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Haptic guidance has been shown to have both facilitatory and interfering effects on motor learning. Interfering effects have been hypothesized to result from the particular dynamic environment, which supports a passive role of the learner, and they should be attenuated by fading guidance. Facilitatory effects, in particular for dynamic movement characteristics, have been hypothesized to result from the high-quality information provided by haptic demonstration. If haptic demonstration provides particularly precise information about target movements, the motor system’s need for such information should more likely increase in the course of motor learning, in which case growing guidance should be more beneficial for learning. We contrasted fading and growing guidance in the course of learning a spatio-temporal motor pattern. To stimulate an active role of the learner, practice trials consisted of three phases, a visual demonstration of the target movement, a guided reproduction, and a reproduction without haptic guidance. Performance was assessed in terms of variable duration errors, relative-timing errors, variable path-length errors, and shape errors. Motor learning with growing and fading guidance turned out to be largely equivalent, so that the notion of an increasing optimal precision of haptic demonstrations, which matches a demand of increasingly precise information on the target movement, found no support. Duration errors declined only with fading, but not with growing guidance. Relative timing revealed a benefit of immediately preceding haptic demonstration, but learning was not different between the two practice protocols. This contrast between absolute and relative timing adds to other evidence according to which acquisition of these two aspects of motor timing involves different learning mechanisms. Whereas relative timing gained from immediately preceding haptic demonstration, but revealed no practice-related improvement in the presence of haptic guidance, the opposite pattern of results was found for the shape error. This finding is consistent with the claim that haptic demonstration is particularly efficient with respect to relative timing, but not with respect to spatial movement characteristics.

We reported previously that concentric or eccentric exercise can lead to errors in human limb position sense. Our data led us to conclude that the errors, post-exercise, were not due to an altered responsiveness of the proprioceptive afferents, and we proposed that they resulted from central changes in the processing of the afferent input. However, it remained uncertain what was responsible for triggering those changes, the volume of afferent traffic during the exercise or the developing fatigue. The afferent traffic hypothesis was tested by subjects carrying out a series of 250 lightly loaded concentric contractions of elbow flexors that produced little fatigue (6 %). This did not lead to significant position errors. In a second experiment, a series of fatiguing isometric contractions, which kept movements of the muscle to a minimum, led to a 24 % fall in force and significant position errors (3°, direction of extension). In the third experiment, at 24 h after eccentric exercise, when the short-term effects of fatigue and accumulated metabolites were gone, but force was still 28 % below control values, this was accompanied by significant position errors in the direction of extension, 3.2° in the relaxed arm and 3.3° in the self-supported arm. It is concluded that it is the fall in force accompanying exercise which is responsible for disturbing limb position sense. It is suggested that the exercise effects are generated in the brain, perhaps as a result of an alteration of the body map, triggered by the fall in force.

Pre- and postsynaptic fluxes of Ca2+ and K+ were determined concurrently in CA1 of the hippocampus in vitro under conditions where synaptic transmission was blocked. The Ca2+ entry blocker, Ni2+, abolished both pre- and postsynaptic Ca2+-influx and reduced presynaptic K+-efflux by about 20%. Postsynaptic K+-efflux was reduced nearly by 60%. 4-aminopyridine (4-AP) strongly enhanced presynaptic Ca2+-influx but only marginally increased Ca2+-entry into the postsynaptic neurones. At the same time, total K+-efflux from presynaptic sites was increased by about 30% but was unaltered postsynaptically. Finally, tetraethylammonium (TEA) enhanced both pre- and postsynaptic Ca2+-influx. Postsynaptic influx was more profoundly affected than presynaptic. Neither pre- nor postsynaptic efflux of K+ was altered by TEA. The results indicate a complex balance between inward Ca2+ and outward K+ currents in CA1 and that this balance may differ pre- and postsynaptically.

This study examines to which extent developing dentate granule cells grafted into excitotoxic lesions of the adult rat fascia dentata can be appropriately innervated by the host brain. The lesions were induced by focal injections of ibotenic acid (IA) and resulted in localized dentate and hippocampal neuronal cell death, but sparing of the afferent connections, now deprived of their targets. One week later pieces of fascia dentata from new-born rats were grafted into the lesions. After 6 weeks to 9 months the recipient brains were processed and analyzed by cell stain, histochemistry, immunohistochemistry, anterograde nerve fiber degeneration methods, and electron microscopy. Dentate grafts survived well in the lesion area and became organo-typically organized. They contained the normal nerve cell types of the fascia dentata and hilus (CA4), including the peptidergic somatostatin-, cholecystokinin- and enkephalin-reactive ones. The grafts were innervated by AChE-positive, cholinergic fibers from the host septum, and perforant path fibers from the host entorhinal area. The presence of the latter were demonstrated by Timm staining and light and electron microscopy of anterograde axonal degeneration. When the extent and density of the host perforant path innervation was examined and mapped at the electron microscopical level the grafts in the IA-lesions were found to receive a more extensive and denser host innervation than grafts placed in the normal fascia dentata of adult rats without a preceding axon-sparing ibotenic acid lesion. In this way the results demonstrate that certain lesion types can enhance the innervation of intracerebral grafts by already mature neural pathways of the point-to-point type.

Transcranial magnetic stimulation (TMS) interventions that modulate cortical plasticity may achieve a more functional benefit if combined with neuro-rehabilitation therapies. With a TMS protocol targeting I-wave dynamics, it is possible to deliver stimuli while a subject performs a motor task, and this may more effectively target functional networks related to the task. However, the efficacy of this intervention during a simple task such as a low-level voluntary contraction is not known. We delivered paired-pulse TMS at an inter-pulse interval (IPI) of 1.5 ms for 15 min while subjects performed a 10 ± 2.5% voluntary contraction of the first dorsal interosseous (FDI) muscle and made motor evoked potential (MEP) amplitude and short-interval intracortical facilitation (SICF) curve measurements. Pre-intervention SICF curves showed only a single peak at 1.3–1.5 ms IPI. During the intervention, MEP amplitude steadily increased (P < 0.001) to 137 ± 13% of its initial value. After the intervention, SICF curves were increased in amplitude (P < 0.001) and later peaks emerged at 2.8 and 4.3 ms IPIs. A control experiment, replacing paired-pulse stimulation with single-pulse stimulation showed no effect on MEP amplitude (P = 0.951). We conclude that the I-wave intervention can be administered concurrently with a simple motor task and that it acts by increasing trans-synaptic efficacy across a number of I-waves. The ability to perform a motor task simultaneously with a TMS intervention could confer a degree of specificity to the induced excitability changes and may be beneficial for functional neuro-rehabilitation programs built around motor learning and retraining.

Size constancy is the ability to perceive objects as remaining constant in size regardless of their distance from the observer. Emmert’s law demonstrates that viewing distance determines the perceived size of afterimages according to the amount of depth cues that are available. Using an afterimage paradigm, we examined to what extent removing stereopsis and other depth cues affects size–distance scaling. Thirty participants ‘projected’ afterimages onto a surface presented at different distances under binocular, monocular, and eyes-closed conditions. The perceived size of the afterimages closely followed the size–distance scaling predictions made by Emmert’s law under binocular testing conditions, when all depth cues were available. In contrast, monocular testing decreased adherence to Emmert’s law, while the eyes-closed condition resulted in a greater breakdown of size–distance scaling. Because we used an afterimage paradigm, this study provides the first demonstration of how perceived size is modulated by the availability of depth cues under conditions with a constant retinal image stimulus.

We report on experiments which were undertaken in an attempt to clarify mechanisms underlying the contrast effects of chromatic surround illumination on spectral responsiveness of cells in the parvocellular layers of the LGN (P-LGN-cells), that had been demonstrated under standard conditions in the preceding companion paper. The experiments were done in anesthetized macaques (Macaca fascicularis). In some neurons, S-potentials were recorded together with the post-synaptic action potentials, and all effects seen in P-LGN-cells were present already in their retinal afferents indicating their retinal origine. The responsiveness of the cells for center stimuli of different wavelengths and during illumination of the receptive field center or the outer surround was determined. Continuous outer surround illumination alered maintained dicharge rate (MDR), sensitivity and gain of P-LGN and retinal ganglion cells in the same way and empirically not distinguishable from direct illumination of the receptive field. Responses to surround flashes showed the same dependence on spectral composition as those to center flashes. Adaptation and excitation caused by outer surround illumination (inner diameter 5°, outer diameter 20°) were, in the average, ten times weaker than those exerted by light of the same spectral composition shone directly into the receptive field. Surround effects decreased proportional to r-2. Excitation by outer surround flashes was reduced by adaptation of the receptive field center in the same manner as responses to center flashes. The findings indicate that outer surround light has a direct excitatory and adaptive effect on the excitatory or inhibitory cones feeding into the receptive field. This indicates that straylight from the surround into the center could be responsible for the adaptive and excitatory effects of surround illumination. The straylight fraction from the remote surround into the receptive field must be higher, however, than that estimated from the psychophysically determined point spread function. It comes closer to earlier direct straylight measurements in excised eyes, but may be enhanced by chromatic aberration. If a surround of excitatory colour is flashed simultaneously with an excitatory center stimulus, additivity of center and surround excitation is observed only at low center intensities, while at higher center intensities the gain for center excitation is reduced similar to adaptive gain control. This could be explained by lateral interaction through horizontal connections in the retina, which decays within seconds, while adaptation of the cones feeding into the receptive field center is fully effective only after about 3 s. Our findings therefore suggest a two stage model for surround effects, a fast one mediated through horizontal connections controlling the gain of receptorbipolar transmission and a slow one through adaptation by straylight and controlling receptor gain. The fast process is receptor unspecific, i.e. pooling activity from all receptor types, while the second one is receptor specific. During real seeing both processes are simultaneous and complement each other because of continuous eye movements. Perceptual darkness and colour induction by remote surrounds are consistent with this model, which can also be applied to colour constancy. WM-cells (Yellow-minus-blue) show peculiar properties during surround or center illumination with blue light, suggestng an opponent mechanism different from that of other P-LGN-cells.

Behavioural advantages for imitation of human movements over movements instructed by other visual stimuli are attributed to an ‘action observation-execution matching’ (AOEM) mechanism. Here, we demonstrate that priming/exogenous cueing with a videotaped finger movement stimulus (S1) produces specific congruency effects in reaction times (RTs) of imitative responses to a target movement (S2) at defined stimulus onset asynchronies (SOAs). When contrasted with a moving object at an SOA of 533 ms, only a human movement is capable of inducing an effect reminiscent of ‘inhibition of return’ (IOR), i.e. a significant advantage for imitation of a subsequent incongruent as compared to a congruent movement. When responses are primed by a finger movement at SOAs of 533 and 1,200 ms, inhibition of congruent or facilitation of incongruent responses, respectively, is stronger as compared to priming by a moving object. This pattern does not depend on whether S2 presents a finger movement or a moving object, thus effects cannot be attributed to visual similarity between S1 and S2. We propose that, whereas both priming by a finger movement and a moving object induces processes of spatial orienting, solely observation of a human movement activates AOEM. Thus, S1 immediately elicits an imitative response tendency. As an overt imitation of S1 is inadequate in the present setting, the response is inhibited which, in turn, modulates congruency effects.