Functional subdivisions in low-frequency primary auditory cortex (AI)

Abstract

We wished to test the hypothesis that there are modules in low-frequency AI that can be identified by their responsiveness to communication calls or particular regions of space. Units were recorded in anaesthetised guinea pig AI and stimulated with conspecific vocalizations and a virtual motion stimulus (binaural beats) presented via a closed sound system. Recording tracks were mainly oriented orthogonally to the cortical surface. Some of these contained units that were all time-locked to the structure of the chutter call (14/22 tracks) and/or the purr call (12/22 tracks) and/or that had a preference for stimuli from a particular region of space (8/20 tracks with four contralateral, two ipsilateral and two midline), or where there was a strong asymmetry in the response to beats of different direction (two tracks). We conclude that about half of low-frequency AI is organized into modules that are consistent with separate "what" and "where" pathways.

Location of cells giving phase-locked responses to pure tones in the primary auditory cortex

“…The rhythmic structure of the sound bursts in the purr call may also be important in permitting a prolonged timelocked response because recent work in the ferret has shown that the temporal envelope of a complex sound may act in gating the analysis of fine structure at up to a few hundred Hz (Elhilali et al, 2004). We have previously suggested that the responses to the purr stimulus are organized in a columnar fashion (Wallace et al, 2005;Wallace and Palmer, 2009) and our previous results (Wallace et al, 2002) indicate that there is at least a partial overlap between the cells that phase-lock to pure tones and those that accurately represent the structure of the purr. However, there is not a one-to-one correspondence as we were able to record units that gave a strong, time-locked response to the purr call but did not phase-lock to pure tones.…”

“…In our recent study of AI(LF) (Wallace and Palmer, 2009) we suggested that there may be 8 or 9 functional columns e each with a diameter of about 500 mm. These columns were defined in terms of responses to conspecific vocalizations or sensitivity to interaural phase differences.…”

“…In our previous studies of phase-locked responses in AI (Wallace et al, 2002) and in the ventrorostral belt (Wallace et al, 2000b) we only ever recorded from a single group of phaselocked cells. We have subsequently made numerous recordings with both tungsten (Wallace and Palmer, 2009) and silicon multielectrodes without noticing any phase-locked responses, possibly because the phase-locked cells only form one column out of many.…”

“…We define AI(LF) as being the part of AI that contains cells that are sensitive to interaural timing differences. In the guinea pig inferior colliculus timing-sensitive neurons generally have a CF of 1.5 kHz (McAlpine et al, 1996) and a slightly lower cut-off point (1.3 kHz) is present in the cortex (Wallace and Palmer, 2009). Coding for interaural time differences requires a reasonable population of cells with CFs that are less than 1 kHz.…”

“…We denote this as AI(LF). This division has many units that are sensitive to interaural phase differences (Fitzpatrick et al, 2000;Scott et al, 2009;Wallace and Palmer, 2009) and analyzing the time differences that these represent is an important element of localizing sounds in the azimuthal plane. We define AI(LF) as being the part of AI that contains cells that are sensitive to interaural timing differences.…”