Maximal activity limits and the diminishment of prior activities across spatially selective neurons.

Under the edge model, intermediate activities across spatially selective neurons contribute substantially to spatial hearing, whereas minimal and maximal activities activities contribute little or nothing.

This suggests that activities across the neurons may be ‘constrained’ by an upper limit or threshold, so that minimal, maximal and intermediate activity levels are apparent.

This, in turn, suggests that current activities and contributions to spatial hearing may diminish naturally over time, and that the rate of diminishment may be inversely proportional to the prevailing activity limit.

Activities consistent with these dynamics are illustrated below, where activity increments are modeled across IPD-selective neurons for 30 sequential interaural phase differences (IPD), starting at -0.15 cycles and ending at 0.15 cycles. Each increment is then followed by a proportional decrement or decay of activities across neurons (once the specified activity limit is surpassed). Compounded decrements are recorded in right-hand figures to show each prior IPD’s contribution to current activities.

Notes:

• • Activity increments are integers in the above demonstrations because IPD magnitude does not exceed 0.15 cycles. Non-integer activity ‘weights’ are also omitted, corresponding, for instance, to waveform amplitude fluctuations.
• • This is relevant because the ‘rate’ of diminishment or decay over time depends on the size of each activity increment in addition to the current activity limit. Peripheral IPDs beyond ±0.15 cycles, for instance, would evoke smaller activity increments, prompting slower decay rates. 

• • The rate with which activities diminish or decay over time can be calculated as the number of IPDs modeled per waveform cycle divided by frequency. If only one IPD is estimated per waveform cycle, for instance, the abscissa of each righthand figure is equivalent to 300 ms at 100 Hz, 60 ms at 500 Hz, or 30 ms at 1000 Hz.

• • By default, activities in left-hand figures are plotted versus IPD-selective neurons ‘labeled’ by IPDs evoking  half-maximal responses along their medial edges, in accord with the edge model. The ‘Maximal’ button changes this so that neurons are ‘labeled’ by IPDs evoking maximal responses, in accord with the maximal model.

Igor Pro experiment used to generate figures.