Networks of oscillators produce vital activity in diverse natural systems. The dynamics of these networks are frequently studied via computational models that assume weak coupling, yet this assumption has not been experimentally validated. We applied weak stimuli to neuronal oscillators in Aplysia californica and deconvolved infinitesimal phase response curves (IPRCs) that describe the phase response of a neuron. We show that these IPRCs reliably predict the phase response for weak stimuli, independent of the stimulus waveform used. These weak stimuli are in the range of normal synaptic activity for these neurons, suggesting that weak coupling is a likely mechanism.