The frontal eye field (FEF) is a critical region for the deployment of overt and covert spatial attention. Although investigations in the macaque continue to provide insight into the neural underpinnings of the FEF, due to its location within a sulcus, the macaque FEF is virtually inaccessible to electrophysiological techniques such as high-density and laminar recordings. With a largely lissencephalic cortex, the common marmoset (Callithrix jacchus) is a promising alternative primate model for studying FEF microcircuitry. Putative homologies have been established with the macaque FEF on the basis of cytoarchitecture and connectivity; however, physiological investigation in awake, behaving marmosets is necessary to physiologically locate this area. Here, we addressed this gap using intracortical microstimulation in a broad range of frontal cortical areas in three adult marmosets (two males, one female). We implanted marmosets with 96-channel Utah arrays and applied microstimulation trains while they freely viewed video clips. We evoked short-latency fixed vector saccades at low currents (<50 μA) in areas 45, 8aV, 8C, and 6DR. We observed a topography of saccade direction and amplitude consistent with findings in macaques and humans: small saccades in ventrolateral FEF and large saccades combined with contralateral neck and shoulder movements encoded in dorsomedial FEF. Our data provide compelling evidence supporting homology between marmoset and macaque FEF and suggest that the marmoset is a useful primate model for investigating FEF microcircuitry and its contributions to oculomotor and cognitive functions.
Visual extinction has been characterized by the failure to respond to a visual stimulus in the contralesional hemifield when presented simultaneously with an ipsilesional stimulus (Corbetta and Shulman, 2011). Unilateral damage to the macaque frontoparietal cortex commonly leads to deficits in contralesional target selection that resemble visual extinction. Recently, we showed that macaque monkeys with unilateral lesions in the caudal prefrontal cortex (PFC) exhibited contralesional target selection deficits that recovered over 2-4 months (Adam et al., 2019). Here, we investigated the longitudinal changes in functional connectivity (FC) of the frontoparietal network after a small or large right caudal PFC lesion in four macaque monkeys. We collected ultra-high field resting-state fMRI at 7-T before the lesion and at weeks 1-16 post-lesion and compared the functional data with behavioural performance on a free-choice saccade task. We found that the pattern of frontoparietal network FC changes depended on lesion size, such that the recovery of contralesional extinction was associated with an initial increase in network FC that returned to baseline in the two small lesion monkeys, whereas FC continued to increase throughout recovery in the two monkeys with a larger lesion. We also found that the FC between contralesional dorsolateral PFC and ipsilesional parietal cortex correlated with behavioural recovery and that the contralesional dorsolateral PFC showed increasing degree centrality with the frontoparietal network. These findings suggest that both the contralesional and ipsilesional hemispheres play an important role in the recovery of function. Importantly, optimal compensation after large PFC lesions may require greater recruitment of distant and intact areas of the frontoparietal network, whereas recovery from smaller lesions was supported by a normalization of the functional network.
Resting-state functional MRI (RS-fMRI) is widely used to assess how strongly different brain areas are connected. However, this connection obtained by RS-fMRI, which is called functional connectivity (FC), simply refers to the correlation of blood oxygen level-dependent (BOLD) signals across time it has yet to be quantified how accurately FC reflects cellular connectivity (CC). In this study, we elucidated this relationship using RS-fMRI and quantitative tracer data in marmosets. In addition, we also elucidated the effects of distance between two brain regions on the relationship between FC and CC across seed region. To calculate FC, we used full correlation approach that is considered to reflect not only direct (monosynaptic connections) but also indirect pathways (polysynaptic connections). Our main findings are that: (1) overall FC obtained by RS-fMRI was highly correlated with tracer-based CC, but correlation coefficients varied remarkably across seed regions; (2) the strength of FC decreased with increase in the distance between two regions; (3) correlation coefficients between FC and CC after regressing out the effects of the distance between two regions still varied across seed regions, but some regions have strong correlations. These findings suggest that although FC reflects the strength of monosynaptic pathways, it is strongly affected by the distance between regions.