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Gap detection threshold in the rat before and after auditory cortex ablation
Hearing Research 172 (2002) 151^159 www.elsevier.com/locate/heares
Gap detection threshold in the rat before and after auditory cortex ablation J. Syka a
a;
, N. Rybalko a , J. Mazelova¤ a , R. Druga
b
Institute of Experimental Medicine, Academy of Sciences of the Czech Republic, V|¤den›ska¤ 1083, 142 20 Prague 4, Czech Republic b Department of Functional Anatomy, 2nd Medical Faculty, Charles University, Prague, Czech Republic Received 4 March 2002; accepted 27 June 2002
Abstract Gap detection threshold (GDT) was measured in adult female pigmented rats (strain Long^Evans) by an operant conditioning technique with food reinforcement, before and after bilateral ablation of the auditory cortex. GDT was dependent on the frequency spectrum and intensity of the continuously present noise in which the gaps were embedded. The mean values of GDT for gaps embedded in white noise or low-frequency noise (upper cutoff frequency 3 kHz) at 70 dB sound pressure level (SPL) were 1.57 6 0.07 ms and 2.9 6 0.34 ms, respectively. Decreasing noise intensity from 80 dB SPL to 20 dB SPL produced a significant increase in GDT. The increase in GDT was relatively small in the range of 80^50 dB SPL for white noise and in the range of 80^ 60 dB for low-frequency noise. The minimal intensity level of the noise that enabled GDT measurement was 20 dB SPL for white noise and 30 dB SPL for low-frequency noise. Mean GDT values at these intensities were 10.6 6 3.9 ms and 31.3 6 4.2 ms, respectively. Bilateral ablation of the primary auditory cortex (complete destruction of the Te1 and partial destruction of the Te2 and Te3 areas) resulted in an increase in GDT values. The fifth day after surgery, the rats were able to detect gaps in the noise. The values of GDT observed at this time were 4.2 6 1.1 ms for white noise and 7.4 6 3.1 ms for low-frequency noise at 70 dB SPL. During the first month after cortical ablation, recovery of GDT was observed. However, 1 month after cortical ablation GDT still remained slightly higher than in controls (1.8 6 0.18 for white noise, 3.22 6 0.15 for low-frequency noise, P 6 0.05). A decrease in GDT values during the subsequent months was not observed. ; 2002 Elsevier Science B.V. All rights reserved. Key words: Gap detection; Temporal resolution; Bilateral ablation; Auditory cortex; Rat
1. Introduction The measurement of gap detection threshold (GDT) is an e⁄cient tool for evaluating the time resolution abilities of the auditory system. The estimation of GDT in man has been the subject of many studies, beginning with the paper by Plomp (1964). Numerous experiments have shown that gap detection ability is dependent on the frequency spectrum and intensity of the continuous noise in which the gaps are embedded (Penner, 1977; Giraudi et al., 1980; Tyler et al., 1982;
* Corresponding author. Tel.: +42 (02) 4106 2700; Fax: +42 (02) 4106 2787. E-mail address: [email protected] (J. Syka). Abbreviations: GDT, gap detection threshold; SPL, sound pressure level; Te, temporal; Par, parietal; rf, rhinal ¢ssure
Florentine and Buus, 1984; Shailer and Moore, 1983; Fitzgibbons, 1983; Snell et al., 1994). Minimal GDTs in human were recorded when the noise signal encompassing the gap was presented at 30^40 dB SL and included frequencies above 5^6 kHz. GDT in this case amounted to about 2^3 ms (Plomp, 1964; Fitzgibbons, 1983). Clinical observations have shown that neurological patients with various types of cerebral lesions have di⁄culties in temporal resolution tasks, such as reduced gap detection abilities (Buchtel and Stewart, 1989; Albert and Bear, 1974; Efron et al., 1985; Phillips and Farmer, 1990). Neuronal correlates of gap detection have been studied at di¡erent levels of the auditory system (for review see Frisina, 2001). The fundamental mechanism of coding sound gaps at the level of the auditory nerve ¢bres is through a cessation of activity in response to a gap (Relkin and Turner, 1988; Zhang et al., 1990),
0378-5955 / 02 / $ ^ see front matter ; 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 5 9 5 5 ( 0 2 ) 0 0 5 7 8 - 6
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whereas at the level of cochlear nuclei and the inferior colliculus the mechanism switches to an increase in the spike ¢ring rate at the conclusion of the gap (Boettcher et al., 1990; Kaltenbach et al., 1993; Palombi et al., 1994; Walton et al., 1997; Barsz et al., 1998). Walton et al. (1997) concluded that the gap encoding capabilities of inferior colliculus neurones in the CBA mouse were similar to the behavioural gap functions. Neural correlates of gap detection were studied in the auditory cortex of the cat by Eggermont (1995, 1999). In these experiments gaps inserted either early (after 5 ms) or late (after 500 ms) in a 1 s duration noise burst were used. The neural detection threshold for the ‘late gap’ was found to be similar to the psychophysical gap threshold (around 5 ms) and was not signi¢cantly different from that found in the inferior colliculus (Walton et al., 1997). Signi¢cantly larger minimum late-gap durations were found in the anterior auditory ¢eld in comparison with the primary and secondary auditory ¢elds. The measurement of GDT may be used for investigating pathologies of temporal resolution ability and for estimating their severity. For this purpose, studies dealing with the in£uence of experimentally produced impairments in gap detection in animals may be of importance. The aim of such studies is usually to determine the involvement of individual structures of the auditory system in temporal resolution. Only a few papers on gap detection in animals with a cortical lesion have been published so far (in rat: Ison et al., 1991 ; in ferret: Kelly et al., 1996). The purpose of this study was to examine the role of the auditory cortex in the ability to detect gaps in noise. GDT was measured in pigmented rats before and after bilateral ablation of the auditory cortex.
2. Materials and methods 2.1. Subjects The ability to detect gaps in a continuously present noise was tested in ¢ve adult (8^13 months old) pigmented female rats (strain Long^Evans) with no primary pathology, before and after bilateral lesions of the primary auditory cortex. GDT measurements after a bilateral lesion of the auditory cortex began on the ¢fth day after surgery and continued for 2 months. In addition, the dependence of GDT on the frequency spectrum and intensity of the carrier noise was studied in three other rats.
tioning procedure with food reinforcement, therefore the rats had free access to water but were restricted in food intake. Twenty-three hours before a training or testing session, the rats were completely deprived of food. Food-deprived animals were trained to detect the presence of gaps in a continuous noise. The test apparatus and environment were similar to those described previously (Syka et al., 1996). The experiments were conducted in a grid (1 cm stainless steel mesh) test box (size 20U20U20 cm) with two levers (starting and responding) and a foodcup. The test apparatus was placed in an anechoic sound-proof room. The foodcup connected with a food dispenser was placed in the centre of the front wall; starting and responding levers (3.5 cm length) were mounted 6 cm above the box £oor 2 cm to the right and left of the foodcup. Noise was presented from a loudspeaker located next to the food dispenser outside the experimental box, 50 cm in front of the wall with the levers. The continuous noise was switched on before the rat was put in the test box. The experiment was designed so that 0.5^5 s after pressing the starting lever, test signals consisting of ¢ve gaps were triggered. Pressing the responding lever during the time window, when ¢ve gaps were presented, and up to 1 s after its end, was scored as a hit reaction and the rat was rewarded with a pellet (Fig. 1). Using ¢ve gaps as test stimuli instead of one gap shortened the training procedure and did not signi¢cantly in£uence the testing results. In preliminary experiments we demonstrated that the values of GDT were the same when the signal consisted of ¢ve or two gaps. When only one gap in a continuous noise was presented, GDTs increased by about 0.4 ms (P 6 0.05, tested with the paired t-test). In this case, the increased GDTs were caused mainly by an increased number of accidental responses. For the purpose of controlling the stimulus (stimulus control determines the degree of conjunction of the rat’s response with the test stimulus ^ Hendricks, 1966; Kelly and Masterton, 1977), in 33% of trials in a series the pressing of the starting lever did not result in the appearance of test signals. These were so-called ‘catch trials’. The rat’s response during the catch trial, when gaps were missing, was classi¢ed as a false alarm. False alarms allowed us to estimate the number of guessed responses among all hit responses. False alarms were punished with a 4 s ‘time-out’ period (when the starting lever and the whole system were without function). A small cue light placed above the starting lever was switched on when the system was in operation, i.e., when a new trial could be initiated by pressing the starting lever.
2.2. Behavioural apparatus and procedures 2.3. Stimulus parameters The rats were housed in a plexiglass cage (2^3 rats in a cage). GDTs were investigated by an operant condi-
GDT was measured before and after lesion using
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Fig. 1. Schema of the experimental arrangement for estimating GDT.
gaps embedded in continuous white noise or continuous low-frequency noise (upper cuto¡ frequency 3 kHz) of 70 dB sound pressure level (SPL) intensity. Five ¢xedduration gaps (rise and decay times below 20 Ws) with a 150 ms interval between them were used as test stimuli. In three rats, GDT was also measured at various noise intensities, i.e. at 10, 20, 30, 40, 50, 60, 70, and 80 dB SPL, using both white noise and low-frequency carrier noise. The acoustical system consisted of an RFT 03004 noise generator, an RFT 01013 frequency ¢lter, an attenuator, a gating device producing gaps, an ampli¢er and a two-way loudspeaker system (Tesla ARV 3604 and ARZ 4604). Acoustical calibration was carried out with a 0.5 inch Bru«el and Kjaer 4133 condenser microphone, a 2619 preampli¢er, and a 2606 measuring ampli¢er. The microphone was placed at the typical location of a subject’s head (near the foodcup). During GDT measurement we used gaps of a duration between 1.5 and 70 ms. Stimulus presentation and data acquisition were controlled by a custom-programmed PC. 2.4. Behavioural testing The threshold of gap detection was ¢rst estimated by a method of limits. Gap duration during one experimental series was gradually reduced from 70 ms in dec-
rements of 10, 5, 2, 1 or 0.5 ms until no response occurred and then increased again until the response reappeared. An identical testing procedure was used for each rat until the thresholds of three successive testing series di¡ered by less than 1.5 ms. Final GDT values were obtained by the method of constant stimuli using ¢ve or six di¡erent stimulus durations. Each gap duration was presented 20 times in a test series. The test series consisted of 20 randomly presented test blocks. In each block ¢ve test trials with gaps of the same duration and 2^3 catch trials were grouped (so that for every 20 trials with gaps of identical duration there were 10 catch trials). Minimal gap duration was shorter than the threshold value of gap duration determined by the method of limits, and maximal gap duration was chosen so that the performance exceeded 90%. Performance at each tested gap duration was represented by the hit rate value, de¢ned as the ratio of the number of correct responses to the number of signal trials. Final performance was expressed as a hit rate corrected by the false alarm rate according to the formula proposed by He¡ner and He¡ner (1988) : hit rate3(hit rateUfalse alarm rate). The GDT was de¢ned as the duration of a gap corresponding to a 0.5 hit rate corrected for false alarms.
Fig. 2. GDT as a function of the intensity level of the carrier noise for white noise and low-frequency noise in normal rats; individual data.
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2.5. Ablation procedure Rats were anaesthetized with an intraperitoneal injection of a ketamine (Narkamon 5%, Spofa) and xylazine (Rompun 2%, Bayer) mixture (ratio 3:1, dose 0.1 ml/100 g b.wt.). Following the introduction of anaesthesia, the rats were placed in a stereotaxic apparatus. The skin and underlying muscles were retracted. After identi¢cation of bregma, the bone of the skull covering the temporal auditory cortex was bilaterally removed between AP 33.0 mm and AP 37.8 mm. Laterally the opening exposed the rhinal ¢ssure and medially reached up to the sagittal plane 6.0 mm. The dura mater was removed, and an ablation of the auditory cortex was performed with a sharp knife under an operating microscope. The extent of the cortical lesion corresponded to the trepanation opening with the exception of its lateral margin, where it terminated 1^1.5 mm above the rhinal ¢ssure. The stereotaxic coordinates were derived from the atlas of Paxinos and Watson (1986). At the end of the experiments, the rats were deeply anaesthetized and perfused transcardially with saline followed by 4% paraformaldehyde in 0.1 M phosphate bu¡er (pH 7.4). The animals were decapitated, the brains were removed and post¢xed in the same ¢xative for 5^7 days. Coronal sections of 40 Wm thickness were cut with a freezing microtome. Every third section was stained with cresyl violet and mounted on slides. In order to analyse the three-dimensional extent of the cortical lesion, selected serial sections were redrawn and the boundaries of the cortical lesions transferred to standard sections of the Paxinos and Watson (1986) stereotaxic atlas. Delineation and nomenclature of the cortical areas were based on the atlas of Paxinos and Watson and the cytoarchitectonic criteria of Zilles (1985). The care and use of animals reported on in this study were approved by the Ethics Committee of the Institute of Experimental Medicine and followed the guidelines of the Declaration of Helsinki.
3. Results Control experiments revealed that the GDT depends on the spectral characteristics and intensity levels of the continuous noise in which the gaps are embedded. The mean values of GDT measured in eight normal rats when the gaps were embedded in a continuous white noise of 70 dB SPL were signi¢cantly smaller (P 6 0.05, tested with the paired t-test) than when the gaps were embedded in a low-frequency noise of the same intensity; the values were 1.57 6 0.07 ms and 2.9 6 0.34 ms, respectively. The results of an analysis of the e¡ect of noise intensity on GDT indicated that a decrease in
Fig. 3. Localization of the cortical lesions. (A) Lateral view of the rat cerebral cortex indicating the location of the auditory cortex, situated in the dorsal part of the parietal (Par2) area and in three temporal (Te1, Te2, Te3) areas, according to Zilles (1985). (B) The extent of the cortical lesions in the left and right hemispheres of ¢ve rats (animals marked as 1R, 1M, 2M, 1C, 2C) reconstructed from histological slides on the basis of an atlas of the rat brain (Paxinos and Watson, 1986).
noise intensity from 80 dB SPL to 10 dB SPL was accompanied by a signi¢cant increase in GDT (Fig. 2). In the intensity range of 80^50 dB SPL for white noise and 80^60 dB SPL for low-frequency noise, GDT
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Fig. 4. Schematic diagram of coronal sections through the rat hemisphere according to the atlas of Paxinos and Watson (1986) from rostral (bregma 33.8 mm) to caudal (bregma 37.3) showing the extent of cortical lesions (bold black line) in the right hemispheres of rat R1 (upper row) and rat C2 (lower row).
almost did not change with the changes in the noise intensity. The lowest noise intensities at which GDT could be evaluated were 20 dB SPL for white noise and 30 dB for low-frequency noise. The mean GDT values at these intensities were 10.6 6 3.9 ms and 31.3 6 4.2 ms for white noise and low-frequency noise, respectively. It must be noted that although the rats responded to gaps embedded in noise of an intensity lower than 20 dB SPL, the gap thresholds were impossible to estimate due to the high number of false alarm reactions. The false alarm rate was minimal, reaching 0^0.1, when easily distinguishable stimuli were presented and increased up to 0.4 when gap duration approached threshold values. The intersubject variability of GDT increased with decreasing noise intensity (Fig. 2). Bilateral ablation of the primary auditory cortex resulted in increased GDT values. Fig. 3 demonstrates the localization and extent of the cortical lesions of ¢ve rats
reconstructed from histological sections. Light microscopic analysis of the sections revealed full destruction of area 1 of the temporal cortex (Te1) and partial destruction of areas 2 and 3 of the temporal cortex (Te2 and Te3) according to Zilles (1985) in rats 1R, 1M, and 2M (the size of the lesion in the last case was larger than in the ¢rst two cases). In rats 1C and 2C the lesion comprised the Te1, Te2 and Te3 areas and extended to the occipital cortex. In the majority of animals, the cortical lesion also extended into areas 1 and 2 of the parietal cortex (Par1, Par2). The perirhinal cortical area was only slightly a¡ected in some animals. In all animals the cortical lesion was sharply demarcated from the intact cortex and a¡ected the whole thickness of the cortex, while in some animals the lesion slightly involved the subcortical white matter. Subcortical structures (striatum, hippocampus) were intact in all animals (Figs. 4 and 5). Fig. 4 demonstrates with a bold black line the extent of the lesion in two animals (1R and 2C)
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4. Discussion
Fig. 5. Photomicrograph of a coronal section through the rat (1M) temporal cortex stained with cresyl violet, demonstrating the extent of the surgical lesion (bregma 35.8). Bar = 1 mm; rf, rhinal ¢ssure.
on a schematic diagram of coronal sections through the hemisphere. Similarly, the extent of the lesion in another animal (1M) is apparent in Fig. 5. On the ¢fth day after cortical ablation, all rats were capable of gap detection ; however, the number of false alarm reactions increased. The false alarm rate in response to stimuli well above threshold (with hit rate around 0.9) changed from 0^0.1 under normal conditions to 0.4^0.7 (Fig. 6). The values of GDT amounted to 250^300% of the preoperative levels and were on average 4.2 6 1.1 ms for white noise of 70 dB SPL and 7.4 6 3.1 ms for low-frequency noise of the same intensity (Fig. 7). This ¢gure demonstrates GDTs in ¢ve rats (individual data), before and after bilateral ablation of the auditory cortex, measured in white and low-frequency noise at 70 dB SPL. In rats with larger lesions (2M, 1C and 2C), the threshold shifts in the ¢rst week after ablation were larger than in the other rats. Seven days after ablation, all rats showed an improved performance due to a decreased rate of false alarm reactions. A gradual decrease in the GDT was observed over the course of the following 2 weeks for white noise and the following 3 weeks for low-frequency noise. The slope of the recovery curve was steeper in cases of a larger ablation area (animals 2M, 1C and 2C). One month after cortical ablation, gap detection ability had almost recovered. However, GDT values remained slightly higher than before ablation (1.8 6 0.18 ms for white noise, 3.22 6 0.15 ms for low-frequency noise ^ P 6 0.05, tested with the paired t-test). A further decrease of GDT during the subsequent month was not observed.
The lowest GDTs for rats, measured in our experiments under optimal stimulus conditions (in a broadband noise at an intensity above 40 dB SPL) by an operant conditioning method, £uctuated about 1.6 ms and were slightly smaller than those obtained in previous investigations using a startle amplitude reduction paradigm (Ison, 1982; Ison et al., 1991; Leitner et al., 1993). The examination of Ison (Ison, 1982; Ison et al., 1991) of acoustic startle re£ex inhibition by brief gaps in noise showed that the GDT values in rats range from 2 to 4 ms. According to Leitner et al. (1993), who used the same method, rats reliably detect 2 ms gaps. The results of behavioural tests of gap detection abilities in other vertebrates have shown that GDT is 2 ms in CBA mice (Walton et al., 1997), ranges from 2.6 to 3 ms in chinchillas (Giraudi et al., 1980; Salvi and Arehole, 1985), amounts to about 10 ms in ferrets (Kelly et al., 1996) and reaches 2.5 ms in two bird species (budgerigars and zebra ¢nches) (Okanoya and Dooling, 1990). In man, GDTs have been observed in the range of 2^4 ms (Plomp, 1964; Penner, 1977; Fitzgibbons, 1983). Low values of GDT in rats may be linked to the fact that rats are capable of perceiving a broader spectrum of high-frequency acoustical signals. The upper threshold of the hearing range in rats reaches 70 kHz (He¡ner et al., 1994; Syka and Rybalko, 2000), while in ferrets, chinchillas and humans it does not exceed 44 kHz (Kelly et al., 1986), 33 kHz (He¡ner and He¡ner, 1991) and 20 kHz, respectively. Indirect support for this explanation comes from studies reporting an elevated GDT in cases of high-frequency hearing loss (in humans: Florentine and Buus, 1984 ; in chinchillas : Salvi and Arehole, 1985). The lower GDT values in rats obtained in the present study in comparison with the results of Ison (Ison, 1982; Ison et al., 1991) may be caused by di¡erences in the measuring methods employed in the two studies and the use of a series of ¢ve gaps as the test stimulus in our experiments. As reported in Section 2 of methods, in pilot experiments the GDT value for a single gap presentation was approximately 0.4 ms longer than when ¢ve gaps were presented. It will be noted that in the majority of GDT psychophysical measurements in laboratory animals reported in the literature, the authors used more than one gap as well (Giraudi et al., 1980; Salvi and Arehole, 1985; Kelly et al., 1996). The ability to detect gaps in noise depends on the frequency spectrum and intensity of the noise in which the gaps are embedded. GDT in low-frequency noise is always higher than in broadband noise. These data are in good agreement with studies dealing with gap detection abilities in band-limited noise. The decrease of the centre or cuto¡ frequency of noise results in both hu-
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Fig. 6. Performance curves (A) and curves of hit rate (B) and false alarm rate (C) of one experimental animal (1C) before and 5 days after ablations (detection of gap in white noise).
Fig. 7. GDT before (open symbols) and after (¢lled symbols) bilateral ablation of the auditory cortex for white noise and low-frequency carrier noise of 70 dB SPL; individual data.
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mans and animals in an elevation of the GDT (Florentine and Buus, 1984; Fitzgibbons, 1983; Snell et al., 1994). GDT increases with decreased intensity of the carrier noise. The largest e¡ect is observed in the range of intensities adjacent to the hearing threshold. In our study a decrease in noise intensity from 50 dB SPL to 20^30 dB SPL resulted in a GDT increase of about 9.1 ms and 28.4 ms for broadband and low-frequency noise, respectively. The extent to which GDT changes along with changes in noise intensity, as reported in the literature, is di¡erent for di¡erent species. In humans a decrease in the level of broadband noise from 40 to 10 dB SL resulted in an increase in GDT of about 20 ms (Plomp, 1964; Penner, 1977; Fitzgibbons, 1983). In chinchillas the change in intensity of broadband noise from 40 to 20 dB SL led to a GDT increase of about 5 ms (Giraudi et al., 1980). Kelly et al. (1996) found that a decrease in 8 kHz bandpass noise intensity from 70 to 10 dB SPL produced a GDT increase of approximately 10^18 ms in ferrets. In our experiments, it was possible to measure GDT at noise intensities as low as 20 dB SPL for white noise and 30 dB for lowfrequency noise. A high rate of false alarm reactions did not allow an estimation of GDT at lower noise intensities. Over the intensity range used in our experiments, the GDT increased by 9 ms and 28 ms when intensity decreased from 80 to 20 dB SPL for white noise and from 80 to 30 dB SPL for low-frequency noise, respectively. The large inter-subject variability of GDT at low levels of noise pressure in psychophysical experiments apparently re£ects the di¡erences in rats’ individual capabilities for solving tasks under complicated conditions. Bilateral ablation of the auditory cortex in rats was accompanied by an elevation in the GDT. Five days after surgery all rats were able to discriminate gaps in the noise. The GDT in white noise amounted to about 4 ms (preoperative GDT was approximately 1.6 ms). Five to 7 days after ablation the GDT values were related with the extent of the auditory cortex destruction (the larger the ablated area, the bigger the GDT shift, Figs. 3 and 7). During the ¢rst month after cortical ablation, GDTs gradually approached normal preablation values, but even 2 months after ablation the GDT remained slightly but signi¢cantly higher in comparison with pre-ablation values. Several studies have reported that a normally functioning and undamaged neocortex is necessary for gap detection ability. Measurements of the gap-induced inhibition of the acoustic startle re£ex in rats following functional decortication by KCl application demonstrated an inability to detect a gap of 15 ms duration (Ison et al., 1991). Kelly et al. (1996) reported that bilateral destruction of the auditory cortex in ferrets resulted in a deterioration in
GDT. The authors found increased GDT values, shifted by, on average, from 10 to 20 ms. In contrast to our experiments, in which the retesting of the rats began 5 days after surgery, postoperative tests in the ferrets were conducted 1 month after surgery and required a period of 1 week for retraining. The reason of di¡erences between our results and the results of Kelly et al. (1996) may be the di¡erences in the species used and the methods employed. It is also possible that the GDT decline observed in our experiments during 1 month after ablation was produced by instantaneous retraining of the task in this period. The increased false alarm rate after ablation in trained animals may be due to di⁄culty in discriminating the stimuli, which would cause the animal to adopt a strategy of increasing false alarms in order to obtain more rewards. Our results together with the results of other authors (Ison et al., 1991; Kelly et al., 1996) support the assumption that the auditory cortex plays a speci¢c role in temporal resolution. Auditory cortex ablation may also result in the impairment of mechanisms that assure the retention of memory traces and the integration of sensory information with speci¢c motor responses. This explanation is connected with the role of the auditory cortex in auditory learning (Aitkin, 1990; Ehret, 1997; Scheich et al., 1997). The large increase in the false alarm rate in rats after auditory cortex destruction in our experiments complicated considerably the measurement of GDT and might also indicate a change in the rats’ ability to respond correctly to test stimuli. This observation is in good agreement with studies dealing with the e¡ects of lesions before and after training on an animal’s responses. It was demonstrated that auditory cortex destruction in trained animals leads to an increased false alarm rate (Kelly and Whit¢eld, 1971; Kelly, 1973; Ohl et al., 1999). In these experiments, frequency-modulated tone discrimination was studied in the cat (Kelly and Whit¢eld, 1971) and the Mongolian gerbil (Ohl et al., 1999). According to Kelly and Whit¢eld (1971), the increase in the false alarm rate in trained animals after lesion was accompanied by reduced hit rates. Ohl et al. (1999) reported that lesions in trained animals caused an increased false alarm rate without any e¡ects on the hit rate. Lesions in naive animals before training led, on the contrary, to a reduced hit rate and had no e¡ect on the false alarm rate (Kelly, 1973; Ohl et al., 1999). The results of our experiments suggest that the complete destruction of the Te1 and the partial destruction of the Te2 and Te3 areas of the auditory cortex in rats substantially in£uence not only temporal resolution abilities, but also learning and memory mechanisms that assure the correct behavioural response of the rat during testing.
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Acknowledgements This work was supported by grants of the Grant Agency of the Czech Republic (309/01/1063) and the Grant Agency of the Ministry of Health (NK/6454-3).
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Kelly, J.B., 1973. The e¡ects of insular and temporal lesions in cats on two types of auditory pattern discrimination. Brain Res. 62, 71^87. Kelly, J.B., Kavanagh, G.L., Dalton, J.C., 1986. Hearing in the ferret (Mustela putorius): thresholds for pure tone detection. Hear. Res. 24, 269^275. Kelly, J.B., Rooney, B.J., Phillips, D.P., 1996. E¡ects of bilateral auditory cortical lesions on gap-detection thresholds in the ferret (Mustela putorius). Behav. Neurosci. 110, 542^550. Kelly, J.B., Masterton, B., 1977. Auditory sensitivity of the albino rat. J. Comp. Physiol. Psychol. 91, 930^936. Leitner, D.S., Hammond, G.R., Springer, C.P., Ingham, K.M., Mekilo, A., Bodison, P.R., Aranda, M.T., Shawaryn, M.A., 1993. Parameters a¡ecting gap detection in the rat. Percept. Psychophysiol. 54, 395^405. Ohl, F.W., Wetzel, W., Wagner, T., Rech, A., Scheich, H., 1999. Bilateral ablation of auditory cortex in Mongolian gerbil a¡ects discrimination of frequency modulated tones but not of pure tones. Learn. Mem. 6, 347^362. Okanoya, K., Dooling, R.J., 1990. Detection of gaps in noise by budgerigars (Melopsittacus undulatus) and zebra ¢nches (Poephila guttata). Hear. Res. 50, 185^192. Palombi, P.S., Backo¡, P.M., Caspary, D.M., 1994. Paired tone facilitation in dorsal cochlear nucleus neurons: a short-term potentiation model testable in vivo. Hear. Res. 75, 175^183. Penner, M., 1977. Detection of temporal gaps in noise as a measure of the decay of auditory sensation. J. Acoust. Soc. Am. 61, 552^557. Phillips, D.P., Farmer, M.E., 1990. Acquired word deafness, and the temporal grain of sound representation in the primary auditory cortex. Behav. Brain Res. 40, 85^94. Paxinos, G., Watson, C., 1986. The Rat Brain in Stereotaxic Coordinates. Academic Press, Sydney. Plomp, R., 1964. Rate of decay of auditory sensation. J. Acoust. Soc. Am. 36, 227^282. Relkin, E.M., Turner, C.W., 1988. A re-examination of forward masking in the auditory nerve. J. Acoust. Soc. Am. 84, 584^591. Salvi, R.J., Arehole, S., 1985. Gap detection in chinchillas with temporary high-frequency hearing loss. J. Acoust. Soc. Am. 77, 1173^ 1177. Shailer, M.J., Moore, B.C., 1983. Gap detection as a function of frequency, bandwidth, and level. J. Acoust. Soc. Am. 74, 467^473. Scheich, H., Stark, H., Zuschratter, W., Ohl, F.W., Simonis, C.E., 1997. Some functions of primary auditory cortex in learning and memory formation. Rev. Adv. Neurol. 73, 179^193. Snell, K.B., Ison, J.R., Frisina, D.R., 1994. The e¡ects of signal frequency and absolute bandwidth on gap detection in noise. J. Acoust. Soc. Am. 96, 1458^1464. Syka, J., Rybalko, N., Broz›ek, G., Jilek, M., 1996. Auditory frequency and intensity discrimination in pigmented rats. Hear. Res. 100, 107^113. Syka, J., Rybalko, N., 2000. Threshold shifts and enhancement of cortical evoked responses after noise exposure in rats. Hear. Res. 139, 59^68. Tyler, R.S., Summer¢eld, Q., Wood, E.J., Fernandes, M.A., 1982. Psychoacoustic and phonetic temporal processing in normal and hearing-impaired listeners. J. Acoust. Soc. Am. 72, 740^752. Walton, J.P., Frisina, R.D., Ison, J.R., O’Neill, W.E., 1997. Neural correlates of behavioral gap detection in the inferior colliculus of the young CBA mouse. J. Comp. Physiol. 181, 161^176. Zhang, W., Salvi, R.J., Saunders, S.S., 1990. Neural correlates of gap detection in auditory nerve ¢bers of the chinchilla. Hear. Res. 46, 181^200. Zilles, K., 1985. The Cortex of the Rat. A Stereotaxic Atlas. Springer Verlag, Berlin, pp. 1^121.
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Gap detection threshold in the rat before and after auditory cortex ablation J. Syka a
a;
, N. Rybalko a , J. Mazelova¤ a , R. Druga
b
Institute of Experimental Medicine, Academy of Sciences of the Czech Republic, V|¤den›ska¤ 1083, 142 20 Prague 4, Czech Republic b Department of Functional Anatomy, 2nd Medical Faculty, Charles University, Prague, Czech Republic Received 4 March 2002; accepted 27 June 2002
Abstract Gap detection threshold (GDT) was measured in adult female pigmented rats (strain Long^Evans) by an operant conditioning technique with food reinforcement, before and after bilateral ablation of the auditory cortex. GDT was dependent on the frequency spectrum and intensity of the continuously present noise in which the gaps were embedded. The mean values of GDT for gaps embedded in white noise or low-frequency noise (upper cutoff frequency 3 kHz) at 70 dB sound pressure level (SPL) were 1.57 6 0.07 ms and 2.9 6 0.34 ms, respectively. Decreasing noise intensity from 80 dB SPL to 20 dB SPL produced a significant increase in GDT. The increase in GDT was relatively small in the range of 80^50 dB SPL for white noise and in the range of 80^ 60 dB for low-frequency noise. The minimal intensity level of the noise that enabled GDT measurement was 20 dB SPL for white noise and 30 dB SPL for low-frequency noise. Mean GDT values at these intensities were 10.6 6 3.9 ms and 31.3 6 4.2 ms, respectively. Bilateral ablation of the primary auditory cortex (complete destruction of the Te1 and partial destruction of the Te2 and Te3 areas) resulted in an increase in GDT values. The fifth day after surgery, the rats were able to detect gaps in the noise. The values of GDT observed at this time were 4.2 6 1.1 ms for white noise and 7.4 6 3.1 ms for low-frequency noise at 70 dB SPL. During the first month after cortical ablation, recovery of GDT was observed. However, 1 month after cortical ablation GDT still remained slightly higher than in controls (1.8 6 0.18 for white noise, 3.22 6 0.15 for low-frequency noise, P 6 0.05). A decrease in GDT values during the subsequent months was not observed. ; 2002 Elsevier Science B.V. All rights reserved. Key words: Gap detection; Temporal resolution; Bilateral ablation; Auditory cortex; Rat
1. Introduction The measurement of gap detection threshold (GDT) is an e⁄cient tool for evaluating the time resolution abilities of the auditory system. The estimation of GDT in man has been the subject of many studies, beginning with the paper by Plomp (1964). Numerous experiments have shown that gap detection ability is dependent on the frequency spectrum and intensity of the continuous noise in which the gaps are embedded (Penner, 1977; Giraudi et al., 1980; Tyler et al., 1982;
* Corresponding author. Tel.: +42 (02) 4106 2700; Fax: +42 (02) 4106 2787. E-mail address: [email protected] (J. Syka). Abbreviations: GDT, gap detection threshold; SPL, sound pressure level; Te, temporal; Par, parietal; rf, rhinal ¢ssure
Florentine and Buus, 1984; Shailer and Moore, 1983; Fitzgibbons, 1983; Snell et al., 1994). Minimal GDTs in human were recorded when the noise signal encompassing the gap was presented at 30^40 dB SL and included frequencies above 5^6 kHz. GDT in this case amounted to about 2^3 ms (Plomp, 1964; Fitzgibbons, 1983). Clinical observations have shown that neurological patients with various types of cerebral lesions have di⁄culties in temporal resolution tasks, such as reduced gap detection abilities (Buchtel and Stewart, 1989; Albert and Bear, 1974; Efron et al., 1985; Phillips and Farmer, 1990). Neuronal correlates of gap detection have been studied at di¡erent levels of the auditory system (for review see Frisina, 2001). The fundamental mechanism of coding sound gaps at the level of the auditory nerve ¢bres is through a cessation of activity in response to a gap (Relkin and Turner, 1988; Zhang et al., 1990),
0378-5955 / 02 / $ ^ see front matter ; 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 5 9 5 5 ( 0 2 ) 0 0 5 7 8 - 6
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whereas at the level of cochlear nuclei and the inferior colliculus the mechanism switches to an increase in the spike ¢ring rate at the conclusion of the gap (Boettcher et al., 1990; Kaltenbach et al., 1993; Palombi et al., 1994; Walton et al., 1997; Barsz et al., 1998). Walton et al. (1997) concluded that the gap encoding capabilities of inferior colliculus neurones in the CBA mouse were similar to the behavioural gap functions. Neural correlates of gap detection were studied in the auditory cortex of the cat by Eggermont (1995, 1999). In these experiments gaps inserted either early (after 5 ms) or late (after 500 ms) in a 1 s duration noise burst were used. The neural detection threshold for the ‘late gap’ was found to be similar to the psychophysical gap threshold (around 5 ms) and was not signi¢cantly different from that found in the inferior colliculus (Walton et al., 1997). Signi¢cantly larger minimum late-gap durations were found in the anterior auditory ¢eld in comparison with the primary and secondary auditory ¢elds. The measurement of GDT may be used for investigating pathologies of temporal resolution ability and for estimating their severity. For this purpose, studies dealing with the in£uence of experimentally produced impairments in gap detection in animals may be of importance. The aim of such studies is usually to determine the involvement of individual structures of the auditory system in temporal resolution. Only a few papers on gap detection in animals with a cortical lesion have been published so far (in rat: Ison et al., 1991 ; in ferret: Kelly et al., 1996). The purpose of this study was to examine the role of the auditory cortex in the ability to detect gaps in noise. GDT was measured in pigmented rats before and after bilateral ablation of the auditory cortex.
2. Materials and methods 2.1. Subjects The ability to detect gaps in a continuously present noise was tested in ¢ve adult (8^13 months old) pigmented female rats (strain Long^Evans) with no primary pathology, before and after bilateral lesions of the primary auditory cortex. GDT measurements after a bilateral lesion of the auditory cortex began on the ¢fth day after surgery and continued for 2 months. In addition, the dependence of GDT on the frequency spectrum and intensity of the carrier noise was studied in three other rats.
tioning procedure with food reinforcement, therefore the rats had free access to water but were restricted in food intake. Twenty-three hours before a training or testing session, the rats were completely deprived of food. Food-deprived animals were trained to detect the presence of gaps in a continuous noise. The test apparatus and environment were similar to those described previously (Syka et al., 1996). The experiments were conducted in a grid (1 cm stainless steel mesh) test box (size 20U20U20 cm) with two levers (starting and responding) and a foodcup. The test apparatus was placed in an anechoic sound-proof room. The foodcup connected with a food dispenser was placed in the centre of the front wall; starting and responding levers (3.5 cm length) were mounted 6 cm above the box £oor 2 cm to the right and left of the foodcup. Noise was presented from a loudspeaker located next to the food dispenser outside the experimental box, 50 cm in front of the wall with the levers. The continuous noise was switched on before the rat was put in the test box. The experiment was designed so that 0.5^5 s after pressing the starting lever, test signals consisting of ¢ve gaps were triggered. Pressing the responding lever during the time window, when ¢ve gaps were presented, and up to 1 s after its end, was scored as a hit reaction and the rat was rewarded with a pellet (Fig. 1). Using ¢ve gaps as test stimuli instead of one gap shortened the training procedure and did not signi¢cantly in£uence the testing results. In preliminary experiments we demonstrated that the values of GDT were the same when the signal consisted of ¢ve or two gaps. When only one gap in a continuous noise was presented, GDTs increased by about 0.4 ms (P 6 0.05, tested with the paired t-test). In this case, the increased GDTs were caused mainly by an increased number of accidental responses. For the purpose of controlling the stimulus (stimulus control determines the degree of conjunction of the rat’s response with the test stimulus ^ Hendricks, 1966; Kelly and Masterton, 1977), in 33% of trials in a series the pressing of the starting lever did not result in the appearance of test signals. These were so-called ‘catch trials’. The rat’s response during the catch trial, when gaps were missing, was classi¢ed as a false alarm. False alarms allowed us to estimate the number of guessed responses among all hit responses. False alarms were punished with a 4 s ‘time-out’ period (when the starting lever and the whole system were without function). A small cue light placed above the starting lever was switched on when the system was in operation, i.e., when a new trial could be initiated by pressing the starting lever.
2.2. Behavioural apparatus and procedures 2.3. Stimulus parameters The rats were housed in a plexiglass cage (2^3 rats in a cage). GDTs were investigated by an operant condi-
GDT was measured before and after lesion using
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Fig. 1. Schema of the experimental arrangement for estimating GDT.
gaps embedded in continuous white noise or continuous low-frequency noise (upper cuto¡ frequency 3 kHz) of 70 dB sound pressure level (SPL) intensity. Five ¢xedduration gaps (rise and decay times below 20 Ws) with a 150 ms interval between them were used as test stimuli. In three rats, GDT was also measured at various noise intensities, i.e. at 10, 20, 30, 40, 50, 60, 70, and 80 dB SPL, using both white noise and low-frequency carrier noise. The acoustical system consisted of an RFT 03004 noise generator, an RFT 01013 frequency ¢lter, an attenuator, a gating device producing gaps, an ampli¢er and a two-way loudspeaker system (Tesla ARV 3604 and ARZ 4604). Acoustical calibration was carried out with a 0.5 inch Bru«el and Kjaer 4133 condenser microphone, a 2619 preampli¢er, and a 2606 measuring ampli¢er. The microphone was placed at the typical location of a subject’s head (near the foodcup). During GDT measurement we used gaps of a duration between 1.5 and 70 ms. Stimulus presentation and data acquisition were controlled by a custom-programmed PC. 2.4. Behavioural testing The threshold of gap detection was ¢rst estimated by a method of limits. Gap duration during one experimental series was gradually reduced from 70 ms in dec-
rements of 10, 5, 2, 1 or 0.5 ms until no response occurred and then increased again until the response reappeared. An identical testing procedure was used for each rat until the thresholds of three successive testing series di¡ered by less than 1.5 ms. Final GDT values were obtained by the method of constant stimuli using ¢ve or six di¡erent stimulus durations. Each gap duration was presented 20 times in a test series. The test series consisted of 20 randomly presented test blocks. In each block ¢ve test trials with gaps of the same duration and 2^3 catch trials were grouped (so that for every 20 trials with gaps of identical duration there were 10 catch trials). Minimal gap duration was shorter than the threshold value of gap duration determined by the method of limits, and maximal gap duration was chosen so that the performance exceeded 90%. Performance at each tested gap duration was represented by the hit rate value, de¢ned as the ratio of the number of correct responses to the number of signal trials. Final performance was expressed as a hit rate corrected by the false alarm rate according to the formula proposed by He¡ner and He¡ner (1988) : hit rate3(hit rateUfalse alarm rate). The GDT was de¢ned as the duration of a gap corresponding to a 0.5 hit rate corrected for false alarms.
Fig. 2. GDT as a function of the intensity level of the carrier noise for white noise and low-frequency noise in normal rats; individual data.
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2.5. Ablation procedure Rats were anaesthetized with an intraperitoneal injection of a ketamine (Narkamon 5%, Spofa) and xylazine (Rompun 2%, Bayer) mixture (ratio 3:1, dose 0.1 ml/100 g b.wt.). Following the introduction of anaesthesia, the rats were placed in a stereotaxic apparatus. The skin and underlying muscles were retracted. After identi¢cation of bregma, the bone of the skull covering the temporal auditory cortex was bilaterally removed between AP 33.0 mm and AP 37.8 mm. Laterally the opening exposed the rhinal ¢ssure and medially reached up to the sagittal plane 6.0 mm. The dura mater was removed, and an ablation of the auditory cortex was performed with a sharp knife under an operating microscope. The extent of the cortical lesion corresponded to the trepanation opening with the exception of its lateral margin, where it terminated 1^1.5 mm above the rhinal ¢ssure. The stereotaxic coordinates were derived from the atlas of Paxinos and Watson (1986). At the end of the experiments, the rats were deeply anaesthetized and perfused transcardially with saline followed by 4% paraformaldehyde in 0.1 M phosphate bu¡er (pH 7.4). The animals were decapitated, the brains were removed and post¢xed in the same ¢xative for 5^7 days. Coronal sections of 40 Wm thickness were cut with a freezing microtome. Every third section was stained with cresyl violet and mounted on slides. In order to analyse the three-dimensional extent of the cortical lesion, selected serial sections were redrawn and the boundaries of the cortical lesions transferred to standard sections of the Paxinos and Watson (1986) stereotaxic atlas. Delineation and nomenclature of the cortical areas were based on the atlas of Paxinos and Watson and the cytoarchitectonic criteria of Zilles (1985). The care and use of animals reported on in this study were approved by the Ethics Committee of the Institute of Experimental Medicine and followed the guidelines of the Declaration of Helsinki.
3. Results Control experiments revealed that the GDT depends on the spectral characteristics and intensity levels of the continuous noise in which the gaps are embedded. The mean values of GDT measured in eight normal rats when the gaps were embedded in a continuous white noise of 70 dB SPL were signi¢cantly smaller (P 6 0.05, tested with the paired t-test) than when the gaps were embedded in a low-frequency noise of the same intensity; the values were 1.57 6 0.07 ms and 2.9 6 0.34 ms, respectively. The results of an analysis of the e¡ect of noise intensity on GDT indicated that a decrease in
Fig. 3. Localization of the cortical lesions. (A) Lateral view of the rat cerebral cortex indicating the location of the auditory cortex, situated in the dorsal part of the parietal (Par2) area and in three temporal (Te1, Te2, Te3) areas, according to Zilles (1985). (B) The extent of the cortical lesions in the left and right hemispheres of ¢ve rats (animals marked as 1R, 1M, 2M, 1C, 2C) reconstructed from histological slides on the basis of an atlas of the rat brain (Paxinos and Watson, 1986).
noise intensity from 80 dB SPL to 10 dB SPL was accompanied by a signi¢cant increase in GDT (Fig. 2). In the intensity range of 80^50 dB SPL for white noise and 80^60 dB SPL for low-frequency noise, GDT
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Fig. 4. Schematic diagram of coronal sections through the rat hemisphere according to the atlas of Paxinos and Watson (1986) from rostral (bregma 33.8 mm) to caudal (bregma 37.3) showing the extent of cortical lesions (bold black line) in the right hemispheres of rat R1 (upper row) and rat C2 (lower row).
almost did not change with the changes in the noise intensity. The lowest noise intensities at which GDT could be evaluated were 20 dB SPL for white noise and 30 dB for low-frequency noise. The mean GDT values at these intensities were 10.6 6 3.9 ms and 31.3 6 4.2 ms for white noise and low-frequency noise, respectively. It must be noted that although the rats responded to gaps embedded in noise of an intensity lower than 20 dB SPL, the gap thresholds were impossible to estimate due to the high number of false alarm reactions. The false alarm rate was minimal, reaching 0^0.1, when easily distinguishable stimuli were presented and increased up to 0.4 when gap duration approached threshold values. The intersubject variability of GDT increased with decreasing noise intensity (Fig. 2). Bilateral ablation of the primary auditory cortex resulted in increased GDT values. Fig. 3 demonstrates the localization and extent of the cortical lesions of ¢ve rats
reconstructed from histological sections. Light microscopic analysis of the sections revealed full destruction of area 1 of the temporal cortex (Te1) and partial destruction of areas 2 and 3 of the temporal cortex (Te2 and Te3) according to Zilles (1985) in rats 1R, 1M, and 2M (the size of the lesion in the last case was larger than in the ¢rst two cases). In rats 1C and 2C the lesion comprised the Te1, Te2 and Te3 areas and extended to the occipital cortex. In the majority of animals, the cortical lesion also extended into areas 1 and 2 of the parietal cortex (Par1, Par2). The perirhinal cortical area was only slightly a¡ected in some animals. In all animals the cortical lesion was sharply demarcated from the intact cortex and a¡ected the whole thickness of the cortex, while in some animals the lesion slightly involved the subcortical white matter. Subcortical structures (striatum, hippocampus) were intact in all animals (Figs. 4 and 5). Fig. 4 demonstrates with a bold black line the extent of the lesion in two animals (1R and 2C)
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4. Discussion
Fig. 5. Photomicrograph of a coronal section through the rat (1M) temporal cortex stained with cresyl violet, demonstrating the extent of the surgical lesion (bregma 35.8). Bar = 1 mm; rf, rhinal ¢ssure.
on a schematic diagram of coronal sections through the hemisphere. Similarly, the extent of the lesion in another animal (1M) is apparent in Fig. 5. On the ¢fth day after cortical ablation, all rats were capable of gap detection ; however, the number of false alarm reactions increased. The false alarm rate in response to stimuli well above threshold (with hit rate around 0.9) changed from 0^0.1 under normal conditions to 0.4^0.7 (Fig. 6). The values of GDT amounted to 250^300% of the preoperative levels and were on average 4.2 6 1.1 ms for white noise of 70 dB SPL and 7.4 6 3.1 ms for low-frequency noise of the same intensity (Fig. 7). This ¢gure demonstrates GDTs in ¢ve rats (individual data), before and after bilateral ablation of the auditory cortex, measured in white and low-frequency noise at 70 dB SPL. In rats with larger lesions (2M, 1C and 2C), the threshold shifts in the ¢rst week after ablation were larger than in the other rats. Seven days after ablation, all rats showed an improved performance due to a decreased rate of false alarm reactions. A gradual decrease in the GDT was observed over the course of the following 2 weeks for white noise and the following 3 weeks for low-frequency noise. The slope of the recovery curve was steeper in cases of a larger ablation area (animals 2M, 1C and 2C). One month after cortical ablation, gap detection ability had almost recovered. However, GDT values remained slightly higher than before ablation (1.8 6 0.18 ms for white noise, 3.22 6 0.15 ms for low-frequency noise ^ P 6 0.05, tested with the paired t-test). A further decrease of GDT during the subsequent month was not observed.
The lowest GDTs for rats, measured in our experiments under optimal stimulus conditions (in a broadband noise at an intensity above 40 dB SPL) by an operant conditioning method, £uctuated about 1.6 ms and were slightly smaller than those obtained in previous investigations using a startle amplitude reduction paradigm (Ison, 1982; Ison et al., 1991; Leitner et al., 1993). The examination of Ison (Ison, 1982; Ison et al., 1991) of acoustic startle re£ex inhibition by brief gaps in noise showed that the GDT values in rats range from 2 to 4 ms. According to Leitner et al. (1993), who used the same method, rats reliably detect 2 ms gaps. The results of behavioural tests of gap detection abilities in other vertebrates have shown that GDT is 2 ms in CBA mice (Walton et al., 1997), ranges from 2.6 to 3 ms in chinchillas (Giraudi et al., 1980; Salvi and Arehole, 1985), amounts to about 10 ms in ferrets (Kelly et al., 1996) and reaches 2.5 ms in two bird species (budgerigars and zebra ¢nches) (Okanoya and Dooling, 1990). In man, GDTs have been observed in the range of 2^4 ms (Plomp, 1964; Penner, 1977; Fitzgibbons, 1983). Low values of GDT in rats may be linked to the fact that rats are capable of perceiving a broader spectrum of high-frequency acoustical signals. The upper threshold of the hearing range in rats reaches 70 kHz (He¡ner et al., 1994; Syka and Rybalko, 2000), while in ferrets, chinchillas and humans it does not exceed 44 kHz (Kelly et al., 1986), 33 kHz (He¡ner and He¡ner, 1991) and 20 kHz, respectively. Indirect support for this explanation comes from studies reporting an elevated GDT in cases of high-frequency hearing loss (in humans: Florentine and Buus, 1984 ; in chinchillas : Salvi and Arehole, 1985). The lower GDT values in rats obtained in the present study in comparison with the results of Ison (Ison, 1982; Ison et al., 1991) may be caused by di¡erences in the measuring methods employed in the two studies and the use of a series of ¢ve gaps as the test stimulus in our experiments. As reported in Section 2 of methods, in pilot experiments the GDT value for a single gap presentation was approximately 0.4 ms longer than when ¢ve gaps were presented. It will be noted that in the majority of GDT psychophysical measurements in laboratory animals reported in the literature, the authors used more than one gap as well (Giraudi et al., 1980; Salvi and Arehole, 1985; Kelly et al., 1996). The ability to detect gaps in noise depends on the frequency spectrum and intensity of the noise in which the gaps are embedded. GDT in low-frequency noise is always higher than in broadband noise. These data are in good agreement with studies dealing with gap detection abilities in band-limited noise. The decrease of the centre or cuto¡ frequency of noise results in both hu-
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Fig. 6. Performance curves (A) and curves of hit rate (B) and false alarm rate (C) of one experimental animal (1C) before and 5 days after ablations (detection of gap in white noise).
Fig. 7. GDT before (open symbols) and after (¢lled symbols) bilateral ablation of the auditory cortex for white noise and low-frequency carrier noise of 70 dB SPL; individual data.
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mans and animals in an elevation of the GDT (Florentine and Buus, 1984; Fitzgibbons, 1983; Snell et al., 1994). GDT increases with decreased intensity of the carrier noise. The largest e¡ect is observed in the range of intensities adjacent to the hearing threshold. In our study a decrease in noise intensity from 50 dB SPL to 20^30 dB SPL resulted in a GDT increase of about 9.1 ms and 28.4 ms for broadband and low-frequency noise, respectively. The extent to which GDT changes along with changes in noise intensity, as reported in the literature, is di¡erent for di¡erent species. In humans a decrease in the level of broadband noise from 40 to 10 dB SL resulted in an increase in GDT of about 20 ms (Plomp, 1964; Penner, 1977; Fitzgibbons, 1983). In chinchillas the change in intensity of broadband noise from 40 to 20 dB SL led to a GDT increase of about 5 ms (Giraudi et al., 1980). Kelly et al. (1996) found that a decrease in 8 kHz bandpass noise intensity from 70 to 10 dB SPL produced a GDT increase of approximately 10^18 ms in ferrets. In our experiments, it was possible to measure GDT at noise intensities as low as 20 dB SPL for white noise and 30 dB for lowfrequency noise. A high rate of false alarm reactions did not allow an estimation of GDT at lower noise intensities. Over the intensity range used in our experiments, the GDT increased by 9 ms and 28 ms when intensity decreased from 80 to 20 dB SPL for white noise and from 80 to 30 dB SPL for low-frequency noise, respectively. The large inter-subject variability of GDT at low levels of noise pressure in psychophysical experiments apparently re£ects the di¡erences in rats’ individual capabilities for solving tasks under complicated conditions. Bilateral ablation of the auditory cortex in rats was accompanied by an elevation in the GDT. Five days after surgery all rats were able to discriminate gaps in the noise. The GDT in white noise amounted to about 4 ms (preoperative GDT was approximately 1.6 ms). Five to 7 days after ablation the GDT values were related with the extent of the auditory cortex destruction (the larger the ablated area, the bigger the GDT shift, Figs. 3 and 7). During the ¢rst month after cortical ablation, GDTs gradually approached normal preablation values, but even 2 months after ablation the GDT remained slightly but signi¢cantly higher in comparison with pre-ablation values. Several studies have reported that a normally functioning and undamaged neocortex is necessary for gap detection ability. Measurements of the gap-induced inhibition of the acoustic startle re£ex in rats following functional decortication by KCl application demonstrated an inability to detect a gap of 15 ms duration (Ison et al., 1991). Kelly et al. (1996) reported that bilateral destruction of the auditory cortex in ferrets resulted in a deterioration in
GDT. The authors found increased GDT values, shifted by, on average, from 10 to 20 ms. In contrast to our experiments, in which the retesting of the rats began 5 days after surgery, postoperative tests in the ferrets were conducted 1 month after surgery and required a period of 1 week for retraining. The reason of di¡erences between our results and the results of Kelly et al. (1996) may be the di¡erences in the species used and the methods employed. It is also possible that the GDT decline observed in our experiments during 1 month after ablation was produced by instantaneous retraining of the task in this period. The increased false alarm rate after ablation in trained animals may be due to di⁄culty in discriminating the stimuli, which would cause the animal to adopt a strategy of increasing false alarms in order to obtain more rewards. Our results together with the results of other authors (Ison et al., 1991; Kelly et al., 1996) support the assumption that the auditory cortex plays a speci¢c role in temporal resolution. Auditory cortex ablation may also result in the impairment of mechanisms that assure the retention of memory traces and the integration of sensory information with speci¢c motor responses. This explanation is connected with the role of the auditory cortex in auditory learning (Aitkin, 1990; Ehret, 1997; Scheich et al., 1997). The large increase in the false alarm rate in rats after auditory cortex destruction in our experiments complicated considerably the measurement of GDT and might also indicate a change in the rats’ ability to respond correctly to test stimuli. This observation is in good agreement with studies dealing with the e¡ects of lesions before and after training on an animal’s responses. It was demonstrated that auditory cortex destruction in trained animals leads to an increased false alarm rate (Kelly and Whit¢eld, 1971; Kelly, 1973; Ohl et al., 1999). In these experiments, frequency-modulated tone discrimination was studied in the cat (Kelly and Whit¢eld, 1971) and the Mongolian gerbil (Ohl et al., 1999). According to Kelly and Whit¢eld (1971), the increase in the false alarm rate in trained animals after lesion was accompanied by reduced hit rates. Ohl et al. (1999) reported that lesions in trained animals caused an increased false alarm rate without any e¡ects on the hit rate. Lesions in naive animals before training led, on the contrary, to a reduced hit rate and had no e¡ect on the false alarm rate (Kelly, 1973; Ohl et al., 1999). The results of our experiments suggest that the complete destruction of the Te1 and the partial destruction of the Te2 and Te3 areas of the auditory cortex in rats substantially in£uence not only temporal resolution abilities, but also learning and memory mechanisms that assure the correct behavioural response of the rat during testing.
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Acknowledgements This work was supported by grants of the Grant Agency of the Czech Republic (309/01/1063) and the Grant Agency of the Ministry of Health (NK/6454-3).
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