neurosciences Paris Île de France to KSK, and
by Swiss National Science Foundation Grant No. P2BEP3_195283 to BK.
Competing interests
The authors declare they have no relevant financial or non-financial interests to disclose.
Author contributions
All authors contributed to the study conception and design. Material preparation and data
collection were performed by DH, JL, KSK. Analyses were performed by BK, DH and JL. The
first draft of the manuscript was written by PB and all authors commented on previous versions
of the manuscript. All authors read and approved the final manuscript.
Data availability
Data will be made available on reasonable request.
Ethics approval
All subjects gave written consent according to the Declaration of Helsinki. The study was
promoted by the Inserm (C13-41) and approved by the Ile-de-France I IRB committee.
Consent to participate
Informed consent was obtained from all individual participants included in the study.
Consent to publish
Consent to publish has been received from all participants.
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