# Two-Dimensional Sensory Input

**URL:** <http://discourse.iapct.org/t/two-dimensional-sensory-input/11850>\
**Category:** CSG2002\
**Created:** [November 12, 2002, 8:24pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850 "2002-11-12T20:24:51Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Fred\_Nickols2](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/f/53a042/32.png) [@Fred\_Nickols2](http://discourse.iapct.org/u/Fred_Nickols2)\
**Post date:** [November 12, 2002, 8:24pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/1 "2002-11-12T20:24:51Z")

</div>

I was participating on another list when someone made mention of ecological  
psychology. That led me to some web pages, from one of which I extracted  
the following:

\<snip\>

"Gibson, James Jerome

"In his last book, The Ecological Approach to Visual Perception, James  
Gibson (1904-1979) concluded with a plea that the terms and concepts of his  
theory "...never shackle thought as the old terms and concepts have!" He  
was referring to the framework of traditional perception, as was reflected,  
for example, in the classical problem of space perception Bishop Berkeley  
posed more than three hundred years ago (Berkeley 1963). How is it possible  
to perceive three-dimensional space when the input to our senses is a  
two-dimensional retinal surface in the case of vision, or a skin surface in  
the case of touch? Logically, it seemed this inadequate stimulation had to  
be supplemented somehow to account for our ordinary perception of a  
three-dimensional world. There have been two general proposals for the  
nature of this supplementation. An empiricist proposal, advocated by  
Berkeley himself, based the supplementation in the prior experience of the  
individual. The alternative nativist proposal based the supplementation in  
the innate functioning of the mental apparatus which intrinsically imposes  
a three-dimensional structure on two-dimensional stimulation. These two  
alternatives in only slightly modified forms persist to this day."

\<end snip\>

I'm probably way out of my league here but it seems to me that the  
preceding paragraph doesn't stand up to the slightest scrutiny. It doesn't  
seem to me that the retinal surface is two dimensional; it has depth or  
thickness. Nor is the skin a two-dimensional surface. In fact (and I know  
I use that word loosely), it seems to me that there are no true  
two-dimensional surfaces; all surfaces have a third dimension, even the  
archetypical sheet of paper. Or, in different terms, a true  
two-dimensional surface is really just an abstraction made for the purposes  
of analytical thinking or mathematicaly reasoning but doesn't really have  
any counterpart in physical reality (or whatever it is of so-called  
physical reality that we can perceive).

Anyway, I thought the members of this list might have some thoughts about  
the basic question posed above; namely, " How is it possible to perceive  
three-dimensional space when the input to our senses is a two-dimensional  
retinal surface in the case of vision, or a skin surface in the case of touch?"

Regards,

Fred Nickols  
[nickols@safe-t.net](mailto:nickols@safe-t.net)

---

<div class="post-metadata">

**Author:** ![Abbott\_Bruce](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/a/eb9ed0/32.png) [@Abbott\_Bruce](http://discourse.iapct.org/u/Abbott_Bruce)\
**Post date:** [November 12, 2002, 9:15pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/2 "2002-11-12T21:15:52Z")

</div>

[From Bruce Abbott (2002.11.12.16015 EST)]

> Fred Nickols (2002.11.12) --

> I'm probably way out of my league here but it seems to me that the  
> preceding paragraph doesn't stand up to the slightest scrutiny. It doesn't  
> seem to me that the retinal surface is two dimensional; it has depth or  
> thickness. Nor is the skin a two-dimensional surface. In fact (and I know  
> I use that word loosely), it seems to me that there are no true  
> two-dimensional surfaces; all surfaces have a third dimension, even the  
> archetypical sheet of paper. Or, in different terms, a true  
> two-dimensional surface is really just an abstraction made for the purposes  
> of analytical thinking or mathematicaly reasoning but doesn't really have  
> any counterpart in physical reality (or whatever it is of so-called  
> physical reality that we can perceive).

The quoted material was referring to the fact that the retina or skin  
surface registers only a two-dimensional image of the three-dimensional  
world. The fact that the receptive surfaces themselves have depth is  
irrelevant.

With respect to vision, information from which depth can be reconstructed  
is available from a variety of sources. The most potent of these is  
retinal disparity -- the differences between the two-dimensional images  
formed on each retina owing to the fact that the two eyes view the scene  
before them from somewhat different positions. The closer an object is to  
the viewer, the greater is the apparent difference in the apparent position  
of the object, relative to a distant background. This disparity is used by  
the visual system to reconstruct the distance of the object from the viewer  
and is the basis for the "3-D" or pop-out effect one gets while viewing a  
scene with both eyes open and which disappears when one eye is closed. It  
would be of considerable interest to know how the visual cortex performs  
the computations, which are facilitated by the fact that the neural  
representations of the left visual fields of both eyes lie next to one  
another in the right occipital lobe and similarly for the right visual  
field of both eyes in the left occipital lobe. The process that  
accomplishes this is innately wired into the brain but requires appropriate  
visual input if the wiring is to be properly maintained (i.e., "use it or  
loose it"). For this reason it is recommended that children whose eyes  
don't align properly (e.g., cross-eyed) have their condition corrected as  
soon as practical, as later correction of the physical problem not correct  
the problem in depth perception.

Bruce A.

---

<div class="post-metadata">

**Author:** ![Fred\_Nickols2](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/f/53a042/32.png) [@Fred\_Nickols2](http://discourse.iapct.org/u/Fred_Nickols2)\
**Post date:** [November 12, 2002, 9:40pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/3 "2002-11-12T21:40:23Z")

</div>

[From Fred Nickols (2002.11.12.1632)] --

> [From Bruce Abbott (2002.11.12.16015 EST)]
> 
> > Fred Nickols (2002.11.12) --
> 
> > I'm probably way out of my league here but it seems to me that the  
> > preceding paragraph doesn't stand up to the slightest scrutiny. It doesn't  
> > seem to me that the retinal surface is two dimensional; it has depth or  
> > thickness. Nor is the skin a two-dimensional surface. In fact (and I know  
> > I use that word loosely), it seems to me that there are no true  
> > two-dimensional surfaces; all surfaces have a third dimension, even the  
> > archetypical sheet of paper. Or, in different terms, a true  
> > two-dimensional surface is really just an abstraction made for the purposes  
> > of analytical thinking or mathematicaly reasoning but doesn't really have  
> > any counterpart in physical reality (or whatever it is of so-called  
> > physical reality that we can perceive).
> 
> The quoted material was referring to the fact that the retina or skin  
> surface registers only a two-dimensional image of the three-dimensional  
> world. The fact that the receptive surfaces themselves have depth is  
> irrelevant.

Uh, hmm, gee, well, I dunno. I guess that's the essence of my question. I  
think I get the bit about binocular vision but is the image on the retinal  
surface truly a two-dimensional image or is there more to it than that?

Fred Nickols  
nickols@safe-t.net

---

<div class="post-metadata">

**Author:** ![Richard\_Marken2](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/r/edb3f5/32.png) [@Richard\_Marken2](http://discourse.iapct.org/u/Richard_Marken2)\
**Post date:** [November 13, 2002, 2:23pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/4 "2002-11-13T14:23:31Z")

</div>

[From Rick Marken (2002.11.13.0920)]

> I'm probably way out of my league here but it seems to me that the  
> preceding paragraph doesn't stand up to the slightest scrutiny. It doesn't  
> seem to me that the retinal surface is two dimensional; it has depth or  
> thickness. Nor is the skin a two-dimensional surface.

The "classical problem of space perception" is really only a "problem" if  
perception is conceived of as a process of correctly representing the reality  
that is presumed to exist outside of oneself. This problem doesn't exist in PCT  
because the role of perception in PCT is not to provide an accurate map of  
reality; in PCT the role of perception is to provide a controllable construction  
of reality. Indeed, in PCT it is "reality" that is the mystery, not perception.

"Reality", in the form of a physical model of the environment, is one of the  
assumptions (guesses) made by the PCT model. The PCT model essentially says "If  
reality is assumed to be, say, the projection on a two dimensional retinal surface  
of a ball moving in three dimensional space, and if a control system constructs a  
perception of that reality in terms of the two dimensional projection of the angle  
of the ball relative to a fixed point in three space and if the system moves in  
that reality so as to keep that perception constantly increasing then the system  
will intercept the ball". In PCT, the question isn't how "veridical"  
representations of reality are constructed. In PCT the question is "what is the  
nature of the representation of reality that \_is\_ constructed". What, that is, is  
the function that relates what we perceive to what we \_assume\_ (based on "physics  
and chemistry") to be the reality that is the "argument" of that function. We  
study this aspect of perception using the test for the controlled perceptual  
variable.

Best

Rick

> **···**
>
> --  
> Richard S. Marken, Ph.D.  
> The RAND Corporation  
> PO Box 2138  
> 1700 Main Street  
> Santa Monica, CA 90407-2138  
> Tel: 310-393-0411 x7971  
> Fax: 310-451-7018  
> E-mail: rmarken@rand.org

---

<div class="post-metadata">

**Author:** ![Richard\_Marken2](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/r/edb3f5/32.png) [@Richard\_Marken2](http://discourse.iapct.org/u/Richard_Marken2)\
**Post date:** [November 13, 2002, 4:27pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/5 "2002-11-13T16:27:28Z")

</div>

[From Rick Marken (2002.11.13.1130)]

> Wolfgang Zocher (2002.11.13.1857 CET)
> 
> \> [From Rick Marken (2002.11.13.0920)]  
> \>
> 
> Thanks for this wonderful and "constructivistish" description of  
> perception. It's really GREAT!!

Gee, thanks Wolfgang! Great to hear from you. I hope the fact that you are able to post  
to CSGNet means that you are feeling better! It would be so great if you could come to  
the conference in LA.

Anyway, best regards to you and Marion.

Best

Rick

> **···**
>
> --  
> Richard S. Marken, Ph.D.  
> The RAND Corporation  
> PO Box 2138  
> 1700 Main Street  
> Santa Monica, CA 90407-2138  
> Tel: 310-393-0411 x7971  
> Fax: 310-451-7018  
> E-mail: rmarken@rand.org

---

<div class="post-metadata">

**Author:** ![Wolfgang\_Zocher](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/w/b487fb/32.png) [@Wolfgang\_Zocher](http://discourse.iapct.org/u/Wolfgang_Zocher)\
**Post date:** [November 13, 2002, 6:00pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/6 "2002-11-13T18:00:29Z")

</div>

[From Wolfgang Zocher (2002.11.13.1857 CET)

> [From Rick Marken (2002.11.13.0920)]

Thanks for this wonderful and "constructivistish" description of  
perception. It's really GREAT!!

Wolfgang

> **···**
>
> > The "classical problem of space perception" is really only a "problem" if  
> > perception is conceived of as a process of correctly representing the reality  
> > that is presumed to exist outside of oneself. This problem doesn't exist in PCT  
> > because the role of perception in PCT is not to provide an accurate map of  
> > reality; in PCT the role of perception is to provide a controllable construction  
> > of reality. Indeed, in PCT it is "reality" that is the mystery, not perception.
> > 
> > "Reality", in the form of a physical model of the environment, is one of the  
> > assumptions (guesses) made by the PCT model. The PCT model essentially says "If  
> > reality is assumed to be, say, the projection on a two dimensional retinal surface  
> > of a ball moving in three dimensional space, and if a control system constructs a  
> > perception of that reality in terms of the two dimensional projection of the angle  
> > of the ball relative to a fixed point in three space and if the system moves in  
> > that reality so as to keep that perception constantly increasing then the system  
> > will intercept the ball". In PCT, the question isn't how "veridical"  
> > representations of reality are constructed. In PCT the question is "what is the  
> > nature of the representation of reality that \_is\_ constructed". What, that is, is  
> > the function that relates what we perceive to what we \_assume\_ (based on "physics  
> > and chemistry") to be the reality that is the "argument" of that function. We  
> > study this aspect of perception using the test for the controlled perceptual  
> > variable.
> > 
> > Best
> > 
> > Rick  
> > --  
> > Richard S. Marken, Ph.D.  
> > The RAND Corporation  
> > PO Box 2138  
> > 1700 Main Street  
> > Santa Monica, CA 90407-2138  
> > Tel: 310-393-0411 x7971  
> > Fax: 310-451-7018  
> > E-mail: rmarken@rand.org
> 
> --  
> Wolfgang Zocher
> 
> -------------------------------------------------------------------  
> www: [http://wzocher.bei.t-online.de/](http://wzocher.bei.t-online.de/)  
> -------------------------------------------------------------------

---

<div class="post-metadata">

**Author:** ![DAVID\_WOLSK1](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/d/13edae/32.png) [@DAVID\_WOLSK1](http://discourse.iapct.org/u/DAVID_WOLSK1)\
**Post date:** [November 15, 2002, 7:12am UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/7 "2002-11-15T07:12:20Z")

</div>

Fred, the rotating trapezoid illusion provides some interesting experiences  
in this regard  
David Wolsk.

> **···**
>
> ----- Original Message -----  
> From: "Fred Nickols" \<nickols@SAFE-T.NET\>  
> To: \<CSGNET@listserv.uiuc.edu\>  
> Sent: Tuesday, November 12, 2002 12:24 PM  
> Subject: Two-Dimensional Sensory Input
> 
> > Anyway, I thought the members of this list might have some thoughts about  
> > the basic question posed above; namely, " How is it possible to perceive  
> > three-dimensional space when the input to our senses is a two-dimensional  
> > retinal surface in the case of vision, or a skin surface in the case of
> 
> touch?"
> 
> > Regards,
> > 
> > Fred Nickols  
> > nickols@safe-t.net

---

<div class="post-metadata">

**Author:** ![Bill\_Powers1](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@Bill\_Powers1](http://discourse.iapct.org/u/Bill_Powers1)\
**Post date:** [November 15, 2002, 3:49pm UTC](http://discourse.iapct.org/t/two-dimensional-sensory-input/11850/8 "2002-11-15T15:49:57Z")

</div>

[From Bill Powers (2002.11.15.0828 MST)]

Fred Nickols" \<nickols@SAFE-T.NET\> --

\> Anyway, I thought the members of this list might have some thoughts about

> \> the basic question posed above; namely, " How is it possible to perceive  
> \> three-dimensional space when the input to our senses is a two-dimensional  
> \> retinal surface in the case of vision, or a skin surface in the case of  
> touch?"

Has anyone given a direct answer to this question? Maybe Bruce Abbott did.  
Anyway, the answer is that the eyes provide two images from (slightly)  
different points of view, and this is enough to permit judging depth from  
differences in the relative positions of objects in the two eyes. This  
works up to 20 feet or so, depending on the sharpness of vision.

It's also possible to judge depth using only one eye if the point of view  
is shifted laterally (at right angles to the line of sight). Doing this  
shows the relative positions of objects changing as the point of view is  
moved. As noted in B:CP, it is even possible to see depth when one image is  
presented to one eye on one day, and the shifted image is presented to the  
other eye a day later -- at least one woman was able to do this! The test  
was to pick out an elevated patch in a two fields of random dots normally  
viewed binocularly. This woman was obviously able to store images at a very  
low order of perception, perhaps second order, and compare a remembered  
image with a real one.

In the Little Man Version Two, the simulation perceives the radial  
distances of the hand and of the target by computing the disparity of  
retinal positions between the two eyes. Optical computations find the  
points on the retina where the fingertip and the target will appear in each  
eye, relative to the center of the retina. A simple perceptual computation  
then produces a nonlinear and approximate measure of distance from the  
image disparity. Reaching out to touch the target is controlled by making  
the fingertip image disparity the same as the target image disparity, so  
the errors in computing actual physical distance don't matter -- the same  
errors and nonlinearities occur in computing both distances.

Actually, the eyes also individually move to center the target image, so  
the target image disparity is reduced to zero. But the convergence of the  
eyes needed to do that directly adds to or subtracts from the disparity  
between images of the fingertip, so the depth control system still works  
the same way, by making the fingertip image disparity the same as the  
target image disparity -- in this case, zero.

I don't know how the brain actually computes distance from the two sets of  
image disparities, but the Little Man simulations clearly shows that this  
is possible to do by at least one means.

Best,

Bill P.

.