Implications of dominant inhibition of LR neurons from
neighboring FS cells
In contrast to the lack of local connectivity of LR cells, there is
dense connectivity between FS cells and their neighboring excitatory
neurons, with the probability of FS-to-LR connectivity reaching 52%
(Figure 6), higher than that reported in other regions of the neocortex.
For example, connectivity probabilities between FS and pyramidal neurons
in superficial layers of the mouse visual cortex was reported as roughly
35%, of the rat visual cortex as 46.5%, of the mouse somatosensory
cortex as only 19%, and of the rat barrel cortex as 44% (Beierlein et
al., 2003; Jiang et al., 2015; Packer and Yuste, 2011; Yoshimura and
Callaway, 2005). This, coupled with the complete lack of local
excitatory connections onto LR cells (0%; Figure 6), indicates that the
superficial layers of the RSG are a network dominated by strong local
inhibition. Therefore, the activity of LR neurons in the superficial RSG
is likely to be strongly influenced by feedforward inhibition from
neighboring FS neurons.
In strong concordance with our results, a recent study quantifying the
response of pyramidal neurons in the superficial RSD to contralateral
stimulation has shown that layer 2/3 is characterized by
inhibition-dominated feed-forward dynamics (Geijo-barrientos et al.,
2019; Sempere-Ferrà ndez et al., 2018). Also, in response to
photostimulation of subicular axons which project extensively to layer 3
of the RSC, the firing of superficial pyramidal neurons is dominated by
strong feed-forward inhibition coming from local interneurons (Corcoran
et al., 2018). Therefore, similar to the other cortical regions
(Avermann et al., 2012; Mateo et al., 2011), the superficial RSG could
implement a sparse neuronal code dominated by local inhibition from
interneurons. Given the well-known role of the RSC in memory and spatial
navigation related functions, such sparse population codes can greatly
boost the pattern storage and recognition capabilities of the system
(Marr, 1971). Indeed, sparse neuronal codes representing sequential,
place-cell like firing have been reported in the neurons of the
superficial RSC during spatially-guided movements (Mao et al., 2017).
Our results start to explain how the local cells and circuitry of the
superficial retrosplenial cortex can support these unique computational
functions.