Neural mechanisms of directional hearing in the pigeon

The directional sensitivity of single auditory neurons in the midbrain (Nucleus mesencephalicus lateralis pars dorsalis) of the pigeon (Columba livia) was studied, using acoustic free-field stimulation (usually pure tones) in the frontal hemifield. Of a total of 337 units, 84.6% showed statistically significant changes of their responses as a function of sound azimuth. Of these, most units respond maximally to sounds in a particular azimuthal range, each has its "best area". These neurons were classified into four classes according to the properties of their best areas: (1) contralateral neurons (53.4%); (2) ipsilateral neurons (6.2%); (3) frontal neurons (18.1%); and (4) complex neurons (3.3%). The first two showed only one border of the best area within the frontal hemifield, with an increase of response strength towards the contralateral and the ipsilateral side, respectively; with frontal neurons, the best area was bounded towards both sides within the frontal hemifield, whereas the complex neurons had two or more separated best areas or extensive frontal inhibitory areas. In the remaining units (3.6%), termed weakly directional neurons, changes of their discharge rate depending on sound azimuth were statistically significant, but too poor to determine any best areas. There was a significant under-representation of best frequencies in the mid-frequency range (1-2 kHz) with a minimum in the relative number of MLD neurons recorded from at 2 kHz. However, the directional sensitivity of the neurons quantified by analysing different parameters of the directional diagrams (dynamic range, roll-off steepness, best area width) was undiminished in the mid-frequency range. In several experiments, in addition to the neurons' directional sensitivity in free-field sound, their sensitivity to interaural ongoing time (phase) differences (OTDs) and interaural intensity differences (IIDs) were also tested, using dichotic stimulation (pure tones) by headphones. Directional sensitive neurons tuned to low frequencies (best frequency less than 2 kHz) were either sensitive exclusively to OTDs or to both OTDs and IIDs; the ranges of best OTDs were correlated significantly with the azimuthal position of the best area. "High frequency" units (best frequency greater than 2 kHz) were sensitive to IIDs but not to OTDs.(ABSTRACT TRUNCATED AT 400 WORDS)