GABA-MEDIATED INHIBITION CONTRIBUTES TO NEURONAL SELECTIVITY FOR INTERAURAL TIME DIFFERENCE IN THE OWL'S INFERIOR COLLICULUS.

  The barn owl uses interaural time differences (LTD) for localizing the azimuthal position of sounds.  Neuronal selectivity for ITD first appears in the nucleus laminaris and improves in the central nucleus "core" (ICc ) and the external nucleus (ICx ) of the inferior colliculus.  Tonal stimuli cause both ICc and ICx neurons to respond maximally not only to one particular ITD (the characteristic delay), dt, but also to dt + nT, where T is the tonal period and n an integer.  This phenomenon, phase ambiguity, does not occur when ICx neurons are stimulated with noise.
  Iontophoretically applied bicuculline methiodide (BMI, a selective GABAA antagonist)  decreased the ITD selectivity of ICc neurons.  The effects were identical for tone- and noise-evoked responses.  In ICx, BMI decreased ITD selectivity to tones only in neurons tuned to frequencies below 5 kHz.  BMI led to loss of ability of ICx neurons to signal uniquely their dt.  The results suggest that under physiological conditions GABAergic inhibition sharpens ITD selectivity in ICc neurons and ICx neurons tuned to below 5 kHz, and eliminates phase ambiguity in ICx by interaction between the converging frequency bands.  (Supported by NIH Uehara Memorial Foundation.)


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