Long-term deficits after somatosensory cortical lesions in rats.

Rats having either sham operations or one-stage bilateral lesions of the two somatosensory areas of the cortex were tested for acquisition of five tactile discriminations after postoperative recovery intervals of 14, 35, 180, 365 or 730 days. The group with lesions performed worse than its time-matched control group in every instance, and there was no evidence for recovery of function with the longer postoperative recovery periods. These results suggest that time per se is not a significant determinant of restitution after somatosensory cortical ablations.     

Learning in escape/avoidance tasks in female rats does not vary with reproductive condition

To determine whether the development of novel stimulus-response associations by the mother during the periparturient period is attributable to a general facilitation of learning produced by the hormonal milieu during that period, learning ability under various reproductive conditions was assessed in two tasks unrelated to the periparturitional situation. The two tasks, selected because they equalized the various groups for motivation and performance variables, were acquisition of a water-maze escape (including two reversals), and acquisition and retention of an unsignalled shuttlebox shock avoidance. The groups tested in the water maze were a midpregnant group, an immediately prepartum group, and an immediately postpartum group. In the shuttlebox, the same conditions (different rats) were compared, together with a nonpregnant estrus condition, and a nonpregnant diestrus condition. The results of both experiments indicate that although learning occurred, the characteristics of acquisition and retention were not influenced by reproductive condition.     

Aging effects on spatial tuning of hippocampal place cells in mice

One reason the electrophysiological correlates of hippocampal neurons are of interest is the possibility that they reflect their representational properties, presumably spatial/relational ones. Stable spatial representations, based on activity of ensembles of hippocampal place cells, initially develop through a series of short-episodic spatial tunings. Hence these short-episodic spatial tunings are important for understanding the establishment of stable place fields. Studies of age-related changes in place cell activities traditionally focus on place fields. In the present study, we characterized the short-episodic spatial tunings (1-min bins) of hippocampal CA1 place cells of freely moving mice in a familiar cylinder arena, and compared these functions in young and old mice. Spatial tuning was expressed by spatial selectivity, which we found fluctuated across a 16-min recording session in both young and old mice. High spatial selectivity, which is mainly due to the low firing of a place cell out of the place field in young mice, was significantly higher in old mice. The high firing rate out of the place field was the main factor contributing to significantly lower spatial selectivity in old mice. In addition, young mice showed a broad peak in the spatial selectivity between 4 and 10 min. In contrast old mice showed no peak in the spatial selectivity during this time period. The stability of place fields after a 24-h interval was also lower in old mice than in young mice. The low spatial tuning and unstable place fields suggest that a hippocampal-based spatial representation was impaired in the old mice. Furthermore, we speculate that the age-related impairment in hippocampal inhibition system may be involved in the impaired spatial representation of hippocampal CA1 place cells in old mice. Electronic Publication     

Latent variable discovery in classification models

The naive Bayes model makes the often unrealistic assumption that the feature variables are mutually independent given the class variable. We interpret a violation of this assumption as an indication of the presence of latent variables, and we show how latent variables can be detected. Latent variable discovery is interesting, especially for medical applications, because it can lead to a better understanding of application domains. It can also improve classification accuracy and boost user confidence in classification models.     

Amnesic effects of bilateral lesions placed in the hyperstriatum ventrale of the chick after imprinting

Summary The purpose of this study was to investigate whether bilateral lesions to a part of the hyperstriatum ventrale (IMHV) impair retention if they are placed after chicks have been imprinted. Domestic chicks were hatched and reared in darkness and exposed to an imprinting (training) stimulus for 2 h commencing 22 h post hatch. The chicks were then anaesthetised and bilateral lesions placed in IMHV (N = 16) birds, hyperstriatum accessorium (HA; N = 16) or the lateral part of the cerebral hemispheres (LCA; N = 16). Forty-eight sham-operated chicks served as controls. Chicks were returned to the dark incubator, and, 15–20 h after the operation, their approach towards the training stimulus and to a second novel stimulus was measured. The controls and the chicks with lesions in HA and LCA showed a strong preference for the training stimulus and hence a high level of retention. The preferences of these three experimental groups did not differ significantly from one another. The mean preference of chicks with lesions in IMHV was significantly less than that of the sham-operated controls (P<0.01) and of chicks lesioned in HA (P<0.05). Bilateral lesions to IMHV therefore selectively impair retention of a preference acquired through imprinting. This impairment is unlikely to be a non-specific consequence of defective sensory processing or motor performance because the four groups did not differ from each other in (i) the time taken accurately to peck a rocking bead, (ii) the accuracy of pecking millet seeds and (iii) the performance of a simultaneous visual discrimination task involving heat reinforcement.Supported by grants from the Science Research Council, the Leverhulme Trust, the Wellcome Trust and FAPESP (Brazil)     

Learning and retention of a shock avoidance task by rat using deafferented hind limb

Learning and retention of a shock avoidance task were studied in the absence of proprioceptive or other topographic feedback. Rats with a deafferented hind limb were trained using the instrumental shock avoidance and retention paradigm of Horridge. Rats received shock whenever they lowered an electrode attached to a deafferented hind foot into an electrolyte bath; yoked-control rats received the shocks to a deafferented hind food along with the experimental animals. Experimental animals consistently held their feet above the electrolyte (p<0.01). During the early minutes of the second phase (a testing situation, in which the experimental and control animals were both shocked for leg lowering) the experimentals received fewer shocks than the controls (p<0.001); the controls eventually withheld their feet (p<0.001). These results indicate that rats with deafferented hind limbs provide a simple system for the study of learning and retention in the absence of proprioceptive or other topographic feedback.     

The human force:velocity relationship; activity in the knee flexor and extensor muscles before and after eccentric practice

The human voluntary force:velocity relationship frequently fails to demonstrate the expected high eccentric forces. Possible explanations include unique activation strategies which might be affected by neural learning mechanisms. We investigated the effect of practicing eccentric contractions on (1) the force: velocity relationship of the human knee extensor muscles and (2) the extent of agonist and antagonist muscle activity. Eight healthy adults [seven women, group mean age 31 (SEM 5) years ± ] practised twice a week for 4 weeks using their non-dominant legs. Each session comprised three isokinetic concentric and eccentric maximal voluntary contractions (MVC) at randomised angular velocities of 100, 200 and 300° · s⁻¹. Before and after, the force:velocity relationship was determined bilaterally (angular velocities 0–300° · s⁻¹). There were no significant differences in the forces generated or relative electromyogram (EMG) activity after practice, although there was a trend for dynamic forces to increase. Beforehand, the bilateral eccentric MVC forces were lower than isometric (P < 0.0025); afterwards they were broadly similar. The agonist EMG was similar during isometric and eccentric contractions, but lower during concentric (P < 0.03). Antagonist EMG activity showed considerable individual variation, was similar during all contraction types and tended to be greater during dynamic contractions. These data indicate that neither central learning mechanisms nor total muscle activation strategies underlie the human failure to produce the expected high eccentric voluntary forces in humans. Accepted: 19 September 2000     

On-line compensation for perturbations of a reaching movement is cerebellar dependent: support for the task dependency hypothesis

Although the cerebellum has been shown to be critical for the acquisition and retention of adaptive modifications in certain reflex behaviors, this structures role in the learning of motor skills required to execute complex voluntary goal-directed movements still is unclear. This study explores this issue by analyzing the effects of inactivating the interposed and dentate cerebellar nuclei on the adaptation required to compensate for an external elastic load applied during a reaching movement. We show that cats with these nuclei inactivated can adapt to predictable perturbations of the forelimb during a goal-directed reach by including a compensatory component in the motor plan prior to movement initiation. In contrast, when comparable compensatory modifications must be triggered on-line because the perturbations are applied in randomized trials (i.e., unpredictably), such adaptive responses cannot be executed or reacquired after the interposed and dentate nuclei are inactivated. These findings provide the first demonstration of the condition-dependent nature of the cerebellums contribution to the learning of a specific volitional task.     

Dorsal bundle extinction effect: Motivation or attention?

Male albino Wistar rats were injected bilaterally with 4 micrograms of 6-hydroxydopamine into the dorsal noradrenergic bundle to deplete forebrain noradrenaline to less than 5% of control values. Acquisition learning of a fixed interval schedule or a continuously reinforced schedule was not altered but resistance to extinction was seen after food reinforced training on either schedule but not after water reinforced training. A possible increase in food motivation was tested by the use of preloading with free food prior to a fixed interval session but both control and lesioned rats reacted similarly to this manipulation thus appearing to exclude an increase in food motivation. An attentional explanation is proposed and tested by the demonstration that resistance to extinction does not occur after a partially (variable ratio 4), as opposed to a continuously, reinforced schedule. Further evidence in favour of an attentional mechanism comes from the finding that on both a fixed interval and a continuously reinforced schedule the lesion has to be present during the acquisition phase to result in subsequent resistance to extinction. Intact animals trained on either schedule and subsequently subjected to the lesion failed to show an increased resistance to extinction.     

Selective impairment of paired associate learning after administration of a centrally-acting adrenergic agonist (clonidine)

Eight normal volunteers had IV infusions of 200 g clonidine (a centrally-acting adrenergic agonist which reduces noradrenaline release), and saline in a double-blind cross-over design. Clonidine reduced subjective estimates of arousal but did not affect performance on the Digit Symbol Substitution Test. Clonidine impaired pairedassociate learning, but it did not affect performance on a number of measures of short and long term memory. The findings suggest either 1) that there is a specific (adrenergic) mechanism involved in the acquisition of novel associations, but not in other types of learning, or 2) that paired associate learning is more vulnerable than other learning tasks to disruption of adrenergic transmission.