Secondary Motor Cortex: Where ‘Sensory’ Meets ‘Motor’ in the Rodent Frontal Cortex

Abstract

In rodents, the medial aspect of the secondary motor cortex (M2) is known by other names including medial agranular cortex, precentral cortex, and frontal orienting field. As a subdivision of the medial prefrontal cortex, M2 can be defined by a distinct set of afferent and efferent connections, microstimulation responses, and lesion outcomes. However, the behavioral role of M2 remains mysterious. Here, we focus on evidence from rodent studies, highlighting recent findings of early and context-dependent choice-related activity in M2 during voluntary behavior. Based on the current understanding, we suggest that a major function for M2 is to flexibly map such antecedent signals as sensory cues to motor actions, thereby enabling adaptive choice behavior.

Thalamic afferents to prefrontal cortices from ventral motor nuclei in decision‐making

“…For example, in the dorsolateral prefrontal cortex (Kim & Shadlen, 1999), supplementary motor cortex (Okano & Tanji, 1987) dentate deep cerebellar nucleus (Ohmae et al, 2017) and VL nucleus of the ventral motor thalamic nuclei (Tanaka, 2007). In rats, ramp-up responses have been observed in a few cases; for example, in the prefrontal cortex (Bregma anterior 2 mm, lateral 1.3 mm) in a memory guided orienting task (Erlich et al, 2011) and in the secondary motor cortex, in a delay-based decision-making task (Barthas & Kwan, 2017). In mice, reports have been more frequent than in rat studies thanks to headfixed recording procedures in awake animals performing whiskertactile behaviors designed by Guo et al (2014a).…”

Prefrontal connections of the perirhinal and postrhinal cortices in the rat

“…Given the substantial differences in anatomical connections, it should not be surprising that the PER and POR support different functions, or that the two regions interact to support some cognitive tasks [6]. Available evidence suggests that prefrontal cortical regions (PFC) are also involved in context-guided and novelty-guided behavior [reviewed in 7, 8–11]. Thus understanding the PFC connections of the PER and POR will facilitate studies of the functional circuitry underlying the use of context and novelty to guide appropriate behavior.…”

“…Interestingly, MOs in the rodent plays an important role in processing sensory information in order to guide appropriate goal-directed behavior [22]. This region has been studied under a variety of names, including secondary motor cortex (M2, AGm, and Fr2), shoulder cortex, and rat frontal eye fields [reviewed in 7]. Damaging MOs seems to disrupt motor responses driven by sensory input [23] and impairs motor learning, but not execution of motor responses [24].…”

Secondary Motor Cortex Transforms Spatial Information into Planned Action During Navigation

“…Previous work has considered a role for M2 in orienting decisions [39][40][41][42][43][44] . If the primary function of 30 this region lies in decision-making as opposed to action planning and execution, the ability to 31 discriminate left versus right turning action may disappear at locations where no left versus right 8 of 24 choice exists.…”

“…These two regions contain neurons representing information in multiple 7 egocentric and allocentric reference frames 9,[25][26][27][28][29][30][31][32][33] and boast dense projections to secondary 8 motor cortex (M2) [34][35][36] . In turn, M2 projects strongly to primary motor cortex and brainstem and 9 spinal regions involved in motor control 37,38 and could therefore be a structure important to the 10 transition of integrated spatial information into action as part of the larger navigational 11 process 35,39 . 12 M2 has been the subject of much experimental work aimed at determining critical 13 neurophysiological components of decision-making processes, rule implementation, and action 14 planning and execution as instructed by single modality sensory cues [40][41][42][43][44][45][46][47] .…”