The edge model of spatial hearing proposes that individual neurons are ‘labeled’ by spatial cues evoking half-maximal activities and the representations of auditory space amount to activities evoked across many neurons having slightly different ‘labels.’ In the examples below, five recurrent interaural level differences (ILD) are considered: -12, -6, 0, 6, or 12 dB.

The greatest contributions to spatial hearing, under the edge model, arise from activities closest to half of the maximum activity level achieved at any one moment, i.e., the same normalized level used to label individual neurons. A more intuitive graphical scheme, demonstrated below, is thus to transform values exceeding half-maxima by reflecting them downward so that minimal and maximal activities are both viewed as contributing equally little to spatial hearing. Under this graphic scheme, ‘peaks’ are used to indicate where contributions to spatial hearing from half-maximal activities are highest. Such peaks may also be viewed as ‘relief points’ along a topographic contour marking where contributions to spatial hearing are highest.

There are two important differences between activity patterns evoked by ITDs and ILDs.

1)  One difference is that half-maximal activities evoked by peripheral ITDs become spatially ambiguous as frequency increases. In contrast, analogous activities evoked by peripheral ILDs do not. Moreover, incorporating ILD-evoked activities into the edge model is sufficient to eliminate spatial ambiguities at the half-maximal activity level, as we will soon see...

2)  A second difference is that activities in the left and right hemispheres are similar under conditions prompting ILD to fluctuate over time. This differs markedly from fluctuations of ITD, where half-maximal activities in the left and right hemispheres split apart, prompting listeners to hear two distinct images or edges when the fluctuations are broad enough.