Re: Reply to Wolf



There are, as far as I know, no "edge detectors" in human retinas. You are
probably thinking of the so called "simple cells" of the primary visual
cortex (V1, aka "striate cortex", aka "area 17"), some of which, following
the work of Hubel and Wiesel in the 1960's, were thought of for a while as
"edge detectors". By the 1980's the limitations and inaccuracies of that
view had made it untenable, though it lives on in the imaginations of some
individuals, none of them neuroscientists, and none of them students of (the
interdisciplinary topic) vision science.

You might enjoy Helga Kolb's survey article: "How The Retina Works"

http://webvision.med.utah.edu/



The matter may be "closed" in some circles, but if you follow down
the link given, you find .....

http://webvision.med.utah.edu/ -->

http://webvision.med.utah.edu/VisualCortex.html -->

http://webvision.med.utah.edu/VisualCortex.html#columns
==============
Orientation and Direction Selectivity.

V1 is the first site where strong orientation and direction
selectivities are observed in the macaque monkey (Hubel and Wiesel,
1968). While the vast majority of V1 cells show some degree of
orientation selectivity, only approximately 25-35% of V1 cells are
strongly directionally selective (Schiller et. al., 1976; DeValois et
al., 1982). The classic method for testing orientation and direction
selectivity is to measure the spike rate of a single cell in response
to drifting oriented luminance bars and/or drifting luminance spots
(see Figure 21).



Figure 21. A tuning curve and corresponding polar plot obtained from
two macaque V1 cells in response to drifting luminance bars of
systematically varied orientation and direction. The responses of one
orientation selective cell and one nonselective cell are provided for
comparison. Histograms surrounding the polar plots demonstrate the
cellular response as a function of time. Orientation bias (OB) and
direction bias (DB) are measures of how selective a cell is, where
0.1 is significant, and 0.3 is approximately an 8:1 maximum firing
rate to minimum firing rate ratio. From Schmolesky et al. (2000). (27K
jpeg image).

The concept of an orientation column can be easily appreciated by
examining figures 20 and 21. When an electrode is lowered into V1 at
an angle relatively parallel to the cortical layers (see Fig. 22) the
orientation selectivities of the cells encountered vary
systematically, where adjacent cellular regions share approximate
orientation preferences.



Figure 22. One extensive electrode penetration in macaque V1. The
short, near vertical lines represent recording sites, and polar plots
for each site are indicated. This figure shows the preferred
orientation of each cell in relation to the cortical layers, CO
compartments (a filled circle indicates a blob; lack of this circle
indicates an interblob) and color selectivity (C=color selective;
B=broadband) in macaque V1. From Leventhal et al. (1995). (27 K jpeg
image)

Such recordings led Hubel and Wiesel to propose models of functional
organization like the one shown below (Figure 23).



Figure 23. The ice-cube model of the cortex. It illustrates how the
cortex is divided, and the same time, into two kinds of slabs, one set
of ocular dominance (left and right) and one set for orientation. The
model should not be taken literally: Neither set is as regular as
this, and the orientation slabs especially are far from parallel or
straight. (27 K jpeg image)

Recently, the orientation columns of V1 first described by Hubel and
Wiesel have also been recast into more complex geometries such as
partial columns ("slabs") and pinwheels as dictated by the increasing
volumes of evidence (e.g. Bonhoeffer and Grinvald, 1991).
====================

.

Relevant Pages

  • Re: Reply to Wolf
    ... cells in macaque visual cortex. ... orientation tuning, retinal eccentricity, and cell type. ... Spatial-frequency organization in primate striate cortex. ... Orientation and Direction Selectivity. ...
    (comp.ai.philosophy)
  • Re: Aligning Gridlines at 45 degrees (Orientation)
    ... The text in the cells must be aligned to 45 degress in order to get slanted ... borders. ... You cannot change the orientation of the cell itself, ... The first row is a higher level group of attributes in which each ...
    (microsoft.public.excel.newusers)
  • Re: Aligning Gridlines at 45 degrees (Orientation)
    ... borders. ... You cannot change the orientation of the cell itself, ... The first row is a higher level group of attributes in which each ... that the outer boundaries of the cells in the first row align with the 45 ...
    (microsoft.public.excel.newusers)
  • RE: How do I rotate text in a word document
    ... You can change the text orientation in drawing objects, such as text boxes, ... shapes and callouts, or in table cells so that the text is displayed ...
    (microsoft.public.word.docmanagement)
  • Re: Reply to Wolf
    ... V1 may be doing a local spatial frequency analysis of input images is ... I did not claim that the notion that so-called simple cells in V1 can be ... analyzers any more than orientation selectivity makes them "edge detectors" ...
    (comp.ai.philosophy)