The evolution of whisker-mediated somatosensation in mammals: Sensory processing in barrelless S1 cortex of a marsupial, Monodelphis domestica.

Movable tactile sensors in the form of whiskers are present in most mammals, but sensory coding in the cortical whisker representation has been studied almost exclusively in mice and rats. Many species that possess whiskers lack the modular "barrel" organization found in the primary somatosensory cortex (S1) of mice and rats, but it is unclear how whisker-related input is represented in these species. We used single-unit extracellular recording techniques to characterize receptive fields and response properties in S1 of Monodelphis domestica (short-tailed opossum), a nocturnal, terrestrial marsupial that shared its last common ancestor with placental mammals over 160 million years ago. Short-tailed opossums lack barrels and septa in S1 but show active whisking behavior similar to that of mice and rats. Most neurons in short-tailed opossum S1 exhibited multiwhisker receptive fields, including a single best whisker (BW) and lower magnitude responses to the deflection of surrounding whiskers. Mean tuning width was similar to that reported for mice and rats. Both symmetrical and asymmetrical receptive fields were present. Neurons tuned to ventral whiskers tended to show broad tuning along the rostrocaudal axis. Thus, despite the absence of barrels, most receptive field properties were similar to those reported for mice and rats. However, unlike those species, S1 neuronal responses to BW and surround whisker deflection showed comparable latencies in short-tailed opossums. This dissimilarity suggests that some aspects of barrel cortex function may not generalize to tactile processing across mammalian species and may be related to differences in the architecture of the whisker-to-cortex pathway. J. Comp. Neurol. 524:3587-3613, 2016. © 2016 Wiley Periodicals, Inc.

Ramamurthy DL ，Krubitzer LA 《-》

Neural Coding of Whisker-Mediated Touch in Primary Somatosensory Cortex Is Altered Following Early Blindness.

Sensory systems do not develop and function independently of one another, yet they are typically studied in isolation. Effects of multisensory interactions on the developing neocortex can be revealed by altering the ratios of incoming sensory inputs associated with different modalities. We investigated neural responses in primary somatosensory cortex (S1) of short-tailed opossums (Monodelphis domestica; either sex) after the elimination of visual input through bilateral enucleation very early in development. To assess the influence of tactile experience after vision loss, we also examined naturally occurring patterns of exploratory behavior. In early blind (EB) animals, overall levels of tactile experience were similar to those of sighted controls (SC); locomotor activity was unimpaired and accompanied by whisking. Using extracellular single-unit recording techniques under anesthesia, we found that EB animals exhibited a reduction in the magnitude of neural responses to whisker stimuli in S1, coupled with spatial sharpening of receptive fields, in comparison to SC animals. These alterations manifested as two different effects on sensory processing in S1 of EB animals: the ability of neurons to detect single whisker stimulation was decreased, whereas their ability to discriminate between stimulation of neighboring whiskers was enhanced. The increased selectivity of S1 neurons in EB animals was reflected in improved population decoding performance for whisker stimulus position, particularly along the rostrocaudal axis of the snout, which aligns with the primary axis of natural whisker motion. These findings suggest that a functionally distinct form of somatosensory plasticity occurs when vision is lost early in development.SIGNIFICANCE STATEMENT After sensory loss, compensatory behavior mediated through the spared senses could be generated entirely through the recruitment of brain areas associated with the deprived sense. Alternatively, functional compensation in spared modalities may be achieved through a combination of plasticity in brain areas corresponding to both spared and deprived sensory modalities. Although activation of neurons in cortex associated with a deprived sense has been described frequently, it is unclear whether this is the only substrate available for compensation or if plasticity within cortical fields corresponding to spared modalities, particularly primary sensory cortices, may also contribute. Here, we demonstrate empirically that early loss of vision alters coding of sensory inputs in primary somatosensory cortex in a manner that supports enhanced tactile discrimination.

Ramamurthy DL ，Krubitzer LA 《-》

The evolution of active vibrissal sensing in mammals: evidence from vibrissal musculature and function in the marsupial opossum Monodelphis domestica.

Grant RA ，Haidarliu S ，Kennerley NJ ，Prescott TJ ... -  《-》

Spatial gradients and inhibitory summation in the rat whisker barrel system.

Brumberg JC ，Pinto DJ ，Simons DJ 《JOURNAL OF NEUROPHYSIOLOGY》

Structure, function, and cortical representation of the rat submandibular whisker trident.

Thé L ，Wallace ML ，Chen CH ，Chorev E ，Brecht M ... -  《-》