Interpreting the notion of simultaneous confinement in space and
spatial frequency in terms of confinement, not variance, yields a rich
class of intrinsically two-dimensional patterns
Jonathon Victor1,2,3 and B.W. Knight2,3
1Department of Neurology and Neuroscience,
Weill Medical College of Cornell University
2The Rockefeller University
3Department of Biomathematical Sciences,
Mount Sinai School of Medicine
Well-established theoretical reasons lead to consideration of
functions that are limited in space and spatial frequency as the
starting point for modeling receptive fields of visual neurons.
Limited spatial spread economizes on connection length and allows for
spatial localization. Limited spread in spatial frequency allows for
analysis at a particular spatial scale. In the traditional view,
spread is measured by variance. As Daugman (1985) showed via analogy
with the uncertainty principle, the Gabor functions have the smallest
joint spread in space and spatial frequency. While it is interesting
to note that Gabor functions resemble receptive fields in V1, this
view provides little insight into receptive fields in other visual
areas, and also does not provide an indication of why typical V1
receptive fields have only a few lobes.
Here we present an alternative construction of the notion of "limited
in space and spatial frequency," based on the concept of confinement.
From this notion, a much richer set of functions, the 2-dimensional
Hermite functions, emerge. These functions form a natural hierarchy.
The first levels of this hierarchy contain functions that resemble
Gabor functions with a small number of lobes, and thus resemble V1
receptive fields. Further down in the hierarchy are intrinsically
two-dimensional functions, some of which resemble the non-Cartesian
gratings, to which some V4 neurons respond preferentially (Gallant,
1996). In addition to their many interesting mathematical properties,
the two-dimensional Hermite functions allow for efficient ("sparse")
local synthesis of images.
While we make no claim that this view suffices to account for visual
receptive field structure, we suggest that it provides a framework for
a principled study of receptive fields, and that it is useful to think
of receptive fields (along the V1-to-V2-to-V4 pathway) as not only
expanding, but also increasing in their combined space-bandwidth