Corticospinal Neurons in Macaque Ventral Premotor Cortex with Mirror Properties: A Potential Mechanism for Action Suppression?

Abstract

SummaryThe discovery of “mirror neurons” in area F5 of the ventral premotor cortex has prompted many theories as to their possible function. However, the identity of mirror neurons remains unknown. Here, we investigated whether identified pyramidal tract neurons (PTNs) in area F5 of two adult macaques exhibited “mirror-like” activity. About half of the 64 PTNs tested showed significant modulation of their activity while monkeys observed precision grip of an object carried out by an experimenter, with somewhat fewer showing modulation during precision grip without an object or grasping concealed from the monkey. Therefore, mirror-like activity can be transmitted directly to the spinal cord via PTNs. A novel finding is that many PTNs (17/64) showed complete suppression of discharge during action observation, while firing actively when the monkey grasped food rewards. We speculate that this suppression of PTN discharge might be involved in the inhibition of self-movement during action observation.

The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations

“…Mirror neurons do not fire in response to a simple presentation of objects, including food. Most of them do not respond or respond only weakly to the observation of the experimenter performing a motor act (for example, grasping) without a target object 7 .…”

Do mirror neurons really mirror and do they really code for action goals?

“…The remaining MNs respond to the observation and execution of dissimilar actions (e.g., to the sight of food being placed on a surface, but the act of grasping food to eat) and have been termed logically-related MNs. The existence of suppression MNs -units that fire during action performance, but are inhibited during the observation of similar actions (e.g., Kraskov et al, 2009) -further complicates matters. Rather than 'mirror' observed actions, such units appear to systematically prevent mirroring.…”

“…Moreover, examination of the early data reveals that 'pantomimed' actions (performed without objects) evoked responses in many MNs, albeit more weakly than object-directed actions (Gallese et al, 1996). Indeed, a more recent study found that 79% of MNs modulated their firing rate in response to pantomimed actions (Kraskov et al, 2009). …”

Sensorimotor learning and the ontogeny of the mirror neuron system

“…Here, for the sake of completeness, I will briefly mention other interesting response characteristics which have been observed. These include specificity for mouth actions [21], sensitivity to whether actions are performed close to or further away from the monkey [6], response suppression to observed actions [42], sensitivity to mimed grasps [42], viewdependence [5] and sensitivity to the grasped object's reward value [4].…”

“…Indeed, until recently, a clear difference between the macaque mirror neuron data and human data from TMS and neuroimaging experiments was that the former demonstrate mirror responses to goal-directed actions, while the latter show mirror responses to actions both with and without a goal. However, Ferrari et al [21] described mouth mirror neurons which responded to actions without a goal, while Kraskov et al [42] found mirror neurons which responded to mimed actions; thus the conclusion that mirror neurons only respond to goal-directed actions is no longer secure. The sensorimotor account of mirror neuron properties explains this aspect of the difference between monkey and human data by comparing their learning histories:…”