question: why is it easier to see something in the dark when you’re not looking directly at it?
comment: Because the light sensitive cells in your eye are just slightly out of center. You have more color focused cells there instead.
passages: Averted vision works because there are virtually no rods (cells which detect dim light in black and white) in the fovea: a small area in the center of the eye. The fovea contains primarily cone cells, which serve as bright light and color detectors and are not as useful during the night. This situation results in a decrease in visual sensitivity in central vision at night. Based on the early work of Osterberg (1935), and later confirmed by modern adaptive optics, the density of the rod cells usually reaches a maximum around 20 degrees off the center of vision. 
   Section::::Ultimate Causation Hypotheses.:Long-distance foliage hypothesis.
  
  This hypothesis suggests that trichromacy has evolved to adapt to distinguishing objects from the background foliage in long distance viewing. This hypothesis is based upon the fact that there is a larger variety of background S/(L+M) and luminance values under long-distance viewing.
  
  Section::::Ultimate Causation Hypotheses.:Short-distance foliage hypothesis.
   Since the fovea does not have rods, it is not sensitive to dim lighting. Hence, in order to observe dim stars, astronomers use averted vision, looking out of the side of their eyes where the density of rods is greater, and hence dim objects are more easily visible.
   D. H. Foster (2011) states, "in the natural environment, the source itself may not be well defined in that the illumination at a particular point in a scene is usually a complex mixture of direct and indirect [light] distributed over a range of incident angles, in turn modified by local occlusion and mutual reflection, all of which may vary with time and position." The wide spectrum of possible illuminances in the natural environment and the limited ability of the human eye to perceive color means that color constancy plays a functional role in daily perception. Color constancy allows for humans to interact with the world in a consistent or veridical manner and it allows for one to more effectively make judgements on the time of day.
   Information for pre-attentive processing is detected through the five senses. In the visual system, the receptive fields at the back of the eye (retina) transfer the image via axons to the thalamus, specifically the lateral geniculate nuclei. The image then travels to the primary visual cortex and continues on to be processed by the visual association cortex. At each stage, the image is processed with increasing complexity. Pre-attentive processing starts with the retinal image; this image is magnified as it moves from retina to the cortex of the brain. Shades of light and dark are processed in the lateral geniculate nuclei of the thalamus. Simple and complex cells in the brain process boundary and surface information by deciphering the image's contrast, orientation, and edges. When the image hits the fovea, it is highly magnified, facilitating object recognition. The images in the periphery are less clear but help to create a complete image used for scene perception.
   Central vision is relatively weak in the dark (scotopic vision) since cone cells lack sensitivity at low light levels. Rod cells, which are concentrated further away from the fovea, operate better than cone cells in low light. This makes peripheral vision useful for detecting faint light sources at night (like faint stars). Because of this, pilots are taught to use peripheral vision to scan for aircraft at night. 
   Section::::Perceptions of light and shadows.
   The pupillary response to light is not purely reflexive, but is modulated by cognitive factors, such as attention, awareness, and the way visual input is interpreted. For example, if a bright stimulus is presented to one eye, and a dark stimulus to the other eye, perception alternates between the two eyes (i.e., binocular rivalry): Sometimes the dark stimulus is perceived, sometimes the bright stimulus, but never both at the same time. Using this technique, it has been shown the pupil is smaller when a bright stimulus dominates awareness, relative to when a dark stimulus dominates awareness. This shows that the pupillary light reflex is modulated by visual awareness. Similarly, it has been shown that the pupil constricts when you covertly (i.e., without looking at) pay attention to a bright stimulus, compared to a dark stimulus, even when visual input is identical. Moreover, the magnitude of the pupillary light reflex following a distracting probe is strongly correlated with the extent to which the probe captures visual attention and interferes with task performance. This shows that the pupillary light reflex is modulated by visual attention and trial-by-trial variation in visual attention. Finally, a picture that is subjectively perceived as bright (e.g. a picture of the sun), elicits a stronger pupillary constriction than an image that is perceived as less bright (e.g. a picture of an indoor scene), even when the objective brightness of both images is equal. This shows that the pupillary light reflex is modulated by subjective (as opposed to objective) brightness.
   The parafoveal magnification paradigm compensates for how visual acuity drops off as a function of retinal eccentricity. On each fixation and in real time, parafoveal text is magnified to equalize its perceptual impact with that of concurrent foveal text.
   Section::::Debate over role of attention.:More recent evidence reasserting the need for focused attention.
   Section::::In marine animals.:Mechanism.:Matching light intensity and wavelength.
  
  At night, nocturnal organisms match both the wavelength and the light intensity of their bioluminescence to that of the down-welling moonlight and direct it downward as they swim, to help them remain unnoticed by any observers below.
   Section::::Function.
  
  The magnocellular pathway cannot provide finely detailed or colored information, but still provides useful static, depth, and motion information. The M pathway has high light/dark contrast detection, and is more sensitive at low spatial frequencies than high spatial frequencies. Due to this contrast information, M cells are essential for detecting changes in luminance, and performing visual search tasks and detecting edges.
   As with a photographic camera, it is easier to make the eye focus on an object when there is intense ambient light. With intense lighting, the eye can constrict the pupil, yet allow enough light to reach the retina. The more the eye resembles a pinhole camera, the less it depends on focusing through the lens. In other words, the degree of decoupling between focusing and convergence needed to visualize an autostereogram is reduced. This places less strain on the brain. Therefore, it may be easier for first-time autostereogram viewers to "see" their first 3D images if they attempt this feat with bright lighting.
   Cones are responsible for colour vision. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, they connect through adjoining cells within the retina to send an electrical signal to the optic nerve fibres. The optic nerves send off impulses through these fibres to the brain.
   In 1933, Stiles and Crawford discovered that the light sensitivity of the fovea differs significantly for rays entering the eye through the center of the pupil versus rays entering from its peripheral regions. They observed that the usual "intensity multiplied by aperture" rule did not apply in foveal vision and that rays entering the eye via peripheral regions of the pupil were less efficient by roughly a factor of five. This effect is now known as the Stiles-Crawford effect and also has implications for the reverse chromostereoptic effect.
  
  Section::::Theory.
   Both of the Stiles–Crawford effects of the first and second kind are highly wavelength-dependent, and they are most evident under photopic conditions. There are several factors that contribute to the Stiles–Crawford effect, though it is generally accepted that it is primarily a result of the guiding properties of light of the cone photoreceptors. The reduced sensitivity to light passing near the edge of the pupil enhances human vision by reducing the sensitivity of the visual stimulus to light that exhibits significant optical aberrations and diffraction.
  
  Section::::Discovery.
   Each of the three types of cones in the retina of the eye contains a different type of photosensitive pigment, which is composed of a transmembrane protein called opsin and a light-sensitive molecule called 11-cis retinal. Each different pigment is especially sensitive to a certain wavelength of light (that is, the pigment is most likely to produce a cellular response when it is hit by a photon with the specific wavelength to which that pigment is most sensitive). The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) wavelengths respectively.
   Teller's initial research studies were focused on the spatial and temporal properties of a phenomenon first described by Professor Gerald Westheimer and sometimes referred to as the “Westheimer effect” or the “Westheimer function.” In this phenomenon, which Teller termed “spatial sensitization,” the detectability of a test light is first decreased and then increased by adding light to successively larger surrounding or annular regions. Westheimer proposed that the increase in detectability with surround illumination might represent the manifestation of lateral inhibition within the retina, and this possibility motivated Teller's interest in the phenomenon.
  
  Section::::Professional career.:Research interests.:Visual development.
   The cones respond to bright light and mediate high-resolution colour vision during daylight illumination (also called photopic vision). The rod responses are saturated at daylight levels and don't contribute to pattern vision. However, rods do respond to dim light and mediate lower-resolution, monochromatic vision under very low levels of illumination (called scotopic vision). The illumination in most office settings falls between these two levels and is called mesopic vision. At mesopic light levels, both the rods and cones are actively contributing pattern information. What contribution the rod information makes to pattern vision under these circumstances is unclear.
   Section::::Physiology.
  
  Form perception is a demanding task for the brain because a retina has a significant blind spot and retinal veins that obstruct light from reaching cells that detect light, or photoreceptor cells. The brain handles the blind spots through boundary processes, includes perceptual grouping, boundary completion, and figure-ground separation, and through surface processing, including compensation for variable illumination (“discounting the illuminant”), and filling blank areas with the surviving illuminant-discounted signals.
   Recent research has attempted to extend the basis for the traditional chromostereoptic theory, including work done by Stiles and Crawford. In 1933, Stiles and Crawford accidentally discovered that the light sensitivity differed for rays entering through center versus those entering from peripheral regions of the eye. The efficiency of the rays is less when the rays enter via the peripheral region because the shape of the cone cells that collect the incident quanta are different from cone receptors in the center of the eye. This effect can cause both positive and negative chromostereopsis depending on the position of the pupil. If the pupil is centered on optical axis, it causes positive chromostereopsis. However, if the pupil is significantly off-center from the optical axis, negative chromostereopsis will ensue. Because most people have a point of maximum luminous efficiency that is off-center, the Stiles-Crawford Effects generally will have antagonistic chromostereoptic effects. Therefore, instead of seeing red in front of blue, blue will be seen in front of red and the effect will be reversed. The Stiles-Crawford effect also explains why positive chromostereopsis is decreased when illumination is lowered. At lower illumination, the dilation of pupil increases the pupillary peripheral region and therefore increases the magnitude of the Stiles-Crawford effect.
   Sensitivity to light is modulated by changes in intracellular calcium ions and cyclic guanosine monophosphate.
  
  The sensitivity of the rod pathway improves considerably within 5–10 minutes in the dark. Color testing has been used to determine the time at which rod mechanism takes over; when the rod mechanism takes over colored spots appear colorless as only cone pathways encode color.
  
  Four factors affect dark adaptation:
   The retinal must diffuse from the vision cell, out of the eye, and circulate via the blood to the liver where it is regenerated. In bright light conditions, most of the retinal is not in the photoreceptors, but is outside of the eye. It takes about 45 minutes of dark for "all" of the photoreceptor proteins to be recharged with active retinal, but most of the night vision adaptation occurs within the first five minutes in the dark. Adaptation results in maximum sensitivity to light. In dark conditions only the rod cells have enough sensitivity to respond and to trigger vision.
   The "tapetum lucidum" functions as a retroreflector which reflects light directly back along the light path. This serves to match the original and reflected light, thus maintaining the sharpness and contrast of the image on the retina. The "tapetum lucidum" reflects with constructive interference, thus increasing the quantity of light passing through the retina. In the cat, the "tapetum lucidum" increases the sensitivity of vision by 44%, allowing the cat to see light that is imperceptible to human eyes.
  
  It has been speculated that some flashlight fish may use eyeshine both to detect and to communicate with other flashlight fish.
   Rods, which are extremely abundant (about 120 million), are in the periphery of the human retina. Rods respond only to faint levels of light and are very light sensitive, therefore, completely useless in daylight. Cones have three types of pigments with different color sensitivities, whereas rods only have one and so are achromatic (colorless). Because of the distribution of rods and cones in the human eye, people have good color vision near the fovea (where cones are) but not in the periphery (where the rods are).
  