Heavy ion Nd³⁺ is introduced into the electrolyte layer to study frequency selectivity of a semiconducting polymer/electrolyte double-layer cell. This cell exhibits long-term depression under low-frequency stimulations and potentiation under high-frequency stimulations by positive triangular pulses, suggesting a conventional learning protocol, i.e., spike-rate-dependent plasticity. The frequency selectivity depends significantly on the input shape due to large ionic size and mass. The input threshold of the frequency selectivity is around the voltage inducing a negative differential resistance (V_NDR) influenced by the loading rate. The typical value of V_NDR is 0.3 V for a loading rate of 100 V s⁻¹, but V_NDR disappears when the loading rate exceeds 1000 V s⁻¹. Besides, the frequency selectivity has not been observed under rectangular pulse input. Moreover, the possibility of bidirectional signal transfer has been tested simply by anti-connecting two individual cells. Our study suggests the possibility to realize signal pruning and synthetizing by changing ionic types.