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A MATLAB package is now available that implements a simple recurrent model
of orientation selectivity in the primary visual cortex. It can be run in
seconds on a personal computer, and allows one to observe the responses of
recurrent models to a variety of visual stimuli. The model is described in
the attached abstract. The address for downloads is
http://cns.nyu.edu/home/matteo/v1ori.html.


  PREDICTIONS OF A RECURRENT MODEL OF ORIENTATION SELECTIVITY
  Matteo Carandini and Dario L. Ringach
  Vision Research, submitted (1996)


  Recurrent models of orientation selectivity in the visual cortex
  postulate that an initially broad tuning given by the pattern of
  geniculate afferents is substantially sharpened by intracortical
  feedback.

  We show that these models can be tested on the basis of their
  predicted responses to certain visual stimuli, without the need for
  pharmacological or physiological manipulations.  First, we consider
  a detailed recurrent model proposed by Somers, Nelson and Sur (1995)
  and show that it can be simplified to a single equation: a
  center-surround feedback filter in the orientation domain.  Then, we
  explore the responses of the simplified model to stimuli containing
  two or more orientations.  We find that the model exhibits peculiar
  responses to stimuli containing two orientations, such as plaids or
  crosses: if the component orientations differ by less than
  45 degrees the model cannot distinguish between them; if the
  orientations differ by more than 45 degrees the model
  overestimates their angle by as much as 30 degrees.  Moreover, the
  model cannot signal the presence of three orientations separated by
  60 degrees (it responds as if there were only two orientations),
  and the addition of two-dimensional visual noise to an oriented
  stimulus results in strong spurious responses at the orthogonal
  orientation.

  We argue that the effects of attraction and repulsion between
  orientations and the emergence of responses at off-optimal
  orientations are common to a wide class of feedback models of
  orientation selectivity.  These models could thus be tested by
  measuring the visual responses of cortical neurons to stimuli
  containing multiple orientations.


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Matteo Carandini
http://cns.nyu.edu/home/matteo

Howard Hughes Medical Institute
Center for Neural Science
New York University
4 Washington Place #809
New York, NY 10003