Similarly, the intermediate layers of the superior colliculus iSC , which are traditionally not considered part of the pupil pathways, inhibit the EWN; presumably, these inhibitory connections drive the rapid pupil dilation that accompanies the orienting response C. The effect of light is twofold. First, and most directly, light activates the constriction pathway, causing the pupil to constrict. But light also induces wakefulness, and activates the dilation pathway, via a connection through the suprachiasmatic nucleus SCN , which is part of the hypothalamus, and the dorsomedial hypothalamus DMH to the LC.
In other words, light drives pupil constriction through a direct pathway, and pupil dilation through an indirect pathway. The pupil light response PLR , also called the pupil light reflex, is the constriction of the pupil in response to brightness, and the dilation of the pupil in response to darkness.
The profile of a typical pupil light response. The x axis indicates time since stimulus onset. The y axis indicates pupil size as a proportion of pre-stimulus pupil size. Errors bars reflect the standard error. All data shown in this figure and others is available through the URL provided at the end of the article. The exact latency depends on many factors, such as stimulus intensity latencies decrease with stimulus intensity and age latencies increase with age Ellis, This unconstriction, when it occurs, is sometimes called pupil escape ; whether it occurs depends on the color of the light: blue light leads to sustained constriction, whereas red light leads to pupil escape.
This difference results from the different photoreceptors that are sensitive to blue and red light, as described below. Dilation due to light offset occurs much more slowly than constriction due to light onset. It can take many seconds for the pupil to fully recover, and recovery is faster for red than blue light, again because of the photoreceptors that are sensitive to red and blue light.
After a high-intensity blue light, the pupil remains slightly constricted for many minutes. The PLR is driven by all known types of photoreceptors: rods, cones, and intrinsically photosensitive retinal ganglion cells ipRGCs Kardon, ; Markwell et al.
Cones are sensitive to color, in the sense that there are three types of cones that are maximally responsive to different colors, and the relative activation of these different cone types allows us to distinguish between different colors.
Cone density is highest in the fovea, and compared to rods cones require intense light to become active; therefore, cones dominate central vision, and vision in medium-to-bright levels of light.
Rods are not sensitive to color, in the sense that all rods are maximally sensitive to the same shade of blueish green, and can therefore not distinguish between different colors.
Rods are mostly absent from the fovea, and compared to cones respond also to weak light; therefore, rods dominate peripheral vision in darkness. In addition to their role in the PLR as described below , ipRGCs project to the suprachiasmatic nucleus SCN of the hypothalamus sometimes called the biological clock to maintain the circadian day-night rhythm.
That is, we are not consciously aware of the effect that light has on synchronizing our circadian rhythm. However, some studies suggest that ipRGCs may also play a minor role in conscious visual perception Ecker et al.
Rods and cones drive the initial pupil constriction of the PLR 0. However, input from rods and cones desensitizes quickly; therefore, if the PLR was based only on rod and cone vision, the pupil would rapidly unconstrict even while the light was still on.
Simply put: because of rods and cones, the pupil rapidly constricts in response to sudden light increases; but because of ipRGCs, the pupil stays constricted throughout the day Gooley et al.
Historically, the PLR was considered a purely reflexive response to the amount of light that falls on the retina. However, recent studies—and also some recently rediscovered older studies—have shown that the PLR is not merely a reflex, but is affected by how visual input is selected i.
This different visual input to both eyes induced binocular rivalry: participants sometimes consciously perceived the horizontal line, and sometimes the vertical line, but rarely both. In addition, the researchers occasionally flashed a dim light into one of the eyes, and recorded whether this flash triggered a measurable pupil constriction or not.
Crucially, they found that a measurable pupil constriction was more likely to be triggered when the light was flashed into the eye that, at that moment, dominated visual awareness. Phrased differently, the PLR was strongest for light sources that were consciously perceived—a clear demonstration of how high-level cognition influences the PLR. Without moving their eyes, participants shifted their attention to the cued side, which could be either dark or bright.
Crucially, we found that pupils were smaller when participants covertly attended to the bright side of the display, compared to the dark side of the display. In other words, covertly attending to something that is bright or dark causes a PLR, just like although much more weakly than directly looking at something that is bright or dark. The FEF is a brain area in the prefrontal cortex PFC , and is involved in eye movements and covert shifts of attention: strong suprathreshold FEF stimulation triggers eye movements; weak subthreshold stimulation triggers covert shifts of attention.
Given the results discussed above, a natural question is whether this presaccadic shift of attention results in a preparatory pupil light response; that is, when you make an eye movement toward a lamp, does your pupil already begin to constrict before your eyes start to move? Crucially, we found that the pupil began to respond weakly to the brightness of the cued side already while the eyes were still in motion. And it also makes ecological sense, considering that humans make about three eye movements per second e.
Rayner, By reducing the effective latency of the PLR, preparation may allow the sluggish PLR to keep up with our eye movements.
Crucially, they found that images that contained a sun triggered stronger pupil constriction than images that did not contain a sun, even when eye position and objective luminance were controlled for. This effect disappeared when images were flipped vertically, presumably because vertically flipped images are difficult to interpret. From this finding, the authors concluded that the PLR reflects subjective interpretation: an image that we interpret as being very bright such as an image of the sun causes a strong pupil constriction, even when the objective luminance of the image is modest.
In the studies described above, pupil responses were always triggered by visual stimuli even though the strength of the pupil response was modulated by cognitive influences. To make sure that participants processed the meaning of the words, they were instructed to press spacebar whenever the word was an animal name.
Some of the non-animal words were associated with brightness e. And, plausibly, this mental image then causes the pupil to constrict. In contrast, the studies described in the current section address a different question: can we tell what someone is keeping in memory by looking at pupil size?
More specifically, can we use the PLR to track whether someone keeps something bright or something dark in working memory? We found that the pupil was smaller when participants memorized a location on a bright, compared to a dark, background.
This suggests that memorizing a location is similar to covertly attending to a location, in line with previous research e. In other words, the effect of spatial working memory on the PLR is presumably mediated by covert visual attention. Olmos-Solis et al. In their study, participants memorized a color, which defined the target stimulus during a subsequent visual-search task.
In the retention interval before the visual-search task, an irrelevant probe was briefly presented. Crucially, the color of this probe either matched, or did not match, the memorized color. This finding is again mediated by visual attention; more specifically, it is mediated by the fact that stimuli that match the contents of visual working memory tend to capture attention e. However—and somewhat to our own surprise—we found no evidence that, after the stimuli had been removed from the display, maintaining bright or dark stimuli in working memory affected pupil size.
This apparent discrepancy illustrates that we do not yet fully understand how high-level cognition in general, and mental imagery in particular, modulate the PLR. How does the PLR help visual perception? The benefit of a dilated pupil is clear: A large pupil lets a lot of light into the eye, thus increasing the amount of visual information that the brain receives, thus increasing visual sensitivity; therefore, a dilated pupil, compared to a constricted pupil, allows you to better detect faint stimuli.
Imagine a cat in the dark, looking for small moving stimuli mice ; this cat would probably benefit from large pupils that capture as much light as possible. One answer that may come to mind is that pupil constriction serves to protect the retina from damage due to overexposure.
And when you look directly at the sun which can damage the retina , retinal light flux is so intense that a constricted pupil offers little protection.
But if the PLR does not primarily serve to protect the retina from overexposure, then what is it good for? When you then go from brightness into darkness, rods and cones need to adapt.
Dark adaptation is a gradual process that, especially for rods, takes tens of minutes. During the period that you are in darkness while dark adaptation is not yet complete, vision is impaired.
Because the pupil can dilate much more rapidly than rods and cones can adapt, pupil dilation reduces the amount of dark adaptation that is necessary, thus compared to a static pupil improving vision during the first few minutes of dark adaptation.
Light adaptation is a very rapid process, and does not appear to benefit as much, or at all, from the PLR. A small lens, compared to a large lens, suffers less from optical distortions that reduce depth of field. In most laboratory settings, stimuli are presented on a computer display at a fixed distance, and depth of field is therefore of little importance. However, the real world is three dimensional and contains many relevant objects at many different distances; therefore, in real life, the increased depth of field offered by a constricted pupil may be very important.
Analogous to depth of field, this is because a constricted pupil, and thus a small lens, suffers less from optical distortions spherical and chromatic abberations that cause optical blur. When the pupil becomes too small, a different set of optical distortions diffractions emerge that impair, rather than improve, visual acuity; this may be why the minimum size of the human pupil is about 2 mm, just above the size at which these distortions come into play.
The benefits of pupil constriction on visual acuity and depth of field are often described as two separate phenomena, but they are really two sides of the same coin: when the pupil constricts, focus improves for all objects except those that are already in perfect focus.
For poorly focused objects, such as objects that are far beyond the point of fixation, the benefit is especially pronounced; this is why pupil constriction leads to a pronounced increase in depth of field.
For objects that are fixated and are therefore already in good focus, the benefit is more subtle; therefore, pupil constriction leads to only a subtle increase in visual acuity at fixation. Therefore, pupil constriction always improves visual acuity, although the effect can be very small.
The PLR thus likely helps vision by reducing dark adaption, and by optimizing the trade-off between visual acuity and depth of field which benefit from a small pupil and visual sensitivity which benefits from a large pupil, especially in darkness.
In humans, these effects seem to be small, and vision would likely not be strongly impaired if the pupil would not respond to light. Although this is difficult to prove, because there seems to be no naturally occurring condition in which pupil size is fixed while vision is otherwise healthy.
However, other animals have far larger ranges of pupil sizes than humans, and thus may benefit more from the PLR. For example, the area of the slit pupil of the Gecko can change by a factor of Denton, The pupil near response PNR , also called the pupil near reflex, is the constriction of the pupil in response to looking at a nearby object, and the dilation of the pupil in response looking at a far-away object.
The PNR is certainly the least studied, and perhaps the least understood of all pupil responses. The profile of a typical pupil near response. The x axis indicates time since the onset of the auditory cue to shift gaze. This shift of gaze was such that, except for vergence eye movements, eye position did not change. The profile of the resulting PNR looks as follows:. This time comprises both the time it takes to process the cue and to shift focus from far to near, and the latency of the PNR proper.
There is no notable pupil escape in the PNR. The PNR is part of three eye movements, the near triad, that usually but not necessarily, e. Which cortical areas are involved in the PNR is not entirely clear; however, there are projections from the frontal eye fields FEF and parietal cortex to the EWN that are involved in vergence movements Gamlin, Because of the strong association between vergence and the PNR, and the central role of the EWN in pupil constriction, it is possible that these projections also play a role in the PNR.
The results of these studies are interesting yet puzzling, and more research is certainly needed. Enright asked participants to look at two-dimensional drawings of three-dimensional boxes i. Participants viewed these drawings monocularly, with one eye covered. Participants fixated either on the corner of the box that appeared nearby, or on the corner that appeared far away. Even though the drawing was in a single plane, participants nevertheless made vergence movements as if they looked at the nearby or far-away corner of a real three-dimensional box.
But there were no corresponding pupil-size changes: the pupil was not smaller when participants looked at the nearby corner, indicating that vergence was not accompanied by a PNR.
However, the results were very different when participants viewed so-called Neckercubes. A Neckercube is an ambiguous box-like drawing, in which the same corner can be perceived as either nearby or far away.
Participants looked at this corner, and indicated whether they subjectively perceived it as the nearby or far-away corner. The results for vergence movements were more-or-less the same as for the regular box drawings although slightly weaker , but vergence was now accompanied by exceptionally large pupil responses: the pupil was smaller when the corner was subjectively nearby as compared to when it was subjectively far away.
However, while the direction of the effect was consistent with a PNR, the size of the effect was unrealistically large, casting doubt on whether it truly was a PNR, or rather an artifact of some unrecognized confound. Participants either covertly attended to the nearby or far-away display while keeping gaze on the central display , or, in a different condition, made an eye movement to the nearby or far-away display.
In the covert-attention condition, we found that pupil size did not change as a function of whether participants covertly attended to the nearby or far-away display; this suggests that, unlike the PLR e. Binda et al. In the eye-movement condition, we found that the pupil responded to the distance of the to-be-looked-at display, and, importantly, did so with an extremely low latency; this is reminiscent of the effect of eye-movement preparation on the PLR Ebitz et al.
However, these results should be interpreted with caution, because distance is related to other factors that might also affect pupil size, such as brightness and size i. In conclusion, it is unclear whether the PNR is affected by cognitive influences. The main function of the PNR is likely to increase depth of field for near vision. The reason that a large depth of field is especially useful for near vision, is that depth of field is much smaller for nearby than far-away objects; that is, you can simultaneously and sharply see two objects at ten and eleven meters distance, but you cannot simultaneously and sharply see two objects at half and one-and-a-half meters distance.
The pupil dilates after an arousing stimulus, thought, or emotion. Here I call this the psychosensory pupil response PPR , because this term aptly indicates that this response is driven by both sensory and psychological stimuli.
But the same response is sometimes also referred to as reflex dilation, arousal-related dilation , or effort-related dilation. The profile of a typical psychosensory response to sound. This figure shows data of myself while hearing a 1 s burst of auditory white noise. The y axis indicates pupil size as a proportion of pre-stimulus pupil size, and is intentionally kept identical to the other figures to illustrate the size of the effect.
The profile of a typical psychosensory response during working-memory maintenance. The x axis indicates time since the onset of the retention interval. Attempts to link specific cognitive processes to the PPR not mediated by processing load are therefore, in my view, doomed to fail. Sudden events sounds, movements, painful touch, etc. It is small compared to the large changes in pupil size induced by the pupil light response PLR and the pupil near response PNR —or at least it is when triggered by the relatively mild stimuli that are generally used in psychological studies.
In humans, the pupil orienting response is characterized by a fast pupil dilation that peaks between 0. This type of PPR is endogenous i. They observed that pupil size reflected the difficulty of the calculation: the harder the calculation, the larger the pupil.
Many different terms have been used, but the general finding is clear: whatever activates the mind causes the pupil to dilate. These studies show that pupil size reflects arousal. Importantly, whether arousal is triggered by something negative or positive makes little or no difference e. Pupil size also correlates with other physiological measures of arousal, such as skin conductance Bradley et al.
In terms of pupil size, the effects of arousal and mental effort appear to be similar: both activate the mind, and both cause the pupils to dilate.
Since the seminal studies by Hess and Polt , ; Hess et al. But pupillometry has since become a standard tool to measure cognitive processing in a wide variety of situations. Head injuries can cause severe and even life-threatening complications, even if the effects are not immediately apparent. Anyone who has suffered a head injury should seek immediate medical attention. Research suggests that accidental exposure to pesticides organophosphates can cause pinpoint pupils in some cases.
The person may also show signs of:. Anyone who thinks they or someone else has been poisoned by a pesticide, household cleaner, or other toxic product requires immediate medical attention.
If a person is unconscious or vomiting, it is important to roll them onto their side if possible and keep their head tilted slightly downward. This position will ensure any vomit can escape without the person choking on it. Pinpoint pupils are not a disease on their own, but they can indicate an underlying medical problem. Anyone experiencing pinpoint pupils with no apparent cause should see a doctor as soon as possible.
Many of the causes of pinpoint pupils are serious medical conditions, such as opioid dependency or pesticide poisoning. Early intervention can help prevent life-threatening complications. In this article, we examine the symptoms of poisoning from organophosphate, a form of insecticide. We also look at the risks and treatment options.
Cognitive behavioral therapy CBT is a short-term talking therapy where a professional counselor or therapist works with an individual to help them….
Hypertension or high blood pressure can lead to heart disease, stroke, and death and is a major global health concern. A range of risk factors may…. In this article, learn about some common causes of red eyes, including conjunctivitis, corneal ulcers, dry eye syndrome, and subconjunctival…. A brain hemorrhage is bleeding in the brain. Both pupils may be the same size, but some people have pupils of different sizes anisocoria. This condition may be perfectly normal.
In some cases, however, anisocoria can be a sign of a serious problem, such as a viral infection, an issue with the nervous system or an eye injury or condition. Pupil size changes naturally with age — this is totally normal. In newborns, for instance, pupil size is much smaller, though size increases as a child grows, becoming largest in adolescence. During the teen years and throughout adult life, pupils shrink. In older adults, pupils are smaller when at rest in the dark and slower to dilate in response to light.
One study measured pupil size at different light levels and across different age groups. The results show that, in the right eye, average pupil size ranges from 2. A variety of conditions and health issues ranging from eye injury to drug use can result in an abnormally constricted pupil.
Small pupils may mean that you have an underlying condition. Some of the conditions that can cause constricted pupils include:.
Cluster headaches — Cluster headaches are a rare type of neurovascular head pain. Eye-related symptoms may include miosis, eyelid swelling and conjunctivitis pink eye on the same side as the pain.
Horner's syndrome — Symptoms of Horner's syndrome typically affect one side of the face, and include a constricted pupil, a drooping eyelid and an inability to sweat. That allows more light in, which improves night vision. Pupil constriction and dilation are involuntary reflexes.
Other than lighting, pupils can change size in reaction to other stimuli. Some drugs can cause your pupils to get bigger, while others make them get smaller. In adults, pupils normally measure between 2 and 4 millimeters in bright light. In the dark, they usually measure between 4 and 8 millimeters.
One of the most likely reasons someone might have pinpoint pupils is the use of narcotic pain medications and other drugs in the opioid family, such as:. Pinpoint pupils are a symptom, not a disease. Symptoms will depend on how much of the drug you take and how often you take it. In the longer term, opioid use can reduce lung function.
Signs that you might be addicted to opioids include:. Intracerebral hemorrhage may cause severe headache, nausea, and vomiting, and may be followed by loss of consciousness. If your pinpoint pupils are due to Horner syndrome, you might also have a drooping eyelid and decreased sweating on one side of your face.
Babies with Horner syndrome might have one iris that is lighter in color than the other. Additional symptoms of anterior uveitis include redness, inflammation, blurred vision, and light sensitivity. However, it can be a symptom of one. The diagnosis will guide your treatment options.
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