Complexity Digest 2000.32

07-Aug-2000

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Content

  1. Structure and Dynamics of Complex Interactive Networks, SFI Workshop Video Notes
  2. Transition From Coherence To Bistability In A Model Of Financial Markets
  3. Prior Information in Motor and Premotor Cortex: Activity During the Delay Period and Effect on Pre-Movement, J. Neurophysiol.
  4. Learning Motor Synergies Makes Use of Information on Muscular Load, Learn. Mem.

  1. Structure and Dynamics of Complex Interactive Networks, SFI Workshop Video Notes Next Article Bookmark and Share

    The website of this workshop contains important information and links to the work of the participants. All talks were video-taped and can be ordered from the Santa Fe Institute. We have recorded a number of video clips from speakers and participants who give a summary of their work and how it relates to the theme of the workshop. (Click on the word "Video" in the table of content above to view the video. You need a viewer from www.real.com to view the videos. More information can be obtained from the workshop website or via e-mail directly from the researchers.

  2. Transition From Coherence To Bistability In A Model Of Financial Markets Next Article Bookmark and Share

    Abstract: We present a model describing the competition between information transmission and decision making in financial markets. The solution of this simple model is recalled, and possible variations discussed. It is shown numerically that despite its simplicity, it can mimic a size effect comparable to a crash. Two extensions of this model are presented that allow to simulate the demand process. One of these extensions has a coherent stable equilibrium and is self-organized, while the other has a bistable equilibrium, with a spontaneous segregation of the population of agents. A new model is introduced to generate a transition between those two equilibria. We show that the coherent state is dominant up to an equal mixing of the two extensions. We focus our attention on the microscopic structure of the investment rate, which is the main parameter of the original model. A constant investment rate seems to be a very good approximation.

  3. Prior Information in Motor and Premotor Cortex: Activity During the Delay Period and Effect on Pre-Movement, J. Neurophysiol. Next Article Bookmark and Share

    In instructed-delay (ID) tasks, instructional cues provide prior information about the nature of a movement to execute after a delay. Neuronal responses in dorsal premotor cortex (PMd) during the instructed-delay period (IDP) between the CUE and subsequent GO signals are presumed to reflect early planning stages initiated by the prior information. In contrast, in multiple-choice reaction-time (RT) tasks, all motor planning and execution processes must occur after the GO signal. These assumptions predict that neuronal planning correlates recorded during the IDP of ID trials should share common features with early post-GO activity in RT trials, and that those response components need not be recapitulated after the GO signal of ID trials. These two predictions were tested by comparing activity recorded in RT and ID tasks from 503 neurons in PMd and caudal (MIc) and rostral (MIr) primary motor cortex. The incidence and strength of directionally tuned IDP activity declined progressively from PMd to MIc. The directional tuning of activity during the IDP of ID trials was more similar to that in the reaction-time epoch (RTE) of RT trials than after movement onset, especially in PMd. A modulation of post-GO activity was often observed between RT and ID trials and was confined mainly to the RTE. This effect was also most prominent in PMd. The most common change was a reduction in intensity of short-latency phasic responses to the GO signal between RT and ID trials, especially in PMd cells with a short-latency phasic response to CUE signals. However, the largest group of cells in each area showed no large change in peak RTE activity between RT and ID trials, whether they were active in the IDP or not. Since early phasic CUE-related responses are least likely to be recapitulated after the GO signal in ID trials, they may be a neuronal correlate of an early planning stage such as response selection. Tonic IDP responses, which are not as strongly associated with a post-GO reduction in activity, may be related to other aspects of motor planning and preparation. Finally, a major component of the movement-related activity in both MI and PMd is not susceptible to modification by prior information and is indivisibly coupled temporally to movement execution.


  4. Learning Motor Synergies Makes Use of Information on Muscular Load, Learn. Mem. Bookmark and Share

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    Prism adaptation, a form of procedural learning, requires the integration of visual and motor information for its proper acquisition. Although the role of the visual feedback has begun to be understood, the nature of the motor information necessary for the development of the adaptation remains unknown. In this work we have tested the idea that modifying the arm load at different stages of the adaptation process, and the ensuing change of motor information perceived by the subjects, would modify the final properties of the adaptation. We trained a set of subjects to throw balls to a target while wearing prism glasses and varied the weight of their arms at different time points during the task. We observed that the acquisition of the adaptation was not affected by the change in load. However, its persistence (i.e., the aftereffect) was reduced when tested under a weight condition different from the training trials. Furthermore, when the training weight conditions were restored later during testing, a second, late aftereffect was unmasked, suggesting that the missing aftereffect did not disappear but had remained latent. Our results show that the internal representation of a motor memory incorporates information about load conditions and that the memory stored under a specific weight condition can be fully retrieved only when the original training condition is restored.

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