# How does the brain create straight lines from wiggly stuff ? **URL:** \ **Category:** Great Debates\ **Created:** [July 9, 2013, 12:22am UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990 "2013-07-09T00:22:16Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![MrQwertyasd](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@MrQwertyasd](https://boards.straightdope.com/u/MrQwertyasd)\ **Post date:** [July 9, 2013, 9:30pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/21 "2013-07-09T21:30:02Z")
hmm, 4- (4/4+8/4) =1 8- (4/4+8/4) =5 But somehow those seperate cell signals become one continuous line ? Or do they ? Do my eyes deceive me or is there a graininess to vision ? In fact, wouldn’t it be right to say that the signal received by the eyes is quantised (photons), or digital, but the perception we have of lines and edges is variable or analogue ? ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [July 9, 2013, 10:06pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/22 "2013-07-09T22:06:06Z")
> [@MrQwertyasd](#): > > In fact, wouldn’t it be right to say that the signal received by the eyes is quantised (photons), or digital, but the perception we have of lines and edges is variable or analogue ? The signal received by your eyes is quantized. Photons hitting your retina are translated into a set of pulses flowing along your optic nerve. But it would be a mistake to think of this quantized information being projected onto a digital screen inside your skull. The pulses from your optic nerve cascade through the layers of your visual cortex. These layers perform operations like edge detection and motion detection. What emerges is a highly abstracted set of neural impulses encoding what your visual cortex has judged to be the salient information within your visual field. This trickle of information is then used to update the internal model of the world that is running inside your head. What you experience as “seeing” is merely a convenient “user interface” of the physical world constructed by your brain from the salient traces that your visual cortex has extracted from the light impinging on your retina. ---
**Author:** ![MrQwertyasd](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@MrQwertyasd](https://boards.straightdope.com/u/MrQwertyasd)\ **Post date:** [July 9, 2013, 10:20pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/23 "2013-07-09T22:20:30Z")
> [@](#): > > But it would be a mistake to think of this quantized information being projected onto a digital screen inside your skull. Sure seems like it. Seems like there is a watcher and a watched - but does current neuroscience say that there is no ‘I’ watching the input from the senses ? So who is aware of it all ? Is the phrase ‘I see’ another useful fiction like ‘I think of a straight line’ ? ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [July 10, 2013, 6:41pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/24 "2013-07-10T18:41:55Z")
> [@MrQwertyasd](#): > > Sure seems like it. Seems like there is a watcher and a watched - but does current neuroscience say that there is no ‘I’ watching the input from the senses ? If you posit a homunculus in your head watching an internal screen, then all you’ve done is \*displaced \*the problem of how sensation yields conscious experience. You still haven’t explained how your homunculus translates the internal screen into what you experience. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [July 10, 2013, 7:00pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/25 "2013-07-10T19:00:09Z")
By “second derivative,” you guys just mean that a perceived _edge_ of some sort looks sharp not just because of the change in value (brightness, say) of a transect crossing the edge (delta), nor just because of the change in the _abruptness_ of that change in value (first derivative), but because of a change in…what…the abruptness of the abruptness of that change in value? Something like that? ---
**Author:** ![MrQwertyasd](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@MrQwertyasd](https://boards.straightdope.com/u/MrQwertyasd)\ **Post date:** [July 10, 2013, 8:00pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/26 "2013-07-10T20:00:49Z")
> [@](#): > > By “second derivative,” Was wondering about that - acceleration of value change over a perceived border? ---
**Author:** ![MrQwertyasd](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@MrQwertyasd](https://boards.straightdope.com/u/MrQwertyasd)\ **Post date:** [July 10, 2013, 8:02pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/27 "2013-07-10T20:02:58Z")
The homunculus has a homunculus. It’s homunculi, homunculi, homunculi all the way down. Reiterating, fractal homonculi going on for infinity. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [July 10, 2013, 8:21pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/28 "2013-07-10T20:21:25Z")
> [@MrQwertyasd](#): > > Was wondering about that - acceleration of value change over a perceived border? I think it must be the acceleration of the acceleration which someone has shown our brains perceive as most “edgy”, but I’ll let the experts confirm this. This sort of makes sense. We see a sky as a gradual wash from blue to white, say. That’s continuous change (analogous to “position” WRT movement) – no edge. If there’s a cloud in the sky, its edge will be soft, but will represent a quickening of the change across space (analogous to first derivative – “movement” – WRT movement). So, some perceived edge, but a rather blurry one. But if the edge some other cloud has a truly sharp edge within that blurry boundary, that’s because the quickening of change is itself quickening there (analogous to _second_ derivative – “acceleration” – WRT movement). Or am I way off here? ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [July 10, 2013, 8:28pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/29 "2013-07-10T20:28:48Z")
> [@JKellyMap](#): > > By “second derivative,” you guys just mean that a perceived _edge_ of some sort looks sharp not just because of the change in value (brightness, say) of a transect crossing the edge (delta), nor just because of the change in the _abruptness_ of that change in value (first derivative), but because of a change in…what…the abruptness of the abruptness of that change in value? Delta = first derivative. Say you’ve got a function that transitions linearly from dark to light over a narrow interval. The first derivative of this function will be zero everywhere except inside the interval where it’s a constant value equal to the slope of the transition. The second derivative is a narrow upward spike at the beginning of the interval and a narrow downward spike at the end of the interval. Your visual cortex detects those spikes. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [July 10, 2013, 8:29pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/30 "2013-07-10T20:29:05Z")
Wait, scratch that. It’s simpler, I’m guessing. “Position” would just be, say, the sky being one color – say, blue. “First derivative” would be a gradual shift from, say, blue to white across the sky. “Second derivative” is obviously when there’s a quickening of that change – say, when you reach a cloud’s edge, fuzzy or not. Our brain obviously picks up on these edges readily, and the movement of those edges (which it reifies as some real thing moving in space). ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [July 10, 2013, 8:38pm UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/31 "2013-07-10T20:38:01Z")
Hmmm…Thanks, Hamster King. Looks like I was closer to it the first time. A typical cloud has a pretty fast, but even, shift from gray to white within it. The sky has a slow, but also even, shift from blue to white. Pretty much everything we see, in other words, is “airbrushed” (closer or further from a light source, etc., etc.) But the change from one “speed of change” (the sky) to another “speed of change” (the cloud) isn’t what we notice as the edge – it’s the fact that THIS change is so abrupt, at least in a typical \*cumulus \*cloud. In other words, if we have a typical _cirrus_ cloud, which offers a smooth transition from the sky’s rate of change to the cloud’s rate of change, no edge. ---
**Author:** ![MrQwertyasd](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@MrQwertyasd](https://boards.straightdope.com/u/MrQwertyasd)\ **Post date:** [July 11, 2013, 11:31am UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/32 "2013-07-11T11:31:26Z")
For primates is edge detection chiefly about finding branches in trees ? Monkeys and apes can do some pretty spectacular brachiation, any mistake would be a long fall so the ability to keenly detectthe edge of a branch would be essential. Just guessing, then it’s a couiple of more evolutionary steps and suddenly humanity is talking about straight lines and Euclidian geometry and such based on the perception of edges ? ---
**Author:** ![MrQwertyasd](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@MrQwertyasd](https://boards.straightdope.com/u/MrQwertyasd)\ **Post date:** [July 11, 2013, 11:40am UTC](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990/33 "2013-07-11T11:40:57Z")
[Spectaculer brachiation](http://www.youtube.com/watch?v=XLSVt9CWSpc)- you’d need to know where those edges are! [Previous page](https://boards.straightdope.com/t/how-does-the-brain-create-straight-lines-from-wiggly-stuff/662990.md?page=1)