Shapes and level tolerances of frequency tuning curves in primary auditory cortex: quantitative measures and population codes.

The shape and level tolerance of the excitatory frequency/intensity tuning curves (eFTCs) of 160 cat primary auditory cortical (A1) neurons were investigated. Overall, A1 cells were characterized by tremendous variety in eFTC shapes and symmetries; eFTCs were U-shaped ( approximately 20%), V-shaped ( approximately 20%), lower-tail-upper-sharp ( approximately 15%), upper-tail-lower-sharp (<2%), slant-lower ( approximately 10%), slant-upper (<3%), multipeaked ( approximately 10%), and circumscribed ( approximately 20%). Quantitative analysis suggests that eFTC are best thought of as forming a continuum of shapes, rather than falling into discrete categories. A1 eFTCs tended to be more level tolerant than eFTCs from earlier stations in the ascending auditory system as inferred from other studies. While individual peaks of multipeaked eFTCs were similar to single peaked eFTCs, the overall eFTC of multipeaked neurons (spanning the range of all peaks) tended to have high-frequency tails. Measurements of shape and symmetry indicate that A1 eFTCs, on average, tended to have greater area on the low-frequency side of characteristic frequency (CF) than on the high-frequency side. A1 cells showed a relationship between CF and the inverse slope of low-frequency edges of eFTCs, but not for high-frequency edges. These data demonstrate that frequency tuning, particularly along the eFTC low-frequency border, sharpens along the lemniscal pathway to A1. The results are consistent with studies in mustached bats (Suga 1997) and support the idea that spectral decomposition along the ascending lemniscal pathway up to A1 is a general organizing principle of mammalian auditory systems. Altogether, these data suggest that A1 neurons' eFTCs are shaped by complex patterns of inhibition and excitation accumulating along the auditory pathways, implying that central rather than peripheral filtering properties are responsible for certain psychophysical phenomena.