Bilateral Ablation of Auditory Cortex in Mongolian Gerbil Affects Discrimination of Frequency Modulated Tones but not of Pure Tones

Abstract

This study examines the role of auditory cortex in the Mongolian gerbil in differential conditioning to pure tones and to linearly frequency-modulated (FM) tones by analyzing the effects of bilateral auditory cortex ablation. Learning behavior and performance were studied in a GO/NO-GO task aiming at avoidance of a mild foot shock by crossing a hurdle in a two-way shuttle box. Hurdle crossing as the conditioned response to the reinforced stimulus (CR+), as false alarm in response to the unreinforced stimulus (CR−), intertrial activity, and reaction times were monitored. The analysis revealed no effects of lesion on pure tone discrimination but impairment of FM tone discrimination. In the latter case lesion effects were dependent on timing of lesion relative to FM tone discrimination training. Lesions before training in naive animals led to a reduced CR+ rate and had no effect on CR− rate. Lesions in pretrained animals led to an increased CR− rate without effects on the CR+ rate. The results suggest that auditory cortex plays a more critical role in discrimination of FM tones than in discrimination of pure tones. The different lesion effects on FM tone discrimination before and after training are compatible with both the hypothesis of a purely sensory deficit in FM tone processing and the hypothesis of a differential involvement of auditory cortex in acquisition and retention, respectively.

Bottom–up and top–down influences on auditory scene analysis: Evidence from event-related brain potentials.

“…has been shown that bilateral lesions of auditory cortical areas impair frequency-modulated tone discrimination but have little effect on pure tone discrimination (Ohl, Wetzel, Wagner, Rech, & Scheich, 1999). Thus, neurons in primary or secondary auditory cortices may provide the neural substrates for parsing co-occurring sounds based on frequency periodicity, and they probably contribute to the ERP responses elicited by perceptually segregated sounds.…”

“…Evidence from single-cell recordings in nonhuman primates (for a review, see Rauschecker, 1997) and neuromagnetic recordings in humans (Langner, Sams, Heil, & Schulze, 1997) suggests that frequency periodicity is represented within the primary or secondary auditory cortices, or both (Langner et al, 1997). In animals, it has been shown that bilateral lesions of auditory cortical areas impair frequency-modulated tone discrimination but have little effect on pure tone discrimination (Ohl, Wetzel, Wagner, Rech, & Scheich, 1999). Thus, neurons in primary or secondary auditory cortices may provide the neural substrates for parsing co-occurring sounds based on frequency periodicity, and they probably contribute to the ERP responses elicited by perceptually segregated sounds.…”

What can we learn from inactivation studies? Lessons from auditory cortex

“…With such differences in the ability to manipulate inactivation spatially and temporally, each method has its own advantages and limitations ( Table 1 ). Timescale itself is a significant factor since the duration of loss of AC function ( Figure 2 ) shapes the magnitude of the observed deficit: in many cases, performance on a task continues to evolve for a long time after the inactivation, with most severe impairments occurring in the short term [ 4. , 5.…”

“…In most cases, AC inactivation does not impact the performance of tasks requiring discrimination of simple features. Severe deficits are reported only when reversible inactivation methods are used, that likely target the whole AC [ 24 , 43 ] or for a limited period following a lesion [ 4. , 5.…”

Cortical Responses in Rats Predict Perceptual Sensitivities to Complex Sounds.

“…Previous studies indicate that the auditory cortex may play an important role in the processing of these time-varying features of sounds. For example, gerbils with total bilateral ablation of the auditory cortex are no longer capable of discriminating the direction of change of frequency-modulated (FM) sounds, even though their capacity to discriminate pure tone stimuli remains intact after such ablation (Ohl, Wetzel, Wagner, Rech, & Scheich, 1999; Wetzel, Ohl, Wagner, & Scheich, 1998). Similarly, electrophysiological studies performed in various species indicate that auditory cortical neurons are highly sensitive to FM sounds.…”