Diverse cortical codes for scene segmentation in primate auditory cortex.

Characteristics of neurons in auditory cortex of monkeys performing a simple auditory task.

Reliability and Representational Bandwidth in the Auditory Cortex

Properties of sequence-sensitive neurons in primary auditory cortex of cats were explored in detail. Stimuli were sequences of two tones, in which the frequency and intensity of the first tone and the temporal separation between the first and second, or probe, tone were parametrically varied. After presentation of the first tone, the responses of 32 single units and 48 multiunits to the probe tone were found to be enhanced up to 140-5270% (median 340%) above the response obtained in the single-tone condition. Probe tone enhancement was induced from a considerable number of sequence conditions and depended on the frequency and intensity of the first tone and on the temporal separation between the onsets of the first and the probe tone. On average, the maximally enhanced response occurred when the first tone was 1 octave below or above the probe tone and its intensity was 14 dB louder than the probe tone. The most effective temporal separation of the tones for an enhancement effect was approximately 100 ms. The range of enhancing tones was largely outside the excitatory tuning curve of a neuron. Results extend previous findings of properties of sequence-sensitive neurons in the auditory cortex of echolocating bats and non-echolocating mammals, and suggest that sequence-sensitive neurons are quite common and involved in the cortical representation of spectrotemporal patterns of acoustic signals.

AILDD1 AC'1 The responses of neurons in the mustached bat's auditory cortex are specialized to extract particular information from biosonar signals. For this study, we mapped response properties across entire hemispheres in several animals. These experiments enabled us to construct a standard map that aided in determining the connections among the areas, as described subsequently. The mapping also yielded quantitative data regarding the relative sizes of areas and the proportion of cortex devoted to different response types. We identified six response types that were distributed in 11 areas. Eight areas, comprising two-thirds of the auditory cortex, contained neurons sensitive to particular components in biosonar signals. Most were facilitated by combinations of frequency modulated and constant frequency biosonar signal components (FMs and CFs, respectively). There were three major types of combination-sensitive neurons: FM-FM, CF/CF, and FM-CF. Each type of combination sensitivity occurred in multiple areas. The largest proportion were FM-FM neurons (approximately 30% of all neurons in auditory cortex), followed by FM1-CF2 (approximately 23%) and CF/CF (approximately 11%). In the other three areas comprising approximately one-third of the auditory cortex, most neurons responded well to frequencies not contained in biosonar signals.

Over the last 50 yr, environmental enrichment has been shown to generate more than a dozen changes in brain anatomy. The consequences of these physical changes on information processing have not been well studied. In this study, rats were housed in enriched or standard conditions either prior to or after reaching sexual maturity. Evoked potentials from awake rats and extracellular recordings from anesthetized rats were used to document responses of auditory cortex neurons. This report details several significant, new findings about the influence of housing conditions on the responses of rat auditory cortex neurons. First, enrichment dramatically increases the strength of auditory cortex responses. Tone-evoked potentials of enriched rats, for example, were more than twice the amplitude of rats raised in standard laboratory conditions. Second, cortical responses of both young and adult animals benefit from exposure to an enriched environment and are degraded by exposure to an impoverished environment. Third, housing condition resulted in rapid remodeling of cortical responses in <2 wk. Fourth, recordings made under anesthesia indicate that enrichment increases the number of neurons activated by any sound. This finding shows that the evoked potential plasticity documented in awake rats was not due to differences in behavioral state. Finally, enrichment made primary auditory cortex (A1) neurons more sensitive to quiet sounds, more selective for tone frequency, and altered their response latencies. These experiments provide the first evidence of physiologic changes in auditory cortex processing resulting from generalized environmental enrichment.

The frequency resolution of neurons throughout the ascending auditory pathway is important for understanding how sounds are processed. In many animal studies, the frequency tuning widths are about 1/5th octave wide in auditory nerve fibers and much wider in auditory cortex neurons. Psychophysical studies show that humans are capable of discriminating far finer frequency differences. A recent study suggested that this is perhaps attributable to fine frequency tuning of neurons in human auditory cortex (Bitterman Y, Mukamel R, Malach R, Fried I, Nelken I. Nature 451: 197-201, 2008). We investigated whether such fine frequency tuning was restricted to human auditory cortex by examining the frequency tuning width in the awake common marmoset monkey. We show that 27% of neurons in the primary auditory cortex exhibit frequency tuning that is finer than the typical frequency tuning of the auditory nerve and substantially finer than previously reported cortical data obtained from anesthetized animals. Fine frequency tuning is also present in 76% of neurons of the auditory thalamus in awake marmosets. Frequency tuning was narrower during the sustained response compared to the onset response in auditory cortex neurons but not in thalamic neurons, suggesting that thalamocortical or intracortical dynamics shape time-dependent frequency tuning in cortex. These findings challenge the notion that the fine frequency tuning of auditory cortex is unique to human auditory cortex and that it is a de novo cortical property, suggesting that the broader tuning observed in previous animal studies may arise from the use of anesthesia during physiological recordings or from species differences.

Vocal communication plays an important role in the lives of both humans and many animal species. Ensuring accurate communication, however, requires auditory self-monitoring to control vocal production and rapidly compensate for errors in vocal output. Despite the importance of this process, the underlying neural mechanisms are relatively unknown. Previous work has demonstrated that neurons in the auditory cortex are suppressed during vocal production, while simultaneously maintaining their sensitivity to vocal feedback, suggesting a role in auditory self-monitoring. The behavioral role of auditory cortex in vocal control, however, remains unclear. We investigated the function of auditory cortical activity during vocal self-monitoring and feedback-dependant vocal control in marmoset monkeys. Using real-time frequency-shifted feedback during vocalization, we demonstrate that marmosets exhibit rapid compensatory changes in vocal production, a feedback-dependent behavior that is predicted by the activities of neurons in auditory cortex. We further establish the role of auditory cortex in vocal control using electrical microstimulation to evoke rapid changes in produced vocalizations. These findings suggest a causal role for the auditory cortex in vocal self-monitoring and feedback-dependent vocal control, linking mechanisms of production and perception, and have important implications for understanding human speech motor control.

Objective To explore the possible mechanism of miR-34a/Bcl-2 apoptotic pathway in the mouse model of age-related hearing loss. Methods A C57BL/6 mouse model of age-related hearing loss was conducted, and 4-week-old and 12-month-old mice were considered as the objects. The auditory brainstem response (ABR) was used to test the hearing function. The TdT-mediated dUTP nick end labeling (TUNEL) was used to observe the apoptosis of neuron in auditory cortex. The mRNA and protein levels of miR-34a, Bcl-2 and caspase 3 were detected by real-time PCR and Western bloting, respectively. Results The ABR showed that the hearing threshold level at 4, 8, 16, 32 kHz were higher in 12-month-old mice than in 4-week-old mice〔(80.0±2.5) dBHL vs. (32.0±4.5) dBHL, (74.0±3.5) dBHL vs. (51.0±1.2) dBHL, (86.0±4.6) dBHL vs. (51.0±3.5) dBHL, (87.0±6.6) dBHL vs. (56.0±1.5) dBHL, all P<0.05〕. Compared with 4-week-old mice, the total number of neurons in auditory cortex was decreased, the number of apoptosis neurons was increased, the expressions of miR-34a (t=6.02, P=0.001), Bax (t=6.51, P=0.012) and Caspase-3 (P=0.023) rised, and the expression of Bcl-2 (t=7.12, P=0.032) declined in 12-month-old mice. Conclusions The activation of miR-34a/Bcl-2 apoptotic pathway may be one of the mechanisms of age-related hearing loss. Key words: Presbycusis; Apoptosis

Azimuth tuning of high-frequency neurons in the primary auditory cortex (AI) is known to depend on binaural disparity and monaural spectral (pinna) cues present in broadband noise bursts. Single-unit response patterns differ according to binaural interactions, strength of monaural excitatory input from each ear, and azimuth sensitivity to monaural stimulation. The latter characteristic has been used as a gauge of neural sensitivity to monaural spectral directional cues. Azimuth sensitivity may depend predominantly on binaural disparity cues, exclusively on monaural spectral cues, or on both. The primary goal of this study was to determine whether each cortical response pattern corresponds to a similar pattern in the medial geniculate body (MGB) or whether some patterns are unique to the cortex. Single-unit responses were recorded from the ventral nucleus (Vn) and lateral part of the posterior group of thalamic nuclei (Po), tonotopic subdivisions of the MGB. Responses to free-field presentation of noise bursts that varied in azimuth and sound pressure level were obtained using methods identical to those used previously in field AI. Many units were azimuth sensitive, i.e., they responded well at some azimuths, and poorly, if at all, at others. These were studied further by obtaining responses to monaural noise stimulation, approximated by reversible plugging of one ear. Monaural directional (MD) cells were sensitive to the azimuth of monaural noise stimulation, whereas binaural directional (BD) cells were either insensitive to its azimuth or monaurally unresponsive. Thus BD and MD cells show differential sensitivity to monaural spectral cues. Monaural azimuth sensitivity could not be used to interpret the spectral sensitivity of predominantly binaural cells that exhibited strong binaural facilitation because they were either unresponsive or poorly responsive to monaural stimulation. The available evidence suggests that some such cells are sensitive to spectral cues. The results do not indicate the presence of any response types in AI that are not present in the MGB. Vn and Po contain similar classes of MD and BD cells. Because Po neurons project to the anterior auditory field, neurons in this cortical area also are likely to exhibit differential sensitivity to binaural disparity and monaural spectral cues. Comparison of these MGB data with a published report of cochlear nucleus (CN) single-unit azimuth tuning shows that MGB sensitivity to spectral cues is considerably stronger than CN sensitivity.

The effects of conditioning on the discharges of single neurons in primary auditory cortex (AI) were determined during acquisition of the pupillary conditioned response in chronically prepared cats. Acoustic stimuli (1-s white noise or tone) were presented with electrodermal stimulation unpaired during a sensitization control phase followed by pairing during a subsequent conditioning phase. Stimulus constancy at the periphery was ensured by the use of neuromuscular blockade. Discharge plasticity developed rapidly for both evoked and background activity, the former attaining criterion faster than the latter. The pupillary dilation conditioned response was acquired at the same rate as were changes in evoked activity (i.e., 10-15 trials) and faster than background activity (i.e., 20-25 trials). Increases in background activity were correlated with increasing level of tonic arousal, as indexed by pretrial size of the pupil.

Frequency-specific plasticity of the auditory cortex elicited by thalamic stimulation in the rat

M-channels modulate the intrinsic excitability and synaptic responses of layer 2/3 pyramidal neurons in auditory cortex

Mapping of stimulus features and meaning in gerbil auditory cortex with 2-deoxyglucose and c-Fos antibodies

Event Abstract Back to Event Causal role of auditory cortex in mediating attention to moments in time Attention is the means by which the brain selects and highlights important sensory information for further processing, ignoring or suppressing the rest. The earliest single unit studies revealed that neural activity in the first stages of cortical sensory processing is modulated by attention (Hubel et al., 1959). To date, most of what is known about the neural mechanisms underlying attention is based on correlations between neural activity and behavior. Here we establish for the first time that not only is neural activity in the auditory cortex correlated with attention, but that the auditory cortex is causally involved in mediating the behavioral advantages conferred by attention. We first developed a rodent behavioral paradigm in which we could demonstrate that attention enhanced performance on a sensory discrimination task. The task was an auditory two-alternative choice paradigm, modeled after a related visual paradigm in humans (Coull and Nobre, 1998), in which we manipulated attention to moments in time. Rats were required to detect a frequency-modulated target sound immersed in a train of pure tone distractors. Performance varied systematically with task difficulty, manipulated by the varying modulation depth of the target. Behavioral analysis showed a clear improvement in both reaction times and correct discrimination whenever the target appeared at an expected moment, compared to the same target appearing unexpectedly. These results show that attention improved performance. We next used tetrodes to record responses from single neurons in the primary auditory cortex of rats performing this task. The majority of responsive neurons showed an increase evoked response to tones immediately preceding the expected appearance of the target when compared against responses to the same tones occurring long before the expected target location. These results show that attention to moments in time modulates changes in neural activity in the auditory cortex. Finally, we assessed the effect on performance of pharmacological inactivation of the auditory cortex. We used the GABA-A receptor agonist muscimol to reversibly inactivate the auditory cortex bilaterally. Muscimol inactivation impaired performance compared with control application of saline, implying that the auditory cortex plays an important role in this task under normal conditions. Furthermore, inactivation of the auditory cortex significantly reduced the improvements in performance associated with attention. These results suggest that the auditory cortex is required to mediate the behavioral advantage conferred by attention. In summary, we have shown that: (1) attention to moments in time can lead to improved performance on a two-alternative choice auditory detection/discrimination task in rats; (2) neural activity in the primary auditory cortex is correlated with attention to moments in time; and (3) the auditory cortex plays a causal role in mediating the behavioral advantage conferred by attention.

The technology for large-scale electrical recordings is rapidly improving, and extracellular recordings with multielectrode arrays now offer a unique window into neural activity at the population level. The high-frequency part of the recorded signal (MUA; multi-unit activity) is a measure of action-potential firing of neurons in the immediate vicinity of the electrode contacts, while the low-frequency part (LFP; local-field potential) appears to mainly be a measure of subthreshold dendritic activity of surrounding neurons [1,2]. The interpretation of MUA and LFP signals in terms of neural circuit activity is not straightforward, however, and new analysis methods are needed to take full advantage of the technological developments. Laminar population analysis (LPA) [1] was introduced a few years ago as a new method for extracting information about population activity. Based on physiologically-constrained joint modeling of the MUA and LFP signals, both the (i) laminar organization and time-resolved firing rates of the main cortical populations, and (ii) the functional synaptic connection patterns between these populations, were estimated. Later, the population firing-rates extracted by means of LPA were used to specify mathematical neural-network models describing thalamocortical and intracortical connections between neural populations in the rat somatosensory (barrel) system [3]. In Ref. [1] LPA was used to analyze stimulus-averaged laminar (linear) electrode data from the rat barrel system, and the predicted time-resolved population firing rates and synaptic connection patterns were found to be in qualitative agreement with previous experimental findings. Here we apply LPA to recently recorded laminar-electrode data from the primary auditory cortex of rats [4]. In this study the rats were exposed to a large variety of different auditory stimuli to investigate the phenomenon known as ‘stimulus-specific adaptation’, i.e., that neurons respond stronger to unexpected stimuli. Such rich data sets where the cortical neural circuits of interest potentially exhibit a large variety of activation patterns, are particularly well suited for analysis by means of LPA. Results from LPA analysis will be presented, highlighting the differences and similarities seen between the sensory-evoked cortical response patterns in primary auditory and somatosensory cortices of rats.