An investigation into blocking of filial imprinting in the chick during exposure to a compound stimulus

The occurrence of "blocking" was investigated in jungle fowl chicks (Gallus gallus spadiceus B.) in an imprinting situation. In Experiment 1, chicks were simultaneously exposed to two stationary coloured cylinders, either two red cylinders (Group RR), a yellow and a red cylinder (YR), or two yellow cylinders (YY). After six days of exposure, the cylinders were removed from the cages and replaced by a yellow and a blue cylinder (i.e. YB) for each chick. This second phase of the experiment lasted for seven days. When the blue cylinder was presented alone during tests at different stages in Phase 2, the RR birds spent significantly more time with this cylinder and emitted fewer shrill calls than the chicks in the YR and YY groups. In Experiment 2, RR and YY birds were reared as in Experiment 1, except that in the second phase of the experiment they were exposed to a blue cylinder only. In this experiment the development of an attachment to the novel blue cylinder proceeded similarly in the RR and YY birds. In Experiment 3, it was found that chicks that were reared with a yellow and a red cylinder preferred the latter stimulus. Thus, although in the first phase of Experiment 1 the RR birds had been exposed to a more attractive stimulus, in tests during the second phase they spent more time with a novel stimulus (B) than the YY birds. These results are consistent with the suggestion that imprinting to a novel stimulus is "blocked" to some extent when that stimulus is presented in compound with another stimulus to which the animal has previously been exposed.     

Spontaneous generalization of abstract multimodal patterns in young domestic chicks

From the early stages of life, learning the regularities associated with specific objects is crucial for making sense of experiences. Through filial imprinting, young precocial birds quickly learn the features of their social partners by mere exposure. It is not clear though to what extent chicks can extract abstract patterns of the visual and acoustic stimuli present in the imprinting object, and how they combine them. To investigate this issue, we exposed chicks (Gallus gallus) to three days of visual and acoustic imprinting, using either patterns with two identical items or patterns with two different items, presented visually, acoustically or in both modalities. Next, chicks were given a choice between the familiar and the unfamiliar pattern, present in either the multimodal, visual or acoustic modality. The responses to the novel stimuli were affected by their imprinting experience, and the effect was stronger for chicks imprinted with multimodal patterns than for the other groups. Interestingly, males and females adopted a different strategy, with males more attracted by unfamiliar patterns and females more attracted by familiar patterns. Our data show that chicks can generalize abstract patterns by mere exposure through filial imprinting and that multimodal stimulation is more effective than unimodal stimulation for pattern learning.     

Domestic chicks perceive stereokinetic illusions

Stereokinetic illusions occur when certain 2-D patterns are set in slow rotation in a plane perpendicular to the line of sight. Such phenomena have never been investigated in animal species other than our own. We used the domestic chick (Gallus gallus) to check whether these illusions are experienced by non-human species, taking advantage of filial imprinting. Newly hatched visually naive chicks were individually exposed for 4 h to 2-D stimuli producing, to a human observer, the perception of a stereokinetic cone (experiment 1) or of a stereokinetic cylinder (experiment 2). Thereafter, each chick underwent a free-choice test between a solid 3-D cone and a solid 3-D cylinder. A control group of newly hatched but not imprinted chicks underwent the same testing procedure, to check for the presence of any spontaneous preference for one or other of the two solid objects. Imprinted chicks approached the 3-D stimulus closely resembling the stimulus they had been exposed to during imprinting (the cone in experiment 1 and the cylinder in experiment 2). Non-imprinted chicks did not show any preference. These results suggest that domestic chicks experience stereokinetic illusions.     

Anchor effects on the discrimination of pitch

Two experiments considered the effects of introducing an extreme stimulus (anchor) upon the differential perception of tonal stimuli. In the first experiment, in which Ss rated series stimuli from 1,000 to 2,000 Hz or from 2,000 to 3,000 Hz, the presence of a 750-Hz anchor apparently disrupted discrimination. A second experiment involved testing for a difference threshold (method of limits) in which the standard was a tone of 1,500 Hz. The uncertainty interval (Iu) was larger when the interval between paired stimulus presentations was filled with a 750-Hz tone, again suggesting that discrimination is impaired by the introduction of a low-frequency anchor. Results are discussed in terms of theories relating range extent and discriminability.     

Call recognition in chicks of the Australian brush-turkey (Alectura lathami)

Most birds rely on imprinting and experience with conspecifics to learn species-specific recognition cues. Australian brush-turkeys (Alectura lathami) do not imprint and form no bonds with parents. They hatch asynchronously, disperse widely and meet juvenile conspecifics at an unpredictable age. Nevertheless, in captivity, hatchlings respond to other chicks. A recent study, which involved the use of robotic models, found that chicks prefer to approach robots that emit specific visual cues. Here, we evaluated their response to acoustic cues, which usually play an important role in avian social cognition. However, in simultaneous choice tests, neither 2-day-old nor 9-day-old chicks preferred the choice arm with playback of either chick or adult conspecific calls over the arm containing a silent loudspeaker. Chicks of both age classes, however, scanned their surroundings more during chick playback, and the response was thus consistent in younger and older chicks. We also presented the chicks with robotic models, either with or without playback of chick calls. They did not approach the calling robot more than they did the silent robot, indicating that the combination of visual and acoustic cues does not evoke a stronger response. These results will allow further comparison with species that face similar cognitive demands in the wild, such as brood parasites. Such a comparative approach, which is the focus of cognitive ecology, will enable us to further analyse the evolution and adaptive value of species recognition abilities.     

Effects of imprinting and isolation on aggressive responses in chicks

The importance of isolation and imprinting as separate factors influencing early aggressive responses in chicks (Gallus gallus) was studied in two separate experiments. In addition the effect of the presence or absence of the imprinting stimulus in the test situation was examined in Experiment 1. Neither imprinting per se, nor the presence of the imprinting stimulus significantly affected aggressiveness. Rearing conditions (isolation vs social rearing) did, however, influence aggressive responses. Isolated chicks were significantly more aggressive than socially reared buds. The adaptedness of selective aggression in socially (naturally) reared chicks, as well as the indiscriminant aggressiveness caused by social deprivation is discussed.     

Directed attention influences the modification of startle reflex probability

The present study was designed to investigate the effect of directed attention on elicitation and modification of the startle reflex. 30 adult human subjects received 90 dB(A) broadband noise startle stimuli either alone or preceded by a 60 dB(A) prepulse (either 2000-Hz tone, 1000-Hz tone, or broadband noise). Subjects were instructed to attend to one of the three prepulses during half the trials and to ignore all stimuli during the rest of the trials. The probability of responding while attending to a prepulse was significantly lower than the probability of responding while ignoring the prepulses. Responding to the prepulses was also more probable while subjects were attending to the prepulses, and these effects were more pronounced for tone than for noise prepulses. These results suggest that directed attention can influence the probability of the startle reflex without influencing startle amplitude or latency.     

Human central auditory plasticity associated with tone sequence learning

We investigated learning-related changes in amplitude, scalp topography, and source localization of the mismatch negativity (MMN), a neurophysiological response correlated with auditory discrimination ability. Participants (n = 32) underwent two EEG recordings while they watched silent films and ignored auditory stimuli. Stimuli were a standard (probability = 85%) and two deviant (probability = 7.5% each for high [HD] and low [LD]) eight-tone sequences that differed in the frequency of one tone. Between recordings, subjects practiced discriminating the HD or LD from the standard for 6 min. The amplitude of the LD MMN increased significantly across recordings in both groups, whereas the amplitude of the HD MMN did not. The LD was easier to discriminate than was the HD. Thus, practicing either discrimination increased the MMN for the easier discrimination. Learning and changes in the LD MMN amplitude were highly correlated. Source localizations of event-related potentials (ERPs) to all stimuli revealed bilateral sources in superior temporal regions. Compared with the standard ERP, the LD ERP revealed a stronger source in the left superior temporal region in both recordings, whereas the right-sided source became stronger after learning. Consistent with prior studies of auditory plasticity in animals and humans, tone sequence learning induced rapid neurophysiological plasticity in the human central auditory system. The results also suggest that there is asymmetric hemispheric involvement in tone sequence discrimination learning and that discrimination difficulty influences the time course of learning-related neurophysiological changes.     

The ontogeny of auditory frequency generalization in the chicken.

To determine if there is an ontogenetic change in stimulus coding, chickens between the day of hatching and 9-10 days old were tested using a habituation-generalization paradigm. Experiment 1 indicated that 1 day-old and 3--4-day-old chicks show similar habituation of an eye-opening response to auditory stimuli in the 800--1,200-Hz range. In Experiment 2 the eye-opening response to a 1,000-Hz stimulus was habituated and then immediately tested using stimuli which varied between 800 and 1,200 Hz. Each age-group (1 day, 3--4 days, and 9--10 days) showed a symmetrical stimulus generalization gradient around the 1,000-Hz stimulus and the 1-day-old chicks displayed a reliably flatter gradient than either of the older groups, which did not differ. In a third experiment, the position of the gradients relative to the baseline was shifted without altering the relative shapes. These results allow general arousal, general auditory responsiveness, overall error rate, and metric characteristics of the independent and dependent variables to be eliminated as possible sources of the age differences in gradient shape. The changes in stimulus generalization, therefore, support the view that during normal development there is a sharpening of perceptual coding processes.     

Perceptual learning during filial imprinting: Evidence from transfer of training studies

Chicks were first imprinted by exposing them to a moving training stimulus, B or C, that was projected onto a screen at one end of an experimental cabinet. Subsequently, subjects selectively approached the stimulus to which they had been exposed. On the following day, the chicks were placed into a chilled experimental cabinet (15°C) and received trials on which two stimuli (A and B) were projected onto screens located at opposite ends of the cabinet. If the subject approached Stimulus A, a stream of warm air was delivered; if it approached B, the trial terminated, and no heat was presented. For subjects that had been imprinted with Stimulus B, those in Group B, Stimulus A was novel, and Stimulus B was familiar. For chicks that had been imprinted with Stimulus C, Group C, both A and B were novel. In two experiments, Group B acquired the discrimination more rapidly than did Group C. This observation, made using a novel training procedure, was taken to support the suggestion that imprinting results in a form of perceptual learning in which the familiar imprinting object has become more discriminable from other novel objects.     

Verbal processing deficits in schizophrenia

The authors reported that a subgroup of schizophrenic patients performed well on a tone serial position task but was impaired on an auditory word serial position task (Wexler, Stevens, Bowers, Cerniak, & Goldman-Rakic, 1998). This study assessed 30 schizophrenic and 32 controls (matched for comparable tone discrimination) on 4 versions of the verbal serial position tasks and 2 tone serial position tasks. Patients performed poorly on all verbal tasks but performed comparably to controls when tones served as stimuli. Proactive interference and visual presentation further compounded the verbal deficits. Deficits persisted with pronounceable nonword stimuli. These findings provide evidence of specific deficits in language-related processing, although the authors could not rule out the possibility that the differential effects that were observed between the tone and word tasks, and particularly among the verbal tasks, may result from differing discriminating power of the different tests.     

The role of stimulus comparison in perceptual learning: An investigation with the domestic chick

In two experiments an imprinting procedure was used to familiarize chicks with two stimuli, A and B, that subsequently served as the discriminanda in a simultaneous discrimination. On the first day of each experiment, subjects either received presentations of A and B that were intermixed within a session (mixed exposure) or presentations of A in one session and of B in another (separate exposure). For half of the subjects in each of the exposure conditions, A and B differed in both colour and form; for the remainder A and B differed in form alone. On the second day of the experiments, the chicks were placed into a cool test apparatus and given training in which approaching A was rewarded by the delivery of a stream of warm air, but approaching B was not. Acquisition of this discrimination was more rapid when A and B differed in two respects than when they differed in form alone. When A and B differed in both colour and form, the heat-reinforced discrimination was acquired more rapidly after separate exposure than after mixed exposure; but when A and B differed in form alone, discrimination learning was more rapid following mixed exposure than separate exposure. The latter finding, that the opportunity to compare stimuli differing in only one dimension facilitates subsequent discrimination learning, is consistent with earlier suggestions (Gibson, 1969) regarding the conditions that promote perceptual learning.     

Perception of partly occluded objects by young chicks

Completion of partly occluded objects is a ubiquitous phenomenon in human visual perception. It is unclear, however, whether it occurs at all in other species: Studies on visual discrimination learning have revealed that animals usually attend to parts and features of the discriminative stimuli rather than to global object properties. We provide here the first demonstration of recognition of partly occluded objects in a bird species, the domestic chickGallus gallus, using the naturalistic setting made available by filial imprinting, a process whereby young birds form attachments to their mothers or some artificial substitute. In Experiment 1, newborn chicks were reared singly with a red cardboard triangle, to which they rapidly imprinted and therefore treated as a social partner. On Day 3 of life, the chicks were presented with pairs of objects composed of either isolated fragments or occluded parts of the imprinting stimulus. Chicks consistently chose to associate with complete or with partly occluded versions of the imprinting object rather than with separate fragments of it. Similarly, in Experiment 2, chicks reared with a partly occluded triangle chose to associate with a complete triangle rather than with a fragmented one, whereas chicks reared with a fragmented triangle chose to associate with a fragmented triangle and not with a complete one. Newborn chicks thus appear to behave as if they could experience amodal completion.     

Imprinting, Conditioning, and Within-event Learning

Three experiments investigated whether or not the temporal relationship between the visual (V) and auditory (A) components of an imprinting stimulus influenced the acquisition of a visual preference in domestic chicks (Gallus gallus domesticus). In Experiments 1a and 1b, chicks given simultaneous presentations of V and A showed a more marked tendency to approach V than those given sequential exposure to V and A. In Experiments 2 and 3, chicks received exposure to two visual stimuli, V1 and V2. During training, presentations of V1 preceded, followed, or were separated by 30 sec from presentations of a simultaneous compound comprising V2 and A. Chicks in each condition subsequently showed an equivalent preference to approach V2 rather than V1. These results show that filial imprinting, unlike Pavlovian conditioning but like within-event learning, proceeds most readily when training involves the simultaneous presentation of stimuli.     

Maternal hen calls modulate memory formation in the day-old chick: the role of noradrenaline

Facilitation of memory for discriminative learning in young chicks is enhanced following exposure to a synthesized rhythmic auditory stimulus. Increased arousal, mediated by noradrenergic activation, is believed to underlie this effect. In this report we examine whether ethologically relevant rhythmic auditory stimuli produce the same noradrenaline-mediated memory enhancement in neonate chicks (Gallus gallus domesticus). Maternal hen attraction calls which contained a rhythmic pattern were found to facilitate retention. Intracerebral injection of noradrenaline or the beta(2)-adrenergic antagonist propranolol demonstrated that this enhancement is likely to be mediated by noradrenergic activation of central beta(2)-noradrenergic receptors. In contrast, a rhythmic alarm call inhibited retention. Subcutaneous injection of the alpha(1)-adrenergic antagonist prazosin revealed that this impairment may be due to higher arousal levels resulting in activation of alpha(1)-noradrenergic receptors. It is concluded that the maternal hen calls of domestic chickens can influence the memory ability of the offspring via noradrenaline release in the brain. The current data suggest that call meaning and rhythmicity interact to yield the appropriate levels of beta(2)-adrenergic activation required to facilitate retention for a discriminative task.     

Behavioral thresholds for tones during infancy

An adaptive up-down tracking procedure was used in combination with a visually reinforced head turn response to examine auditory sensitivity for 500, 2000, and 8000-Hz tone bursts in infants 6 to 18 months of age. Six- and 10-month-old infants were tested with headphone presentation of stimuli, while 10-, 14, and 18-month-olds were tested in sound field. Infant threshold estimates for both headphone and sound field were within 15 dB of adult comparisons for all frequencies and age groups. Six-month-olds were significantly less sensitive to the 8000-Hz tone than to either of the lower frequency stimuli, but older infants demonstrated approximately equal sensitivity for all three frequencies tested.     

Stimulus control of heart rate by auditory frequency and auditory pattern in pigeons

A new method was used to investigate auditory discrimination in pigeons. Basically, the method involves the repeated presentation of one stimulus preceding the single presentation of a different stimulus that is followed by shock. Stimulus control is assessed by the increase in heart rate that accompanies the presentation of the second stimulus. In Experiment 1, the efficiency of the method was explored by determining the frequency difference thresholds of pigeons at 500, 1000, 2000, and 4000 Hz. Weber fractions comparable to those reported in an earlier study using the conditioned suppression method were obtained. Experiment 2 demonstrated that, contrary to results of earlier studies, auditory temporal patterns can exercise differential stimulus control in pigeons. One stimulus consisted of the presentation (once per second) of a 1000-Hz pure tone of 150 msec duration followed by a 2000-Hz pure tone of equal duration; the other was the same except for the reversed order of the frequency components. Results indicated that the frequency pattern and not the loudness pattern of the stimuli was the cue controlling heart-rate changes.     

Mediating factors in chick social pecking: 1. Effects of isolation,imprinting, and peer contact

Two experiments examined the influence of periods of social isolation on rates of intersubject pecking in pairs of domestic chicks. Of central interest was the effect of an imprinting condition wherein single birds were reared with either a red or green Styrofoam object. In the first experiment, imprinted subjects were given social pecking tests in the absence of the imprinting object, while in the second study the tests for social pecking were conducted in the presence of one of the imprinting objects. In the latter test the object was familiar to one bud of a pair, but was novel to the other animal. The results of both experiments showed that the rate of intersubject pecking of the imprinted subjects was intermediate to, and statistically different from both the low pecking rate of chicks reared in pairs and the high pecking rate of chicks reared in total isolation. The presence of a familiar object during tests in the second study seemed to serve as a secure base since chicks that had been reared with that object exhibited more social pecking than did chicks that were unfamiliar with the object. However, regardless of the provision of the object the social pecking of the imprinted chicks again fell on a point intermediate to the other groups. Hypotheses concerning a) intolerance of figureground movement and b) sheer stress of isolation were advanced to account for these results.     

Cardiac responses to acoustic playback experiments in the captive bottlenose dolphin (Tursiops truncatus)

Acoustic recordings were used to investigate the cardiac responses of a captive dolphin (Tursiops truncatus) to sound playback stimuli. A suction-cup hydrophone placed on the ventral midline of the dolphin produced a continuous heartbeat signal while the dolphin was submerged. Heartbeats were timed by applying a matched-filter to the phonocardiogram. Significant heart rate accelerations were observed in response to playback stimuli involving conspecific vocalizations compared with baseline rates or tank noise playbacks. This method documents that objective psychophysiological measures can be obtained for physically unrestrained cetaceans. In addition, the results are the 1st to show cardiac responses to acoustic stimuli from a cetacean at depth. Preliminary evidence suggests that the cardiac response patterns of dolphins are consistent with the physiological defense and startle responses in terrestrial mammals and birds.     

Stimulus configuration and valence-enhanced pecking by neonatal chicks

Valence-enhanced pecking occurs when hatchling chicks preferentially peck at releasing stimuli that have been assigned increased releasing power or valence. Enhanced valence is assigned by another releasing stimulus—here a “pecking” arrow. Pecking movements by the arrow have two separate effects: the stimulation or release of pecking by chicks and releasing valence enhancement, that is, pecking directed to particular stimuli. Exposure of chicks to the moving arrow yields pecking significantly increased over that accruing to a stationary arrow. A distinctive stimulus placed on the floor at the tip of the arrow, or, most effective, attached to the tip of the moving arrow, produces enhanced valence: pecking by observing chicks that is selectively increased to matching stimuli.