Processing of Communication Calls in Guinea Pig Auditory Cortex

Abstract

Vocal communication is an important aspect of guinea pig behaviour and a large contributor to their acoustic environment. We postulated that some cortical areas have distinctive roles in processing conspecific calls. In order to test this hypothesis we presented exemplars from all ten of their main adult vocalizations to urethane anesthetised animals while recording from each of the eight areas of the auditory cortex. We demonstrate that the primary area (AI) and three adjacent auditory belt areas contain many units that give isomorphic responses to vocalizations. These are the ventrorostral belt (VRB), the transitional belt area (T) that is ventral to AI and the small area (area S) that is rostral to AI. Area VRB has a denser representation of cells that are better at discriminating among calls by using either a rate code or a temporal code than any other area. Furthermore, 10% of VRB cells responded to communication calls but did not respond to stimuli such as clicks, broadband noise or pure tones. Area S has a sparse distribution of call responsive cells that showed excellent temporal locking, 31% of which selectively responded to a single call. AI responded well to all vocalizations and was much more responsive to vocalizations than the adjacent dorsocaudal core area. Areas VRB, AI and S contained units with the highest levels of mutual information about call stimuli. Area T also responded well to some calls but seems to be specialized for low sound levels. The two dorsal belt areas are comparatively unresponsive to vocalizations and contain little information about the calls. AI projects to areas S, VRB and T, so there may be both rostral and ventral pathways for processing vocalizations in the guinea pig.

Descending and tonotopic projection patterns from the auditory cortex to the inferior colliculus

“…1, where VRB placements were more lateral and ventral than AI placements. These locations are similar to those previously determined in guinea pig (Wallace et al, 2000;Grimsley et al, 2012). The identification of locations was confirmed by differences in the first-spike latency (FSL) responses, which are significantly longer in VRB (Wallace et al, 2000).…”

“…Anatomical analysis using multiple staining preparations in guinea pig found that the ventral (i.e., deeper) border of layer V is approximately 1190 lm below the ventral border of layer II for AI, though this value could vary between 580 and 1470 lm when comparing the distance between the closest and farthest borders of the cortical layers (Wallace and Palmer, 2008). Though it is unclear what depths correspond to specific layers in the VRB, cortical thickness has typically been found to be thinner in belt regions, though thickness could vary from 1.6 to 2.2 mm in comparison to the typical thickness of 2 mm in AI (Grimsley et al, 2012). The consistent range of values in our study with these expected anatomical depths supports that the CSD analysis is correctly identifying sites in layer V.…”

“…Located ventral and lateral to AI, neurons in VRB respond to frequency tones with low thresholds and are organized in a tonotopic pattern parallel to that of AI, thus causing the tonotopy to appear as a continuum of AI (Wallace et al, 2000). Originally distinguished from AI because of its long latencies and poor responses to noise (Wallace et al, 1999), VRB has also been shown to have the most units of all cortical fields that can discriminate conspecific vocalizations using a rate code (Grimsley et al, 2012). In terms of histological labeling, there are no clear transitions between AI and VRB for either myelin or cytochrome oxidase (CYO) staining (Wallace et al, 2000).…”

Neural processing of natural sounds

“…Historically, the first use of natural sounds in auditory neuroscience came from neuro-ethologists who investigated how conspecific vocalizations or communication signals were selectively processed in the auditory system of auditory specialists. These investigations in model systems led to the discovery of cricket-song selective neurons and their contribution to the females' phonotaxis behavior [77], of call selective neurons in frogs [78] and guinea pigs [79], of song selective neurons in songbirds [7,[80][81][82], of neurons selective to the echolocation signal in bats [5], and of brain regions selective for conspecific calls in primates [83]. Selectivity for conspecific communication calls can be reflected not only in the mean rate of single neurons but also (and sometimes only) in time-varying responses [84,85] or ensemble responses [86,87].…”

A function for binaural integration in auditory grouping and segregation in the inferior colliculus

“…Harmonic complexes are important components of speech, music, and some animal communications (including guinea pig calls : Berryman 1976;Grimsley et al 2012). Harmonic complexes create a perception of pitch, a perceptual grouping feature that has received particular attention (reviewed in Plack et al 2005;Wang and Bendor 2010).…”