![]() More recently, optogenetic suppression experiments in other neural systems have shown that acute suppression can reveal involvement of a brain structure in specific tasks even when lesions of the same structure have no effect ( Goshen et al., 2011 Kumar et al., 2013 Otchy et al., 2015 Hong et al., 2018). Lesion studies typically include at least several days of recovery after surgery, which could allow time for cortical or subcortical plasticity to eventually allow alternative structures or pathways to mediate frequency discrimination. The fact that some transient inactivation studies observed complete impairment suggests that auditory cortex could potentially be involved in frequency discrimination, and that the effects of lesions could differ from those of transient inactivation because of cortical reorganization or some other long-term recovery or compensatory processes. The effects of transient inactivation on pure tone discrimination (for example, with local muscimol application) have been inconsistent, with some studies reporting no effect while others report complete impairment ( Talwar et al., 2001 Gimenez et al., 2015). Discrimination of more spectrotemporally complex sounds such as frequency-modulated tones is impaired by lesions of auditory cortex, suggesting that auditory cortex is recruited when task demands require spectral or temporal integration ( Ohl et al., 1999). Although there are conflicting results, lesion studies have generally shown that frequency discrimination of pure tones is not affected by ablation of auditory cortex, even after extensive lesions of all known auditory cortical fields ( Meyer and Woolsey, 1952 Butler et al., 1957 Thompson, 1960 Goldberg and Neff, 1961 Sellick, 1983 Buser and Imbert, 1992 Ohl et al., 1999 Talwar et al., 2001 Rybalko et al., 2006 Porter et al., 2011 Gimenez et al., 2015). Is auditory cortex involved in the perception and discrimination of sound frequencies? For pure tones, across a wide range of species and behavioral paradigms, the consensus view has been that the answer appears to be no. Neurons in auditory cortex are well tuned for frequency and are organized into multiple tonotopic maps across the cortical surface. These results indicate a fundamental difference between the effects of brain lesions and optogenetic suppression, and suggest the existence of a rapid compensatory process possibly induced by injury. Yet this alternative pathway is insufficient for task performance when auditory cortex is intact but has its activity suppressed. This suggests that when auditory cortex is destroyed, an alternative pathway is almost immediately adequate for mediating frequency discrimination. In contrast, bilateral electrolytic lesions of auditory cortex had no effect on performance of the identical task, even when tested only 4 h after lesion. We found that transient bilateral optogenetic suppression partially but significantly impaired discrimination performance. ![]() Here, we compared the effects of lesions and optogenetic suppression of auditory cortex on frequency discrimination in mice. This suggests the possibility that successful tone discrimination after recovery from lesion surgery could arise from long-term reorganization or plasticity of compensatory pathways. However, transient pharmacological inactivation has been reported to impair frequency discrimination. Lesion studies have shown that auditory cortex is not essential for frequency discrimination of pure tones. The auditory cortex is topographically organized for sound frequency and contains highly selective frequency-tuned neurons, yet the role of auditory cortex in the perception of sound frequency remains unclear.
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