Using the common marmoset for neurophysiological studies of neocortical functions

The common marmoset is a small New World monkey highlighted as an experimental primate model species to which genetic engineering techniques have become applicable (Sasaki et al. 2009). This species could also have another advantage as an animal model for neurophysiological studies. In an article by Tia et al. (2017) in this issue of The Journal of Physiology, the authors implanted high density multichannel (64‐channel) electrocorticography (ECoG) array electrodes in the epidural space above the premotor, primary motor and somatosensory cortex of marmosets and recorded the local field potentials (LFPs) during a variety of reaching and grasping tasks. ECoG has advantages over the commonly used electroencephalography recorded outside the skull, because ECoG enables recording of the high frequency γ‐band signals which are known to carry important information about processing in the brain, but are difficult to record through the skull. In the marmoset, an ECoG array can directly cover all the above‐mentioned cortical motor areas because this species has a lissencephalic brain. In addition, it is possible to record high quality LFP signals from the epidural space in the marmoset because the dura is relatively thin, which makes the implantation surgery less invasive and reduces the possibility of infection and damage to the brain tissue after surgery. In macaque monkeys, which have been used more widely for the studies of higher brain functions, a large part of the above‐mentioned cortical motor areas are inside the central sulcus and difficult to access with ECoG. Because the dura is thick in this species, it is also necessary to implant the ECoG electrodes in the subdural space to record the high quality signals. The marmoset lacks direct connections from the motor cortex to spinal motor neurons, which is different from humans and macaque monkeys, and also exhibits different grasping behaviours. Even so, the authors could clearly record the event‐related desynchronization (ERD) of β‐band activity and event‐related synchronization of γ‐band activity during reaching and grasping behaviours. Furthermore, the authors applied the phase‐slope index to analyse the signal flow between different cortical areas. Results showed that whole‐hand and scissor grips triggered stronger β‐band ERD than finger grip. Task epochs clearly modulated γ‐power, especially for finger and scissor grips. Comparison of the effective connectivity clarified that the finger and scissor grips evoked stronger outflow from the primary motor to premotor cortex, whereas whole‐hand grip displayed the opposite pattern. These findings suggest that fundamental control mechanisms of reach and grasp movements, that is, adjustments of cortical activity and connectivity, are conserved across primates. Because of direct accessibility to all these cortical areas, such analysis would be more straightforward in marmosets than in macaque monkeys.

Because epidural ECoG recording promises long‐term stability, novel ways of long‐term longitudinal follow‐up of the learning/developmental plasticity, ageing‐related changes and development of mental disorders will become possible in the future.

All these results suggest the usefulness of the common marmoset to study cortical network operation related to a variety of sensorimotor and cognitive functions and their pathophysiology, especially if in future combined with disease model animals genetically engineered for neuropsychiatric disorders.

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Competing interests

None declared.