Figure 6. Dominant local inhibition in the superficial layers of the retrosplenial cortex. A. Table indicating the percentage of connectivity between all types of pairs tested. No FS-FS pairs were tested in this study. FS→LR connections existed in 11 of the 21 pairs recorded (52%). FS→RS connections existed in 1 of the 2 pairs recorded (50%). LR→FS connections existed in 4 of the 23 pairs recorded (17%). There were no RS→FS connections recorded and no E→E connections recorded (LR→LR, LR→RS, RS→LR, RS→RS). The heat map to the right indicates the probability of connections between the neuron types indicated in each cell of the table. Deeper gold indicates connection probabilities of near 50%, while lighter gold indicates lower probabilities and white indicates a connection probability of 0. B. Bar graph representing the total connectivity probability between all inhibitory to excitatory directional pairs (52%) and all inhibitory to excitatory directional pairs (16%). Bootstrap resampling followed by a t-test revealed a significant difference in probability to observe I→E connections versus E→I connections. C. Representative trace of the connection between a layer 3 FS cell (held at -65 mV) and layer 3 LR cell (held at -55 mV). The neurons were 27 um apart with the LR cell located superficial to the FS cell. Schematic shows the patched pair in which the FS cell is being stimulated to spike at 10 Hz and the responses of the LR cell are being recorded. The purple trace is the responses of the LR cell to each FS cell spike (indicated by the orange arrows). D. Representative trace of the connection between a layer 3 LR cell (held at -65 mV) and layer 3 FS cell (held at -55 mV). The neurons were 80 um apart with the LR cell located superficial to the FS cell. Schematic shows the patched pair in which the LR cell is being stimulated to spike at 10 Hz and the responses of the FS cell are being recorded. The orange trace is the responses of the FS cell to each LR cell spike (indicated by the purple arrows). E. Bar graph showing the average amplitude of the IPSPs recorded from the FS→LR pairs (red) and the EPSPs recorded from the LR→FS pairs (blue). Error bars are standard error. F. Bar graph showing the average latency to onset of the IPSPs recorded from the FS→LR pairs (red) and the EPSPs recorded from the LR→FS pairs (blue). Error bars are standard error. Latency to onset was calculated as the time from the peak of the presynaptic action potential to the beginning of the postsynaptic IPSP/EPSP. G. Bar graph showing the average latency to peak of the IPSPs recorded from the FS→LR pairs (red) and the EPSPs recorded from the LR→FS pairs (blue). Error bars are standard error. Latency to peak was calculated as the time from the onset of the postsynaptic IPSP/EPSP to the peak of the postsynaptic IPSP/EPSP. H. Group synaptic dynamics for FS→LR connections (n=9). Inhibition onto LR cells exhibited strong short-term depression. I. Group synaptic dynamics for LR→FS connections (n=2). Excitation onto FS cells did not exhibit short-term depression or facilitation.