Coding of free field intensity in the auditory midbrain of the leopard frog. I. Results for tonal stimuli

Single unit recordings in the auditory midbrain of the leopard frog were obtained in response to free field, random frequency, tone pip stimulation. Five sequences of 81 logarithmically distributed frequencies from 100 Hz to 3200 Hz inclusive at stimulus levels between 94 dB SPL and the unit's threshold were presented. Sigmoidal response-intensity functions were fit through the data points for monotonic and nearly monotonic units (53 in total) at all frequencies for which the units were responsive. From the fitcurves a generalized slope and inflection point were obtained and converted into a dynamic range and a threshold value. Single unit dynamic ranges at CF frequencies were between 5 and 80 dB, and thresholds were between 20 dB and 90 dB. For the population of units the dynamic ranges were positively correlated with the threshold values. A population response was computed by adding the response at each intensity-frequency combination, and again curve fitting with the sigmoidal function. The population dynamic range was about 70 dB for the basilar papilla range (around 1200 Hz) and somewhat less, about 45 dB, for the amphibian papilla neurons. The amount of synchronization of the firings of the individual units with the stimulus was calculated on basis of the shifted auto-coincidence function. The stimulus synchronization appeared to be largely independent of stimulus intensity. Inter-neuron synchronization was computed on basis of the cross-coincidence function of two neuronal spike trains. Also this synchronization was stimulus independent. It was concluded that synchronization or other forms of multiplicative action cannot on its own code for stimulus intensity. From the application of signal detection theory it is concluded that a rate coding on basis of a modest population of units has an acceptable dynamic range, roughly covering 30-100 dB SPL, produces a more than adequate sensitivity for intensity differences and may be used as a code for intensity.