Physical and physiological components of the graviresponses of wild-type and mutant Paramecium Tetraurelia.
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Wild-type and the morphological mutant kin 241 of Paramecium tetraurelia showed improved orientation away from the centre of gravity (negative gravitaxis) when accelerations were increased from 1 to 7 g. Gravitaxis was more pronounced in the mutant. A correlation between the efficiency of orientation and the applied g value suggests a physical basis for gravitaxis. Transiently enhanced rates of reversal of the swimming direction coincided with transiently enhanced gravitaxis because reversals occurred more often in downward swimmers than in upward swimmers. The results provide evidence of a physiological modulation of gravitaxis by means of the randomizing effect of depolarization-dependent swimming reversals. Gravity bimodally altered propulsion rates of wild-type P. tetraurelia so that sedimentation was partly antagonized in upward and downward swimmers (negative gravikinesis). In the mutant, only increases in propulsion were observed, although the orientation-dependent sensitivity of the gravikinetic response was the same as in the wild-type population. Observed swimming speed and sedimentation rates in the wild-type and mutant cells were linearly related to acceleration, allowing the determination of gravikinesis as a linear (and so far non-saturating) function of gravity. (+info)
Place recognition monitored by location-driven operant responding during passive transport of the rat over a circular trajectory.
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Spatial memory of animals is usually tested in navigation tasks that do not allow recognition and recall processes to be separated from the mechanisms of goal-directed locomotion. In the present study, place recognition was examined in rats (n = 7) confined in an operant chamber mounted on the periphery of a slowly rotating disk (diameter 1 m, angular velocity 9 degrees /s). The animals were passively transported over a circular trajectory and were rewarded for bar pressing when they passed across a 60 degrees -wide segment of the path. This segment was recognizable with reference to room landmarks visible from the operant box. Responding defined in the coordinate system of the room increased when the chamber entered the 60 degrees -wide approach zone, culminated at the entrance into the reward sector, was decreased inside it by eating the available reward, and rapidly declined to zero at the exit from this zone. When reward was discontinued, the skewed response distribution changed into a symmetric one with a maximum in the center of the reward sector. With advancing extinction, the response peak in the reward sector decreased in most rats proportionally to the overall decline of bar pressing. The rewarded and nonrewarded response patterns indicate that passively transported rats can recognize their position in the environment with an accuracy comparable to that of actively navigating animals and that location-driven operant responding can serve as a useful tool in the analysis of the underlying neural mechanisms. (+info)
Visual space from visual motion: turn integration in tethered flying Drosophila.
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Organisms navigating by path integration need to continuously measure their forward movement and their angular orientation with respect to an external reference. How they do it is little understood. Tethered flies at the flight simulator "navigate" in an artificial visual landscape without forward movement. They can return to a previously held orientation if the panorama provides a singularity (landmark) as reference. Surprisingly, in a regularly striped drum without singularities, they can use a temporal cue instead. In this experiment the arena is illuminated with only one color that is either green or blue. The arena is virtually divided into four quadrants. Whenever a quadrant boundary moves past an arbitrary point, the color of the arena light changes. When a fly is heated with one color it acquires a preference for the other one. Subsequently, it avoids the borders toward the potentially 'hot' quadrants even without touching them. The only way to achieve this is by turn integration, that is, by adding and subtracting all the turns it performs once it crosses the border. The color switch defining the border crossing resets the turn integrator, using the orientation of the arena at this moment as reference. In contrast, landmarks or, if it were available, the skylight compass enable the fly to establish by pattern learning any orientation as a reference. If the reference orientation coincides with the desired orientation, that is, if the animal stores the pattern while being oriented toward the goal, it can maintain its orientation without recourse to turn integration (which may be error prone). (+info)
Canal-otolith interactions after off-vertical axis rotations I. Spatial reorientation of horizontal vestibuloocular reflex.
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We examined the three-dimensional (3-D) spatial orientation of postrotatory eye velocity after horizontal off-vertical axis rotations by varying the final body orientation with respect to gravity. Three rhesus monkeys were oriented in one of two positions before the onset of rotation: pitched 24 degrees nose-up or 90 degrees nose-up (supine) relative to the earth-horizontal plane and rotated at +/-60 degrees /s around the body-longitudinal axis. After 10 turns, the animals were stopped in 1 of 12 final positions separated by 30 degrees. An empirical analysis of the postrotatory responses showed that the resultant response plane remained space-invariant, i.e., accurately represented the actual head tilt plane at rotation stop. The alignment of the response vector with the spatial vertical was less complete. A complementary analysis, based on a 3-D model that implemented the spatial transformation and dynamic interaction of otolith and lateral semicircular canal signals, confirmed the empirical description of the spatial response. In addition, it allowed an estimation of the low-pass filter time constants in central otolith and semicircular canal pathways as well as the weighting ratio between direct and inertially transformed canal signals in the output. Our results support the hypothesis that the central vestibular system represents head velocity in gravity-centered coordinates by sensory integration of otolith and semicircular canal signals. (+info)
Local infusion of scopolamine into intraparietal cortex slows covert orienting in rhesus monkeys.
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There is accumulating evidence to suggest that cholinergic neurotransmission may play an important role in visuospatial attention, but the brain sites at which acetylcholine modulates attention are not well understood. The present work tested the hypothesis that the cholinergic influences within the intraparietal cortex are necessary for normal attentional shifting (covert orienting) in nonhuman primates. Two rhesus monkeys were trained to perform a visual, cued target detection task for liquid reinforcement. The animals pressed a lever to produce a visual display in which a central fixation point was flanked by two circles. Shortly after fixation was established, one of the circles brightened (cue), and a target appeared subsequently within one of the circles. Detection was signaled by a manual response and the reaction time to the appearance of the target was recorded. Four types of trials were presented. For valid cue trials, the cue and target were at the same spatial location; for invalid cues, cue and target were in opposite hemifields; for double cues, both cues were brightened but the target appeared in either the left or right circle; in no-cue trials, the cue was omitted. We localized the intraparietal region by recording attention-related, cellular activity with intracerebral microelectrodes. Among visually responsive cells in this area, valid cues presented to the receptive fields of visual neurons enhanced the responses to target stimuli in about half the cells and inhibited those responses in the remainder. In addition, some cells showed longer response latencies to invalid cues than to valid cues. We then infused scopolamine into attention-related activity sites and assessed its effect on performance. Scopolamine produced a dose-dependent increase in reaction times and decrease in performance accuracy that lasted more than 1 h. Neither vehicle injections in the same locations nor scopolamine outside the physiologically defined area produced any significant change in behavior. Under our conditions of measurement, we conclude that activity mediated by muscarinic cholinergic receptors within the intraparietal cortex is necessary for normal covert orienting. (+info)
Attention to both space and feature modulates neuronal responses in macaque area V4.
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Attention is the mechanism with which we select specific aspects of our environment for processing. Psychological experiments have shown that attention can be directed to a spatial location or to a particular object. Electrophysiological studies in trained macaque monkeys have found that attention can strengthen the responses of neurons in cortical area V4. Some of these studies have attributed these effects to spatial attention, whereas others have suggested that feature-directed attention may modulate the neuronal response. Here we report that neuronal correlates for both spatial and feature-directed attention exist in individual neurons in area V4 of behaving rhesus monkeys. (+info)
Fetal spinal cord tissue in mini-guidance channels promotes longitudinal axonal growth after grafting into hemisected adult rat spinal cords.
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Solid fetal spinal cord (FSC) tissue, seeded into semipermeable mini-guidance channels, was tested for the ability to promote axonal growth across the gap created by a midthoracic (T8) hemisection in adult rats. Fetal thoracic spinal cords, at embryonic days 13 to 15, were harvested and gently aspirated into mini-guidance channels (1.25 mm in diameter and 3.0 mm in length). Care was taken to maintain the rostro-caudal orientation of the FSC. In control rats, the FSC-channel construct was exposed to 5 freeze/thaw cycles to produce non-viable grafts before implantation into the hemisected cord. All cases revealed intact tissue cables of various diameters spanning the rostro-caudal extent of the lesion cavity, with integration of host-graft tissues at both interfaces. Immunofluorescence results indicated that numerous neurofilament-positive axons were present within the FSC tissue cable. Double-labeling of a subpopulation of these axons with calcitonin gene-related peptide indicated their peripheral nervous system (PNS) origin. Descending serotonergic and noradrenergic axons were found in the proximity of the rostral host-graft interface, but were not observed to grow into the FSC-graft. Anterograde tracing of propriospinal axons with Phaseolus vulgaris-leucoagglutinin demonstrated that axons had regenerated into the FSC-graft and had traveled longitudinally to the distal end of the channel. Few axons were observed to cross the distal host-graft interface to enter the host spinal cord. Cross-sectional analysis at the midpoint of the tissue cable stained with toluidine blue demonstrated a significant increase (P < 0.01) in myelinated axons in viable FSC grafts (1455 +/- 663, mean +/- S.E.M.; n = 6) versus freeze-thaw control grafts (155 +/- 50; n = 5). In addition to the myelinated axons, many unmyelinated axons were observed in the tissue cable at the electron microscopic level. Areas resembling the PNS with typical Schwann cells, as well as those resembling the central nervous system with neurons and central neuropil, were also seen. In freeze-thaw control grafts, neither viable neurons nor central neuropil were observed. Retrograde tracing with Fast Blue and Diamidino Yellow demonstrated that neurons within the FSC graft extended axons into the host spinal cord at least for 2 mm from both the rostral and caudal host-graft interfaces. We conclude that viable FSC grafts within semipermeable guidance channels may serve both as a permissive bridge for longitudinally directed axonal growth and a potential relay for conveying information across a lesion site in the adult rat spinal cord. (+info)
Response features determining spike times.
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Interpreting messages encoded in single neuronal responses requires knowing which features of the responses carry information. That the number of spikes is an important part of the code has long been obvious. In recent years, it has been shown that modulation of the firing rate with about 25 ms precision carries information that is not available from the total number of spikes across the whole response. It has been proposed that patterns of exactly timed (1 ms precision) spikes, such as repeating triplets or quadruplets, might carry information that is not available from knowing about spike count and rate modulation. A model using the spike count distribution, the low-pass filtered PSTH (bandwidth below 30 Hz), and, to a small degree, the interspike interval distribution predicts the numbers and types of exactly-timed triplets and quadruplets that are indistinguishable from those found in the data. From this it can be concluded that the coarse (< 30 Hz) sequential correlation structure over time gives rise to the exactly timed patterns present in the recorded spike trains. Because the coarse temporal structure predicts the fine temporal structure, the information carried by the fine temporal structure must be completely redundant with that carried by the coarse structure. Thus, the existence of precisely timed spike patterns carrying stimulus-related information does not imply control of spike timing at precise time scales. (+info)