The brain is remarkable for its plasticity, the ability to change and adapt to environmental, behavioral, and metabolic conditions. However, the most studied forms of plasticity occur at very short timescales while behavioral adaptations to the environment occur over months, seasons, and years. 

Seasonal behavioral patterns such as hibernation, migration, and mating are well known, but their effects on brain activity and plasticity are poorly understood. Some small mammals, such as shrews, have developed an extreme adaptation to changing environmental conditions and food availability - they dramatically shrink their organs, including the brain, in winter. 

Researchers from the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS), in collaboration with colleagues from Germany and Israel, have now revealed the seasonal changes in the brain of the smallest terrestrial mammal, the Etruscan shrew. Their study was published in PNAS on Dec. 15.

Using magnetic resonance imaging and microstructural analysis of brain sections, the researchers found that seasonal volume changes in the cerebral cortex were correlated with changes in the thickness of the cortex, in particular in layer 4, the major target for sensory input to the cortex. 

Etruscan shrews mainly rely on their sense of touch mediated by long facial hairs - whiskers - to identify prey and navigate their environment. Thus, using in-vivo two-photon calcium imaging, the researchers focused on recording activity in the somatosensory cortex, which is critical for processing tactile stimuli.

They found that in summer few neurons were activated by touch and many were suppressed, while in winter more neurons were activated by touch and fewer were suppressed, suggestive of a lower threshold for detecting sensory stimuli in winter. The seasonal differences in neural responses were also most pronounced in layer 4, indicating that coordinated changes in anatomy and physiology contribute to long-term brain plasticity. 

While the brain volume changes studied by the researchers were subtler than those observed in wild animals, detailed studies of brain plasticity in other species may reveal that seasonal changes in brain structure and function were more widespread than currently known.

More generally, long-term changes in cortical structure are frequently observed in the human brain during learning or disease processes but remain poorly understood at the level of neural activity.