Auditory spatial responses of young guinea pigs (Cavia porcellus) during and after ear blocking

Newborn guinea pigs were tested to determine their ability to approach an auditory stimulus early in development. Observations of the behavior of 1-4-day-old animals in a circular eight-choice maze revealed a pronounced tendency to orient toward and approach a tape-recorded signal of guinea pig vocalizations. The occurrence of approach responses was reduced to chance in animals tested with one ear occluded by wax ear plugs which attenuated but did not totally eliminate sound. The effect of monaural ear blocks was more severe than binaural blocks, which reflects the importance of binaural cues in the maintenance of approach responses to sound. In a second study, the ability of older animals, 11-31 days of age, was examined. Directional approach responses to sound were also evident at this age, and ear plugs disrupted performance only under monaural conditions. Furthermore, in animals raised from birth with monaural ear blocks but tested without ear plugs, there was a subsequent disruption of performance for at least 21 days. These results indicate the importance of binaural cues in the development of early auditory spatial responses and suggest the need for appropriate binaural experience for subsequent localization of sounds.     

Undernutrition during the brain growth period of the rat significantly delays the development of processes mediating Pavlovian trace conditioning

Pavlovian trace conditioning procedures allow one to assess the ability of animals to associate events that are temporally separated because the conditioned stimulus terminates prior to the occurrence of the unconditioned stimulus. We report that undernutrition during Postnatal Days 2-18 significantly delays the development of trace conditioning to a visual stimulus. Previously undernourished 20-day-old rats conditioned when no temporal interval separated the termination of the CS and US onset (0-s trace interval). However, it was not until the undernourished pups were 30 days old that they conditioned when the trace interval was 10 or 30 s. In contrast, control pups only 20 days old were able to condition when a 10-s trace interval separated the CS and US events, and 25-day-old control pups conditioned when the interval was either 10 or 30 s. These results suggest that undernutrition delays the development of processes that enable the rat to sustain a representation of a visual CS during the trace interval.     

Heart-rate responses of infant rats and guinea pigs to stimulus onset and offset: tests of the generality of a bidirectional effect

To test the generality, across modality and across species, of a bidirectional effect of photic stimulation in infant rats found in previous experiments, two experiments examined the effects on heart rate (HR) of onset and offset of auditory stimulation in rats and of photic stimulation in guinea pigs. Experiment 1 demonstrated a unidirectional, decelerative response in rats 17 and 21 days of age to the onset and offset of a 1-kHz tone, at both low (80 dB) and high (100 dB) intensity levels and at 14 days to the onset of the high-intensity tone. Experiment 2 demonstrated that infant guinea pigs, which unlike infant rats are precocial and not nocturnal, responded to the offset but not to the onset of light, and that this response was decelerative. These results indicated a lack of generality of the bidirectional HR effect, and were discussed in terms of the orienting/defensive reflex distinction.     

Housing pregnant mice (Mus musculus) in small groups facilitates the development of odor-based homing in offspring

Infant mice (Mus musculus) born to dams housed in isolation throughout pregnancy (IsoPreg) begin differentially approaching homenest bedding over clean bedding on Postnatal Day 6. Offspring of dams housed with 2 other potentially pregnant conspecifics (SocPreg) display such homing behavior on Day 4. Earlier onset of homing reflects facilitated olfactory responsiveness in SocPreg pups, rather than qualitative or quantitative differences in IsoPreg versus SocPreg nest odors, body growth, or motoric capabilities. Exposing pregnant IsoPreg dams to SocPreg bedding also accelerated homing onset in the offspring, though not to the same extent as the full social context. Thus, it appears that the facilitation of homing is mediated through the pregnant dam by a combination of chemical cues and other social stimuli.     

Control of responding by the location of sound: role of binaural cues.

In auditory localization experiments, where the subject observes from a fixed position, both relative sound intensity and arrival time at the two ears determine the extent of localization performance. The present experiment investigated the role of binaural cues in a different context, the sound-position discrimination task, where the subject is free to move and interact with the sound source. The role of binaural cues was investigated in rats by producing an interaural imbalance through unilateral removal of the middle auditory ossicle (incus) prior to discrimination training. Discrete trial go-right/go-left sound-position discrimination of unilaterally incudectomised rats was then compared with that of normal rats and of rats with the incus of both sides removed. While bilateral incus removal affected binaural intensity and arrival times, the symmetry of sound input between the two ears was preserved. Percentage of correct responses and videotaped observations of sound approach and exploration showed that the unilateral rats failed to localize the sounding speaker. Rats with symmetrical binaural input (normal and bilaterally incudectomised rats) accurately discriminated sound position for the duration of the experiment. Previously reported monaural localization based upon following the intensity gradient to the sound source was not observed in the unilaterally incudectomised rats of the present experiment. It is concluded that sound-position discrimination depends upon the use of binaural cues.     

Acute hunger of rat pups elicits increased kyphotic nursing and shorter intervals between nursing bouts: implications for changes in nursing with time postpartum

Earlier findings, based on limited behavioral observations, indicate that nursing behavior in rats declines dramatically in duration over time postpartum-despite increasing ingestion of milk by rat pups to meet their growth and metabolic needs-although hungry pups elicit more nursing than do well-nourished pups. The authors compared the nursing pattern in detail for 6 hr on Days 7 and 14 and induced hunger in pups acutely with mammary-duct-ligated dams unable to provide milk. Compared with Day 7, on Day 14, supine nursing and the interval between nursing bouts increased, whereas hovering over pups and kyphotic nursing decreased. When pups were increasingly hungry, these age-related changes were counteracted. Thus, the ingestive motivation of pups largely regulates the nursing pattem over time.     

Directional responses to sounds in young gerbils (Meriones unguiculatus)

Three experiments were conducted to determine the ability of infant gerbils to approach an auditory stimulus. In the first experiment, gerbil pups, 16-23 days of age, were tested in a circular apparatus with a central start area and a movable sound source located at one of eight positions around the perimeter. Stimuli included high- and low-intensity presentations of a tape-recorded gerbil social call, a broad-band white noise stimulus, and a no-stimulus control condition. The subjects showed a strong tendency to approach the low-intensity social call and a less pronounced tendency to approach the white noise. In the second experiment, gerbil pups were tested in the same apparatus with or without ear blocks to determine the role of binaural cues in directional approach responding. The tendency to approach a low-intensity vocalization was disrupted by obstruction of one ear but not by blocking both ears. Thus, binaural balance was shown to be important for early sound localization. In the third experiment, the tendency to approach a social call was compared at different ages, 12-15, 16-19, 20-23, and 24-27 days after birth. Approach responses were first seen at 16-19 days. The responses continued during the 20-23-day period but began to wane at 24-27 days of age.     

The additivity of loudness across critical bands: A conjoint measurement approach

Five subjects were required in each trial to compare directly two sounds and to indicate which sound was louder. Each of the 64 sounds employed consisted of a combination of one of eight intensity levels of a 2-kHz tone and one of eight intensities of a 5-kHz tone. If, as Fletcher and Munson (1933) argued, loudness is additive for tone combinations in which the frequencies are widely separated, then subjects’ judgments should reflect the summed loudnesses of the 2- and 5-kHz tones in a two-tone combination. Judgments of individual subjects were shown to satisfy the conditions for an additive structure, and individual loudness scales were constructed. These loudness scales varied from subject to subject. Since this paired comparison procedure minimized response biases, the results suggest substantial individual differences in the sensory representation of sound intensity. The relations among sensory scales derived from other structured sensory judgments, such as binaural loudness, are discussed.     

Stimulus control of maternal responsiveness to Norway rat (Rattus norvegicus) pup ultrasonic vocalizations

Mother rats (Rattus norvegicus; 6 to 8 days postpartum) approach and maintain proximal orientation to a pup that is emitting ultrasonic vocalizations (USVs) far more than do virgin females (W. J. Farrell & J. R. Alberts, 2002). We used a playback regimen to examine the roles of acoustic and nonacoustic cues in regulating maternal proximal orientation toward vocalizing pups. When presented with recorded USVs, mothers of 6- to 8-day-old pups and nulliparous virgin females exhibited equivalent levels of proximal orientation toward the playback speaker. Mothers did show enhanced proximal orientation toward recorded USVs, however, if a silent pup was positioned below the speaker. Pup odors appear to be crucial for the maternal response to vocalizing pups, as peripherally induced anosmia attenuated maternal proximal orientation toward a vocalizing pup. Furthermore, spatial contiguity between olfactory and auditory stimuli was required for a maximal maternal response.     

Neuroanatomical basis of binaural phase-difference analysis for sound localization: a comparative study.

Four varieties of mammals whose medial superior olives range from large to none at all were tested for their ability to localize single, brief tone pips at various frequencies. Although each animal could localize high-frequency tone pips, their ability to localize middle- and low-frequency tone pips corresponded to the size of their medial superior olive (MSO). Since this latter range of frequencies is the one in which binaural phase-difference cues predominate, this anatomical-behavioral correspondence supports the idea that MSO is the chief binaural time-analyzing center for sound localization.     

Sound localization in a predatory rodent, the northern grasshopper mouse (Onychomys leucogaster)

A comparison of the ability of mammals to localize sound revealed that among the animals examined to date, none of the rodents have been able to localize as accurately as the carnivores. Because all of these rodents are prey animals, the question arises as to whether their poor localization acuity is a phyletic trait of Rodentia or whether it is a trait common to prey species that may be under less selective pressure than predators to localize sound accurately. To answer this question, sound localization acuity was determined in a species that is both predatory and a rodent, the northern grasshopper mouse. Localization thresholds for a single 100-ms noise burst were determined for three grasshopper mice using a conditioned avoidance procedure. Their 50% discrimination threshold of 19 degrees is larger than that of any of the previously tested carnivores and well within the range of other rodents. However, calculations of the binaural sound localization cues available to rodents (based on their head size) suggest that the grasshopper mouse may make more efficient use of the available locus cues than other rodents. Thus, although the grasshopper mouse cannot localize as accurately as carnivores, it appears to be more accurate than predicted for a nonpredatory rodent of its size.     

Temperature sensitivity: One subjective continuum or two?

An experiment is reported on the effect of six consecutive days of exposure to distortions of binaural time and intensity cues on the localization function for tones of 350, 1000, and 4000 Hz. Tests were conducted under “free-field” conditions. The changes in auditory spatial orientation indicate that adjustments to binaurally distorted temporal and intensive cues can occur. Reorientation does not appear to depend upon the existence of veridical auditory cues.     

Orientation and lever responding in auditory discriminations in squirrel monkeys.

Head orientation during auditory discriminations was studies in squirrel monkeys using a two-lever trial-by-trial procedure. Animals were studied using auditory discriminations based on the position of the sound and on the spectral content differences between a pure tone and a noise. After the percentage of correct responses reached asymptote, head orientation was measured using videotape recordings. Orientation occurred on virtually every trial and was under the control of the position of the sound under all conditions. Lever responding was controlled by the same parameters of the sound under some conditions, and by different parameters in others. Orientation and lever responding were correlated (a level response could be predicted from the direction of orientation) when both responses were under the control of the same parameters of the sound. The two responses were uncorrelated when they were controlled by different parameters of the sound. Orientation and lever responding were not functionally related.     

Prenatal expression of species-typical action patterns in the rat fetus (Rattus norvegicus)

We investigated sensory and behavioral responsiveness of the rat fetus. On Days 19, 20, or 21 of gestation, rat fetuses received intraoral infusions of a biologically important stimulus, milk, or a novel chemical stimulus, lemon. Using a technique to directly observe behavior in utero, we found that rat fetuses discriminate between intraoral infusions of milk and lemon, exhibiting different levels and patterns of overall activity after infusion. Milk was found to evoke a low magnitude, delayed increase in overall fetal activity from Day 19 through Day 21, whereas lemon evoked a high-magnitude, spiked pattern of activity that diminished from Day 19 to Day 21. Late in gestation these two stimuli elicited species-typical action patterns. Milk infusions elicited a stretch response much like the one shown by pups at the nipple; lemon infusions elicited face wiping typical of older pups and adults exposed to aversive gustatory stimuli.     

Maternal aggression in rats: Changes over pregnancy and lactation in a sprague-dawley strain

Maternal aggression in a Sprague-Dawley strain of laboratory rat (Charles River CD) was explored from Day 1 after mating on Day 0 through Day 24 of lactation (L0-L24). Aggression toward unfamiliar male “intruders” during 10-min tests was low among nonpregnant, nonlactating females and during the first 10 days of gestation. Frequency of attack on intruders increased moderately but significantly by Gestation Day 16 (G16; Experiment 1) or G21 (Experiment 2), a prepartum phenomenon not previously reported in laboratory rats. Levels of aggression were highest, however, during the first 9 days of lactation, when attacks averaged more than 6 per 10-min session. Attacks declined sharply in frequency after L14 and by L24 did not exceed levels observed among nonpregnant females. Experiment 3 examined the importance of the test site (home cage with nest and pups, if any, vs unfamiliar cage without nest or pups) to agonistic behavior associated with pregnancy and lactation. Late pregnant females tested in a novel cage were not aggressive; however, females tested shortly after delivering their pups were highly aggressive, averaging over four attacks per 5-min session. In novel cage tests frequency of attack remained high through L4 but declined significantly by L7. These data are consistent with the hypothesis that maternal aggression at its onset is primarily under hormonal control, but becomes increasingly dependent upon external factors, presumably pup stimulation, during the postpartum period.     

Prepartum changes in maternal responsiveness and nest defense in Rattus norvegicus

Changes in maternal responsiveness during late pregnancy were traced by exposing females to foster pups (15-30-min tests) under conditions favoring the rapid initiation of maternal behavior. Groups were tested when nonpregnant or on Days 17, 20, 21, or 22 of pregnancy (Day 22 = parturition). Nulliparous females were compared with primiparous and with "experienced breeders." Nest defense was observed by introducing unfamiliar males (2-min tests) on Day 22. Results focused attention on three periods: (a) Days 17-20, when maternal responsiveness was lower than in the nonpregnant condition, (b) Day 21, when maternal responsiveness returned to or rose above nonpregnant levels, and (c) on Day 22, the 3.5 hr prior to delivery, during which 90% of females almost immediately retrieved, gathered, and tended foster pups and during which 92% attacked the unfamiliar intruders. (Attacks were rare earlier.) Maternally experienced females were more responsive to pups than nulliparous females when nonpregnant and throughout late pregnancy, but both groups were equally likely to show prepartum aggression.     

Litter gender composition affects maternal behavior of the primiparous mouse dam (Mus musculus)

Thirty-nine litters of the outbred Swiss CD-1 mouse strain were reduced at birth to 6 pups, according to three conditions: MM (all males), MF (3 males and 3 females), or FF (all females). The maternal behavior of their respective dams (frequency and duration of nursing, licking, nest-building, and other activities such as eating, drinking, sniffing, grooming, and undirected locomotor activity) were scored during a single 10-min session on Days 2, 3, 5, 7, 10, 14, and 18. Dams rearing MM litters showed higher scores of maternal care on Days 2 and 18 in the case of nursing, on Days 2, 7, and 10 in the case of nest-building, and on Day 18 in the case of licking. These results confirm previous rat data, which indicated that male and female pups are exposed early in infancy to a different pattern of maternal behavior. Concomitant measurements of pup development (body-weight gain and achievement of a number of physiological and neurobehavioral landmarks) indicated that these effects of litter gender composition are not mediated by gross changes in the maturational trends of the pups.     

Ontogenetic change in the auditory conditioned stimulus pathway for eyeblink conditioning

Two experiments examined the neural mechanisms underlying the ontogenetic emergence of auditory eyeblink conditioning. Previous studies found that the medial auditory thalamus is necessary for eyeblink conditioning with an auditory conditioned stimulus (CS) in adult rats. In experiment 1, stimulation of the medial auditory thalamus was used as a CS in rat pups trained on postnatal days (P) 17-18, 24-25, or 31-32. All three age groups showed significant acquisition relative to unpaired controls. However, there was an age-related increase in the rate of conditioning. Experiment 2 examined the effect of inactivating the medial auditory thalamus with muscimol on auditory eyeblink conditioning in rats trained on P17-18, 24-25, or 31-32. Rat pups trained on P24-25 and P31-32, but not P17-18, showed a significant reduction in conditioned responses following muscimol infusions. The findings suggest that the thalamic contribution to auditory eyeblink conditioning continues to develop through the first postnatal month.     

Olfactory stimulation induces filial preferences for huddling in rat pups.

Rat pups of all ages huddle with conspecifics, but the senosory control of contact behavior changes ontogenetically. Thermal cues control huddling until about Day 15, at which time species' odors become the dominant stimulus. The present experiments indicate that the filial response to conspecifics is dependent on olfactory experience. A synthetic chemical scent was added to the smells of the dam from Day 1 to Day 20 postpartum. Standardized videographic tests were used to assess the development of huddling preference. Preferences for nest-typical smells emerged by Day 15 in pups from both scented and nonscented litters. Pups from scented nests preferred to huddle with a scented stimulus rat, whereas control pups preferred a nonadulterated rat stimulus. Additional testing indicated that the affiliative preferences were specific to rearing odor and were not based on decreased aversion to test scents, or on disrupted olfactory discrimination. The ontogeny of species-typical contact behavior is discussed in terms of the induction of a perceptual preference that is based on early odor stimulation.     

The effect of a flashing visual stimulus on the auditory continuity illusion

The effect of a visual stimulus on the auditory continuity illusion was examined. Observers judged whether a tone that was repeatedly alternated with a band-pass noise was continuous or discontinuous. In most observers, a transient visual stimulus that was synchronized with the onset of the noise increased the limit of illusory continuity in terms of maximum noise duration and maximum tone level. The smaller the asynchrony between the noise onset and the visual stimulus onset, the larger the visual effect on this illusion. On the other hand, detection of a tone added to the noise was not enhanced by the visual stimulus. These results cannot be fully explained by the conventional theory that illusory continuity is created by the decomposition of peripheral excitation produced by the occluding sound.