From postsynaptic potentials to spikes in the genesis of auditory spatial receptive fields.

Space-specific neurons in the owl's inferior colliculus respond only to a sound coming from a particular direction, which is equivalent to a specific combination of interaural time difference (ITD) and interaural level difference (ILD). Comparison of subthreshold postsynaptic potentials (PSPs) and spike output for the same neurons showed that receptive fields measured in PSPs were much larger than those measured in spikes in both ITD and ILD dimensions. Space-specific neurons fire more spikes for a particular ITD than for its phase equivalents (ITD +/- 1/F, where F is best frequency). This differential response was much less pronounced in PSPs. The two sides of pyramid-shaped ILD curves were more symmetrical in spikes than in PSPs. Furthermore, monaural stimuli that were ineffective in eliciting spikes induced subthreshold PSPs. The main cause of these changes between PSPs and spikes is thresholding. The spiking threshold did not vary with the kind of acoustic stimuli presented. However, the thresholds of sound-induced first spikes were lower than those of later sound-induced and spontaneous spikes. This change in threshold may account for the sharpening of ITD selectivity during the stimulus. Large changes in receptive fields across single neurons are not unique to the owl's space-specific neurons but occur in mammalian visual and somatosensory cortices, suggesting the existence of general principles in the formation of receptive fields in high-order neurons.