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1Bio-Imaging Lab and 2Vision Lab, University of Antwerp, Antwerp, Belgium; 3Sensory Ecology and Neuroethology Lab ENES EA3988, Université Jean Monnet, Saint-Etienne & Laboratoire Traitement du Signal Instrumentation, Centre National de la Recherche Scientifique, Unité Mixte de Recherche (CNRS UMR) 8620, Université Paris XI, Orsay, France; and 4Hubert Curien Lab CNRS UMR 5516, Université Jean Monnet, Saint-Etienne, France
Submitted 29 April 2007; accepted in final form 18 September 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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). The communicability of auditory signals provided by bird vocalizations is dependent on many factors present in the natural environment, not least of which are wind noise and foliage density. The capacity to segregate auditory signals in unfavorable auditory environments requires peripheral filtering at the level of the cochlea (Evans 1992), but auditory recognition and memorization are the product of more central structures, such as the caudomedial nidopallium (NCM), a higher-order auditory region of the telencephalon (Bolhuis et al. 2001; Mello and Clayton 1994; Stripling et al. 2001; Terpstra et al. 2004). Neurophysiologic recordings and immediately early gene (IEG) expression have shown that NCM is responsive to the playback of conspecific songs including the bird's own song (BOS) and tutor song (Mello and Clayton 1994; Mello and Ribeiro 1998; Stripling et al. 2001; Terpstra et al. 2004; Velho et al. 2005) and exhibits stimulus-dependent adaptation, which could serve as a mechanism for the memory of familiar conspecific songs (Chew et al. 1996; Mello et al. 1995; Stripling et al. 1997). Recently, stimulus-specific processing in the auditory region NCM was revealed by functional magnetic resonance imaging (fMRI) applied in anesthetized intact songbirds (Van Meir et al. 2005). Functional MRI that relies on blood oxygen level–dependent (BOLD) contrast (Ogawa et al. 1990) is one of the commonly used techniques for imaging brain activity in humans. This technique allows the in vivo noninvasive investigation of local hemodynamic changes during neural activation induced by various simple but also complex tasks such as auditory scene analysis. Using fMRI in the zebra finch, we wanted to investigate the effect of background noise on song-induced activation in auditory regions of the telencephalon of songbirds. We investigated activation in the auditory forebrain of anesthetized adult male zebra finches (Taeniopygia guttata) using fMRI during playback of a conspecific signal mixed with different levels of broadband noise, and tested the behavioral response of zebra finches exposed to the original intact conspecific song stimulus recorded within the magnet, to provide stimuli comparable to those perceived by the birds within the MR scanner. Successful recognition of conspecific song in a noisy background was reflected in the fMRI response of rostral NCM. |
METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Seven adult male zebra finches [Taeniopygia guttata, 12–20 g body weight (b.w.)] served as subjects for this experiment. The birds were obtained from local suppliers and were kept for about 5 months in the laboratory aviaries with unrestricted access to food and water, temperature between 20 and 25°C, and natural light/dark rhythm. Because the birds came in August (summer time) and experiments were done in January (winter time), the natural light/dark rhythm ranged from 15 to 8 h of daylight. Four birds underwent fMRI measurements and three birds were used for behavioral tests on stimulus recognition. Experimental procedures were in agreement with the Belgian laws on the protection and welfare of animals and had been approved by the ethical committee of the University of Antwerp (Belgium).
MOTION CONTROL. Immobilization of the animal's head is critical to allow accurate fMRI measurements. By using anesthesia and a robust stereotaxic device, motion was reduced to a minimum. Zebra finches were initially anesthetized with an intramuscular injection in the pectoral muscles of 25 mg/kg ketamine (Ketalar, 50 mg/ml; Parke-Davis, Zaventem, Belgium) and 2 mg/kg medetomidine (Domitor, 1 mg/ml; Orion Pharma, Espoo, Finland). After 30 min, medetomidine was continuously infused at a rate of 0.02 ml/h through a catheter positioned in the chest muscle. This allowed the birds to be steadily anesthetized for a minimum of 8 h. The anesthetized zebra finches were immobilized in a nonmagnetic, custom-made head holder composed of a beak mask and a circular radio-frequency (RF) surface antenna (diameter 15 mm) tightly placed around the bird's head above both ears and eyes. This allowed accurate and reproducible positioning of the bird within the magnet while at the same time preventing motion. Whole brain and especially the auditory regions of interest were situated in the sensitive region of the RF-receiving circular antenna.
MONITORING PHYSIOLOGY. To maintain an optimal and stable physiological condition during functional brain research, body temperature and respiration were continuously monitored. Body temperature was monitored with a cloacal temperature probe (SA Instruments, Stony Brook, NY) and maintained at 40°C (range: 39.3–41.7°C) by a cotton jacket and a water-heated pad connected to an adjustable heating pump (EX-111; Neslab Instruments, Newington, NH). Respiration rate and amplitude were monitored with a small pneumatic sensor (SA Instruments) positioned under the bird. The respiration rate could not be standardized and showed substantial variation between birds (range: 60–158 cycles/min) but rather small variation within birds (average range: 33 cycles/min). The stability of the expired pCO2 was monitored by a small tube fixed to the stereotaxic mask and connected to a CO2 analyzer (Capstar-100; CWI, Diss Norfolk, UK). The pCO2 fluctuations measured during the experiments were almost nonexistent.
Auditory stimulation
EXPERIMENTAL STIMULI.
The auditory stimuli consisting of conspecific song and noise were provided and were described previously by Mathevon's group (Vignal et al. 2004). The original signal [conspecific signal (CS)] was a sequence of songs and calls recorded in the zebra finch aviary of the respective laboratory. Because our fMRI experiments were performed in zebra finches of our own aviary, the birds of this study were exposed to unfamiliar song. In the recorded 20 s of CS stimulus, 6% represented silence and 94% represented songs and calls. Three stimuli were built by mixing CS with different levels of a continuous masking noise [white noise (WN)] using Syntana software (Aubin 1994). In each mixed signal, all frequencies of the masking noise had equal energy and ranged from 0 Hz to the maximum CS frequency, i.e., 10,000 Hz. The stimuli had different CS/WN intensity level ratios, whereas an equal average sound intensity was maintained. These intensity level ratios were defined as E = 20 log (ACS/AWN), where E represents the emergence level of the CS in dB, ACS is the absolute amplitude of the CS, and AWN is the absolute amplitude of WN. The values of E were –3 dB (stimulus SN-3), –9 dB (stimulus SN-9), and –18 dB (stimulus SN-18) (Fig. 1).
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). A common intensity (loudness) analysis measures the power of a signal using the root-mean-square (RMS) measure. Calculating mean RMS power is well known for its accuracy and allows the discrimination of sound sections that are perceived as loud or weak. To control for loudness effects, we calculated the mean and maximum RMS power of the equalized signals that were recorded in the MR scanner with the electret microphone, described in the data supplements,1 and checked them to match the loudness of original signals. The ratios of these mean and maximum values between original and recorded equalized versions were similar for all stimuli, meaning that the equalization and presentation of stimuli with a different noise level did not affect the loudness of one stimulus more than another.
To quantify the differences between the signals CS, SN-3, SN-9, SN-18, and WN, the correlations were assessed between the amplitude envelope of each stimulus and the CS envelope, and between the frequency spectrum of each stimulus and the CS spectrum. Signal emergence over background noise was also assessed by computing the entropy (Shannon and Weaver 1949). Because the background noise is a constant white noise, a signal lost in the background noise does not significantly modify the distribution of energy over time. On the contrary, a signal that emerges strongly from the background noise modifies the time distribution of energy. To quantify these energy distribution modifications, we measured the SD of the envelope of each experimental signal (CS, SN-3, SN-9, SN-18, WN). The entropy H was then calculated according to the method described in Beecher (1988, 1989): H = log (SDexperimental signal/SDCS). To obtain the normalized entropy H' ranging between 0 and 1, H was divided by its maximum value; thus a value of H' near 1 characterizes a signal almost lost in the background noise. The entropy value of each experimental signal and comparisons of their amplitude envelopes and frequency spectra to those of the CS provide a good picture of the different degradations of the original signal obtained in each stimulus (Table 1). It appeared that all three mixed stimuli differed greatly from the CS. However, the stimulus SN-18 was appreciably degraded, whereas the stimulus SN-3 conserved the main characteristics of CS.
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; Zann 1996) are the sounds most frequently emitted. To assess the bird's response during the playback tests, the number of distance calls emitted during song presentation was determined in the recordings using Syntana software (Aubin 1994) and Goldwave 4.26. STIMULATION PROTOCOL. A block design was used in the fMRI experiment. Auditory stimuli were presented in six repeated blocks consisting of 20 s stimulation and 60 s of rest (no auditory stimulation). Images were collected with a block design paradigm consisting of six cycles of 8 images collected during stimulation, and 24 images collected during rest, resulting in 192 functional images (Fig. 2A). Each experiment, which was preceded by the acquisition of 8 dummy images to allow the signal to reach a steady state, thus took about 8.5 min. In all birds, five consecutive experiments were performed in random order during which the birds were exposed to one of the five different stimuli: CS, SN-3, SN-9, SN-18, and WN. The average song power (average over an entire song) was set at 70 dB SPL (sound pressure level).
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IMAGING SETTINGS. An in vivo 7-T NMR microscope was used with a console from MR Solutions (Guildford, UK) and a magnet from Magnex Scientific (Oxfordshire, UK). The horizontal bore of the magnet is 150 mm wide and the actively shielded gradient-insert (Magnex Scientific) has an inner diameter of 100 mm and a maximum gradient strength of 400 mT/m. A Helmholtz (45 mm) antenna served for transmitting the RF pulses and a circular RF surface antenna (15 mm) was used for MR signal reception.
A set of one parasagittal, one horizontal, and one coronal gradient-echo (GE) scout image and a set of 12 horizontal GE images were first acquired to determine the position of the brain in the magnet. Functional imaging was performed using a T2*-weighted single-slice GE fast low-angle shot (FLASH) sequence [field of view (FOV) = 25 mm, echo time (TE) = 14 ms, repetition time (TR) = 40 ms, flip angle = 11°, gradient ramp time = 1,000 µs, acquisition matrix = 128 x 64, reconstruction matrix 128 x 128, slice thickness = 0.5 mm]. Long gradient ramp times (1,000 instead of 100 µs) reduced the gradient noise to 63 dB. The total acquisition time per image was 2.56 s and a spatial resolution of 195 x 195 µm2 was obtained. As illustrated on Fig. 3, the functional images were acquired on a parasagittal slice in the right hemisphere that was chosen to go through the auditory forebrain regions Field L and NCM. Because the fiber track that defines subregion L2a (Fortune and Margoliash 1992; Vates et al. 1996) can be clearly seen on a structural MR image, and because NCM begins next to the midline as a small circular area and becomes gradually larger in more lateral sections 
1 mm lateral (Mello et al. 1992), the lateral position of our functional slice was chosen to cover a big part of NCM whereby subregion L2a at the rostral side can easily be seen. This was fulfilled for a lateral distance from 0.25 to 0.75 mm from the midline. Anatomical high-resolution imaging was performed at the same position as the functional imaging slice with a T2-weighted spin-echo (SE) sequence (FOV = 25 mm, TE = 45 ms, TR = 2,000 ms, acquisition matrix = 256 x 128, reconstruction matrix 256 x 256, slice thickness = 0.5 mm, and eight averages).
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CLUSTER ANALYSIS AND BRAIN STRUCTURE ASSIGNMENT.
To distinguish adjacent activated regions, significant activated voxels (P < 0.05) were clustered by means of the SI time course during successive stimulation and rest periods. First, a time course was generated by averaging the six consecutive trials to obtain a 32-dimensional vector space, where each average voxel's time course is represented as one point. Amplitude normalization of the signals ensured clustering was done on the shape of the time course, and not only on the amplitude that can be subject to physiological and/or imaging trends. After determination of the significantly activated voxels (t-test, P < 0.05), isolated voxels were removed. Finally, a reduction of the dimensionality of the feature space was obtained by applying principal components analysis (PCA), i.e., by projecting data points onto the most relevant components. Structure in the measurement data was detected by Fuzzy C-Means (FCM), a clustering technique that looks for similarities in the fMRI data feature space (Bezdek et al. 1984; Fadili et al. 2000) based on the Euclidean distance of the individual data points in the cluster space. As a result, a spatial map with two detected clusters and their corresponding time courses was obtained. Cluster analysis was exclusively done for the fMRI data series acquired during exposure to the stimuli CS and WN (Fig. 2C) because primary and secondary auditory processing regions could be discriminated by comparing undegraded biologically relevant signals (CS) with signals that did not contain any relevant information (WN).
Positions of the two clustered regions were compared relative to the location of various landmarks that are visible in the anatomical images. Most pronounced is the darker ellipsoid region that corresponds to the dense fiber track that defines subregion L2a (Fortune and Margoliash 1992; Vates et al. 1996) (Fig. 3). To verify the position of the two clusters of activated pixels, we delineated on the anatomical images the darker region (i.e., subfield L2a) and the border with the cerebellum, and pasted these marks on the clustering maps calculated for the fMRI data series CS and WN (Fig. 2C). Given the wider spread of activity in the rostral–caudal dimension of all measured WN data series compared with the reported size of L2a in the rostral–caudal dimension in parasagittal slices (Fortune and Margoliash 1992; Vates et al. 1996) and to the darker ellipsoid region discerned in the anatomical MR images, we conclude that the BOLD activation on white noise also extends to the neighboring subregions L2b, L3, and subarea L within Field L (see Fig. 3). Therefore all subsequent reportings of "cluster Field L" herein include these different Field L subregions. If cluster analysis discriminated this cluster Field L from a more caudal region that was exclusively activated on CS stimulation, we concluded that there was a second pole of BOLD activation that, based on its anatomical location, could be the secondary auditory region NCM. However, based on observations reported in more detailed histological studies (Fortune and Margoliash 1992), this more caudal area defined here as "cluster NCM" seems to correspond mostly to rostral NCM and to largely overlap with the subregion L as defined in Fortune and Margoliash (1992). Also the very medial extremes of the subregions L2b and L3 might have been included (see Fig. 3). The correspondence between the activated areas and the anatomical landmarks was established in each bird separately.
REGIONAL ANALYSIS. SI changes and Z-scores were subsequently analyzed in defined regions of interest (ROIs). Regional analysis was performed for ROIs at the level of the primary telencephalic auditory region, Field L, and the secondary telencephalic auditory region, NCM. On the basis of the clustering maps with marks at the location of subfield L2a and cerebellum, ROIs were chosen to consist of six connected pixels each [i.e., 6 x (195 µm)2] that could be placed centrally in cluster Field L and cluster NCM with a gap of minimum one row pixels between both ROIs. The ROI in Field L was selected to overlap with the darker region representing subfield L2a, and with the region activated by WN that was discriminated from other activated regions by the CS. The ROI in NCM was located at the center of the caudal extension activated by CS and rostral from the cerebellum. In all the fMRI data series we calculated for both ROIs the mean SI for the 192 time points and the Z-score. Figure 2D illustrates the percentage BOLD SI changes during the stimulation periods of two successive cycles in the ROIs Field L and NCM of one representative bird that was exposed to the original signal CS and WN. These percentage values were calculated relative to the mean SI of the last 16 time points of the rest period in the respective stimulation/rest block. Because discriminatory properties of auditory regions are better characterized by different levels of auditory activation, further analysis started with the Z-scores.
Previously we showed that Field L shows a positive BOLD response to presentation of several different kinds of stimulus (Van Meir et al. 2005). Therefore we controlled for this positive response in cluster Field L during each stimulation period, preventing possible underestimations of the NCM response. If the mean percentage BOLD SI change in ROI Field L during a stimulation period was negative, this respective stimulation/rest block was excluded from further data analysis and the Z-score was recalculated for the remaining time points. The occurrence of the negative mean percentage BOLD SI change in Field L seemed to be minimal. From a total of 120 stimulation/rest blocks (6 blocks x 5 stimuli x 4 birds), we excluded only seven blocks spread among the four different birds. Also the stimulus and number of period repetitions seemed to have no causal connection with the occurrence of the negative Field L response. We excluded two blocks for stimulus SN-18, two blocks for stimulus SN-9, and three blocks for WN.
STATISTICAL ANALYSIS. Statistical analysis of the Z-scores was performed with Statistical Package for Social Sciences (Chicago, IL). Differences in Z-scores were statistically analyzed using an ANOVA (P = 0.05) for repeated measures with the independent factors being the brain region (Field L and NCM) and stimulus type (CS, SN-3, SN-9, SN-18, WN). Correlations between Z-scores and auditory stimuli were statistically analyzed using a linear regression analysis (P = 0.05) with the dependent variables the Z-scores in cluster Field L and cluster NCM, and the independent factors the signal degradation values (the entropy value of each experimental signal, and the comparisons of their amplitude envelopes and their frequency spectra to those of the CS). All data are presented in the corresponding figures as means with SEs.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Behavioral response to auditory stimuli generated in the fMRI environment
Each tested bird answered to the original CS as well as to the equalized version recorded in the magnet by emitting distance calls [(mean number of calls ± SD) = (6 ± 2) and (5 ± 1), respectively], whereas WN did not provoke any vocal response. During WN as well as during the first 40 s of silence in the behavioral test, all birds remained completely silent. Thus the acoustic stimulus significantly influences the number of distance calls emitted by the birds [Friedman ANOVA, 
2(3,3) = 8.111, P < 0.0438], indicating stimulated calling behavior rather than just spontaneous activity. Moreover, the number of distance calls emitted by the bird in response to the original CS and to the equalized version recorded in the magnet did not differ significantly (Wilcoxon matched-pair test, Z = 1.069, P = 0.285). The fact that the recorded equalized song was always recognized reveals that the sounds generated by the magnetless dynamic speakers preserve all relevant information to maintain song recognition by the bird.
fMRI experiments
The mean Z-score (n = 4) for ROIs in cluster Field L and cluster NCM to presentation of CS, SN-3, SN-9, SN-18, and WN is displayed in Fig. 4. One data set from an individual bird with stimulation SN-9 was excluded from data analysis as a result of imaging artifacts that caused considerable phasic SI changes that were not correlated with the functional stimulation protocol.
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2 = 0] and cluster NCM [P = 0.21, F(4,14) = 1.66, R2 = 0.32, 2 = 0.12]. However, because (NCM)2 > 0 and because we observed a clear linear trend in the NCM Z-scores as a function of the signal degradation, we explored the use of linear regression on nonnominal (scale) stimulus parameters. A linear regression analysis (P = 0.05) with the Z-scores of the four birds as dependent variables and the three quantitative signal degradation values of Table 1 as independent factors also revealed a different auditory response in cluster Field L and cluster NCM. We observed no significant correlation in Field L, but indeed a significant correlation in NCM between the Z-score and the signal degradation values representing comparisons between amplitude envelopes (R2 = 0.297, P = 0.016), frequency spectra (R2 = 0.240, P = 0.033), and entropy (R2 = 0.238, P = 0.034). These results demonstrate that the auditory response in Field L subregions, at least at the neuronal population level, is not influenced by the CS/WN ratio of the stimulus, whereas the response in more caudal regions NCM and potentially subregions L2b, L3, and L decreases with more degradation of the CS. The behavioral data also show that the recorded undegraded song was recognized, meaning that the sounds generated within the magnet preserve all relevant information to maintain song recognition by the bird. Moreover, the activation in cluster NCM was significant only for the two experimental stimuli SN-3 and SN-9 that showed significantly more behavioral responses than the more degraded stimuli. This means that only those two conditions are recognized as conspecific song and that the first area within the auditory system where the ability to discern song from masking noise emerges is located in cluster NCM.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Vates et al. 1996). Field L projects to secondary auditory areas in the telencephalon, including the NCM. Besides these auditory inputs into NCM from medial subfield L2a and subfield L3, NCM also receives input from the secondary caudal medial mesopallium (CMM) and from Ov (Vates et al. 1996). The telencephalic regions Field L and NCM are part of the avian analogue of the mammalian auditory cortex. The impact of anesthesia
Most MRI studies in animals are conducted under anesthesia to minimize motion artifacts in the imaging. Previous recordings have shown that auditory responsiveness in the forebrain song system nucleus HVC is affected by changes in the bird's behavioral state (Cardin and Schmidt 2003; Rauske et al. 2003; Schmidt and Konishi 1998). In awake behaving zebra finches, HVC auditory responses are largely suppressed and less selective than responses in anesthetized and sleeping birds. These studies did not show similar effects in the auditory area Field L. Medetomidine, a nonnarcotic sedative and analgesic, is a potent 
2-adrenoreceptor agonist that produces sedation and analgesia. It has been shown that noradrenergic terminals are found throughout the avian auditory and vocal system (Mello et al. 1998) and that -adrenergic receptor blockade abolishes song-induced ZENK induction in zebra finch NCM (Ribeiro and Mello 2000). In medetomidine-anesthetized starlings, however, stimulus-specific fMRI responses were previously revealed in the NCM, but this drug was given at a lower dose (Van Meir et al. 2005) than the one applied in the current study on the zebra finch. Further studies are required to determine the effects that anesthetic agents may have on auditory processing, as compared with fully awake birds.
Comparison of data obtained with fMRI, electrophysiology, and gene expression
Electrophysiological studies in zebra finches have shown that the selectivity for songlike sounds increases in a hierarchical manner along ascending processing stages in the auditory system (Hsu et al. 2004). Woolley et al. (2005) showed that the discrimination of conspecific vocalizations from other sounds results from tuning properties to temporal modulations that differ most across sounds. Most Field L auditory neurons are more selective to either conspecific song or white noise with a much smaller number of neurons showing weak or no selectivity (Grace et al. 2003). These electrophysiological studies suggest that complex natural sounds, such as conspecific song, are preferentially represented in the neural activity of the auditory forebrain relative to other background sounds that are commonly present in the bird's environment.
Analysis of immediate early gene (IEG) expression has been very useful in generating high-resolution maps of brain activation associated with perceptual and motor aspects of vocal communication in songbirds (Mello 2002, 2004; Mello et al. 2004; Ribeiro and Mello 2000; Velho et al. 2005). By contrast to the electrophysiological results, IEG induction is absent in subfield L2, whereas it is robust in the NCM after song presentation (Gobes and Bolhuis 2007; Mello and Clayton 1994; Mello et al. 1992, 2004; Theunissen and Shaevitz 2006; Velho et al. 2005). Vignal et al. (2004) investigated behavioral responses and IEG expression to the same stimuli (SN-3, SN-9, SN-27, and WN) used in this fMRI study (with the exception of SN-27). The stimuli SN-3 and SN-9 that elicited significant behavioral responses and gene activation in the NCM also elicited a significant fMRI response in the region that appears to constitute at least the rostral part of NCM. These results suggest that successful recognition of relevant information in noise may be reflected in the fMRI response of cluster NCM. In addition, fMRI allowed us to quantify the activity modulations in Field L subregions (see following text), a goal that cannot be achieved by IEG analysis because of the absence of ZENK expression in subfield L2.
Discriminatory properties of Field L and NCM
Vocal communication in songbirds involves the recognition of individuals based on their vocal performance and segregation of these vocalizations in a noisy environment. A behavioral approach in adult male canaries demonstrated that the accuracy of the discrimination between two conspecific song segments progressively declines as a function of the number of masking distractors (Appeltants et al. 2005). Noisy signals have to contain sufficient information to allow successful recognition against noise (Vignal et al. 2004). Several lines of evidence indicate that the regions NCM and CMM mediate auditory processing, with a more specific role for the NCM in facilitating recognition of species-specific song (Bailey et al. 2002; Gentner et al. 2004; Gobes and Bolhuis 2007; Mello et al. 2004; Terpstra et al. 2004). The acoustical features underlying song recognition and discrimination in birds are not well understood, but most suggestions rely on the difference in spectral and/or temporal modulations of sounds.
For all auditory stimuli presented here, cluster Field L showed a substantially larger Z-score than that of cluster NCM, indicating a larger SI difference in Field L between stimulation and rest periods. At the neuronal population level, Field L was shown to be responsive to any auditory input with no significant response modulation between the different degraded stimuli. On the other hand, the fMRI response in cluster NCM varied gradually among the degraded stimuli and showed a significant regression with degradation of the conspecific signal. The lack of a significant effect in the ANOVA might have been due to the small sample size. Nevertheless, the significant regression found in cluster NCM implies that in the songbird the capacity to segregate meaningful auditory signals in unfavorable auditory environments may be an emergent property of NCM and potentially subregions L2b, L3, and L. In combination with the findings of behavioral studies (Appeltants et al. 2005; Vignal et al. 2004), our results suggest that successful recognition of relevant information against noise is reflected in the auditory NCM response. Moreover, data from Van Meir et al. (2005) showed that BOLD responses in Field L and NCM were significantly different with white noise stimuli. Together with our significant correlation result in cluster NCM (Table 1), we suggest that the first area within the auditory system—where the ability to discern song from masking noise emerges—is located in cluster NCM.
Electrophysiology in canary brain revealed that Field L and NCM differ in their electrophysiological response properties to pure-tone stimuli, suggesting differential roles in auditory processing (Terleph et al. 2006). NCM properties, in particular, may allow for response integration across multiple spectrally varying stimulus elements, such as those that occur during birdsong, especially information from multiple Field L2 sites (each tuned to a narrow frequency range) that may converge onto a single site in NCM. Unlike electrophysiology, fMRI reveals neural activity by detecting hemodynamic changes induced by groups of cells. It is important to note, however, that the measured regional heterogeneity between clusters Field L and NCM may be dependent on the resolution of the fMR image. Our image voxels of 195 x 195 x 500 µm3 correspond to hundreds of cells or more and, at this resolution, it may be impractical to distinguish between the separate subfields in Field L. Because cell groups, belonging either to different subregions or to one subregion, may express substantially different response properties to conspecific song, the capacities ascribed to Field L may cover several such cell populations. Likewise, the potential differences between regions may reflect quantitative shifts in the proportions of cells coding separate stimulus features rather than qualitative differences in the information coded in Field L (at large) and NCM.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 The online version of this article contains supplemental data. 
Address for reprint requests and other correspondence: T. Boumans, University of Antwerp, Bio-Imaging Lab, Groenenborgerlaan 171, B-2020 Antwerp, Belgium (E-mail: Tiny.Boumans{at}ua.ac.be)
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