Preferential hyperacuity perimetry (PHP) is a psychophysical test used to identify and quantify visual abnormalities such as

metamorphopsia Metamorphopsia (from , ) is a type of distorted vision in which a grid of straight lines appears wavy and parts of the grid may appear blank. People can first notice they suffer with the condition when looking at mini-blinds in their home. For examp ...

and

scotoma A scotoma is an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is surrounded by a field of normal – or relatively well-preserved – vision. Every normal mam ...

. It is a type of

perimetry A visual field test is an eye examination that can detect dysfunction in central and peripheral vision which may be caused by various medical conditions such as glaucoma, stroke, pituitary disease, brain tumours or other neurological deficits. ...

Background

Vision abnormalities such as metamorphopsia (distortions) and

are symptoms of retinal diseases such as

macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred or no vision in the center of the visual field. Early on there are often no symptoms. Over time, however, som ...

. In advanced stages of the disease,

photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...

s may be irreversibly damaged. Hence, if not treated, macular degeneration may lead to blindness. Awareness to early changes in vision, especially in high risk patients, leads to early treatment (such as intravitreal injection of anti-VEGF factors, e.g.

bevacizumab Bevacizumab, sold under the brand name Avastin among others, is a medication used to treat a number of types of cancers and a specific eye disease. For cancer, it is given by slow injection into a vein (intravenous) and used for colon cancer, l ...

ranibizumab Ranibizumab, sold under the brand name Lucentis among others, is a monoclonal antibody fragment ( Fab) created from the same parent mouse antibody as bevacizumab. It is an anti-angiogenic that is approved to treat the "wet" type of age-related m ...

) and prevents loss of vision. Because of complex brain mechanisms such as

filling-in In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in norma ...

, patients with small and peripheral defects in their vision are often unaware of such changes until late stages in the disease. Another problem is that minute visual aberrations can be normal and therefore should be distinguished from genuine visual abnormalities. Preferential hyperacuity perimetry (PHP) is a technology that bypasses filling-in and quantifies the extent of visual abnormalities.

Technology

PHP takes advantage of visual hyperacuity, also known as

vernier acuity Vernier acuity (from the term "vernier scale", named after astronomer Pierre Vernier) is a type of visual acuity – more precisely of hyperacuity – that measures the ability to discern a disalignment among two line segments or gratings. A subje ...

—the ability to identify the misalignment of visual objects. Visual hyperacuity is at least 10 times more sensitive than

visual acuity Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...

, the ability to separate between distinct objects. Therefore, in retinal diseases such as macular degeneration responses to hyperacuity stimuli may be abnormal long before any changes in visual acuity are observed. Another key element in PHP technology relies on the competition principle. According to this principle, visual attention is ''preferably'' attracted to the larger object. When two stimuli, which are different in size, are simultaneously and briefly displayed, the observer is more likely to perceive the larger of the two, while missing the other.

PHP test

In a PHP test, the

macula The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal avas ...

(central area of the retina) is scanned with a succession of stimuli, each stimulus consisting of a series of dots arranged along a vertical or horizontal axis. In each stimulus, a small number of dots are misaligned, thereby creating an artificial distortion (bump or wave). The examinee's task is to perceive these artificial distortions and mark their locations on the visual field. When a stimulus is projected on a healthy portion of the retina, the examinee identifies the artificial distortion and is likely to mark a correct location. If the stimulus is projected on a damaged region of the retina, a pathological distortion may be perceived instead of the artificial distortion, especially if the pathological distortion is more prominent than the artificial distortion. The examinee may then mark a location that is distant from the artificial distortion, indicating that a pathological distortion may have been perceived. By manipulating the amplitude of artificial distortions, the amplitude of the pathology in the area of interest can be quantified. At the end of test, comparison of the set of erroneous responses against a normative data base is used to determine if test results are within normal limits.

References

{{DEFAULTSORT:Preferential Hyperacuity Perimetry Medical tests Ophthalmology Optometry