Re: Retinotopic Maps

From: HARNAD Stevan (
Date: Mon Jun 03 1996 - 15:40:06 BST

> From: "Saha, Sanna" <>
> Date: Wed, 22 May 1996 20:35:14 GMT
> In brain anatomy and imaging there are parts of the brain that are
> analog copies of the shadow, cast by objects on the retina.

A "shadow" is just a metaphor here. The pattern of the light rays
(photons) bouncing off an object and landing on the retina produces a
kind of "Image" or "shadow" of the object.

> They are
> shadows of shadows. Whenever an object casts a shadow on the retina, it
> casts a shadow on higher retinotopic areas, as well as keeping its
> shape.

How is the shape of the shadow physically passed from the retina to the
higher level analogs of it? There are point-to-point anatomical
connections. This point-to-point mapping, also called a "topographical"
mapping, because it preserves the topography, the shape, of the original
"shadow", is called a retinotopic mapping.

> However, the brain image does not look like the mental image or
> the original object.

It may not LOOK like it, but who looks at brain images? The important
thing is that there is something about the shape of an analog copy that
resembles, i.e., preserves some of the structure, of whatever it is a
copy of. You would have confused kid-sib here...

> This image mapping is known as 'retinotopic
> mapping', which is point-for-point copies of the topography of the
> retina.
> An example is the somatosensory (skin) humunculus, which is analogous
> exactly to the retinotopic map. It is a point-for-point map of your
> skin surface copied to higher levels of your brain, preserving the
> local topography and connectivity.

Both retinotopic and somototopic projections are examples of analog

> Retinotopic mapping occurs due to the principle by which stimuli, that
> are adjacent to each other in the visual world are processed by
> adjacent sets of neurons at the higher levels of the visual system; in
> this way the spatial pattern of light and dark arriving at the retina
> is preserved. PET imaging has been utilised to identify retinotopic
> areas, that are active during imaging.

Actually, retinotopic maps preserve local point-to-point relations in
the retinal projection; how well they preserve the topography of the
object itself depends on the shadow that actually falls on the retina.
As we know, some shadows, at some angles, preserve the shape of whatever
they are shadows of, but others do not.

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