1: Biol Cybern. 2005 Jan;92(1):21-37. Epub 2004 Dec 10. 

A model of the saccade-generating system that accounts for trajectory variations
produced by competing visual stimuli.

Arai K, Keller EL.

The Smith-Kettlewell Eye Research Institute, 2318 Fillmore Street, San
Francisco, CA, 94115, USA, elk@ski.org.

Variable saccade trajectories are produced in visual search paradigms in which
multiple potential target stimuli are present. These variable trajectories
provide a rich source of information that may lead to a deeper understanding of
the basic control mechanisms of the saccadic system. We have used published
behavioral observations and neural recordings in the superior colliculus (SC),
gathered in monkeys performing visual search paradigms, to guide the
construction of a new distributed model of the saccadic system. The new model
can account for many of the variations in saccade trajectory produced by the
appearance of multiple visual stimuli in a search paradigm. The model uses
distributed feedback about current eye motion from the brainstem to the SC to
reduce activity there at physiologically realistic rates during saccades. The
long-range lateral inhibitory connections between SC cells used in previous
models have been eliminated to match recent physiological evidence. The model
features interactions between visually activated multiple populations of cells
in the SC and distributed and topologically organized inhibitory input to the SC
from the SNr to produce some of the types of variable saccadic trajectories,
including slightly curved and averaging saccades, observed in visual search
tasks. The distributed perisaccadic disinhibition of SC from the substantia
nigra (SNr) is assumed to have broad spatial tuning. In order to produce the
strongly curved saccades occasionally recorded in visual search, the existence
of a parallel input to the saccadic burst generators in addition to that
provided by the distributed input from the SC is required. The spatiotemporal
form of this additional parallel input is computed based on the assumption that
the input from the model SC is realistic. In accordance with other recent
models, it is assumed that the parallel input comes from the cerebellum, but our
model predicts that the parallel input is delayed during highly curved saccadic
trajectories.

PMID: 15650897 [PubMed - in process]



2: Biol Cybern. 2004 Dec;91(6):377-87. Epub 2004 Nov 19. 

A dynamical neural network for hitting an approaching object.

Dessing JC, Caljouw SR, Peper PE, Beek PJ.

Institute for Fundamental and Clinical Human Movement Sciences,
Amsterdam/Nijmegen, The Netherlands. joost.dessing@fbw.vu.nl

Besides making contact with an approaching ball at the proper place and time,
hitting requires control of the effector velocity at contact. A dynamical neural
network for the planning of hitting movements was derived in order to account
for both these requirements. The model in question implements continuous
required velocity control by extending the Vector Integration To Endpoint model
while providing explicit control of effector velocity at interception. It was
shown that the planned movement trajectories generated by the model agreed
qualitatively with the kinematics of hitting movements as observed in two recent
experiments. Outstanding features of this comparison concerned the timing and
amplitude of the empirical backswing movements, which were largely consistent
with the predictions from the model. Several theoretical implications as well as
the informational basis and possible neural underpinnings of the model were
discussed.

PMID: 15599591 [PubMed - in process]



3: Biol Cybern. 2004 Dec;91(6):359-76. Epub 2004 Nov 18. 

Frequency-dependent selection of alternative spinal pathways with common
periodic sensory input.

Jilge B, Minassian K, Rattay F, Dimitrijevic MR.

TU-BioMed Association for Biomedical Engineering, Vienna University of
Technology, Wiedner Hauptstrasse 8-10, 1040 Vienna, Austria.

Electrical stimulation of the lumbar cord at distinct frequency ranges has been
shown to evoke either rhythmical, step-like movements (25-50 Hz) or a sustained
extension (5-15 Hz) of the paralysed lower limbs in complete spinal cord injured
subjects. Frequency-dependent activation of previously "silent" spinal pathways
was suggested to contribute to the differential responsiveness to distinct
neuronal "codes" and the modifications in the electromyographic recordings
during the actual implementation of the evoked motor tasks. In the present study
we examine this suggestion by means of a simplified biology-based neuronal
network. Involving two basic mechanisms, temporal summation of synaptic input
and presynaptic inhibition, the model exhibits several patterns of mono- and/or
oligo-synaptic motor output in response to different interstimulus intervals. It
thus reproduces fundamental input-output features of the lumbar cord isolated
from the brain. The results confirm frequency-dependent spinal pathway selection
as a simple mechanism which enables the cord to respond to distinct neuronal
codes with different motor behaviours and to control the actual performance of
the latter.

PMID: 15597176 [PubMed - in process]