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[Lina Massone]

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	The following technical report is available.


		Target-Switching Experiments
	     with a Sequential Neuro-Controller

		     Lina Massone
	  Dept. of Brain and Cognitive Sciences
	  Massachusetts Institute of Technology
	77 Massachusetts Avenue - Cambridge Ma 02139
  
  This paper describes some target-switching experiments simulated
  with a neural system that drives a three-joint redundant limb.
  The system is composed of a controller (a sequential network)
  and a limb emulator. The system was trained to generate aiming
  movements of the limb towards targets specified as sensory stimuli;
  it was not trained to perform the target-switching task itself.
  The experiments demonstrate that the system possesses the ability
  to solve the target-switching task, which requires generalization
  with respect to both initial limb posture and sensory stimulation.
  I performed the experiments under two different perceptual conditions:
  (1) on/off switching of the two stimuli, (2) temporal overlap of the
  two stimuli. The second case refers to a hypothesis proposed
  by many experimental investigators about two different systems
  being involved in the programming of movements: a "where-system"
  that would build an internal representation of the target that
  shifts gradually to the new values, and a "when-system" that
  would start the motor program generator.The "where-system" 
  would be able to account for the observed differences in path, while the
  "when-system" would be able to account for the response-time phenomenon.
  The case of temporal overlap of the two stimuli is a simulation
  of the "where-system". I present a qualitative comparison of data
  generated by the neural system under conditions (1) and (2),
  namely (i) the endpoint paths and velocity profiles,
  (ii) the patterns of muscular activation. Results of the
  comparison show that in presence of the "where-system" the controller
  can account for the variability in paths and the basic two-peak
  structure of the velocity profiles commonly observed in psychophysical
  experiments. In absence of the "where-system" the behavior of the
  controller is, on the contrary, highly stereotyped. Results also
  point out an inadequacy in the network architecture to deal with
  the observed high peak velocities after stimuli are switched.

Please forward all requests to lina at ai.mit.edu