lateralsuppression

Spatial hearing

➜ Edge model

Under the edge model, individual neurons are ‘labeled’ by the spatial cues that evoke half-maximal activities along their medial or ipsilateral edges. Spatial ambiguities therefore arise for interaural time or phase differences (ITD or IPD) when equivalent activities are evoked along their lateral or contralateral edges.

Individual modeled neurons selective for interaural phase differences (IPD). Ipsilateral edges are indicated by dark lines. Contralateral edges are indicated by dashed lines. Equivalent and thus spatially ambiguous activities may be evoked by IPDs corresponding to either edge. Equivalent half-maximal activities may be evoked in the neurons shown below, for instance, by IPDs of 0.02 and -0.4 cycles in the left hemisphere or by IPDs of -0.02 and 0.4 cycles in the right hemisphere.

Activities evoked along the ipsilateral and contralateral edges of individual neurons translate to the opposite edges of activity patterns across neurons. Activities along the contralateral edges of individual neurons contribute to the ipsilateral edges of activity patterns across neurons, for instance, whereas activities along the ‘labeled’ ipsilateral edges of individual neurons contribute to the contralateral edges of activity patterns across neurons.

Activities evoked across modeled neurons for various recurrent interaural phase differences (IPD), indicated in the left panel by open circles. Dashed lines indicate selectivity in excess of each evoked activity (open circle) when the activity is congruent with the contralateral edge of the neuron’s selectivity function. Dashed lines in the right panel indicate that such activities contribute to the ipsilateral edges of activity patterns across neurons.

Spatially ambiguous activities are typically evoked along the ipsilateral edges of activity patterns whereas non-ambiguous activities are evoked along contralateral edges. Although often unnecessary under the edge model when interaural level differences (ILD) are considered, one possible strategy for minimizing spatial ambiguities could be the ‘suppression’ of ipsilaterally edged activity patterns across neurons.

Activities evoked across modeled neurons for various recurrent interaural time differences (ITD). ‘Suppression’ of ipsilateral edges may be toggled ‘on’ which, in this demonstration, sets all activities along ipsilateral edges to zero.

ITD ≈ IPD ∙ frequency

Notes:

• When interaural level differences (ILD) are also considered under the edge model, ‘suppression’ along ipsilateral edges is may not be necessary.

• ‘Suppression’ along ipsilateral edges could minimize spatial ambiguities under the inter-hemispheric channel model, although why the auditory system would not then continue to use the underlying activity patterns is unclear.

• Transitions of activity patterns from ipsilateral to contralateral are not crucial under the edge model because minimal and maximal activities contribute little to spatial hearing under the edge model without suppression. Indeed, representations could be further enhanced if minimal or maximal activities prompted suppression similar to ipsilateral edges, although minimal and maximal activities already contribute little to spatial hearing under the edge model.