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The practical implications of this systematic review extend well beyond academic taxonomy. When researchers pool data from both hemispheres without reporting which hemisphere was manipulated, or without counterbalancing the side of intervention across animals, they may inadvertently dilute or bias their results. A lesion placed on the left hemisphere in every animal of a treatment group, for example, could interact with naturally occurring population-level biases in hemispheric organization, producing effects that appear treatment-related but are actually driven by lateralization. The authors’ three-category framework—intrinsic asymmetries, consequences of unilateral manipulations, and interhemispheric or bilateral responses—offers a practical checklist for experimental design. Studies falling into the second category should ideally randomize or counterbalance the side of intervention, while those in the third category should explicitly measure the untouched hemisphere to capture compensatory or propagating effects.

The epilepsy findings deserve particular attention because they illustrate how model choice shapes lateralized outcomes. In some paradigms, early seizure spread and network excitability are clearly lateralized, suggesting that baseline circuit asymmetries determine where pathological activity first takes hold or how rapidly it propagates. In other paradigms, responses appear more symmetric, which the review attributes partly to genetic background and the specific way seizures are induced. This heterogeneity is not a weakness of the models per se; rather, it reflects the genuine biological diversity of epilepsy syndromes, where focal onset in one hemisphere is clinically common but not universal. For preclinical researchers, the message is that lateralized seizure dynamics may be masked if recordings or histological analyses are restricted to a single hemisphere or averaged across both.

Stroke and Parkinson’s disease models show perhaps the most direct translational parallels. In human stroke, outcomes depend strongly on lesion side: language deficits follow left-hemisphere damage in most patients, while spatial neglect and emotional processing disturbances are more often associated with right-hemisphere lesions. Rodent stroke models, typically produced by middle cerebral artery occlusion on one side, recapitulate some of this side-dependent variability in infarct volume, behavioral deficits, and recovery trajectories. Similarly, Parkinson’s disease frequently begins with unilateral motor symptoms, and rodent models using unilateral dopaminergic lesions—such as 6-hydroxydopamine injection—naturally produce asymmetric motor phenotypes. The review suggests that these unilateral designs, while experimentally convenient, may interact with intrinsic hemispheric differences in dopaminergic circuitry, meaning that the side of lesion could modulate both the severity of deficits and the response to experimental therapies.

Alzheimer’s disease models present a somewhat different picture. Because the pathology in transgenic models often develops bilaterally, lateralized effects are subtler and may emerge as differential regional vulnerability rather than overt side-dependent symptoms. Nonetheless, the review identifies studies in which amyloid deposition, tau pathology, or neuroinflammatory markers show hemispheric biases, raising the possibility that intrinsic asymmetries in connectivity or metabolism influence where degenerative changes accumulate first. This has clinical resonance, as imaging studies in patients sometimes reveal asymmetric atrophy patterns that correlate with the predominance of memory versus language or behavioral symptoms.

A recurring theme throughout the review is inter-individual variability in baseline lateralization. Just as handedness and hemispheric dominance vary across humans, rodents show individual differences in which hemisphere is dominant for particular behavioral or structural measures. If these baseline differences are not measured or accounted for, they contribute noise that reduces statistical power and may obscure genuine treatment effects. Conversely, if baseline lateralization is captured—through behavioral screening, imaging, or neurochemical assays performed before the experimental intervention—it could serve as a stratification variable, allowing researchers to identify subgroups of animals with different disease trajectories or treatment responses. This approach mirrors precision-medicine thinking in clinical neurology, where a patient’s baseline functional organization increasingly informs prognosis and rehabilitation planning.

The methodological critique embedded in the review is constructive rather than merely diagnostic. The authors note that hemisphere of manipulation is often not reported at all in the methods sections of published studies, making it impossible for readers to assess whether lateralized confounds might be present. Simple reporting standards—stating which hemisphere was targeted, how many animals received left versus right interventions, and whether hemisphere was included as a factor in statistical analyses—would cost little and could substantially improve reproducibility. The PRISMA-guided search strategy, which covered PubMed and multiple EBSCO databases through January 2026, was designed to capture this literature comprehensively, yet the fact that only 48 studies met inclusion criteria over decades of rodent neurological research underscores how rarely lateralization is treated as a primary experimental variable.

The neurochemical and metabolic dimensions of lateralized disease effects also merit emphasis. Several of the reviewed studies documented hemisphere-specific changes in neurotransmitter concentrations, receptor densities, or regional glucose metabolism following disease induction. These molecular asymmetries may precede behavioral manifestations, potentially serving as early biomarkers of pathology. In Parkinson’s models, for instance, asymmetric dopaminergic depletion in the striatum is well established, but the review highlights evidence that the degree of asymmetry itself—rather than absolute cell loss—may predict the magnitude of motor asymmetry. This reframing suggests that lateralization indices could be more sensitive outcome measures than pooled bilateral averages.

Interhemispheric communication emerges as a double-edged phenomenon in the reviewed literature. On one hand, callosal transfer allows the intact hemisphere to support function after unilateral injury, underpinning compensatory plasticity that is central to recovery. On the other hand, bilateral propagation of pathological activity—seen in seizure spread and potentially in the spread of protein aggregates in neurodegenerative models—means that the unaffected hemisphere is not truly spared. The review’s third category, interhemispheric or bilateral responses, captures this duality and encourages researchers to think of the two hemispheres as a dynamically coupled system rather than two independent structures. Techniques such as optogenetic inhibition of callosal projections, or longitudinal imaging of both hemispheres in the same animal, could disentangle protective from pathological interhemispheric influences.

Looking forward, the authors argue that incorporating hemispheric analysis systematically into experimental design would enhance the sensitivity, clarity, and clinical relevance of rodent models. This does not necessarily require new technology; in many cases it requires only that existing data, already collected from both hemispheres, be analyzed and reported with lateralization in mind. Where new data collection is warranted, balanced unilateral designs, pre-intervention lateralization assessments, and longitudinal within-animal comparisons would align preclinical practice more closely with the lateralized realities of human neurological disease. Given that translational failures remain common in neurology, attention to a fundamental organizational principle of the vertebrate brain—one that shapes processing efficiency, disease expression, and recovery alike—represents a relatively low-cost, high-potential avenue for improving the predictive value of animal research.

Subject of Research: Hemispheric asymmetries in rodent models of neurological diseases: a systematic review

Article Title: Hemispheric asymmetries in rodent models of neurological diseases: a systematic review

Article References: Schuessler, M., & Mundorf, A. (2026). Hemispheric asymmetries in rodent models of neurological diseases: a systematic review. Frontiers in Integrative Neuroscience, 20, Article 1843068. https://doi.org/10.3389/fnint.2026.1843068

Image Credits: AI Generated

DOI: 10.3389/fnint.2026.1843068

Keywords: hemispheric asymmetry, brain lateralization, rodent models, neurological disease, epilepsy, stroke, Alzheimer's disease, Parkinson's disease, traumatic brain injury, systematic review, preclinical research, 6-OHDA