Science desk
< July 23	<< Jun \| July \| Aug >>	July 25 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

July 24[edit]

How many megapixels equivalent does our eyes have?[edit]

How can you measure the amount of megapixels of a human eye?--YX-1000A (talk) 02:04, 24 July 2015 (UTC)[reply]

I keep hearing the claim "about a megapixel" over the years, and after looking into it again, I still think that's the case. First I should get out of the way the issue of measuring the eyeball's resolution to begin with: the human eye is not like a camera. You didn't ask how the human eye is not like a camera, so I won't get into it, but there are ways to measure things like resolution. For instance, if looking at a grated material, how close do the grates have to be (while viewing from a given distance) to tell them apart. Using techniques like this and citing lots of sources that you may also be interested in, this website does a somewhat detailed derivation of the resolution of human vision. They come to a rather ludicrous value in the hundreds of megapixels, but they asking a rather particular question, "how detailed does an image need to be to recreate what you would see while standing in that scene." Since most of our photoreceptors are located in our fovea centralis, your eye is only focused on a very small region of your field of view at a given time. If you take the angular width of the fovea to be only 3 degrees, the result on that source falls down to 1 megapixel. That is, at any given moment, the image you are seeing in your mind could be represented by a 1 megapixel image. Someguy1221 (talk) 03:09, 24 July 2015 (UTC)[reply]

Modern research places the count of photoactive cells in the retina at over 100 million: Facts and Figures Concerning the Human Retina, published by NIH. The authors have made their entire book available at no cost: WebVision: Anatomy and Physiology or the Retina, from the University of Utah.

Are photoactive retinal cells "equivalent" to pixels? That depends on who you ask!

I work with image sensors as part of my day job (when I'm not too busy reading encyclopedias)... so I know a little bit about "pixels..." To be honest, I still don't know how to count pixels on digital cameras, let alone in biological systems with cognitive perception! Today, reputable sensor-manufacturers sell things called "effective pixels" - what are those!? (Here's how Exmor technology works, at least in cartoon form: there's an "intelligent pixel-by-pixel algorithm" baked right into the silicon! But hey, when I learned about them in school, I don't remember photodiodes running algorithms!) Here's another vendor whose latest and greatest toys will also resample the pixels: 2x2, 3x3... you can get as many pixels as you want from today's camera sensors! And here's another vendor who will reshape your square pixels into half as many rectangles! And all these are consumer toys! If only we could see what pixel weirdness they're surely cooking up in the confidential image sensor technology divisions of BAE or Lockheed - they don't put the details on their websites, because we don't need to see what they can see!

The real answer is, whether you're talking about human eyes or exotic cameras, you should probably stop counting pixels, and start looking at other metrics that meaningfully represent image information content - like the optical transfer function and the signal to noise ratio and the dynamic range.

Nimur (talk) 03:42, 24 July 2015 (UTC)[reply]

The vast majority of those 100+ million cells are rods, which don't contribute to daytime vision. There are only about 6 million cones, according to that page. It's also worth noting that there are only about 1 million axons in the optic nerve. -- BenRG (talk) 07:34, 24 July 2015 (UTC)[reply]

Do you have a reference for rods being inactive in daytime vision? I know that rods are what are responsible for night vision, and that they're almost entirely lacking from the fovea, where sharp-focused vision takes place, but I was unaware that rods didn't contribute to daytime peripheral vision. - By the way, the Fovea centralis article has a little table talking about pixel densities of displays in matching the cone density of the fovea (the whole "retina display" thing), which is another way of looking at the resolution of the eye. -- 160.129.138.186 (talk) 15:42, 25 July 2015 (UTC)[reply]

This contributes some interesting data to the discussion, based on resolving power.--Phil Holmes (talk) 11:42, 24 July 2015 (UTC)[reply]

The question is deeply problematic as phrased as the eye is not designed in any sense to function as a camera. It's doesn't capture an image in the sense that a camera does, and if it did, it wouldn't be very ideal to the manner in which the brain processes and assimilates visual phenomena. I'll get into the actual physiology of the eye tomorrow, and try to make some rough correlates between the biomechanics of the eye and the physics of a camera, but first one has to understand the complexities of what human vision actually is. Intuitively, we tend to assume that the eye is taking a recording that is being projected into the brain, but that's not really what is going on. Rather for visual cognition, the brain creates a narrative (that is disturbingly illusory in many ways) from rather limited data. Well, here, no one explains this kind of stuff better than Susan Blackmore, so... Snow ^{let's rap} 10:38, 25 July 2015 (UTC)[reply]

The references posted by nimur and holmes seem to be valid and address the context of the OP's inquiry. I am dubious of Snow's link... Susan Blackmore seems to be full of New age nonsense like "consciousness is an illusion". The link from Blackmore's video lecture is not scientific, the links posted already are scientific, and I'm border line sure that Blackmore is full of ill-informed, unsubstantiated, unfounded claptrap! Void burn (talk) 16:00, 25 July 2015 (UTC)[reply]

Uh, friend, you need to do your research (or at least watch the video which you are judging) before making such hyperbolic, unsubstaniated statements. Susan Blackmore is a leading scientific authority in these matters. There's nothing that is any sense "new age" about the notion that consciousness is a deeply-problematic concept that has troubled any and every researcher who has tried to quantify it in an empirical manner. This is a well-known issue in cognitive science, known very specifically as the "hard problem of consciousness" and the fact that a certain amount of philosophy inevitably creeps in when we try to tackle such a complex and frustratingly elusive concept of what consciousness really is does not change the scientific rigor with which researchers come at the question. And Blackmore doesn't make any strong or remotely controversial claims in that lecture, she just presents the confusing problems, inconsistencies and logical paradoxes that arise from trying to apply the classical intuitive and/or dualistic notions of a mind or soul, and nothing she says is the least bit new to cognitive science -- these are all conundrums that are decades (or in some cases thousands of years) old in the record of inquiry.

I strongly suspect that what happened here is that you watched a particularly vague thirty seconds of that video and then read our article on Blackmore, saw the word "parapsychology" and decided she was a kook. Quite the opposite in fact; here's the story on her checkered history as a hard-science researchers -- in her youth, Blackmore had an "out of body" experience and was so convinced that it was real that she spent years trying to test notions likes telepathy and astral projection, using hard scientific methods. However, having the outlook of a scientist, she was able to see the writing on the wall and eventually accepted that the evidence was not there for the phenomena she had set out to validate, and then changed course and started studying the mind, and in the decades since has become a respected and leading authority in the field of consciousness studies, even penning the definitive introductory text on the topic, that is now used widely in psychology courses. I admit our article on her does a terrible job of contextualizing the two phases of her activity as a researcher (I've been meaning to put together the sources to fix it for ages), but I think that kind of intellectual honesty should be applauded and her early forays into projection (which she did after-all approach with hard scientific methods, hence the lack of confirmation) should not be held against her. The irony of your comments dismissing her stances on consciousness (aside from the fact that they represent the central, non-controversial territory of this field of inquiry) is that the video in question comes from a talk she was giving to the Skeptics Society, an ideological organization devoted to deconstructing pseudoscientific notions (and her introduction makes clear that she was invited to speak in part because she is a stellar example of a scientist who started out studying dubious phenomena and then moved to topics of real significance).

But if you don't like to accept the existence of the hard problem and what modern research says about it from her lips, perhaps you will be more receptive to David Chalmers, Daniel Dennett, and Donald Hoffman. How about John Searle [1]? I'm just using youtube as a shorthand here so as not to swamp the thread with a tangential discussion. There's a stack of research on this topic taller than Everest converging with the issues Blackmore raises in that video (and again, I reiterate that she doesn't even make any novel strong claims therein, she just points out how questionable intuitive models of consciousness are, and that doesn't even begin to be controversial in the hard study of the mind). But if you feel that you can prove, beyond a shadow of a doubt, that consciousness is an absolute and quantifiable phenomena and can define it in a clear and scientific fashion, I have great news for you -- you're perhaps the greatest thinker in the entire history of philosophy, and you've revolutionized both cognitive science and our fundamental understanding of reality in ways relevant to every last field of human inquiry. But you really need to share that understating, because the rest of us are really struggling with this topic!

All of which is a massive distraction from the OP's question. I didn't link that original video to discuss the notion of consciousness in general; I linked it for (and a put a time parameter in the link to direct to) Blackmore's discussion of visual cognition and how the mind constructs a visual narrative based on the information it receives from the eye (including some mechanics of how the eye itself operates). That is all deeply and essentially relevant to answering the OP's question. You can't understand the eye's "resolution" until you understand the nature of the phenomena of visual cognition and how the brain processes that stimuli. If you have a specific scientific rejection of something she says about human visual cognition in that segment of the lecture which you think might be relevant to the OP's understanding of this topic, by all means, let's discuss it in more detail. But as I see it, you made an exceedingly vague and knee-jerk assessment of a broader topic that was not being addressed, without saying a single thing to address the specific mechanics of vision which we were discussing. I'll let the OP and others here watch the content I linked and judge the academic and empirical rigor of Blackmore's observations (and their relevance to that actual question we're discussing here) for themselves. But with regard to the red herring you've raised here and referenced as "new age nonsense" (specifically, the possibility that consciousness might not conform to our basic intuitions about its fundamental nature), I have to tell you that this is considered an open and inescapable possibility arising from the hardest science you can find in the study of the nature of the mind; this speculation is nowhere near new and hardly considered fringe thinking in consciousness studies. Point in fact, it the opposite view (consciousness is a definite and undeniable phenomena, the existence of which we can test for and verify experimentally) that is presently considered untenable.

But again, I specifically posted that video (and its time parameter) for the content on how the brain contextualizes visual stimuli -- and there's no subfield of perception studies that has more robust data than that of visual cognition, including for the specific phenomena Blackmore mentions (inattention blindness, the biomechanical limitations of saccades, the mapping of photoreceptors on the retina/fovea), none of which are controversial and all of which are necessary in answering the OP's question with any degree of depth and accuracy. Snow ^{let's rap} 19:50, 25 July 2015 (UTC)[reply]

I agree that counting the retinal cells doesn't get you a good answer. For example, our eyes continually vibrate at around 80Hz in Ocular tremor - which allows each cell to successively sample a small area around the point where it's nominally pointing - this produces an effective resolution that's considerably higher than you'd get by counting the cells. When you get tired, this motion decreases and your vision gets noticeably more blurry as a result. But your eyes don't transmit "pixels" to your brain anyway - they send information about higher level concepts like the orientation and motion of high-contrast edges. The number of rod/cone cells give some kind of a lower bound of a couple of megapixels...but if I take a photo of a page from a book with a two megapixel camera and examine the image it produces, it's very obvious that my eyesight is much better than that. SteveBaker (talk) 18:17, 25 July 2015 (UTC)[reply]

If you post your original vision research here, please tag it as "OR" or something so that people realize that it's not supported by the peer-reviewed literature.

Blurred vision is caused by poor focus, not by a lack of eye motion.

If the eye completely ceases to move, the result is complete loss of vision due to neural adaptation, not blurred vision.

Curcio et al say "Foveal cone spacing is commonly assumed to be the limiting factor of visual resolving power. Resolution of gratings consisting of alternating light and dark bars requires that at least one row of unstimulated cones lie between rows of stimulated cones", so as of 1990 vision researchers had not heard of the idea that resolving power is increased by ocular microtremors.

The resolution of the image on the retina is limited by diffraction and lens aberration, and the cone density at the center of the fovea roughly matches that limit. A higher sampling density would not increase resolving power by much, if at all. -- BenRG (talk) 01:57, 26 July 2015 (UTC)[reply]

Nuclear Fission[edit]

What is the equation describes the evolution of the energy (w.r.t time) in nuclear fission? 11:34, 24 July 2015 (UTC) — Preceding unsigned comment added by 5.29.9.245 (talk) 11:34, 24 July 2015‎

At it's most basic, it's an exponential decay, depending on the half-life of the material under question. That will do just fine if you're talking about a simple uncontrolled reaction (eg. in a radioisotope thermoelectric generator as used on spacecraft). More complex reactions (for example, a nuclear chain reaction, or a decay chain with several steps) have more complex equations (although still based on exponential decay). Our article on Nuclear reactor physics has equations showing how neutron production changes over time in a reactor - that's a good proxy to energy (since each fission event releases the same amount of energy), but the variables depend not just on the physical properties of the fuel, but also on the design of the reactor. Smurrayinchester 13:35, 24 July 2015 (UTC)[reply]

Thank you! :) 5.29.9.245 (talk) 14:05, 24 July 2015 (UTC)[reply]

Temperate zones in the Pleistocene[edit]

Roughly where were the boundaries of the permafrost regions during the Pleistocene epoch, and did any parts of the world stay warm all the way through? With snow at the equator, surely all the crocodiles would have frozen to death? 213.205.198.236 (talk) 16:49, 24 July 2015 (UTC)[reply]

There's some information in our Pleistocene article. The glaciers pushed as far as the 40th parallel, with the permafrost extending a few hundred kilometres in North America, and several hundred in Eurasia. Rojomoke (talk) 17:01, 24 July 2015 (UTC)[reply]

Here's a paper that dicusses this, with a map (Figure 1) that matches pretty well with Rojomoke's information above. Mikenorton (talk) 19:52, 24 July 2015 (UTC)[reply]

And another that looks at conditions in equatorial parts of Africa and South America, with estimates of about 5°C reduction in average temperatures compared to the present day. Mikenorton (talk) 20:01, 24 July 2015 (UTC)[reply]

5 C would be a larger than typical change for the tropics (though potentially accurate for some locations). Most of the tropics are more typically estimated as having a 2-3 C cooling [2]. We also have articles on Last Glacial Maximum and CLIMAP which may be interesting. To answer the original question, no there was no snow at the sea level equator. The all-time record low at a place like Guayaquil, Ecuador (a coastal equatorial location) is 15 C. [3] It would take much more extreme cooling than hypothesized for the recent sequence of glaciations before it would be cold enough to snow there. Dragons flight (talk) 13:20, 25 July 2015 (UTC)[reply]

One of the most pronounced differences in the tropics is the hypothesized "permanent El Niño" state during much of the Plasticene. But as far as I know the tropics stayed pretty warm overall, albeit cooler than present at least in some places. I'd be curious to know your source for "snow at the equator" (other than at high elevations). Short Brigade Harvester Boris (talk) 03:59, 25 July 2015 (UTC)[reply]

I think you're getting confused - the plasticene era was the time when Wallace and Gromit walked the Earth. Richerman (talk) 08:34, 25 July 2015 (UTC)[reply]

See plasticine to understand the above comment. StuRat (talk) 16:17, 25 July 2015 (UTC) [reply]