Unraveling the Neurobiological Underpinnings of OCD

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TYPE

Original Research

PAGE NO.

165-173

DOI

10.37547/tajmspr/Volume07Issue03-17

OPEN ACCESS

SUBMITED

22 January 2025

ACCEPTED

25 February 2025

PUBLISHED

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VOLUME

Vol.07 Issue03 2025

CITATION

Shekoian Marianna. (2025). Unraveling the Neurobiological Underpinnings
of OCD. The American Journal of Medical Sciences and Pharmaceutical
Research, 165

–

173.

https://doi.org/10.37547/tajmspr/Volume07Issue03-17

COPYRIGHT

© 2025 Original content from this work may be used under the terms
of the creative commons attributes 4.0 License.

Unraveling the
Neurobiological
Underpinnings of OCD

Shekoian Marianna

Crescenta Valley High School Los Angeles, CA, USA

Abstract:

Obsessive

–

Compulsive Disorder (OCD) is a

heterogeneous psychiatric condition marked by
intrusive obsessions and ritualistic compulsions that
significantly impair functioning and quality of life.
Advances in neuroimaging have substantially clarified
its complex neurobiological basis. Structural findings
frequently demonstrate morphological alterations in
the orbitofrontal cortex, anterior cingulate cortex, and
striatum, linked to both gray and white matter
disruptions. Functional neuroimaging studies highlight
hyperactivity within cortico-striato-thalamo-cortical
(CSTC) loops and aberrant connectivity involving the
parietal cortex, limbic structures, and cerebellum. Task-
based paradigms underscore that different symptom
dimensions (e.g., contamination fears, checking,
hoarding) activate partially distinct yet overlapping
cortical

–

subcortical networks. Integrating these

structural and functional perspectives supports a
connectome-based framework, in which OCD emerges
from dysregulated interactions among diverse brain
systems involved in cognitive control, emotional
regulation, and habit learning. Emerging biomarkers

—

such as caudate volume, anterior cingulate
metabolites, and orbitofrontal connectivity

—

show

promise for predicting response to pharmacotherapy
and cognitive-behavioral therapy. Future investigations
may expand on these findings through larger
longitudinal cohorts, inclusion of pediatric populations,
and implementation of multi-omic approaches that
integrate genetic, epigenetic, and neuroimaging data.
This synthesis of current evidence underscores the
potential

for

refined

diagnostic

stratification,

personalized therapeutic interventions, and enhanced
monitoring of treatment efficacy.

Keywords:

Obsessive

–

Compulsive Disorder (OCD);

Cortico-Striato-Thalamo-Cortical Loop; Neuroimaging;
Structural and Functional Alterations; Connectome;
Biomarkers; Personalized Medicine.

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INTRODUCTION:

Obsessive

–

Compulsive Disorder (OCD) is characterized

by intrusive, recurrent thoughts (obsessions) and
repetitive, ritualized behaviors or mental acts
(compulsions), which individuals feel compelled to
perform in order to reduce anxiety or distress. These
symptoms can substantially interfere with daily
functioning, strain interpersonal relationships, and
negatively impact overall quality of life [1, 2].
Epidemiological studies estimate a lifetime prevalence
of OCD ranging from 1% to 3%, making it one of the
more common psychiatric disorders worldwide [2, 3].
Given its prevalence and the profound burden it
imposes, OCD has become an important subject for
translational research aimed at refining diagnostic
procedures and improving therapeutic outcomes [4].

The clinical presentation of OCD involves obsessions

—

unwanted, persistent thoughts, impulses, or images
that generate significant anxiety

—

and compulsions

—

ritualized behaviors or mental routines performed to
alleviate the distress produced by obsessions. Common
obsessional themes include contamination fears,
aggressive or sexual thoughts, a need for symmetry,
and excessive doubts about safety. Corresponding
compulsions often manifest in repetitive washing,
checking, counting, ordering, or seeking reassurance.
Severity can vary from mild to severe, with many
individuals experiencing substantial impairment in daily
activities, social interactions, and work performance
[1]. Globally, OCD affects millions of individuals across
diverse cultures [2, 3]. Its chronic nature, alongside
early onset, often translates into prolonged distress
and comorbidity with other psychiatric conditions,
thereby compounding functional disability [1].
Consequently,

understanding

its

underlying

neurobiology is vital not only for accurate diagnosis but
also for the development of novel, more effective
treatments that can be tailored to individual clinical
profiles [4, 5].

Exploring the neurobiological mechanisms of OCD has
emerged as a key frontier due to advances in
neuroimaging methodologies [4]. These approaches
offer insights into how structural and functional
alterations in specific circuits may underpin the
emergence and perpetuation of obsessive

–

compulsive

symptoms. Early and accurate identification of such
neurobiological markers has the potential to optimize
both preventive strategies and personalized treatment
approaches, potentially enhancing remission rates and
minimizing unwanted side effects [4, 6, 7]. As the
integration of neuroimaging data into clinical practice
continues to evolve, so does the potential for improved
patient outcomes.

Neurobiological inquiries into OCD can be traced back
to the 1980s, when pioneering studies utilized positron
emission tomography (PET) to detect metabolic
hyperactivity in frontal-striatal circuits [6, 8]. These
foundational investigations helped establish the now-
classic model involving cortico-striato-thalamo-cortical

(CSTC) circuits as a framework for understanding OCD’s

neurobiological underpinnings [4, 6]. Over subsequent
decades, research expanded to encompass various
neuroimaging modalities. Magnetic Resonance Imaging
(MRI) enabled detailed structural analyses, highlighting
gray and white matter abnormalities in cortical and
subcortical regions [9, 10]. Functional MRI (fMRI),
particularly in resting-state paradigms, shed light on
atypical connectivity patterns, while PET and Single
Photon Emission Computed Tomography (SPECT)
continued to reveal valuable data on cerebral
metabolism and neurotransmitter function [11, 12].
More recent methods such as diffusion tensor imaging
(DTI) and magnetic resonance spectroscopy (MRS) have
further enriched understanding of white matter
integrity and metabolic deviations in brain networks
pivotal to obsessive

–

compulsive symptoms [4, 13].

These technological strides, supported by improved
computational and analytic techniques, underscore
that multiple neurocircuits beyond the fronto-striatal

loop contribute to OCD’s heterogeneous presentations

[4, 9, 14].

Against this background, the principal aim of this review
is to synthesize up-to-date findings regarding structural
and

functional

aberrations

underlying

OCD,

emphasizing how various neuroimaging methods
converge to elucidate the pathophysiology of the
disorder [4, 10]. A related goal involves illustrating how
evidence from structural and functional perspectives
intersects, with particular attention paid to cortico-
striato-thalamo-cortical

pathways

and

broader

networks. Finally, this integration of data paves the way
for clinical applications, from facilitating early detection
and guiding individualized interventions to identifying
neurobiological markers that might predict treatment
responses [4, 8, 11]. By disentangling the
neurobiological foundation of OCD, the discussion aims
to advance the drive toward precision medicine while
pointing to forthcoming interdisciplinary investigations
that bridge fundamental neuroscience and clinical
psychiatry.

1.Structural alterations in OCD: a neuroanatomical
perspective

Obsessive

–

compulsive disorder (OCD) has repeatedly

been linked to morphological changes in the
orbitofrontal cortex (OFC), anterior cingulate cortex
(ACC), and the basal ganglia (especially the caudate

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nucleus and putamen). Voxel-based morphometry
(VBM) meta-analyses highlight decreased gray matter
volumes in orbitofrontal and cingulate areas, along with
variable findings in the striatum, where volumetric
increases or relative preservation have been reported
[4, 13, 15]. Surface-based morphometry (SBM) builds
upon these insights by revealing cortical thinning in
frontal, parietal, and temporal cortices, while some
deep gray matter structures may exhibit volumetric
enlargement [16, 17].

Several factors can moderate these structural findings.
First, age of onset may shape the extent of
frontostriatal abnormalities, with early-onset OCD
sometimes showing more pronounced differences [9].
Second, medication status appears critical: samples
comprising medication-naïve or drug-free patients
have documented marked frontal and striatal
alterations, whereas chronically treated individuals
may display comparatively attenuated changes [15,
18]. Third, psychiatric comorbidities, including
depressive or anxiety disorders, potentially influence
gray matter volumes by introducing overlapping
neurobiological alterations [9].

White matter integrity also emerges as a key

component of the disorder’s

neuroanatomical profile.

Diffusion tensor imaging (DTI) studies show
microstructural disruptions in the genu of the corpus
callosum, the cingulum bundle, and the superior

longitudinal fasciculus, which connect frontal, striatal,
and limbic structures [4, 19, 20]. These changes often
correlate with scores on the Yale-Brown Obsessive
Compulsive Scale (Y-BOCS), suggesting that severe
disruptions in frontostriatal and frontal-limbic
connections

may

underlie

more

pronounced

obsessive

–

compulsive symptoms [21]. Furthermore,

partial normalization of white matter abnormalities has
been reported following pharmacological or cognitive-
behavioral therapy, underscoring the dynamic nature
of these neural pathways [19, 22].

Magnetic resonance spectroscopy (MRS) complements
the above measures by quantifying neurometabolites
that reflect neuronal viability (N-acetylaspartate, NAA),
astroglial function (myo-inositol), and excitatory
neurotransmission (glutamate/glutamine). Multiple
studies describe reduced NAA/creatine (Cr) ratios in the
OFC, ACC, and striatum, implying compromised
neuronal integrity in these regions [4, 23, 24].
Conversely, investigations have also reported lower
glutamate or glutamate

–

glutamine levels in the frontal

cortex and thalamus, indicative of disturbed excitatory
balance [25]. The magnitude of these metabolic
abnormalities frequently shows positive or negative
relationships with Y-BOCS severity, suggesting that
neuronal health and excitatory

–

inhibitory balance may

both contribute to symptom intensity [26, 27].

Table 1. Structural findings in OCD: key regions, representative results, and clinical correlations

Region or Tract

Representative Finding

Association with Y-
BOCS

Key Studies

Orbitofrontal Cortex
(OFC)

Reduced

gray

matter

volume

and

cortical

thickness in symptomatic
patients

Greater

volume

reductions

often

correlate with symptom
severity

[4, 13]

Anterior

Cingulate

Cortex (ACC)

Volumetric reductions and
lower NAA levels; altered
WM connectivity

Metabolite levels and
white matter integrity
may track severity

[15, 24]

Caudate Nucleus

Increased

or

decreased

volume; lower NAA/Cr in
unmedicated samples

Mixed results; some
findings show volume
changes linked to Y-
BOCS

[17, 23]

Corpus

Callosum

(Genu)

Reduced

fractional

anisotropy

indicating

disrupted interhemispheric

Pronounced

deficits

often map onto worse
obsessions/compulsions

[20, 22]

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Region or Tract

Representative Finding

Association with Y-
BOCS

Key Studies

connectivity

Cingulum Bundle

Lower

fractional

anisotropy, particularly in
anterior sections

Changes

in

tract

integrity

frequently

correlate with symptom
load

[19, 21]

Glutamate/Glutamine
(MRS)

Decreased concentrations in
medial frontal and thalamic
regions

May

reflect

dysregulated excitatory
neurotransmission

[4, 25]

N-Acetylaspartate
(MRS)

Reduced levels in the OFC,
ACC,

and

striatum,

suggesting

impaired

neuronal health

Lower NAA typically
corresponds to greater
clinical severity

[23, 24]

Although the precise direction and extent of structural
changes can vary, the collective data underscore the
involvement

of

cortico-striato-thalamo-cortical

networks in OCD. Disrupted white matter tracts appear
to exacerbate communication deficits, while metabolic
anomalies

in

frontal

–

subcortical

circuits

may

potentiate specific clinical manifestations. These
multilayered findings lay the groundwork for
subsequent functional imaging analyses, which further
clarify how structural alterations intersect with
dynamic brain activity and symptom expression.

2. Functional alterations and the network perspective

Research employing resting-state functional MRI
(fMRI), positron emission tomography (PET), and
single-photon emission computed tomography (SPECT)
has consistently linked obsessive

–

compulsive disorder

(OCD) to hyperactivity within the cortico-striato-
thalamo-cortical (CSTC) loop. Studies using PET often
detect excessive glucose metabolism in the
orbitofrontal cortex (OFC), anterior cingulate cortex
(ACC), and striatum, findings that correlate with
symptom severity and sometimes normalize following
successful therapy [4, 6, 11]. Resting-state fMRI adds to
this knowledge by revealing elevated functional
connectivity between OFC, ventral striatum, and
thalamus, alongside aberrant activity in related regions
such as the parietal cortex, amygdala, hippocampus,
and cerebellum [28, 29]. This broader network
perspective suggests that OCD symptoms may reflect
maladaptive interactions across diverse circuits
implicated in emotion processing, habit formation, and
cognitive control [9, 14]. Patterns of hyperactivation or
hypoactivation in these regions often correlate with the

severity

of

obsessive

–

compulsive

symptoms,

reinforcing

the

view

that

pathophysiological

disruptions extend beyond a single frontostriatal loop
[12, 30].

Task-based fMRI studies, including paradigms such as
Stroop tasks, N-back working memory, Tower of
London, and exposure to symptom-provocative stimuli,
indicate that specific obsessions and compulsions
activate distinct but overlapping regions. Washers
commonly exhibit pronounced medial prefrontal and
striatal reactivity when confronted with contamination-
related stimuli, whereas checkers display stronger
dorsal frontal and subcortical engagement in response
to potential threat cues [31, 32]. Hoarding behaviors
elicit unique neural activity in orbitofrontal and
sensorimotor areas, while ordering or symmetry
obsessions often involve heightened activation in
premotor

and

parietal

regions

[9,

31].

Pharmacotherapy with selective serotonin reuptake
inhibitors (SSRIs) or psychotherapeutic interventions,
such as cognitive-behavioral therapy (CBT), can
modulate these neural response patterns. For instance,
successful SSRI or CBT treatment has been associated
with reduced OFC and caudate hyperactivation,
decreases in ACC overactivity, and partial normalization
of frontoparietal connectivity [4, 11, 19, 33].

Although fMRI, PET, and SPECT remain primary
methods to investigate functional alterations, near-
infrared spectroscopy (fNIRS) constitutes a promising
adjunct, particularly for task-based assessments. fNIRS
gauges oxygenated hemoglobin fluctuations to infer
localized neuronal activity during cognitive or
behavioral tasks [34]. Portable fNIRS setups allow for

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ecologically valid testing scenarios and potentially more
accessible longitudinal studies, although they often
have shallower penetration depth relative to fMRI and
thus provide a narrower field of view [35]. In OCD, initial
fNIRS data suggest atypical lateral prefrontal activation
during tasks taxing executive control, but a limited

number of studies precludes definitive conclusions [4,
36]. Future research could capitalize on fNIRS
portability to broaden participant recruitment (e.g.,
child/adolescent populations) and include real-time
biofeedback paradigms.

Table 2. Overview of functional neuroimaging modalities in OCD: main findings and target brain regions

Method

Key Findings

Relevant Brain

Regions

References

Resting-

state fMRI

Heightened connectivity within CSTC

loops;

network-level

dysregulations

involving parietal, limbic, and cerebellar

areas

OFC, striatum, ACC,

parietal

cortex,

cerebellum

[4, 28, 29]

PET/SPECT Elevated metabolic activity correlating with

symptom severity; partial normalization

post-therapy

OFC,

ACC,

basal

ganglia, thalamus

[6, 11, 30]

Task-based

fMRI

Distinct

patterns

linked

to

specific

obsessions/compulsions;

therapeutic

interventions alter hyper/hypoactivation

Dorsal/ventral

PFC,

striatum,

parietal

cortex, limbic system

[19, 31, 32]

fNIRS

Emerging modality for cortical activation

measurements in more flexible settings;

limited data in OCD

Primarily lateral PFC

(due to measurement

constraints)

[4, 35, 36]

In summary, functional neuroimaging highlights that
OCD is driven by abnormal interactions within and
beyond the classic orbitofronto-striatal circuitry. The
involvement of parietal regions, limbic structures, and
the cerebellum, along with evidence for treatment-
induced neural plasticity, underscores a network
approach to understanding obsessive

–

compulsive

symptoms.

Continued

advances

in

imaging

technologies, including fNIRS, promise to refine these
insights by enabling a broader range of experimental
designs, ultimately guiding more precise therapeutic
strategies.

3.Integration of data: clinical and therapeutic
implications

Findings from structural imaging, such as gray matter
abnormalities in orbitofrontal and cingulate cortices,
and

functional

investigations,

including

hyperconnectivity within the cortico-striato-thalamo-
cortical (CSTC) loop, collectively point to a
multidimensional pathophysiology of obsessive

–

compulsive disorder (OCD). Morphological alterations
in key regions appear tightly interlinked with aberrant
functional connectivity patterns, implying that
disrupted structural networks may predispose
individuals to the dysregulated activation dynamics
observed in functional imaging studies [4, 9]. This
convergence underlines the utility of a connectome-
oriented perspective, wherein OCD is interpreted as a
network-level disturbance rather than an isolated fault

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in any single cortical or subcortical node [14]. By
examining how interactions among multiple systems

—

encompassing executive control, emotional processing,
habit formation, and sensorimotor coordination

—

converge to drive compulsive behaviors, researchers
can better elucidate the heterogeneity in symptom
profiles and treatment responses [16].

Neuroimaging biomarkers increasingly serve as
predictors of treatment outcomes. Studies suggest that
variations in caudate volume, anterior cingulate
metabolism, and orbitofrontal connectivity may
forecast the efficacy of selective serotonin reuptake
inhibitors (SSRIs) and cognitive-behavioral therapy
(CBT), potentially aiding in tailoring personalized
interventions [11, 12]. Some evidence indicates that
reduced metabolic activity or normalization of
hyperactivity in these regions correlates with better
clinical improvement, while persistent hyperactivation
may predict poorer treatment response [30]. Shifts in
neurometabolites, such as elevated N-acetylaspartate
in the anterior cingulate cortex or altered glutamate
levels in the striatum, also show promise as quantifiable
indices of therapeutic success [24, 27]. Such imaging-

based markers could be integrated into longitudinal
monitoring protocols, allowing clinicians to refine
pharmacological dosages or modify psychotherapeutic
strategies in a more targeted, data-driven manner [4].

Future work will benefit from larger, longitudinal
cohorts that capture the developmental trajectory of
OCD and its clinical subtypes. Incorporating pediatric,
adolescent, and adult samples is crucial for resolving
how neurobiological processes evolve over time [9].
Equally important is leveraging multi-omic approaches
that blend imaging data with genetic, epigenetic, and
transcriptomic profiles to enable the creation of
precision medicine frameworks [5]. By identifying
specific molecular and neural circuit signatures,
interventions can be designed with heightened
specificity, potentially reducing refractory cases and
enhancing remission rates [4]. Although substantial
methodological challenges persist

—

such as managing

the high dimensionality of integrated datasets

—

advances in computational modeling and machine
learning offer robust strategies for extracting clinically
relevant biomarkers from complex neurobiological
information [14].

Table 3. Potential imaging biomarkers and clinical relevance: implications for personalized treatment

Domain

Potential

Biomarker

Clinical Relevance

Key Evidence

Structural
Imaging

Caudate

nucleus

volume, gray matter
in OFC/ACC

Larger or more preserved caudate
volume and reduced orbitofrontal
abnormalities can predict better SSRI
response

[4, 11]

Functional
Connectivity

Hyperactivation

of

CSTC loops, resting-
state

DMN

disruptions

Normalization

of

frontostriatal

activity correlates with symptomatic
improvements

[14, 30]

Metabolite
Profiles
(MRS)

N-acetylaspartate,
glutamate in ACC or
striatum

Changes in neuronal health or
excitatory balance may serve as
dynamic markers of treatment efficacy

[24, 27]

Integrated
Multi-omics

Genetic or epigenetic
markers

combined

with

imaging

measures

Aids in personalized interventions by
linking molecular profiles to specific
neurobiological alterations

[5]

Longitudinal
Neuroimaging

Repeated

MRI,

fMRI,

MRS

assessments

across

treatment course

Captures brain plasticity over time,
helping to adjust interventions and
predict remission or relapse

[4, 16]

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Integrating data from diverse imaging modalities thus

enables a more comprehensive understanding of OCD’s

pathophysiology. The connectome framework situates
local structural or functional abnormalities within large-
scale networks, offering richer insights into how
complex symptom domains emerge. Ongoing research
aimed at validating neuroimaging-based biomarkers
and forging multi-omic strategies will likely advance the
efficacy and precision of OCD treatments, shaping
future directions in both clinical care and translational
neuroscience.

CONCLUSION

The collected div of neuroimaging research affirms
that OCD cannot be reduced to isolated frontal or
subcortical anomalies; rather, it involves an intricate
interplay of multiple neural circuits. Structural findings
indicate volumetric and microstructural changes in
frontal, cingulate, and striatal regions, which appear
interdependent with altered functional connectivity
patterns observed in resting-state and task-based
paradigms. This shift toward a network-centric
understanding allows for more nuanced interpretations
of how different obsessional themes and compulsive
rituals emerge and persist.

Clinically, the search for reliable neuroimaging
biomarkers has begun to yield tangible results,
including

correlations

between

specific

brain

alterations and therapeutic outcomes. Identifying
factors such as caudate volume or metabolic shifts in
the anterior cingulate cortex can potentially guide
clinicians in predicting medication response or tailoring
psychotherapeutic protocols. Incorporating repeated
neuroimaging assessments during the course of
treatment also opens avenues for personalized
management, where interventions may be adapted
based on objective neurobiological indicators of
change.

Nonetheless, bridging these insights to routine clinical
practice demands further large-scale, longitudinal
investigations that account for various OCD subtypes
and comorbid conditions. Multi-omic approaches,
uniting imaging, genetic, and epigenetic data, promise

to refine our grasp of the disorder’s pathophysiology

and enhance therapeutic precision. By harnessing these
integrative strategies, the field moves closer to an era
of truly individualized OCD care, in which treatment
decisions and prognostic estimations can be anchored
in robust neurobiological and molecular evidence.

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