Connection preference of cultured brainstem neurons. A. Concerning reference units, 87% CO2-stimulated units projected to another CO2-stimulated units (A1), while only 29% CO2-unresponsive units projected to CO2-stimulated units (A2). B. With respect to target units, 64% CO2-stimulated units received synaptic input from another CO2-stimulated units (B1). Only 10% CO2-unresponsive received such synaptic input (B2). C. For the reference units, 54% CO2-inhibited neurons projected to another CO2-inhibited units (C1), whereas only 26% CO2-unresponsive units projected to the CO2-inhibited units (C2). D. From the target units, 65% CO2-inhibited neuron received synaptic input from another CO2-inhibited units (D1), but only 46% CO2-unresponsive units received synaptic inputs from another CO2-inhibited units (D2).