Ontogenesis of learning: I. Variation in the rat's reflexive and learned responses to gustatory stimulation

These experiments indicate that by Day 15 after birth, the processes that mediate a number of taste-controlled behaviors in the rat are functional. These include the sensory processes necessary to detect and respond reflexively to sucrose, the event-learning processes that reduce the rat's neophobic reaction to sucrose, and the integrative-learning processes that enable it to learn an aversion to sucrose when paired with lithium toxicosis, even when these events are separated by 1 hr. These capacities, however, did not emerge simultaneously. Those necessary to detect and respond reflexively to sucrose emerged prior to those that contribute to the learned control of taste-guided behaviors. It is argued that these age-related dissociations in behavioral capacities reflect a caudal-to-rostral maturational sequence of components of the ascending gustatory system that are thought to underlie these capacities.     

Ontogenesis of learning: III. Variation in the rat's differential reflexive and learned responses to sound frequencies

The ontogenesis of the rat's differential response to variation in sound frequency was investigated. Two behavioral procedures were employed: (a) a differential Pavlovian conditioning procedure that provided a means of inferring the pup's capacity to learn to respond differentially on the basis of sound frequency (e.g., 2000 Hz vs 900 Hz), and (b) a habituation-generalization task that indexed the capacity to discriminate different sound frequencies from the pup's reflexive behavior. Pups less than 17 days old failed to learn to respond differentially to a 2000-Hz vs 900-Hz tone (Experiment I, IIA, IIB, and III). In contrast, pups only 14 days old discriminated tones that differed by as little as 200 Hz when the reflex-habituation-generalization procedure was employed (Experiment IV). This suggests that the reflexive and learned behaviors, potentially controlled by sound frequency, are dissociated during ontogenesis, the former emerging prior to the latter. These data were interpreted within a neurobiological framework that related ontogenetic changes in behavioral capacity to a caudal-to-rostral maturational sequence in the ascending auditory system of the components that contribute to these capacities.     

Ontogenesis of learning: VI. Learned and unlearned responses to visual stimulation in the infant hooded rat

A Pavlovian fear conditioning procedure was used to examine the ontogeny of the hooded rats' learned responses to visual stimulation. The data suggests a dissociation in the emergence of the processes required to detect visual events and those necessary for learning an association between visual stimulation and shock. Pups did not condition to a visual conditioned stimulus (CS) paired with a shock unconditioned stimulus (US) until they were 17 days old, even though 15-day-olds were clearly able to detect the visual CS. Although 15-day-olds failed to condition to the visual CS, they conditioned successfully when an auditory CS was paired with shock. Thus, the 15-day-olds' failure to condition to the visual CS was not due to a performance deficit or to a general ineffectiveness of shock as an US. These data suggest that the components of the visual system that mediate detection and those required for associative learning mature sequentially.     

Electrical responses of the auditory area of the cerebellar cortex to acoustic stimulation

Summary Single unit activity from the VI and VII lobuli of the cerebellar vermis cortex was studied following acoustical stimulation with sound signals of different parameters. Cerebellar neurons, as compared to those from the auditory system, showed low selectivity to sound frequency, intensity and duration. However, about 2/3 of the neurons were selectively sensitive to interaural time and intensity differences; about 1/3 of neurons showed a specific response to signals simulating sound motion in a definite direction. Thus, cerebellar neurons seem to be mainly responsive to those sound parameters which are essential for sound localization.     

Neuronal responses in the rat's thalamus to scrotal heating

Summary Twenty-one single neurons from the rat's thalamus were recorded as they responded to scrotal skin thermal stimulation. Eighteen of them were warm und three inverse-warm cells. After the injection of Lidocain (Xylocain) into the scrotal skin, the response of each neuron was abolished. Eight neurons could be followed until the drug effect had disappeared; their responsiveness was fully restored. External application of Xylocain had the same effect as intracutaneous injection, apart from longer time intervals before the effect became apparent. From this, conclusions are drawn on the origin of the thalamic responses, and speculations are made on the functional role of warm and inverse-warm cells.This project was supported by the Deutsche Forschungs-gemeinschaft, SFB 114 (BIONACH)     

Ontogenesis of learning: V. Variation in associative and nonassociative control of an operant forelimb response in infant rats

To study ontogenetic variation in the mechanisms that control operant responding, infant rats 4-16 days old were trained to lift a forelimb in order to receive an infusion of a sucrose-milk solution (reinforcer). Although the learning processes that contribute to this behavior were to some extent functional in pups 5-6 days old (Experiment V), there were major age-related constraints on the performance of this response. We were unable to observe selective responding with the reinforced forelimb until pups were 9 days old (Experiment I). This was in part because pups tested when 7, but not 9 days old, were highly activated (as measured by responding with both forelimbs) by both the reinforcer (Experiment II) and cues associated with the reinforcer during training (Experiment IV). Implications of these findings are discussed.     

Neural responses to free field and virtual acoustic stimulation in the inferior colliculus of the guinea pig

Virtual auditory space (VAS) stimuli based on outer ear transfer functions became increasingly important in spatial hearing research. However, few studies have investigated the match between responses of auditory neurons to VAS and free-field (FF) stimulation. This study validates acoustic spatial receptive fields (SRFs) of 183 individual midbrain units using both VAS and FF stimuli. The first-spike latency, which varied systematically across SRFs, was 14.9 +/- 8.3 (SD) ms in FF, and 15.1 +/- 8.3 ms in VAS. Spike-count-based SRFs measured 0-20 dB above the neural threshold covered on average 44.5 +/- 18.0% of the recorded sphere in FF and 45.5 +/- 18.7% in VAS. The average deviation of the centroid position of SRFs using FF and VAS stimuli was 7.4 degrees azimuth and 3.3 degrees elevation. The average spike rate remained unchanged. The SRF overlap recorded using FF and VAS stimuli (mean: 71.3 +/- 12.6%) or repeated FF stimuli (70.2 +/- 14.2%) was high and strongly correlated (r = 0.96; P < 0.05). The SRF match observed with FF and VAS stimuli was not significantly altered over a range of stimulus levels (paired t-test P = 0.51; n = 6). Randomized VAS barely affected SRF sizes, centroids, or maximum spike count but decreased the average minimum response to 59% compared with sequential stimulation (paired t-test; P = 0.05; n = 26). SRF recordings in VAS excluding the acoustic distortions of the recording equipment differed from those in VAS incorporating the equipment (paired t-test P = 0.01; n = 5). In conclusion, neurophysiological recordings demonstrate that individualized VAS stimuli provided a good simulation of a FF environment.     

External inferior colliculus integrates trigeminal and acoustic information: unit responses to trigeminal nucleus and acoustic stimulation in the guinea pig

The external nucleus of the inferior colliculus (ICx) receives ascending projections from both auditory and somatosensory nuclei. In the guinea pig, both the spinal trigeminal nucleus (TN) and the cochlear nucleus converge in the ventrolateral region of ICx. We investigated the function of trigeminal-collicular pathways by electrically stimulating the TN while recording unit responses from ICx. Pairing electrical stimulation with acoustic stimuli allowed us to investigate the function of converging auditory and somatosensory inputs, i.e. multisensory integration. Unit responses were recorded from ICx using a multi channel, single shank electrode. Electrical stimulation of the TN produced small changes above or below spontaneous rate, but resulted in significant suppression or enhancement of sound-evoked responses. Multisensory integration has been demonstrated in the dorsal cochlear nucleus (DCN), superior colliculus and sensory cortices and may play a role in plasticity that occurs after sensory deprivation.     

The sweet taste in the calf. II. Glossopharyngeal nerve responses to taste stimulation of the tongue

Recordings were obtained from the glossopharyngeal nerve in 1-5-week-old calves during stimulation of the circumvallate tongue area with NaCl, quinine hydrochloride, citric acid, and the sweet compounds: acesulfam-K, aspartame, fructose, galactose, glucose, glycine, lactose, maltose, monellin, Na-saccharin, sucrose, thaumatin, and xylitol. All compounds except aspartame, monellin and thaumatin gave a nerve response. Glycine, followed by Na-saccharin, elicited the largest responses. Sucrose gave the largest response among the disaccharides, while there was no significant difference between the monosaccharides. Expressed as percent of the NaCl responses, the responses to glycine, sucrose, xylitol, fructose, galactose, glucose, lactose and maltose were considerably larger in the glossopharyngeal nerve than in the chorda tympani nerve. This can be taken as an indication that the posterior region of the tongue serves as the major receptive area for sweet in cattle.     

Neuronal responses of the anterior commissural nucleus to osmotic stimulation and angiotensin II in hypothalamic slices in the rat.

The firing rate and pattern of activity of neurons in the anterior commissural nucleus (ACN), which was rich in oxytocin-containing neurons, were studied electrophysiologically in hypothalamic slices. Extracellular recording showed that most ACN neurons exhibited irregular or regular continuous spontaneous unit activity. Other neurons showed short burst patterns of activity or were silent. The majority of ACN neurons were activated by bath application of angiotensin II, and a substantial number of them showed inhibitory or excitatory responses to hypertonic bathing medium. These results indicate that magnocellular neurosecretory neurons in the ACN may participate in the regulation of water balance.     

The effects of prior aversive stimulation on the behavioral and physiological responses to intense acoustic stimuli in the rat

Two experiments were conducted to assess the effects of intense, inescapable electric shock (preshock, PS) on the behavioral and physiological responses of rats to intense acoustic stimuli. In Experiment 1 startle amplitude, interstimulus activity and plasma corticosterone levels were compared in control and animals previously subjected to the PS treatment. In Experiment 2 heart rate (HR) responses were similarly compared. Results indicated that (1) startle amplitude was not affected by PS treatment (2) interstimulus activity was markedly suppressed and the plasma corticosterone response to startle testing elevated by the PS treatment, and (3) the initial presentations of startle stimuli elicited a dramatic HR deceleration in animals subjected to the PS treatment. These results were discussed in terms of (1) the relationship between arousal and startle and (2) the nature of the PS effect as revealed by the unusual HR response to the initial stimulus presentations.     

Involvement of the sensory cortex in adrenocortical responses following photic and acoustic stimulation in the rat

In order to examine the possibility that the sensory cortex participates in the mediation of adrenocortical responses following sensory stimuli, the effects of photic and acoustic stimuli on plasma corticosterone were studied in rats with either visual or auditory cortex ablation. In animals with visual cortex ablation, the adrenocortical response to acoustic stimuli was intact; however, it was significantly reduced following photic stimulation. On the other hand, in animals with auditory cortex ablation, the response to acoustic stimulation was significantly reduced, but the response to photic stimulation remained intact. These data demonstrate the participation of the specific sensory cortex in adrenocortical responses following the stimulation of the corresponding sensory modality. The possible mechanisms involved may be either a tonic facilitatory effect of the specific cortex on subcortical mechanisms or the transmission of the specific stimuli in the primary sensory pathways, to achieve a full adrenocortical discharge.     

Augmentation of the rat's acoustic startle reflex by nonreflexogenic stimuli

The size of the rat's acoustic startle reflex was augmented by brief acoustic clicks (which did not themselves elicit startle) presented several milliseconds before the reflex-eliciting stimulus (RS). The same clicks presented after the RS gave relatively weak augmentation that was present in the 1st, but not the 2nd, testing session. Brief footshocks set to 75% of each animal's flinch threshold augmented startle when presented both before and after the RS in both testing sessions. Augmentation by a leading footshock increased with shock intensity and was unaffected by the intensity of the RS. Augmentation by a trailing footshock increased with shock intensity and also with the intensity of the RS. Reflex size is not fixed at the time of reflex elicitation but can be augmented by a later nonreflexogenic stimulus. Reflex augmentation may be caused by the 2nd member of a stimulus pair discharging elements of the reflex pathway that were partially activated by the 1st.     

Cardiovascular responses to rewarding brain stimulation.

In awake rats, rewarding stimulation of 155 electrodes spread throughout the forebrain, hypothalamus, and midbrain led to heart rate (HR) deceleration at almost all sites. Stimulation of those animals with forebrain electrodes that were fitted with a carotid catheter produced mean arterial pressure increases acompanying the HR changes. The relative latencies of the cardiovascular responses suggested that rewarding stimulation, in some instances at least, may produce a mixture of autonomic effects that rarely if ever occur in the normal waking animal in the absence of such stimulation. The data obtained when the rats with forebrain electrodes were anesthetized indicated that stimulation caused mean arterial pressure to fall and HR either not to change or to decelerate slightly. These differences between data from the same animals under waking and anesthetized conditions caution against premature generalizations concerning the effects of brain stimulation on cardiovascular changes.