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R S Turner

Publications and source records attributed to R S Turner.

5 recordsLinked to original sources
PubMed1991

A quantitative analysis of pallidal discharge during targeted reaching movement in the monkey.

Neurons in the globus pallidus have been studied during reaching movements of the arm made at varying speeds. The reaching task is similar to one used in earlier experiments, in which disruption of normal pallidal output caused changes in movement time. The pallidal cells studied were those that showed task-related changes in activity and a modification of discharge when the arm was manipulated outside of the task. Neuronal discharge was assessed to evaluate two possible models, one in which the timing of task-related discharge varied as a function of movement time and the other in which the amplitude (mean firing rate) of the change in discharge varied as movement time varied. The relation between neuronal discharge and movement time was examined quantitatively on a trial-by-trial basis using a statistical algorithm that identified each phase of the change in neuronal discharge on each trial. A non-parametric statistic was used to determine the correlation between movement time and the duration or latency of changes in neural firing or the mean discharge during each phase of the cell's response. For fifty-five percent of the 40 neurons studied, the timing of the cell's response (duration or latency) varied as a function of movement time. For only 10 cells (25%) was there a significant correlation between movement time and the mean firing rate during one or more phases of the cell's response. Both timing and frequency modulation with movement time were limited to cells responsive to manipulation at the wrist or the shoulder.

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PubMed1991

Activity of neurons in cerebellar-receiving and pallidal-receiving areas of the thalamus of the behaving monkey.

1. Thalamic neurons that receive synaptic input from the globus pallidus or the cerebellar nuclei were identified in awake monkeys trained to perform an arm-reaching task. The location of electrophysiologically identified cerebellar-receiving (CR) and pallidal-receiving (PR) neurons was used to identify a total of 264 thalamic neurons in cerebellar (CB) or pallidal (GP) regions of the thalamus. 2. Stimulation in the brachium conjunctivum or white matter adjacent to the cerebellar nuclei excited 85 neurons in the thalamus at short latencies. These CR neurons were located in the oral portion of the ventral posterolateral nucleus (VPLo), in caudal portions of the ventral lateral nucleus (VLc), and in area X. 3. Stimulation in the internal globus pallidus (GPi) inhibited 10 thalamic neurons at short latency. These PR neurons were located in rostral portions of VLc, in the oral part of the ventral lateral nucleus (VLo), and in the parvicellular part of the ventral anterior nucleus (VApc). 4. There was no clear single somatotopic organization of neurons in CB and GP regions of the thalamus, as defined by "free-form" responses to passive manipulation and observation of eye movements. There was, in fact, a tendency for two representations, each, of the head/eye/mouth cells and cells with modifications of activity in response to manipulation of the arm. 5. During the hold period before illumination of a visual target, the mean firing rates and variability of discharge of arm-related CR and PR neurons did not differ significantly. This was also true for the total sample of arm-related neurons in the CB versus GP regions. 6. The activity of many neurons in both the CB and GP regions began to change before the reaching movement and, for some, before the earliest recorded changes in electromyographic (EMG) activity. The initial change was an increase in discharge for greater than 75% of the cells studied in both the CB and GP regions. 7. During the reaching task, there also was no significant difference in the time of the initial change in discharge of neurons in the CB versus GP regions of the thalamus. 8. These data are consistent with the hypothesis that the initial task-related change in discharge of PR thalamic neurons is dominated by input from the cerebral cortex and that pallidal input modulates later phases of their movement-related changes in activity.

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PubMed1977

Hermann von Helmholtz and the empiricist vision.

The philosophical convictions of Hermann von Helmholtz and the empiricist psychology he developed have been extensively discussed in historical literature. This literature has not usually emphasized the tacti assumptions about human physiology that underlaid these convictions nor the way in which Helmholtz's epistemology served as a methodological directive in his research. Helmholtz assumed nerve transmission between sense organs and the mind to be a passive process. Distortion in stimulus patterns occurs physically in the sense organs, which can therefore be treated through mechanical analogies. Stimuli become converted to the perceptions of consciousness through mental processes that are essentially analogous to conscious, inductive inference and that are therefore susceptible, in principle, to introspective investigation. This view of mental function reflected Helmholtz's intellectual debt to German idealism, especially to the philosophical views of J.G. Fichte.

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PubMed1975

A possible model for cell-cell recognition via surface macromolecules.

Alternative possibilities for the establishment of the proper cell distribution during embryogenesis are summarized at the beginning, followed by an assessment of the examples known so far where cell-cell recognition is known to be mediated via cell surface components. In the second part the species-specific recognition process which occurs during the sorting-out of dissociated sponge cells is analysed since it may serve as a possible model for cell-cell recognition in higher animals. Three possible mechanisms for the establishment of proper cell distribution are considered. These include, first, chemotaxis: secondly, guidance of cell or cell sheet movement by extracellular matrix or by surrounding cells and thirdly, random movement followed by recognition at the final point of destination. Recognition is necessary for both of the two latter processes, i.e. for cell guidance as well as for locking the cells into their final position after random movement. Two basically different recognition mechanisms should be distinguished from each other. On the one hand cells may recognize each other with the help of macromolecules situated in or just outside of the plasmamembrane which fit to each other like enzymes and substrates or antibodies and antigens. On the other hand, cells may exchange information by exchanging cytoplasmatic components via vesicles or gap junctions. The species-specific aggregation of dissociated sponge cells is considered to be a possible model for cell-cell recognition in higher animals. A proteoglycan-like intercellular macromolecule called aggregation factor seems to mediate recognition of a given species of cells in the reaggregation process of dissociated cells. The data available at the present time suggest that a monovalent surface macromolecule (baseplate) may mediate the recognition process probably by recognizing the carbohydrate side chains of the multivalent proteoglycan aggregation factor. A cell-free system was devised to mimic this aggregation process. Addition of aggregation factor to baseplate-coated sepharose beads of approximately the size of the original sponge cells has essentially the same characteristics as the cellular system. Macromolecule-coded surface information for the recognition between cells has not been established during the embryogenesis of higher animals and remains an interesting challenge.

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