DEVELOPMENT OF AUDITORY SENSITIVITY OF ALTRICIAL BIRDS: ABSOLUTE THRESHOLDS OF THE GENERATION OF EVOKED POTENTIALS L. I. Aleksandrov and L. P. Dmitrieva
UDC 612.822.3+612.825.55+612.65
The thresholds of the generation of EP in the L field of two to nine day old pied flycatcher nestlings in response to monotonal bursts of varied frequency were investigated. The entire range of auditory sensitivity was divided into three channels on the basis of the character of the age-related dynamics of the thresholds (two to nine days of life): low-frequency (0.3-1.6 kHz), middle- (1.5--4.0 kHz), and high-frequency (5.08.0 kHz). Widening of the auditory range in the direction of high frequencies on the 4th to 5th days of nest life was demonstrated. It was shown that the development of auditory sensitivity continues in all three channels in the post-embryonic period (two to nine days after hatching), and that each of these is characterized by its own thresholds time course. The auditory sensitivity of birds is traditionally assessed on the basis of data obtained through the recording of the microphone potential of the cochlea and the evoked potentials (EP) of the auditory nuclei of the brain stem [1, 2]. However, taking into consideration that the early behavior of altricial birds is based mainly on acoustic afferentation, the dynamics of the EP in the structure of the higher integrative level of the auditory system of birds, the L field of the caudal neostriatum, is of special interest for the purpose of judging the development of the perception of sounds utilized in behavior [14]. In previously published investigations [5, 6] of the development of auditory EP from the L field of nestlings on the basis of indirect criteria (the ume course of the amplitude-temporal parameter, and of the latent periods and the cycles of recovery) which reflect to a greater degree the time course of the maturation of auditory structures than the time course of auditory sensitivity proper, it was discovered that EP in response to tones of various frequencies mature heterochronously. In the first place, the structures associated with the generation of EP in response to tones of the cemral frequencies (1.5--4.0 kI-Iz) complete their formation; then in response to high frequencies (4.5-8.0 kHz); and, finally, in response to low frequencies (0.2-1.0 kHz). This has provided grounds for the hypothesis that there are three relatively isolated frequency channels within the limits of the total range of hearing of the nestlings, which are characterized by a varied rate of formation in ontogenesis. At the same time, the time course of the thresholds of the appearance of EP in the L field over the extent of the entire period of the ontogenetic development of hearing could be a direct criterion of the development of auditory sensitivity. METHODS
An investigation was carried out, for the purpose of quantitatively characterizing the age-related time course of auditory sensitivity on the basis of the criterion of change in the thresholds of the generation of auditory EP in response to monotonal bursts of varied frequency, on 43 two to nine day old pied flycatcher (Ficedula hypoleuca) nestlings, The EP were picked up from field L in awake nestlings, freely moving in the nest, bipotarly through silver ball electrodes. Following amplification and filtration, the signal arrived at an Apple II microcomputer, where it was averaged across 25 realizations (epoch 255 or 510 msec, 1 or 2 msec in channel, respectively). Since the absolute threshold is not some specific value, since the responses to near-threshold stimuli are in essence probabilistic in character [9], the minimal sound pressure level (SPL) at which the averaged response was different (this corresponded to a 50% probability of the occurrence of individual EP) was considered the threshold of the occurrence of the EP. The precision of the determination of the threshold in each specific case was usually _+ 2 dB, but in some cases the threshold SPL was determined unequivocally. The influence of a species song of a male pied flycatcher on the formation of auditory sensitivity was investigated in a special series of experiments on six nestlings. Starting from the first day of life, a species song (playing time of the recordInstitute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 41, No. 2, pp. 384-390, March-April, 1991. Original article submitted June 4, 1990; revision submitted October 31, 1990. 132
0009-3122/91/2701-0132512.50 9 ! 992 Plenum Publishing Corporation
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Fig. 1. Age-reNted time course of the thresholds of auditory EP in response to tones of various frequencies in nestlings. A) Low-frequency range, (0.3-1.0 kHz); B) middle-frequency range, (1.5-4.0 kHz); C) high-frequency range, (5.0-8.0 kHz). Along the abscissa: days of life in nest; along the ordinate: threshold sound pressure level, dB. ing, 30 rain), preliminarily recorded on a tape recorder, was reproduced daily for the nestlings of four hatchings through a wide-band speaker mounted in a hollow. The recording was reproduced for the nestlings two to three times at intervals of 4 to 5 h over the course of each day of life. The pickup electrodes were implanted on the 4th to 5th days in the nestlings and the thresholds of the generation of the auditory EP were investigated. A screened cage with a nest in which a temperature that was comfortable for the nestling was maintained automatically was placed in a sound-proof echo-damping chamber. The monotonal bursts (16 msec, fronts 1.7 msec) from the sound generator were formed by an electronic switch (the pulse controlling the switch served also as the marker for the computer), and were amplified by means of a Brig 1301 amplifier. An isodynamic mount fixed 12 cm above the head of the nestling served as the source of the sound. A Robotron-01012 spectral analyzer, the microphone of which was placed in the nest, and a Brtiel and Kjaer (Denmark) SPL meter were used to assess the stimulation channel in the entire working range (0.3-8.0 kHz; 10-88 dB above the 0.00002 Pa level) on the basis of the acoustic output. The principal tone of stimulation exceeded the harmonics by not less than 40 dB. After the correction of the amplitude-frequency characteristics of the stimuli by means of an equalizer, a linearity of + 2 dB was achieved in the entire frequency range. The maximal SPL was 88 + 2 dB. The shifting of the microphone in the nest led to changes in the SPL value of not more than 1-2 dB (i. e., the nestling moved about in a relatively uniform sound field). The series of stimuli of varying frequency and intensity were presented in random order with intervals between the stimuli of 12-90 sec. The feeding schedule of the nestlings corresponded to the natural schedule for the given age. The nestlings were used in the experiment from one to five days. Mter completion of the experiments a morphological control of the localization of the electrodes was carried out. INVESTIGATION RESULTS The recording of the EP was begun 1-1.5 days following hatching, i.e., on the second day of life. At that age the EP are recorded only in response to tones in the 0.3-5.0 kHz range. At the same time, only 50% of the nestlings responded to tones 133
of 5.0 kHz. Responses were also recorded to 6.0 kHz in one nestling at two and three days of age. The thresholds of the responses were quite high: 10--30 dB above the background SPL (36 dB). They are practically unchanged on the 3rd day. The thresholds at middle frequencies decreased on the 4th day to 8.4 + 1.6 dB, whereas the low-frequency thresholds remain unchanged, tn addition, it is then that responses to a tone of 8.0 kHz are recorded for the first time; the responses to 5.0 kHz become stable (54 _+4.9 dB), while EP are recorded in 43% of the nestlings in response to a tone of 6.0 kHz, although the thresholds are still very variable (67.3 +_+13.9 . dB). Finally, on the 5th day responses are recorded over the entire range of frequencies (0.3-0.8 [sic] kHz). The thresholds of the EP in response to low frequencies remain almost unchanged, while those in response to middle frequencies decrease somewhat (by 4.1 _+1.2 dB). Analysis of the age-related time course of the thresholds of the generation of EP in response to tones of various frequencies (Fig. 1A--C) confirms the reality of the existence of three frequency channels within the limits of the auditory system of altriciat birds, and in addition made it possible not only to characterize quantitatively the development of these channels, but also to reveal the distinctiveness of each of them. The sensitivity to tones of low frequencies which as we may assume exists akeady by the moment of hatching [4], changes only insignificantly (not more than 2 dB/day) over the fh'st four days. However, on the 6th day of life the thresholds of the EP in response to low-frequency sounds begin to fall perceptibly: the value of the threshold SPL decreases between the 6th and 7th days by 13.0 _ 2.1 riB. They decrease on the following day, but already at a substantially lower rate: 2.3 _+ 1.0 dB/day. The average rate of the decrease in the thresholds in the low frequency range was 2.9 _+0.2 dB/day from the 2nd through the 9th days. The sensitivity in the region of the middle frequencies is characterized by a different time course (Fig. t13). EP are already recorded at a relatively low threshold SPL (49 - 51 dB), but remain practically unchanged on the 3rd day; the thresholds decrease quite noticeably by the 4th day (8.4 + 1.6 dB), and then decrease relatively constantly and unitormly with an average rate of 3.77 _+0.68 dB/day over the course of the entire period of the recording. As a result they reach extremely low values on the 9th day, down to 16 dB. At the same time, the thresholds in response to tones of 1.5-4.0 kHz turn out to be lowest and least variable over the course of the entire period of development investigated, as compared with the threshold of the responses to tones of the low- and high-frequency ranges. The thresholds of the generation of the EP in the range of high frequencies, as is the case in the middle-frequency range, decrcase relalively unflbrmly (Fig. IC) over the entire time from the point of the appearance of the responses (the 4th to the 5th days) up until the stabilization of the thresholds on the 9th day, with an average rate of 6.7 +_ 1.5 dB/day, At the same time, somewhat higher initial values of the threshold SPL, and the same final values of the threshold SPL as for the middlefrequency range, are characteristic for responses to tones of 5.0 and 6.0 kHz; however, they decrease at a greater rate, about 7 dB/day. The responses to tones of 7.0 and 8.0 kHz are recorded for the first time with very high thresholds, which remain quite high even on the 8th to 9th days. Analysis of the rate of change in the thresholds (Fig. 2) showed that each of the three frequency channels is characterized by an individual pattern of the time course of this parameter. There are quite insignificant changes for the low frequencies over the course of almost the entire period under investigation, with one dramatically pronounced peak between the 6th and 7th days, 13.0 _+2.1 dB/day. The thresholds of the responses to tones of the middle frequencies are practically constant on the 2nd to the 3rd and the 8th to the 9th days, while they decrease at a rather high rate, not less than 3.6 dB/day, over the course of the entire remaining period. At the same time it can be noted that these changes take place most rapidly between the 3rd and the 4th and between the 6th and the 7th days (8.4 _+ 1.6 and 6.3 +_2.1 dB/day, respectively). A quite rapid and relatively uniform decrease is characteristic for the threshold SPL of the responses to high frequencies, although a significantly greater scatter of the characteristics of the individual frequencies is observed in this frequency range than in the low- and middle-frequency ranges. Despite the quite pronounced unevenness of the rate of change of the thresholds with age, it can be noted that the average (for the entire period of observation) rates of change of the threshold SPL differ across all three frequency ranges: low, 2.9 :t: 0.2; middle, 3.73 _+0.68; and high, 6.17 + 1.5 dB/day (the differences are significant: between low and middle, p < 0.t; between middle and high, p < 0.05; between low and high, p < 0.01). Comparison of the thresholds of the generation of the auditory EP in the control nestlings and the nestlings "sound-exposed" to the species song on the 4th day of life showed (Fig. 3) that the thresholds in the latter are noticeably (by 5-12 dB) lower at practically all frequencies. It should be noted in this connection that the control nestlings which are in the experiment one to three or more days, were also subjected to additional, by comparison with natural conditions, acoustic stimulation: to the constant presentation of monotonal bursts of varied frequency over the extent of 12-14 hours in a day. However, comparison of the thresholds recorded in the nestlings on the first day of the experiment with the thresholds of nestlings of the corresponding age which were in the experiment two to four days did not reveal any differences in the sound pressure levels which are threshold for the generation of the EP. 134
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Fig. 2. Rate of change in the thresholds of auditory EP in nestlings. Designations as in Fig. 1. White circles, low-frequency range; triangles, middle-frequency range; black, high-frequency range. Fig. 3. Influence of species song on the formation of auditory sensitivity of nestlings (4th day after hatching). Along the horizontal: frequency of the sound, kHz; along the vertical: threshold sound pressure level, riB. White columns, control; hatched, "soundexposed" nestlings. *, significant difference (p < 0.05). An earlier appearance of responses to high-frequency sounds (7.0 and 8.0 ld-Iz) was also noted in the "sound-exposed" birds. DISCUSSION OF RESULTS The investigations of the absolute thresholds of auditor3, sensitivity on the basis of the criterion of the generation of EP in field L showed that as early as on the 5th day the range of hearing of the nestlings (0.3-8.0 kHz) and the region of maximal sensitivity (1.5-4.0 kHz) correspond to parameters which are characteristic for adult songbirds [13], but that the auditory thresholds at this time are still noticeably higher. Thus, sensitivity in the high-frequency range probably appears in the post-embryonic period, whereas middle- and lowfrequency sensitivity is already quite well developed by the moment of hatching, and is involved in the accomplishment of early forms of acoustically oriented behavior. At the same time, the frequency ranges differ with respect to the character of the change in thresholds. In the low-frequency range the thresholds change over the course of the greater part of the period under investigation only insignificantly, and decrease noticeably over the course of one day, whereas in the region of the middle frequencies this decrease is relatively uniform over the entire extent of the period under investigation; in addition, the average rates of change in the thresholds of the generation of EP differ in all three hearing ranges. In essence a single point of view, based on data obtained in both behavioral and electrophysiological experiments, has been presented in the literature devoted to the development of auditory sensitivity of the vertebrates: that low-frequency sensitivity matures ftrst of all, and only subsequently, middle- and high-frequency sensinvity [7, 14]. The investigations of the development of the auditory system of mature-born birds (ducks, chickens, etc.) have shown that low-frequency sensitivity is already formed in them by the moment of hatching, while high-frequency sensitivity is formed in the post-embryonic period [8, 9, 10, 12, 14, 15]. However, the results of our investigation suggests that in songbirds the maturation of sensitivity in the region of low and middle frequencies is not entirely completed by the moment of hatching; it continues even throughout a targe part of the nesting period. In other words, low-frequency sensitivity continues to form simultaneously with high- and middle-frequency sensitivity, but somewhat more slowly. Thus, along with the expansion of the frequency range of perceptible sounds in post-embryonic ontogenesis, the formation of the auditory mechanisms of the perception of all frequencies continues. 135
The significance of the species song for intmspecies communication between adult birds has been studied repeatedly over the last several decades, and its principal functions have been analyzed in detail (see, for example, [11]). However, as has been established by Khayutin, et al., [6], the species song is also addressed to the nestlings, substantially influencing the organization of nest behavior. The results of our investigation point to the fact that the species song may, along with a direct influence on the behavior of nestlings, also exert an influence on the development of their auditory sensitivity. It is appropriate to note here that in the literature the influence of a sensorily enriched environment is usually not subdivided into a specific and a nonspecific influence. In this case, a substantial additional sensory influence, the presentation of pure tones of varied frequency over one or several days, does not exert any noticeable influence on the development of hearing. On the contrary, the comparatively brief reproduction of the species song to the nestlings leads to a significant decrease in the thresholds of the generation of the auditory EP in field L. Taking note of this difference, it can be assumed that the species song exerts a specific influence on the development of hearing in the nestlings, but is not simply a factor of an enriched acoustic environment. CONCLUSIONS !. Maturation of auditory sensitivity in altricial nestlings is not completed in the course of the embryonic period, but continues in post-embryonic ontogenesis. 2. Three relatively independent frequency channels (low-frequency, 0.3-1.0 kHz; middle-frequency, 1.5-4.0 kHz; and high-frequency, 5.0-8.0 kHz), which are differentiated on the basis of the rate of change and on the basis of other characteristics of the time course of the thresholds of generation of auditory EP, can be distinguished within the limits of the total auditory range of the nestlings. 3. The presentation of a species-specific song to the nestlings may exert a perceptible specific influence on the formation of auditory sensitivity which is manifested in a decrease in the thresholds of generation of auditory EP in response to monotonal sound bursts. LITERATURE CITED 1. 2.
To B. Golubeva, "The development of the hearing of birds in ontogenesis," in: The Sensory Systems and the Brain of Birds [in Russian], Nauka, Moscow (1980), pp. 113-138. T.B. Golubeva, Comparative Analysis of the Development of the Hearing of Birds, Absu'act of Dissertation for the De-
gree of Doctor of Biological Sciences, MGU, Moscow (1987). 3. S.N. Khayutin and L. P, Dmitrieva, The Organization of the Natural Behavior of Nestlings tin Russian], Nauka, Moscow (1981). 4. S.N. Khayutin and L. P. Dmitrieva, "The functional characterization of the development of the hearing of altricial birds. Association with natural behavior," Sensornye Sistemy, 1, No. 3, 299-307 (1987). 5. S.N. Khayutin and L. P. Dmitrieva, "The role of the species acoustical environment in the development of the hearing of nestlings," Zhurn. Vyssh. Nervn. Deyat., 38, No. 1, 21-29 (1988). 6. S.N. Khayutin, Yu. V. Grinchenko, and L. P. Dmitrieva, "The role of the species song in the organization of the nest behavior of nestlings," ZooL Zhurn., 57, No. 3,413-420 (1978). 7. G. Gottlieb, Development of Species Identification in Birds, Chicago Univ. Press, Chicago (1971). 8. L. Gray and E. W. Rubel, "Development of absolute thresholds in chickens," J. Aeoust. Soc. Amer., 77, No. 3, 1162-1172 (1985). 9. D.M. Green and J. A. Swets, Signal Detection Theory and Psychophysics, Wiley, New York (1966). 10. Mo Konishi, "Development of auditory neuronal responsesin avian embryos," Proc. Acad. Sci. USA, 70, 1795-1798 (1973). 11. M. Konishi, "Birdsong: from behavior to neuron," Ann. Rev. ofNeurosci., 8 (1985). 12. W. Lippe and E. W. Rubel, "Ontogeny of tonotopic organization of brainstem auditory nuclei in the chicken: implication for development of the place principle," J. Comp. Neurol., 237, No. 2, 273-289 (1985). 13. K. Okanoya and R. J. Dooling, "Hearing in passerine and psittacine birds: a comparative study of auditory thresholds," J. Comp. Psychol., 101, No. 1, 7-15 (1987). 14. E.W. Rubel, "Ontogeny of auditory system function," Ann. Rev. Physiol. 46, No. 1,213-229 (1984). 136
15.
J.C. Saunders, "The structural and functional development of the outer and middle ear," in: Development of Auditory and Vestibular Systems, R. Romand (ed.), Acad. Press, New York (1983), pp. 3-25.
REINFORCING EFFECT OF STIMULATION OF THE MESOCEREBRAL REGION OF THE BRAIN OF THE EDIBLE SNAIL
O. A. M a k s i m o v a and P. M. B a l a b a n
UDC 612.821.6+612.822.3
Two regions of the brain of the edible snail were stimulated. The spontaneous movements, either opening or closing, of the opening of the mantle cavity served as the signal for the stimulation. The stimulation of the region of the mesocerebrum of the edible snail in a semi-intact preparation may serve as a positive reinforcement of intercurrent behavior, while stimulation of the rostral portion of the parietal ganglia may serve as a negative reinforcement. Depending upon whether the movement itself or its absence is reinforced, the change in the intercurrent behavior may change sign.
One of the most important aspects of the investigation of the transfer of learning is the problem of the mechanisms of action of reinforcement. At the present, the possibility of the use of extracellular stimulation of various parts of the brain as reinforcement has been demonstrated in behavioral experiments on invertebrates [3]; this opens a path for the study of the cellular bases of reinforcement at the level of individual identified neurons, the functional role of which in the neural network and behavior is well known. The influence of extracellular stimulation of mesocerebrum, which is included in the network which supports sexual behavior [4], and of the region of the parietal ganglia, in which the command neurons of defensive behavior are located [1], on the intercurrent behavior of the animal, was investigated in the present study in a semi-intact preparation of the edible snail.
METHODS The experiments were carried out in a semi-intact preparation of the edible snail. The preparation was made up in the standard manner, with the preservation of the neural connections with all peripheral organs [1]. The neurons of the mesocerebruin, which participate in the organization of sexual behavior, and the command neurons of defensive behavior were stimulated by means of a suction glass macroelectrode with a tip diameter of about 500 ~tm. The movements of the spiracle were recorded by means of a displacement sensor, consisting of an infrared lightguide aimed at the object and an infrared photodiode which reacts to changes upon movement of a reflected beam of radiation, on an ink-writing instrument. The intracellular activity of one of the command neurons (pleural or parietal) was also recorded on the same instrument. Movements of the spiracle (opening or closing) served as the signal for the application of the stimulation. In the norm the spiracle is closed in the semi-intact preparation and spontaneously opens with a certain periodicity. We take the opening movement to be the indicator of intercurrent behavior. The experiments were carried out according to the following schemata: 1) the movement itself of the opening or the closing of the aperture of the mantle cavity was reinforced; 2) a specific level of opening of the spiracle was reinforced; 3) a specific level of closing (the interval between openings) of the spiracle was reinforced. In an experiment the mesocerebrum was stimulated fLrst,then the command neurons. The activity of the command neuron was picked up by a glass microelectrode filled with 3 M KCI, with a tip diameter of about 1 lain and a resistance of 10-20 mi), from the contralateral side in relation to the stimulating electrode when the command neurons were being stimulated. Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 41, No. 2, pp. 391-396, March-April, 1991. Original article submitted March 29, 1990; revision submitted October 31, 1990. 0009-3122/91/2701-0137512.50 9 1992 Plenum Publishing Corporation
137
UDC 612.822.3+612.825.55+612.65
The thresholds of the generation of EP in the L field of two to nine day old pied flycatcher nestlings in response to monotonal bursts of varied frequency were investigated. The entire range of auditory sensitivity was divided into three channels on the basis of the character of the age-related dynamics of the thresholds (two to nine days of life): low-frequency (0.3-1.6 kHz), middle- (1.5--4.0 kHz), and high-frequency (5.08.0 kHz). Widening of the auditory range in the direction of high frequencies on the 4th to 5th days of nest life was demonstrated. It was shown that the development of auditory sensitivity continues in all three channels in the post-embryonic period (two to nine days after hatching), and that each of these is characterized by its own thresholds time course. The auditory sensitivity of birds is traditionally assessed on the basis of data obtained through the recording of the microphone potential of the cochlea and the evoked potentials (EP) of the auditory nuclei of the brain stem [1, 2]. However, taking into consideration that the early behavior of altricial birds is based mainly on acoustic afferentation, the dynamics of the EP in the structure of the higher integrative level of the auditory system of birds, the L field of the caudal neostriatum, is of special interest for the purpose of judging the development of the perception of sounds utilized in behavior [14]. In previously published investigations [5, 6] of the development of auditory EP from the L field of nestlings on the basis of indirect criteria (the ume course of the amplitude-temporal parameter, and of the latent periods and the cycles of recovery) which reflect to a greater degree the time course of the maturation of auditory structures than the time course of auditory sensitivity proper, it was discovered that EP in response to tones of various frequencies mature heterochronously. In the first place, the structures associated with the generation of EP in response to tones of the cemral frequencies (1.5--4.0 kI-Iz) complete their formation; then in response to high frequencies (4.5-8.0 kHz); and, finally, in response to low frequencies (0.2-1.0 kHz). This has provided grounds for the hypothesis that there are three relatively isolated frequency channels within the limits of the total range of hearing of the nestlings, which are characterized by a varied rate of formation in ontogenesis. At the same time, the time course of the thresholds of the appearance of EP in the L field over the extent of the entire period of the ontogenetic development of hearing could be a direct criterion of the development of auditory sensitivity. METHODS
An investigation was carried out, for the purpose of quantitatively characterizing the age-related time course of auditory sensitivity on the basis of the criterion of change in the thresholds of the generation of auditory EP in response to monotonal bursts of varied frequency, on 43 two to nine day old pied flycatcher (Ficedula hypoleuca) nestlings, The EP were picked up from field L in awake nestlings, freely moving in the nest, bipotarly through silver ball electrodes. Following amplification and filtration, the signal arrived at an Apple II microcomputer, where it was averaged across 25 realizations (epoch 255 or 510 msec, 1 or 2 msec in channel, respectively). Since the absolute threshold is not some specific value, since the responses to near-threshold stimuli are in essence probabilistic in character [9], the minimal sound pressure level (SPL) at which the averaged response was different (this corresponded to a 50% probability of the occurrence of individual EP) was considered the threshold of the occurrence of the EP. The precision of the determination of the threshold in each specific case was usually _+ 2 dB, but in some cases the threshold SPL was determined unequivocally. The influence of a species song of a male pied flycatcher on the formation of auditory sensitivity was investigated in a special series of experiments on six nestlings. Starting from the first day of life, a species song (playing time of the recordInstitute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 41, No. 2, pp. 384-390, March-April, 1991. Original article submitted June 4, 1990; revision submitted October 31, 1990. 132
0009-3122/91/2701-0132512.50 9 ! 992 Plenum Publishing Corporation
C A
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Fig. 1. Age-reNted time course of the thresholds of auditory EP in response to tones of various frequencies in nestlings. A) Low-frequency range, (0.3-1.0 kHz); B) middle-frequency range, (1.5-4.0 kHz); C) high-frequency range, (5.0-8.0 kHz). Along the abscissa: days of life in nest; along the ordinate: threshold sound pressure level, dB. ing, 30 rain), preliminarily recorded on a tape recorder, was reproduced daily for the nestlings of four hatchings through a wide-band speaker mounted in a hollow. The recording was reproduced for the nestlings two to three times at intervals of 4 to 5 h over the course of each day of life. The pickup electrodes were implanted on the 4th to 5th days in the nestlings and the thresholds of the generation of the auditory EP were investigated. A screened cage with a nest in which a temperature that was comfortable for the nestling was maintained automatically was placed in a sound-proof echo-damping chamber. The monotonal bursts (16 msec, fronts 1.7 msec) from the sound generator were formed by an electronic switch (the pulse controlling the switch served also as the marker for the computer), and were amplified by means of a Brig 1301 amplifier. An isodynamic mount fixed 12 cm above the head of the nestling served as the source of the sound. A Robotron-01012 spectral analyzer, the microphone of which was placed in the nest, and a Brtiel and Kjaer (Denmark) SPL meter were used to assess the stimulation channel in the entire working range (0.3-8.0 kHz; 10-88 dB above the 0.00002 Pa level) on the basis of the acoustic output. The principal tone of stimulation exceeded the harmonics by not less than 40 dB. After the correction of the amplitude-frequency characteristics of the stimuli by means of an equalizer, a linearity of + 2 dB was achieved in the entire frequency range. The maximal SPL was 88 + 2 dB. The shifting of the microphone in the nest led to changes in the SPL value of not more than 1-2 dB (i. e., the nestling moved about in a relatively uniform sound field). The series of stimuli of varying frequency and intensity were presented in random order with intervals between the stimuli of 12-90 sec. The feeding schedule of the nestlings corresponded to the natural schedule for the given age. The nestlings were used in the experiment from one to five days. Mter completion of the experiments a morphological control of the localization of the electrodes was carried out. INVESTIGATION RESULTS The recording of the EP was begun 1-1.5 days following hatching, i.e., on the second day of life. At that age the EP are recorded only in response to tones in the 0.3-5.0 kHz range. At the same time, only 50% of the nestlings responded to tones 133
of 5.0 kHz. Responses were also recorded to 6.0 kHz in one nestling at two and three days of age. The thresholds of the responses were quite high: 10--30 dB above the background SPL (36 dB). They are practically unchanged on the 3rd day. The thresholds at middle frequencies decreased on the 4th day to 8.4 + 1.6 dB, whereas the low-frequency thresholds remain unchanged, tn addition, it is then that responses to a tone of 8.0 kHz are recorded for the first time; the responses to 5.0 kHz become stable (54 _+4.9 dB), while EP are recorded in 43% of the nestlings in response to a tone of 6.0 kHz, although the thresholds are still very variable (67.3 +_+13.9 . dB). Finally, on the 5th day responses are recorded over the entire range of frequencies (0.3-0.8 [sic] kHz). The thresholds of the EP in response to low frequencies remain almost unchanged, while those in response to middle frequencies decrease somewhat (by 4.1 _+1.2 dB). Analysis of the age-related time course of the thresholds of the generation of EP in response to tones of various frequencies (Fig. 1A--C) confirms the reality of the existence of three frequency channels within the limits of the auditory system of altriciat birds, and in addition made it possible not only to characterize quantitatively the development of these channels, but also to reveal the distinctiveness of each of them. The sensitivity to tones of low frequencies which as we may assume exists akeady by the moment of hatching [4], changes only insignificantly (not more than 2 dB/day) over the fh'st four days. However, on the 6th day of life the thresholds of the EP in response to low-frequency sounds begin to fall perceptibly: the value of the threshold SPL decreases between the 6th and 7th days by 13.0 _ 2.1 riB. They decrease on the following day, but already at a substantially lower rate: 2.3 _+ 1.0 dB/day. The average rate of the decrease in the thresholds in the low frequency range was 2.9 _+0.2 dB/day from the 2nd through the 9th days. The sensitivity in the region of the middle frequencies is characterized by a different time course (Fig. t13). EP are already recorded at a relatively low threshold SPL (49 - 51 dB), but remain practically unchanged on the 3rd day; the thresholds decrease quite noticeably by the 4th day (8.4 + 1.6 dB), and then decrease relatively constantly and unitormly with an average rate of 3.77 _+0.68 dB/day over the course of the entire period of the recording. As a result they reach extremely low values on the 9th day, down to 16 dB. At the same time, the thresholds in response to tones of 1.5-4.0 kHz turn out to be lowest and least variable over the course of the entire period of development investigated, as compared with the threshold of the responses to tones of the low- and high-frequency ranges. The thresholds of the generation of the EP in the range of high frequencies, as is the case in the middle-frequency range, decrcase relalively unflbrmly (Fig. IC) over the entire time from the point of the appearance of the responses (the 4th to the 5th days) up until the stabilization of the thresholds on the 9th day, with an average rate of 6.7 +_ 1.5 dB/day, At the same time, somewhat higher initial values of the threshold SPL, and the same final values of the threshold SPL as for the middlefrequency range, are characteristic for responses to tones of 5.0 and 6.0 kHz; however, they decrease at a greater rate, about 7 dB/day. The responses to tones of 7.0 and 8.0 kHz are recorded for the first time with very high thresholds, which remain quite high even on the 8th to 9th days. Analysis of the rate of change in the thresholds (Fig. 2) showed that each of the three frequency channels is characterized by an individual pattern of the time course of this parameter. There are quite insignificant changes for the low frequencies over the course of almost the entire period under investigation, with one dramatically pronounced peak between the 6th and 7th days, 13.0 _+2.1 dB/day. The thresholds of the responses to tones of the middle frequencies are practically constant on the 2nd to the 3rd and the 8th to the 9th days, while they decrease at a rather high rate, not less than 3.6 dB/day, over the course of the entire remaining period. At the same time it can be noted that these changes take place most rapidly between the 3rd and the 4th and between the 6th and the 7th days (8.4 _+ 1.6 and 6.3 +_2.1 dB/day, respectively). A quite rapid and relatively uniform decrease is characteristic for the threshold SPL of the responses to high frequencies, although a significantly greater scatter of the characteristics of the individual frequencies is observed in this frequency range than in the low- and middle-frequency ranges. Despite the quite pronounced unevenness of the rate of change of the thresholds with age, it can be noted that the average (for the entire period of observation) rates of change of the threshold SPL differ across all three frequency ranges: low, 2.9 :t: 0.2; middle, 3.73 _+0.68; and high, 6.17 + 1.5 dB/day (the differences are significant: between low and middle, p < 0.t; between middle and high, p < 0.05; between low and high, p < 0.01). Comparison of the thresholds of the generation of the auditory EP in the control nestlings and the nestlings "sound-exposed" to the species song on the 4th day of life showed (Fig. 3) that the thresholds in the latter are noticeably (by 5-12 dB) lower at practically all frequencies. It should be noted in this connection that the control nestlings which are in the experiment one to three or more days, were also subjected to additional, by comparison with natural conditions, acoustic stimulation: to the constant presentation of monotonal bursts of varied frequency over the extent of 12-14 hours in a day. However, comparison of the thresholds recorded in the nestlings on the first day of the experiment with the thresholds of nestlings of the corresponding age which were in the experiment two to four days did not reveal any differences in the sound pressure levels which are threshold for the generation of the EP. 134
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Fig. 2. Rate of change in the thresholds of auditory EP in nestlings. Designations as in Fig. 1. White circles, low-frequency range; triangles, middle-frequency range; black, high-frequency range. Fig. 3. Influence of species song on the formation of auditory sensitivity of nestlings (4th day after hatching). Along the horizontal: frequency of the sound, kHz; along the vertical: threshold sound pressure level, riB. White columns, control; hatched, "soundexposed" nestlings. *, significant difference (p < 0.05). An earlier appearance of responses to high-frequency sounds (7.0 and 8.0 ld-Iz) was also noted in the "sound-exposed" birds. DISCUSSION OF RESULTS The investigations of the absolute thresholds of auditor3, sensitivity on the basis of the criterion of the generation of EP in field L showed that as early as on the 5th day the range of hearing of the nestlings (0.3-8.0 kHz) and the region of maximal sensitivity (1.5-4.0 kHz) correspond to parameters which are characteristic for adult songbirds [13], but that the auditory thresholds at this time are still noticeably higher. Thus, sensitivity in the high-frequency range probably appears in the post-embryonic period, whereas middle- and lowfrequency sensitivity is already quite well developed by the moment of hatching, and is involved in the accomplishment of early forms of acoustically oriented behavior. At the same time, the frequency ranges differ with respect to the character of the change in thresholds. In the low-frequency range the thresholds change over the course of the greater part of the period under investigation only insignificantly, and decrease noticeably over the course of one day, whereas in the region of the middle frequencies this decrease is relatively uniform over the entire extent of the period under investigation; in addition, the average rates of change in the thresholds of the generation of EP differ in all three hearing ranges. In essence a single point of view, based on data obtained in both behavioral and electrophysiological experiments, has been presented in the literature devoted to the development of auditory sensitivity of the vertebrates: that low-frequency sensitivity matures ftrst of all, and only subsequently, middle- and high-frequency sensinvity [7, 14]. The investigations of the development of the auditory system of mature-born birds (ducks, chickens, etc.) have shown that low-frequency sensitivity is already formed in them by the moment of hatching, while high-frequency sensitivity is formed in the post-embryonic period [8, 9, 10, 12, 14, 15]. However, the results of our investigation suggests that in songbirds the maturation of sensitivity in the region of low and middle frequencies is not entirely completed by the moment of hatching; it continues even throughout a targe part of the nesting period. In other words, low-frequency sensitivity continues to form simultaneously with high- and middle-frequency sensitivity, but somewhat more slowly. Thus, along with the expansion of the frequency range of perceptible sounds in post-embryonic ontogenesis, the formation of the auditory mechanisms of the perception of all frequencies continues. 135
The significance of the species song for intmspecies communication between adult birds has been studied repeatedly over the last several decades, and its principal functions have been analyzed in detail (see, for example, [11]). However, as has been established by Khayutin, et al., [6], the species song is also addressed to the nestlings, substantially influencing the organization of nest behavior. The results of our investigation point to the fact that the species song may, along with a direct influence on the behavior of nestlings, also exert an influence on the development of their auditory sensitivity. It is appropriate to note here that in the literature the influence of a sensorily enriched environment is usually not subdivided into a specific and a nonspecific influence. In this case, a substantial additional sensory influence, the presentation of pure tones of varied frequency over one or several days, does not exert any noticeable influence on the development of hearing. On the contrary, the comparatively brief reproduction of the species song to the nestlings leads to a significant decrease in the thresholds of the generation of the auditory EP in field L. Taking note of this difference, it can be assumed that the species song exerts a specific influence on the development of hearing in the nestlings, but is not simply a factor of an enriched acoustic environment. CONCLUSIONS !. Maturation of auditory sensitivity in altricial nestlings is not completed in the course of the embryonic period, but continues in post-embryonic ontogenesis. 2. Three relatively independent frequency channels (low-frequency, 0.3-1.0 kHz; middle-frequency, 1.5-4.0 kHz; and high-frequency, 5.0-8.0 kHz), which are differentiated on the basis of the rate of change and on the basis of other characteristics of the time course of the thresholds of generation of auditory EP, can be distinguished within the limits of the total auditory range of the nestlings. 3. The presentation of a species-specific song to the nestlings may exert a perceptible specific influence on the formation of auditory sensitivity which is manifested in a decrease in the thresholds of generation of auditory EP in response to monotonal sound bursts. LITERATURE CITED 1. 2.
To B. Golubeva, "The development of the hearing of birds in ontogenesis," in: The Sensory Systems and the Brain of Birds [in Russian], Nauka, Moscow (1980), pp. 113-138. T.B. Golubeva, Comparative Analysis of the Development of the Hearing of Birds, Absu'act of Dissertation for the De-
gree of Doctor of Biological Sciences, MGU, Moscow (1987). 3. S.N. Khayutin and L. P, Dmitrieva, The Organization of the Natural Behavior of Nestlings tin Russian], Nauka, Moscow (1981). 4. S.N. Khayutin and L. P. Dmitrieva, "The functional characterization of the development of the hearing of altricial birds. Association with natural behavior," Sensornye Sistemy, 1, No. 3, 299-307 (1987). 5. S.N. Khayutin and L. P. Dmitrieva, "The role of the species acoustical environment in the development of the hearing of nestlings," Zhurn. Vyssh. Nervn. Deyat., 38, No. 1, 21-29 (1988). 6. S.N. Khayutin, Yu. V. Grinchenko, and L. P. Dmitrieva, "The role of the species song in the organization of the nest behavior of nestlings," ZooL Zhurn., 57, No. 3,413-420 (1978). 7. G. Gottlieb, Development of Species Identification in Birds, Chicago Univ. Press, Chicago (1971). 8. L. Gray and E. W. Rubel, "Development of absolute thresholds in chickens," J. Aeoust. Soc. Amer., 77, No. 3, 1162-1172 (1985). 9. D.M. Green and J. A. Swets, Signal Detection Theory and Psychophysics, Wiley, New York (1966). 10. Mo Konishi, "Development of auditory neuronal responsesin avian embryos," Proc. Acad. Sci. USA, 70, 1795-1798 (1973). 11. M. Konishi, "Birdsong: from behavior to neuron," Ann. Rev. ofNeurosci., 8 (1985). 12. W. Lippe and E. W. Rubel, "Ontogeny of tonotopic organization of brainstem auditory nuclei in the chicken: implication for development of the place principle," J. Comp. Neurol., 237, No. 2, 273-289 (1985). 13. K. Okanoya and R. J. Dooling, "Hearing in passerine and psittacine birds: a comparative study of auditory thresholds," J. Comp. Psychol., 101, No. 1, 7-15 (1987). 14. E.W. Rubel, "Ontogeny of auditory system function," Ann. Rev. Physiol. 46, No. 1,213-229 (1984). 136
15.
J.C. Saunders, "The structural and functional development of the outer and middle ear," in: Development of Auditory and Vestibular Systems, R. Romand (ed.), Acad. Press, New York (1983), pp. 3-25.
REINFORCING EFFECT OF STIMULATION OF THE MESOCEREBRAL REGION OF THE BRAIN OF THE EDIBLE SNAIL
O. A. M a k s i m o v a and P. M. B a l a b a n
UDC 612.821.6+612.822.3
Two regions of the brain of the edible snail were stimulated. The spontaneous movements, either opening or closing, of the opening of the mantle cavity served as the signal for the stimulation. The stimulation of the region of the mesocerebrum of the edible snail in a semi-intact preparation may serve as a positive reinforcement of intercurrent behavior, while stimulation of the rostral portion of the parietal ganglia may serve as a negative reinforcement. Depending upon whether the movement itself or its absence is reinforced, the change in the intercurrent behavior may change sign.
One of the most important aspects of the investigation of the transfer of learning is the problem of the mechanisms of action of reinforcement. At the present, the possibility of the use of extracellular stimulation of various parts of the brain as reinforcement has been demonstrated in behavioral experiments on invertebrates [3]; this opens a path for the study of the cellular bases of reinforcement at the level of individual identified neurons, the functional role of which in the neural network and behavior is well known. The influence of extracellular stimulation of mesocerebrum, which is included in the network which supports sexual behavior [4], and of the region of the parietal ganglia, in which the command neurons of defensive behavior are located [1], on the intercurrent behavior of the animal, was investigated in the present study in a semi-intact preparation of the edible snail.
METHODS The experiments were carried out in a semi-intact preparation of the edible snail. The preparation was made up in the standard manner, with the preservation of the neural connections with all peripheral organs [1]. The neurons of the mesocerebruin, which participate in the organization of sexual behavior, and the command neurons of defensive behavior were stimulated by means of a suction glass macroelectrode with a tip diameter of about 500 ~tm. The movements of the spiracle were recorded by means of a displacement sensor, consisting of an infrared lightguide aimed at the object and an infrared photodiode which reacts to changes upon movement of a reflected beam of radiation, on an ink-writing instrument. The intracellular activity of one of the command neurons (pleural or parietal) was also recorded on the same instrument. Movements of the spiracle (opening or closing) served as the signal for the application of the stimulation. In the norm the spiracle is closed in the semi-intact preparation and spontaneously opens with a certain periodicity. We take the opening movement to be the indicator of intercurrent behavior. The experiments were carried out according to the following schemata: 1) the movement itself of the opening or the closing of the aperture of the mantle cavity was reinforced; 2) a specific level of opening of the spiracle was reinforced; 3) a specific level of closing (the interval between openings) of the spiracle was reinforced. In an experiment the mesocerebrum was stimulated fLrst,then the command neurons. The activity of the command neuron was picked up by a glass microelectrode filled with 3 M KCI, with a tip diameter of about 1 lain and a resistance of 10-20 mi), from the contralateral side in relation to the stimulating electrode when the command neurons were being stimulated. Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 41, No. 2, pp. 391-396, March-April, 1991. Original article submitted March 29, 1990; revision submitted October 31, 1990. 0009-3122/91/2701-0137512.50 9 1992 Plenum Publishing Corporation
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