Entoptic phenomena (from Greek ἐντός "within" and ὀπτικός "visual") are visual effects whose source is within the eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.)

In Helmholtz's words; "Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called "entoptical"."

Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from optical illusions, which are perceptual effects that arise from interpretations of the image by the brain. Because entoptic images are caused by phenomena within the observer's own eye, they share one feature with optical illusions and hallucinations: the observer cannot share a direct and specific view of the phenomenon with others.

Helmholtz comments on phenomena which could be seen easily by some observers, but could not be seen at all by others. This variance is not surprising because the specific aspects of the eye that produce these images are unique to each individual. Because of the variation between individuals, and the inability for two observers to share a nearly identical stimulus, these phenomena are unlike most visual sensations. They are also unlike most optical illusions which are produced by viewing a common stimulus. Yet, there is enough commonality between the main entoptic phenomena that their physical origin is now well understood.

Distorted vision is a symptom with several different possible causes.

Distortion of vision refers to straight lines not appearing straight, but instead bent, crooked, or wavy. Usually this is caused by distortion of the retina itself. This distortion can herald a loss of vision in macular degeneration, so anyone with distorted vision should seek medical attention by an ophthalmologist promptly. Other conditions leading to swelling of the retina can cause this distortion, such as macular edema and central serous chorioretinopathy.

An Amsler grid can be supplied by an ophthalmologist so that the vision can be monitored for distortion in people who may be predisposed to this problem.

Tunnel vision implies that the peripheral vision, or side vision, is lost, while the central vision remains. Thus, the vision is like looking through a tunnel, or through a paper towel roll. Some disorders that can cause this include:

Glaucoma - severe glaucoma can result in loss of nearly all of the peripheral vision, with a small island of central vision remaining. Sometimes even this island of vision can be lost as well.

Retinitis pigmentosa - This is usually a hereditary disorder which can be part of numerous syndromes. It is more common in males. The peripheral retina develops pigmentary deposits, and the peripheral vision gradually becomes worse and worse. The central vision can be affected eventually as well. People with this problem may have trouble getting around in the dark. Cataract can be a complication as well. There is no known treatment for this disorder, and supplements of Vitamin A have not been proven to help.

Punctate Inner Choroidopathy - This condition is where vessels gro (( material is missing ))

Stroke - a stroke involving both sides of the visual part of the brain may wipe out nearly all of the peripheral vision. Fortunately, this is a very rare occurrence

Management of this condition is surgical and typically involves reducing the strength of the superior rectus muscle or anterior transposition of the inferior oblique muscle of the affected eyes.

Several different surgical procedures exist for the correction of DVD including: inferior oblique anteriorization, inferior oblique anteriorization plus resection, superior rectus recession, superior rectus recession plus posterior fixation suture, and inferior oblique myectomy, though there is insufficient evidence to determine which procedure results in the best outcomes for patients.

Closed-eye hallucinations and closed-eye visualizations (CEV) are a distinct class of hallucination. These types of hallucinations generally only occur when one's eyes are closed or when one is in a darkened room. They can be a form of phosphene. Some people report closed-eye hallucinations under the influence of psychedelics. These are reportedly of a different nature than the "open-eye" hallucinations of the same compounds.

There are five known levels of CEV perception which can be achieved either through chemical stimuli or through meditative relaxation techniques. Level 1 and 2 are very common and often happen every day. It is still normal to experience level 3, and even level 4, but only a small percentage of the population does this without psychedelic drugs, meditation or extensive visualization training.

There is no established treatment for visual snow. It is difficult to resolve visual snow with treatment, but it is possible to reduce symptoms and improve quality of life through treatment.

Medications that may be used include lamotrigine, acetazolamide, or verapamil. But these do not always result in benefits.

DVD is often mistaken for over-action of the inferior oblique extra-ocular muscles. DVD can be revealed on ocular movement testing when one eye is occluded by the nose on lateral gaze. This eye will then elevate, simulating an inferior oblique over action. However, in a unilateral case, overaction of the superior rectus muscle in the unaffected dominant eye, can also be a causing factor as well as causing a V pattern exophoria.

Coloboma of optic nerve, is a rare defect of the optic nerve that causes moderate to severe visual field defects.

Coloboma of the optic nerve is a congenital anomaly of the optic disc in which there is a defect of the inferior aspect of the optic nerve. The issue stems from incomplete closure of the embryonic fissure while in utero. A varying amount of glial tissue typically fills the defect, manifests as a white mass.

Although both optic nerve colobomas and morning glory disc anomaly (MGDA) involve mutations of the PAX6 gene, these two separate diseases represent two distinct causes. An optic nerve coloboma is easily differentiated from morning glory anomaly. Colobomas affect only the inferior aspect of the nerve as it represents an incomplete closure of the embryonic fissure, whereas MGDA encompasses all aspects of the nerve and represents more generally a dysgenesis of the mesoderm.

Some neuro-ophthalmologists believe that visual snow is not a medical condition, but a poorly understood symptom. People report seeing "snow", much like the visual noise on a TV screen after transmission ends. These authors hypothesize that what the patients see as "snow" is their own intrinsic visual noise.

Many report more visual snow in low light conditions. This has a natural explanation. "The intrinsic dark noise of primate cones is equivalent to ~4000 absorbed photons per second at mean light levels below this the cone signals are dominated by intrinsic noise".

In addition to visual snow, many of those affected have other types of visual disturbances such as starbursts, increased afterimages, floaters, trails, and many others.

Riddoch syndrome (also known as the "Riddoch phenomenon") is an ocular affectation often caused by lesions in the occipital lobe which limit the sufferer's ability to distinguish objects. Only moving objects in a blind field are visible, static ones being invisible to the patient. The moving objects are not perceived to have color or detail. The subject may only have awareness of the movement without visual perception of it (gnosanopsia), or the general shape of a moving object may be perceivable as a shadow like outline.

At least one patient was able to use a rocking chair—putting non-moving surroundings in relative motion to her head—to improve her motion perception. She eventually was able to do the same with just voluntary movement of her head.

Palinopsia (Greek: "palin" for "again" and "opsia" for "seeing") is the persistent recurrence of a visual image after the stimulus has been removed. Palinopsia is not a diagnosis, it is a diverse group of pathological visual symptoms with a wide variety of causes. Visual perseveration is synonymous with palinopsia.

In 2014, Gersztenkorn and Lee comprehensively reviewed all cases of palinopsia in the literature and subdivided it into two clinically relevant groups: illusory palinopsia and hallucinatory palinopsia. Hallucinatory palinopsia, usually due to seizures or posterior cortical lesions, describes afterimages that are formed, long-lasting, and high resolution. Illusory palinopsia, usually due to migraines, head trauma, prescription drugs, or hallucinogen persisting perception disorder (HPPD), describes afterimages that are affected by ambient light and motion and are unformed, indistinct, or low resolution.

Research needs to be performed on the efficacy of the various pharmaceuticals for treating illusory palinopsia. It is unclear if the symptoms' natural history and treatment are influenced by the cause. It is also not clear if there is treatment efficacy overlap for illusory palinopsia and the other co-existing diffuse persistent illusory phenomenon such as visual snow, oscillopsia, dysmetropsia, and halos.

Future advancements in fMRI could potentially further our understanding of hallucinatory palinopsia and visual memory. Increased accuracy in fMRI might also allow for the observation of subtle metabolic or perfusional changes in illusory palinopsia, without the use of ionizing radiation present in CT scans and radioactive isotopes. Studying the psychophysics of light and motion perception could advance our understanding of illusory palinopsia, and vice versa. For example, incorporating patients with visual trailing into motion perception studies could advance our understanding of the mechanisms of visual stability and motion suppression during eye movements (e.g. saccadic suppression).

Although treatment may be unnecessary, there may be social implications, especially in young children when venturing from a supportive home environment to a public environment (e.g., starting school). Continued support, including monitoring behavior and educating the child about his or her appearance as seen by others, is encouraged. Gradual or sudden withdrawal from interaction with others is a sign that may or may not be related to such behavior. Studies are being conducted to elucidate these implications.

Inverse Marcus Gunn phenomenon is a rare condition that causes the eyelid to fall upon opening of the mouth. In this case, trigeminal innervation to the pterygoid muscles of the jaw is associated with an inhibition of the branch of the oculomotor nerve to the levator palpebrae superioris, as opposed to stimulation in Marcus Gunn jaw-winking.

Aphantasia is similar to invisible disabilities such as face blindness, word blindness, and tone deafness, though aphantasia itself has not been associated with any functional deficits.

Gestaltzerfall (German for "shape decomposition") refers to a type of visual agnosia and is a psychological phenomenon where delays in recognition are observed when a complex shape is stared at for a while as the shape seems to decompose into its constituting parts. With regards to kanji, a study has shown that delays are most significant when the characters are of the same size. When characters to recognize are of different sizes, delays are observed only when they are of different patterns.

The phenomenon was first described and named by C. Faust in 1947 as a symptom of the bilateral region of the parieto-occipital sulcus after a through and through bullet wound of this region. Afterwards, when the subject stared at a truck for a while the truck seemed to decompose into its motor, chassis, driver cab and the person could only focus on one of these parts until he briefly closed his eyes or looked away which reset the shape to the complete truck again.

The characteristic of orthographic satiation as opposed to semantic satiation is that meaning remains intact. It was suggested that this is different from semantic satiation and from the stimulus familiarization effect because orthographic satiation occurs after the perceivers have access to lexical meaning.

Aphantasia is the suggested name for a condition where one does not possess a functioning mind's eye and cannot visualize imagery. The phenomenon was first described by Francis Galton in 1880, but has remained largely unstudied since. Interest in the phenomenon renewed after the publication of a study conducted by a team led by Prof. Adam Zeman of the University of Exeter, which also coined the term "aphantasia". Research on the subject is still scarce, but further studies are planned.

Given the unknown nature of MES, treatments have been largely dependent on an individual basis. Treatments can vary from being as little as self-reassurance to pharmaceutical medications.

Medications can be helpful, such as antipsychotics, benzodiazepines or antiepileptics, but there is very limited evidence for this. Some case studies have found that switching to a prednisolone steroid after a betamethasone steroid which caused MES helped alleviate hallucinations or the use of the acetylcholinesterase inhibitor, Donepezil, have also found that it successfully treated an individual's MES. However, because of the heterogeneous etiology, these methods cannot be applied as general treatment.

Other than treatment by medicinal means, individuals have also successfully alleviated musical hallucinations by cochlear implants, listening to different songs via an external source, or by attempting to block them through mental effort, depending on how severe their condition is.

As yet, there is no cure available for HPPD. A study presented by Dr. Henry Abraham, at the Annual Meeting of the Biological Psychiatry Society in 2012, showed that two drugs, tolcapone and levocarb that are primarily used in the treatment of Parkinson's disease improved the symptoms of HPPD in one third of the 20 test subjects who had participated in the trial. As tolcapone, and levocarb, are not approved for use in HPPD, the principal treatments that are available seek to reduce distress without treating the underlying cause. Primarily benzodiazepines including clonazepam,

diazepam and alprazolam are prescribed with a fair amount of success. The anticonvulsant drug levetiracetam has been reported to diminish some of the visual symptoms, as well as reduce depersonalization and derealization symptoms, that can occur along with HPPD. The efficacy of levetiracetam in treating HPPD has been documented in a prospective study. Another anticonvulsant, lamotrigine, has also been used to successfully treat HPPD.

Some medications have been contraindicated on the basis of their effects on HPPD or the concurrent mental issues. The atypical antipsychotic risperidone is reported to worsen symptoms of HPPD during the drug's duration in some people.

Those with HPPD are often advised to discontinue all drug use, many of which are thought to increase visuals in the short-term. There are also less concrete factors that may be generally detrimental to those with HPPD. For example, sleep deprivation and stress are thought to increase HPPD symptoms.

It must be emphasized that individuals without HPPD will sometimes notice visual abnormalities. These include floaters (material floating in the eye fluid that appears as black/dark objects floating in front of the eyes and are particularly visible when looking at the bright sky or on a white wall) and the white blood cells of the retinal blood vessels (seen as tiny, fast-moving and quickly disappearing white specks). Likewise, bright lights in an otherwise dark environment may generate trails and halos. Most people don't notice these effects, because they are so used to them. A person fearful of having acquired HPPD may be much more conscious about any visual disturbance, including those that are normal. In addition, visual problems can be caused by migraines, brain infections or lesions, epilepsy, and a number of mental disorders (e.g., delirium, dementia, schizophrenia, Parkinson's disease). For an individual to be diagnosed with HPPD, these other potential causes must be ruled out.

Tullio phenomenon, sound-induced vertigo, dizziness, nausea or eye movement (nystagmus) was first described in 1929 by the Italian biologist Prof. Pietro Tullio. (1881–1941) During his experiments on pigeons, Tullio discovered that by drilling tiny holes in the semicircular canals of his subjects, he could subsequently cause them balance problems when exposed to sound.

The cause is usually a fistula in the middle or inner ear, allowing abnormal sound-synchronized pressure changes in the balance organs. Such an opening may be caused by a barotrauma (e.g. incurred when diving or flying), or may be a side effect of fenestration surgery, syphilis or Lyme disease.

Patients with this disorder may also experience vertigo, imbalance and eye movement set off by changes in pressure, e.g. when nose-blowing, swallowing or when lifting heavy objects.

Tullio phenomenon is also one of the common symptoms of superior canal dehiscence syndrome (SCDS), first diagnosed in 1998 by Dr. Lloyd B. Minor, The Johns Hopkins University, Baltimore, United States.

Mees' lines or Aldrich–Mees' lines, also called leukonychia striata, are white lines of discoloration across the nails of the fingers and toes (leukonychia).