index.md (3592B)
1 --- 2 title: "Headlamp illusion" 3 description: A visual illusion for hikers at night. 4 date: 2015-11-25 5 categories: 6 - Science 7 tags: 8 - Psychology 9 --- 10 11 The shortening days and end of daylight saving time mean that I've been walking 12 a dog in the dark lately. I bought [a 13 headlamp](http://www.rei.com/product/875355/black-diamond-cosmo-headlamp) 14 because my neighborhood has no street lights and I wanted to keep a hand free 15 and find my way around. Since I'm not used to wearing one, I was surprised that 16 some textures had a "shimmering" appearance when illuminated by this lamp. 17 18 After a few walks I noticed that this was limited to certain materials, such as 19 the dog's fur and the abundant pine needles, all of which are made up of fine 20 strands. I haven’t found a conclusive explanation for the effect yet. The 21 shimmering goes away when I close either eye, so I assume that the effect is 22 not caused by optics (e.g., this isn’t due to diffraction), and is also 23 binocular. 24 25 We live in a 3D environment, and because of this a different image is captured 26 by each eye. Our brains are used to this, and in the phenomenon “stereopsis” 27 they take advantage of binocular disparity to make inferences about depth. When 28 different images are presented to each eye through contrived means, interesting 29 things can happen. Devices like the 30 [View-Master](https://en.wikipedia.org/wiki/View-Master) or [Oculus 31 Rift](https://www.oculus.com/) present different 2D images to each eye; when 32 the differences correspond to what’s seen when looking at a real scene, 33 stereopsis occurs and the brain constructs a 3D representation of a single 34 scene. 35 36  37 38 If the differences between the images are weird (in the sense that they aren’t 39 the result of depth) “[binocular 40 rivalry](https://en.wikipedia.org/wiki/Binocular_rivalry)” occurs. When 41 sufficiently different images are presented to each eye, our percept alternates 42 between what is shown to each. These perceptual fluctuations don’t occur at 43 once across the visual field; for large stimuli most observers report seeing 44 one image interspersed with unstable blobs of the other. In areas of different 45 luminance, the percept can appear to shimmer in effect called “binocular 46 luster”. Michael Scroggins has [a good blog 47 post](https://michaelscroggins.wordpress.com/explorations-in-stereoscopic-imaging/retinal-rivalry-and-luster/) 48 about rivalry that mentions luster and includes some demos. 49 50  51 52 When you wear a headlamp at night, the world is illuminated by a light source 53 that’s roughly between your eyes. Based on shadows and specular highlights, 54 your visual cortex determines that your left eye is seeing a world illuminated 55 by a light to its right, while your right eye is seeing a world consistent with 56 a light to its left. This is an uncommon arrangement, so it’s possible that the 57 visual system has enough trouble for rivalry to occur. Most of what you see 58 with a headlamp remains consistent with a familiar 3D world, so stereopsis 59 still happens and you know what you’re looking at. I suspect that the 60 unfamiliar arrangement of shadow and specularity can sometimes induce binocular 61 luster. If binocular luster is responsible for this effect, it’s a rare example 62 of rivalry occurring under (relatively) natural viewing conditions. 63 64 I did some searching and have yet to see another mention this phenomenon. Have 65 you experienced the “headlamp illusion”? I’m pretty sure that this is all in my 66 mind, but is mine the only one? 67