Interaural intensity difference sensitivity based on facilitatory binaural interaction in cat superior colliculus

Sensitivity to interaural intensity difference (IID) has generally been identified as a property of neurons exhibiting inhibitory binaural interaction, viz. contralateral excitatory and ipsilateral inhibitory input (EI cells). In the deep layers of the superior colliculus, however, almost 30% of IID-sensitive cells are characterised by facilitatory or mixed facilitatory/inhibitory interactions. Such cells typically have peaked IID sensitivity functions in contrast to the step functions characteristic of EI cells. There appears to be a continuum in IID sensitivity from pure step functions to sharply-peaked functions. The observation that a given form of IID sensitivity can be associated with patterns of binaural interaction other than that by which it is most commonly produced suggests that IID-sensitive neurons are better classified on the basis of the form of their IID sensitivity than their binaural input pattern. It seems probable that IID sensitivity based on facilitatory and mixed facilitatory/inhibitory binaural interactions is a general characteristic of the primary auditory pathway, although only fragmentary data are so far available.