Fig. 4.

Temporal modulation transfer functions based on response synchronization to AM stimuli or PRR of pulse trains (bullfrog TS: AM noise, green circles; PRR, green squares) (big brown bat IC: PRR, blue diamonds) or on the distribution of best AM rates for AM synchronization and tuning (cat IC: AM, red triangles). For bullfrogs, the AM curve shows mean VS for a sample of 56 neurons in the TS tested at each AM rate (data from Simmons et al. 2000). These neurons differed in their AM tuning as defined by changes in spike rate with AM rate, but showed little difference in VS at any particular rate. For big brown bats, the curve shows mean VS at different pulse rates for eight representative AM low-pass neurons (data from Lu et al. 1998; see also Pinheiro et al. 1991). For domestic cats, the curve shows the normalized proportion of neurons at each best AM rate (data from Langner and Schreiner 1988). For comparison, PRR synchronization is shown for the human AC (posteromedial Heschyl’s gyrus, gray hexagons), expressed as the mean power difference between synchronized and unsynchronized responses obtained from nine human subjects (data from Fig. 5 in Brugge et al. 2009). By these measures, the auditory midbrain of frogs, cats, and bats and part of the AC of humans contain neurons that respond robustly and synchronously to AM stimuli up to 100 Hz, with progressively declining synchronization from 100 to 200–250 Hz. The temporal code for these stimuli thus is preserved from the auditory brainstem to the midbrain or forebrain, indicating that each acoustic event (AM cycle or pulse) triggers its own time-locked response for evaluation of its particular time separation from the previous event