What can we learn from inactivation studies? Lessons from auditory cortex
Authors: Zuzanna A. Slonina, Katarina C. Poole, Jennifer K. Bizley
Journal: Trends in Neurosciences (2021)

Wide variation in the outcome of auditory cortex inactivation has been an impediment to clear conclusions regarding the roles of the auditory cortex in behaviour.Inactivation methods differ in their efficacy and specificity. The likelihood of observing a behavioural deficit is additionally influenced by factors such as the species being used, task design and reward.A synthesis of previous results suggests that auditory cortex involvement is critical for tasks that require integrating across multiple stimulus features, and less likely to be critical for simple feature discriminations.New methods of neural silencing provide opportunities for spatially and temporally precise manipulation of activity, allowing perturbation of individual subfields and specific circuits. Inactivation experiments in auditory cortex (AC) produce widely varying results that complicate interpretations regarding the precise role of AC in auditory perception and ensuing behaviour. The advent of optogenetic methods in neuroscience offers previously unachievable insight into the mechanisms transforming brain activity into behaviour. With a view to aiding the design and interpretation of future studies in and outside AC, here we discuss the methodological challenges faced in manipulating neural activity. While considering AC's role in auditory behaviour through the prism of inactivation experiments, we consider the factors that confound the interpretation of the effects of inactivation on behaviour, including the species, the type of inactivation, the behavioural task employed, and the exact location of the inactivation. Inactivation experiments in auditory cortex (AC) produce widely varying results that complicate interpretations regarding the precise role of AC in auditory perception and ensuing behaviour. The advent of optogenetic methods in neuroscience offers previously unachievable insight into the mechanisms transforming brain activity into behaviour. With a view to aiding the design and interpretation of future studies in and outside AC, here we discuss the methodological challenges faced in manipulating neural activity. While considering AC's role in auditory behaviour through the prism of inactivation experiments, we consider the factors that confound the interpretation of the effects of inactivation on behaviour, including the species, the type of inactivation, the behavioural task employed, and the exact location of the inactivation. Since the location of auditory cortex (AC) in the primate brain was first characterised in 1875 [1.Ferrier D. Experiments on the brain of monkeys.—No. I.Proc. R. Soc. Lond. 1875; 23: 409-430Google Scholar], researchers have been attempting to understand its role in auditory behaviours. Initially, the leading method of studying its function was through lesioning different areas of AC in animal models and observing the resulting impairments, with the aim of identifying behaviours for which AC is necessary. Later on, most investigations of AC function became centred on recording cortical activity associated with specific stimuli and behaviours, shifting the focus to correlational studies of neuronal activity and sensory stimuli. While electrophysiological recordings indicate the involvement of AC in nearly all auditory behaviours, from representing basic acoustic properties to associative learning [2.Weinberger N.M. Specific long-term memory traces in primary auditory cortex.Nat. Rev. Neurosci. 2004; 5: 279-290Google Scholar], the results of silencing studies vary greatly. However, recent advances in available inactivation methods offer increasingly more spatially and temporally precise manipulations (Table 1).Table 1Methods of inactivationMethodMechanismPenetranceCharacteristicsLesionsIrreversible removal of neural tissue [87.Lavond D.G. Steinmetz J.E. Lesion techniques for behavioral experiments.in: Handbook of Classical Conditioning. 1st edn. Springer, 2003: 249-276Google Scholar]HighDependent on how completely the target brain area is removed [88.Lomber S.G. The advantages and limitations of permanent or reversible deactivation techniques in the assessment of neural function.J. Neurosci. Methods. 1999; 86: 109-117Google Scholar]Compensatory plasticity [88.Lomber S.G. The advantages and limitations of permanent or reversible deactivation techniques in the assessment of neural function.J. Neurosci. Methods. 1999; 86: 109-117Google Scholar,89.Depner M. et al.Sensory cortex lesion triggers compensatory neuronal plasticity.BMC Neurosci. 2014; 15: 1-13Google Scholar]Damage to fibres of passageaExcitotoxic lesions which spare fibres of passage. [90.Koo J.W. et al.Selective neurotoxic lesions of basolateral and central nuclei of the amygdala produce differential effects on fear conditioning.J. Neurosci. 2004; 24: 7654-7662Google Scholar]Degeneration of upstream areas (e.g., thalamus) [88.Lomber S.G. The advantages and limitations of permanent or reversible deactivation techniques in the assessment of neural function.J. Neurosci. Methods. 1999; 86: 109-117Google Scholar]PharmacologicalActivation of inhibitory neurons via reagents [82.Martin J.H. Ghez C. Pharmacological inactivation in the analysis of the central control of movement.J. Neurosci. Methods. 1999; 86: 145-159Google Scholar]ModerateDependent on ligand diffusion [82.Martin J.H. Ghez C. Pharmacological inactivation in the analysis of the central control of movement.J. Neurosci. Methods. 1999; 86: 145-159Google Scholar]Area of effect relies on diffusion of reagent which may vary between reagents (e.g., muscimol spreads maximally and γ-aminobutyric acid [GABA] minimally) [82.Martin J.H. Ghez C. Pharmacological inactivation in the analysis of the central control of movement.J. Neurosci. Methods. 1999; 86: 145-159Google Scholar]Difficult to apply to certain brain areasCoolingReduction of cortical temperature to reduce spiking [21.Coomber B. et al.Cortical inactivation by cooling in small animals.Front. Syst. Neurosci. 2011; 5: 53Google Scholar]ModerateDependent on temperature conduction through tissue [7.Griffiths P.E. et al.Measuring causal specificity.Philos. Sci. 2015; 82: 529-555Google Scholar,8.Wolff S.B. Ölveczky B.P. The promise and perils of causal circuit manipulations.Curr. Opin. Neurobiol. 2018; 49: 84-94Google Scholar]Slow but sustained control of inactivation [88.Lomber S.G. The advantages and limitations of permanent or reversible deactivation techniques in the assessment of neural function.J. Neurosci. Methods. 1999; 86: 109-117Google Scholar]Area of effect is dependent on the size of the cooling loop/probe [21.Coomber B. et al.Cortical inactivation by cooling in small animals.Front. Syst. Neurosci. 2011; 5: 53Google Scholar]Can cool nontarget areas via cooled blood vessels [21.Coomber B. et al.Cortical inactivation by cooling in small animals.Front. Syst. Neurosci. 2011; 5: 53Google Scholar]ChemogeneticsActivation of receptors, genetically expressed in target neurons, via ligands [9.Lamas V. et al.Long-term evolution of brainstem electrical evoked responses to sound after restricted ablation of the auditory cortex.PLoS One. 2013; 8e73585Google Scholar]Low to moderateDependent on delivery, either viral vector or transgenic animals [23.Popelář J. et al.Cooling of the auditory cortex modifies neuronal activity in the inferior colliculus in rats.Hear. Res. 2016; 332: 7-16Google Scholar,81.Wood K.C. et al.Acute inactivation of primary auditory cortex causes a sound localisation deficit in ferrets.PLoS One. 2017; 12e0170264Google Scholar]Area of effect is dependent on both diffusion of ligand and expression of receptor [79.Fenno L. et al.The development and application of optogenetics.Annu. Rev. Neurosci. 2011; 34: 389-412Google Scholar]OptogeneticsActivation of photoreceptors, genetically expressed in target neurons, via light [79.Fenno L. et al.The development and application of optogenetics.Annu. Rev. Neurosci. 2011; 34: 389-412Google Scholar]Low to moderateDependent on delivery, either viral vector [91.Zhang F. et al.Optogenetic interrogation of neural circuits: technology for probing mammalian brain structures.Nat. Protoc. 2010; 5: 439-456Google Scholar] or transgenic animals [79.Fenno L. et al.The development and application of optogenetics.Annu. Rev. Neurosci. 2011; 34: 389-412Google Scholar]Differential expression between species [79.Fenno L. et al.The development and application of optogenetics.Annu. Rev. Neurosci. 2011; 34: 389-412Google Scholar]Rapid control of inactivation [92.Wiegert J.S. et al.Silencing neurons: tools, applications, and experimental constraints.Neuron. 2017; 95: 504-529Google Scholar]Area of effect is dependent on both spread of light delivery and expression of receptors [79.Fenno L. et al.The development and application of optogenetics.Annu. Rev. Neurosci. 2011; 34: 389-412Google Scholar]Cell specificity mostly restricted to mouse model [93.Zeng H. Madisen L. Mouse transgenic approaches in optogenetics.in: Waxman S. Progress in Brain Research. Elsevier, 2012: 193-213Google Scholar]Controls needed against heat generation from light application [92.Wiegert J.S. et al.Silencing neurons: tools, applications, and experimental constraints.Neuron. 2017; 95: 504-529Google Scholar]a Excitotoxic lesions which spare fibres of passage. Open table in a new tab This, combined with our improved understanding of the functional anatomy of AC and its subregions (Figure 1A ), puts us in a better position to revisit inactivation as a method for investigating cortical function in the context of auditory cortex. We will start by considering the factors that influence the likelihood of observing an inactivation-induced deficit whilst bringing together the current knowledge of behavioural effects of AC inactivation, with the goal of guiding future research. Finally, we will attempt to synthesise the existing literature to identify functions in which auditory cortical activity is most strongly implicated. Approaches to silencing activity in auditory cortex range in timescale from permanent lesions to reversible inactivation via thermal, pharmacological or chemogenetic means (allowing inactivation from minutes to hours) and optogenetics (enabling millisecond precision, Figure 1B). All inactivation methods are subject to the trade-off between certainty that the entire region of interest is inactivated and the risk of inactivating adjacent areas potentially producing off-target effects. Inactivating neural tissue beyond the border of the target area can result in inflating the impairment that would result from a more focal inactivation. Conversely, it has been shown that sparing portions of the target area can be sufficient to subserve some aspects of the studied behaviour [3.Jenkins W.M. Merzenich M.M. Role of cat primary auditory cortex for sound-localization behavior.J. Neurophysiol. 1984; 52: 819-847Google Scholar], creating the risk of underestimating the role of the inactivated structure. 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.Ohl F.W. et al.Bilateral ablation of auditory cortex in Mongolian gerbil affects discrimination of frequency modulated tones but not of pure tones.Learn. Mem. 1999; 6: 347-362Google Scholar, 5.Deutscher A. et al.Cortical and subcortical sides of auditory rhythms and pitches.Neuroreport. 2006; 17: 853-856Google Scholar, 6.Bowen G.P. et al.Auditory cortex lesions in the rat impair both temporal acuity and noise increment thresholds, revealing a common neural substrate.Cereb. Cortex. 2003; 13: 815-822Google Scholar]. It has been argued that the ability to recover from a permanent lesion is a hallmark of an area with a permissive role (see Glossary) in behaviour, rather than an instructive role [7.Griffiths P.E. et al.Measuring causal specificity.Philos. Sci. 2015; 82: 529-555Google Scholar]. Thus, contrasting the effects of permanent and temporary inactivation can distinguish between these two possibilities [8.Wolff S.B. Ölveczky B.P. The promise and perils of causal circuit manipulations.Curr. Opin. Neurobiol. 2018; 49: 84-94Google Scholar]. Nonetheless, given the scope of plasticity within both brain and behaviour, it seems likely that the role of initially instructive areas could be taken on by other circuits. Adaptation itself can result from processes including recovery from surgery [9.Lamas V. et al.Long-term evolution of brainstem electrical evoked responses to sound after restricted ablation of the auditory cortex.PLoS One. 2013; 8e73585Google Scholar,10.Clarkson C. et al.Long-term functional recovery in the rat auditory system after unilateral auditory cortex ablation.Acta Otolaryngol. 2010; 130: 326-332Google Scholar], adaptive plasticity [11.Gold J.R. Bajo V.M. Insult-induced adaptive plasticity of the auditory system.Front. Neurosci. 2014; 8: 110Google Scholar], relearning the discrimination (i.e., recovering from a memory deficit [2.Weinberger N.M. Specific long-term memory traces in primary auditory cortex.Nat. Rev. Neurosci. 2004; 5: 279-290Google Scholar]) or learning to solve the task in an alternative way, either by using different cues [12.Heffner H. Masterton B. Contribution of auditory cortex to sound localization in the monkey (Macaca mulatta).J. Neurophysiol. 1975; 38: 1340-1358Google Scholar,13.Porter B.A. et al.Discrimination of brief speech sounds is impaired in rats with auditory cortex lesions.Behav. Brain Res. 2011; 219: 68-74Google Scholar] or by recruiting alternative neural circuits [14.Heffner H. Heffner R.S. Effect of unilateral and bilateral auditory cortex lesions on the discrimination of vocalizations by Japanese macaques.J. Neurophysiol. 1986; 56: 683-701Google Scholar,15.Kavanagh G.L. Kelly J.B. Midline and lateral field sound localization in the albino rat (Rattus norvegicus).Behav. Neurosci. 1986; 100: 200-205Google Scholar]. Permanent lesions are likely to have profound effects on areas upstream and downstream of the lesioned site; indeed, retrograde degeneration of the ventral division of the medial geniculate body (MBGv) is a hallmark of a successful A1 lesion [16.Neff W.D. et al.Behavioral studies of auditory discrimination: central nervous system.in: Keidel W.D. Neff W.D. Handbook of Sensory Physiology. Springer, 1975: 307-400Google Scholar]. Thus, the behavioural effects of lesioning may lead to overstated conclusions regarding the role of the target region, as in addition to impacts on connected areas, potential damage to blood supply and fibres of passage within the lesioned area may further perturb activity beyond the target [17.Schwarcz R. et al.Ibotenic acid-induced neuronal degeneration: a morphological and neurochemical study.Exp. Brain Res. 1979; 37: 199-216Google Scholar,18.Pai S.S. et al.Minimal impairment in a rat model of duration discrimination following excitotoxic lesions of primary auditory and prefrontal cortices.Front. Syst. Neurosci. 2011; 5: 74Google Scholar]. By contrast, a reversible inactivation preserves normal activity in between sessions such that connected structures are not severely impacted. During inactivation, it is likely that some residual activity will persist; how that information is interpreted in these structures is unclear. One possibility is that the residual activity is similar enough to physiologically normal activity that it is still treated as viable input, even if it is not informative about the actual sensory stimulus [8.Wolff S.B. Ölveczky B.P. The promise and perils of causal circuit manipulations.Curr. Opin. Neurobiol. 2018; 49: 84-94Google Scholar,19.Otchy T.M. et al.Acute off-target effects of neural circuit manipulations.Nature. 2015; 528: 358-363Google Scholar]. While all reversible inactivation methods are susceptible to being incomplete, improved monitoring of activity in the target area could provide better insight into the extent of inactivation. Temporary inactivation approaches, especially with a rapid onset, are less susceptible to the confounding effects of plasticity (although some degree of plasticity can occur very rapidly [20.O'Sullivan C. et al.Auditory cortex contributes to discrimination of pure tones.eNeuro. 2019; 6 (ENEURO.0340-19.2019)Google Scholar]), allowing for clearer insights into functions that the target area is normally involved in, rather than strictly necessary for. Nonetheless, certain aspects of the spatial precision of reversible inactivations are difficult to control. First, both pharmacological agents and cooling may spread to off-target areas [21.Coomber B. et al.Cortical inactivation by cooling in small animals.Front. Syst. Neurosci. 2011; 5: 53Google Scholar]. The diffusive properties of pharmacological agents differ (Table 1), but can be partially controlled depending on the method of application: infusions and topical applications offer less control in that regard, while the use of slow-releasing polymers (e.g., Elvax [22.Bizley J.K. et al.Role of auditory cortex in sound localization in the midsagittal plane.J. Neurophysiol. 2007; 98: 1763-1774Google Scholar]) can help regulate the extent of diffusion. In addition, the efficacy of inactivation through both cooling and use of pharmacological agents may differ across the cortical layers, with more activity remaining in deeper layers [23.Popelář J. et al.Cooling of the auditory cortex modifies neuronal activity in the inferior colliculus in rats.Hear. Res. 2016; 332: 7-16Google Scholar, 24.Talwar S.K. et al.Role of mammalian auditory cortex in the perception of elementary sound properties.J. Neurophysiol. 2001; 85: 2350-2358Google Scholar, 25.Gimenez T.L. et al.Adaptive categorization of sound frequency does not require the auditory cortex in rats.J. Neurophysiol. 2015; 114: 1137-1145Google Scholar]. Compared with lesions and pharmacological methods, optogenetics and chemogenetics potentially offer a much higher degree of spatial precision. Both methods provide the invaluable opportunity to target neuronal activity in a cell-type-specific manner which, in the case of optogenetics, is coupled with millisecond temporal precision [26.Vlasov K. et al.Optogenetics and chemogenetics.Methods Enzymol. 2018; 603: 181-196Google Scholar]. However, there are certain limitations due to the way in which neuronal subtypes are targeted. Notably, there is a risk that rather than fully inhibiting activity in a given area, activity is only reduced and/or perturbed [8.Wolff S.B. Ölveczky B.P. The promise and perils of causal circuit manipulations.Curr. Opin. Neurobiol. 2018; 49: 84-94Google Scholar]. Furthermore, the method of expression (i.e., viral vectors vs. transgenic expression) will determine the penetrance (lower for viral vectors which is dependent on spread from injection site) and spatial specificity (lower for transgenic animals where the light placement will largely determine specificity) [27.Lerchner W. et al.Injection parameters and virus dependent choice of promoters to improve neuron targeting in the nonhuman primate brain.J. Gene. Ther. 2014; 21: 233-241Google Scholar]. While histological methods can confirm the placement of virus-mediated expression, the spread of light will also affect the extent of inactivation. Challenges in delivering light – to deeper cortical layers and, in the case of larger species, to a sufficiently large area – mean that the efficacy with which the targeted populations are modulated can be variable [26.Vlasov K. et al.Optogenetics and chemogenetics.Methods Enzymol. 2018; 603: 181-196Google Scholar], likely resulting in incomplete inactivation. Cell-type-specific manipulations can in certain situations lead to confounding results. In the case of optogenetics, inactivation is usually achieved in one of two ways. One possibility is to activate inhibitory interneurons (most often parvalbumin-positive [PV+] interneurons), which reduce the firing of principal neurons (PN) by providing them with inhibitory input. Alternatively, one can seek to inhibit the firing of excitatory neurons directly. Due to the dense interconnectivity between both PN–PN and PN–PV+ in the cortex, some of the changes resulting from neuron-subtype-specific manipulations might be unexpected. For example, optogenetic suppression of PV+ neurons would be predicted to cause an increase in excitation. However, suppressing PV+ interneurons in layer 4 led to a net increase in PV+ activity in downstream layers [28.Moore A.K. et al.Rapid rebalancing of excitation and inhibition by cortical circuitry.Neuron. 2018; 97: 1341-1355.e6Google Scholar], indicating that the excitation-inhibition imbalance produced can be corrected within milliseconds from PV+ inactivation. Furthermore, while opsins were expressed in neurons in all cortical layers, the effects of optical illumination suppressed the activity of PV+ neurons only in layer 4. In a similar vein, targeting a specific subtype of neurons defined by its genetic markers can have potential caveats, as different subtypes seem to work as functional units and thus are likely to be tied to specific behaviours [29.Fishell G. Kepecs A. Interneuron types as attractors and controllers.Annu. Rev. Neurosci. 2020; 43: 1-30Google Scholar]. As a result, depending on the subtype of interneurons targeted, the behavioural effects of optogenetic inactivation may differ and may not generalise to the effects of silencing other neuronal subtypes. The observation that the severity of behavioural impairment scales with the size of the cortical area affected has been made in a number of studies testing different behaviours [22.Bizley J.K. et al.Role of auditory cortex in sound localization in the midsagittal plane.J. Neurophysiol. 2007; 98: 1763-1774Google Scholar,30.Heffner H.E. Heffner R.S. Effect of bilateral auditory cortex lesions on sound localization in Japanese macaques.J. Neurophysiol. 1990; 64: 915-931Google Scholar,31.Kelly J.B. Whitfield I.C. Effects of auditory cortical lesions on discriminations of rising and falling frequency-modulated tones.J. Neurophysiol. 1971; 34: 802-816Google Scholar]. However, electrophysiological and anatomical studies make it clear that AC comprises multiple subregions with different response properties to sounds; therefore, the precise subregion affected should determine the type of deficit that may be observed. Despite this knowledge, the area of inactivation is often ill defined. Most studies to date aimed to inactivate primary AC (A1) as it is most clearly stereotaxically- and cytoarchitectonically-defined and is the region with most well-defined homologues in other species [32.Kaas J.H. The evolution of auditory cortex: the core areas.in: Winer J.A. Schreiner C.E. The Auditory Cortex. Springer, 2011: 407-427Google Scholar]. The hierarchical organisation of auditory cortex means that disrupting primary cortex will additionally perturb processing in non‐primary areas [33.Carrasco A. Lomber S.G. Evidence for hierarchical processing in cat auditory cortex: nonreciprocal influence of primary auditory cortex on the posterior auditory field.J. Neurosci. 2009; 29: 14323-14333Google Scholar,34.Kok M.A. et al.Dissociable influences of primary auditory cortex and the posterior auditory field on neuronal responses in the dorsal zone of auditory cortex.J. Neurophysiol. 2015; 113: 475-486Google Scholar]. Despite A1 being the most frequently targeted inactivation site, the trade-off between incomplete inactivation within an area and nonspecific effects beyond a target area make it likely that non‐primary auditory fields (Figure 1A) are usually at least partially affected as well, thus obfuscating the exact role of A1. This was particularly well demonstrated in a study in which primary AC and dorsal zone (DZ), a non‐primary area often inactivated in studies targeting primary AC, were inactivated individually, as well as simultaneously [35.Malhotra S. et al.Sound localization deficits during reversible deactivation of primary auditory cortex and/or the dorsal zone.J. Neurophysiol. 2008; 99: 1628-1642Google Scholar]. The study showed that the extent of behavioural deficits in sound localisation often attributed to loss of function in primary AC is in fact overstated; comparable results can be obtained only when both primary AC and DZ are inactivated simultaneously, but not individually. In larger species silencing distinct regions is more feasible, enabling efforts to identify the particular contributions of specific auditory fields to auditory behaviour [36.Malhotra S. Lomber S.G. Sound localization during homotopic and heterotopic bilateral cooling deactivation of primary and nonprimary auditory cortical areas in the cat.J. Neurophysiol. 2007; 97: 26-43Google Scholar,37.Lomber S.G. Malhotra S. Double dissociation of 'what' and 'where' processing in auditory cortex.Nat. Neurosci. 2008; 11: 609-616Google Scholar]. While formal comparisons of the effects of AC inactivation across species are lacking, there does seem to be systematic variation in the results obtained in studies using primates, carnivores, and rodents (Table 2). It has been speculated that the degree to which auditory perception is dependent on AC is proportional to the acuity of hearing of a given species [24.Talwar S.K. et al.Role of mammalian auditory cortex in the perception of elementary sound properties.J. Neurophysiol. 2001; 85: 2350-2358Google Scholar]. For example, sound localisation, which is much less precise in rodents than in carnivores or primates [15.Kavanagh G.L. Kelly J.B. Midline and lateral field sound localization in the albino rat (Rattus norvegicus).Behav. Neurosci. 1986; 100: 200-205Google Scholar], is fully spared in rodents following AC inactivation [38.Kelly J.B. Kavanagh G.L. Effects of auditory cortical lesions on pure-tone sound localization by the albino rat.J. Behav. Neurosci. 1986; 100: 569-575Google Scholar], while in other species it is profoundly impaired [30.Heffner H.E. Heffner R.S. Effect of bilateral auditory cortex lesions on sound localization in Japanese macaques.J. Neurophysiol. 1990; 64: 915-931Google Scholar,37.Lomber S.G. Malhotra S. Double dissociation of 'what' and 'where' processing in auditory cortex.Nat. Neurosci. 2008; 11: 609-616Google Scholar]. Moreover, specificity and precision in localisation of lesions will vary between animals due to differences in size of cortical regions (Figure 1A).Table 2Degree of impairment on standard auditory tasks following AC inactivationAC regionPrimatesCarnivoresRodentsSound detectionPermanentPermanentTemporaryPermanentTemporaryA1–Mild [94.Kavanagh G.L. Kelly J.B. Hearing in the ferret (Mustela putorius): effects of primary auditory cortical lesions on thresholds for pure tone detection.J. Neurophysiol. 1988; 60: 879-888Google Scholar]–No impairment [89.Depner M. et al.Sensory cortex lesion triggers compensatory neuronal plasticity.BMC Neurosci. 2014; 15: 1-13Google Scholar]Mild (optogenetic upregulation of interneurons, several injections of 20 nl, 1 mm optic fibre [43.Kato H.K. et al.Flexible sensory representations in auditory cortex driven by behavioral relevance.Neuron. 2015; 88: 1027-1039Google Scholar]) to severe (optogenetic inactivation, volume of injection: 120 nl, size of fibre: 400 μm [58.Li J. et al.Primary auditory cortex is required for anticipatory motor response.Cereb. Cortex. 2017; 27: 3254-3271Google Scholar]; topical application of 20 μg of muscimol [24.Talwar S.K. et al.Role of mammalian auditory cortex in the perception of elementary sound properties.J. Neurophysiol. 2001; 85: 2350-2358Google Scholar])A1+Moderate (partial recovery) [14.Heffner H. Heffner R.S. Effect of unilateral and bilateral auditory cortex lesions on the discrimination of vocalizations by Japanese macaques.J. Neurophysiol. 1986; 56: 683-701Google Scholar,95.Heffner H.E. Heffner R.S. Hearing loss in Japanese macaques following bilateral auditory cortex lesions.J. Neurophysiol. 1986; 55: 256-271Google Scholar]Mild [96.Cranford J.L. Detection versus discrimination of brief tones by cats with auditory cortex lesions.J. Acoust. Soc. Am. 1979; 65: 1573-1575Google Scholar]–––Non-primary areas–––––Frequency discriminationPermanentPermanentTemporaryPermanentTemporaryA1––––Severe (chemogenetics, 60 nl virus injection)A1+Moderate (partial recovery) [40.Harrington I.A. et al.An investigation of sensory deficits underlying the aphasia-like behavior of macaques with auditory cortex lesions.Neuroreport. 2001; 12: 1217-1221Google Scholar]Moderate (partial recovery) [96.Cranford J.L. Detection versus discrimination of brief tones by cats with auditory cortex lesions.J. Acoust. Soc. Am. 1979; 65: 1573-1575Google Scholar]–No impairment [4.Ohl F.W. et al.Bilateral ablation of auditory cortex in Mongolian gerbil affects discrimination of frequency modulated tones but not of pure tones.Learn. Mem. 1999; 6: 347-362Google Scholar] to mild impairment [20.O'Sullivan C. et al.Auditory cortex contributes to discrimination of pure tones.eNeuro. 2019; 6 (ENEURO.0340-19.2019)Google Scholar]Mild (muscimol: 400 nl at four sites [25.Gimenez T.L. et al.Adaptive categorization of sound frequency does not require the auditory cortex in rats.J. Neurophysiol. 2015; 114: 1137-1145Google Scholar]) to severe (topical application of 20 μg of muscimol [24.Talwar S.K. et al.Role of mammalian auditory cortex in the perception of elementary sound properties.J. Neurophysiol. 2001; 85: 2350-2358Google Scholar])Non-primary areas–––––Gap detectionPermanentPermanentTemporaryPermanentTemporaryA1No available studies*Moderate