NIPS Preprints available

[Geoff Goodhill]

The following 3 papers from Georgetown University will appear in the
1998 NIPS proceedings, and are now available from
http://www.giccs.georgetown.edu/~alex and ~geoff respectively:


           NEURAL BASIS OF OBJECT-CENTERED REPRESENTATIONS
                 Sophie Deneve and Alexandre Pouget
      Georgetown Institute for Cognitive and Computational Sciences

We present a neural model that can perform eye movements to a
particular side of an object regardless of the position and
orientation of the object in space, a generalization of a task which
has been recently used by Olson and Gettner to investigate the neural
structure of object-centered representations. Our model uses an
intermediate representation in which units have oculocentric receptive
fields-- just like collicular neurons--- whose gain is modulated by
the side of the object to which the movement is directed, as well as
the orientation of the object. We show that these gain modulations are
consistent with Olson and Gettner single cell recordings in the
supplementary eye field. This demonstrates that it is possible to
perform an object-centered task without a representation involving an
object-centered map, viz., without neurons whose receptive fields are
defined in object-centered coordinates. We also show that the same
approach can account for object-centered neglect, a situation in which
patients with a right parietal lesion neglect the left side of objects
regardless of the orientation of the objects.


         A MATHEMATICAL MODEL OF AXON GUIDANCE BY DIFFUSIBLE FACTORS
                       Geoffrey J. Goodhill
      Georgetown Institute for Cognitive and Computational Sciences

In the developing nervous system, gradients of target-derived
diffusible factors play an important role in guiding axons to
appropriate targets.  In this paper, the shape that such a gradient
might have is calculated as a function of distance from the target and
the time since the start of factor production. Using estimates of the
relevant parameter values from the experimental literature, the
spatiotemporal domain in which a growth cone could detect such a
gradient is derived. For large times, a value for the maximum guidance
range of about 1 mm is obtained. This value fits well with
experimental data. For smaller times, the analysis predicts that
guidance over longer ranges may be possible. This prediction remains
to be tested.


                GRADIENTS FOR RETINOTECTAL MAPPING
                       Geoffrey J. Goodhill
      Georgetown Institute for Cognitive and Computational Sciences

The initial activity-independent formation of a topographic map in the
retinotectal system has long been thought to rely on the matching of
molecular cues expressed in gradients in the retina and the tectum.
However, direct experimental evidence for the existence of such
gradients has only emerged since 1995. The new data has provoked the
discussion of a new set of models in the experimental literature.
Here, the capabilities of these models are analyzed, and the gradient
shapes they predict in vivo are derived.