Effects of ear plugging on single-unit azimuth sensitivity in cat primary auditory cortex. II. Azimuth tuning dependent upon binaural stimulation

Abstract

1. Single-unit recordings were carried out in primary auditory cortex (AI) of barbiturate-anesthetized cats. Observations were based on a sample of 131 high-best-frequency (> 5 kHz), azimuth-sensitive neurons. These were identified by their responses to a set of noise bursts, presented in the free field, that varied in azimuth and sound-pressure level (SPL). Each azimuth-sensitive neuron responded well to some levels at certain azimuths, but did not respond well to any level at other azimuths. 2. Unilateral ear plugging was used to infer each neuron's response to monaural stimulation. Ear plugs, produced by injecting a plastic ear mold compound into the external ear, attenuated sound reaching the tympanic membrane by 25-70 dB. The azimuth tuning of a large proportion of the sample (62/131), referred to as binaural directional (BD), was completely dependent upon binaural stimulation because with one ear plugged, these cells were insensitive to azimuth (either responded well at all azimuths or failed to respond at any azimuth) or in a few cases exhibited striking changes in location of azimuth function peaks. This report describes patterns of monaural responses and binaural interactions exhibited by BD neurons and relates them to each cell's azimuth and level tuning. The response of BD cells to ear plugging is consistent with the hypothesis that they derive azimuth tuning from interaural level differences present in noise bursts. Another component of the sample consisted of monaural directional (27/131) cells that derived azimuth tuning in part or entirely from monaural spectral cues. Cells in the remaining portion of the sample (42/131) responded too unreliably to permit specific conclusions. 3. Binaural interactions were inferred by statistical comparison of a cell's responses to monaural (unilateral plug) and binaural (no plug) stimulation. A larger binaural response than either monaural response was taken as evidence for binaural facilitation. A smaller binaural than monaural response was taken as evidence for binaural inhibition. Binaural facilitation was exhibited by 65% (40/62) of the BD sample (facilitatory cells). Many of these exhibited mixed interactions, i.e., binaural facilitation occurred in response to some azimuth-level combinations, and binaural inhibition to others. Binaural inhibition in the absence of binaural facilitation occurred in 35% (22/62) of the BD sample, a majority of which were EI cells, so called because they received excitatory (E) input from one ear (excitatory ear) and inhibitory (I) input from the other (inhibitory ear). One cell that exhibited binaural inhibition received excitatory input from each ear.(ABSTRACT TRUNCATED AT 400 WORDS)

Delayed Maturation and Sensitive Periods in the Auditory Cortex

“…Unfortunately, complete auditory deprivation is more difficult to achieve than visual deprivation. Simple ear plugging leads to hearing loss of around 40 dB, under special circumstances 70 dB [Nixon and van Gierke, 1959;Samson et al, 1994]; bone conduction and vocalizations, breathing, chewing, etc. are not attenuated at all.…”

“…are not attenuated at all. Nonetheless, even with this type of conductive hearing loss, impairment in binaural processing has been demonstrated [Samson et al, 1994].…”

Directional Sensitivity of Neurons in the Primary Auditory (AI) Cortex: Effects of Sound-Source Intensity Level

“…The intensity level of the sound source is particularly notable in this regard (Heil et al 1994;Phillips et al 1994;Schreiner 1998). For example, most high-BF neurons in cat AI cortex are reported to exhibit an azimuthal sensitivity that is dependent on the intensity level of a free-field stimulus Imig et al 1990;Rajan et al 1990;Samson et al 1993Samson et al , 1994. A similar result is inferred when cat AI neurons are studied for the affect of intensity level on their interaural intensity difference sensitivity (Irvine et al 1996;Semple and Kitzes 1993a,b); a major cue for the azimuthal direction of high-frequency sound sources.…”

“…Typically, when there are no competing sounds and the intensity level of the source is at or very near the threshold, an AI spatial receptive field is confined to a small portion of acoustic space (Brugge et al 1994Eisenman 1974;Middlebrooks and Pettigrew 1981;Rajan et al 1990). With few exceptions (see Imig et al 1990;Rajan et al 1990;Samson et al 1993Samson et al , 1994, increasing the stimulus strength by no more than 10 dB over a wide range (40 -80 dB) of suprathreshold intensities results in marked increase in receptive field size whether measured along the azimuth, along the elevation, or along both (Brugge et al 1994Imig et al 1990;Middlebrooks and Pettigrew 1981;Rajan et al 1990). This attribute of AI spatial tuning is observed in other auditory cortical fields as well (Middlebrooks et al 1994(Middlebrooks et al , 1998.…”

Binaural interactions shape binaural response structures and frequency response functions in primary auditory cortex

“…doi:10.1016/j.heares.2008.01.003 source in acoustic space. The coding of azimuth and elevation has been assessed by analyzing responses to a mobile audio speaker or to an array of speakers Imig et al, 1990;Middlebrooks et al, 1994;Samson et al, 1993Samson et al, , 1994Stecker et al, , 2005aXu et al, 1998). The stimulus used in such free field studies is typically broadband or Gaussianshaped noise; responses typically are evoked broadly over the stimulus field.…”