Eyesight differs from person to person due to several factors, including aging, genetics, and lifestyle choices. Human-led technology has aimed to remove the hurdles accompanying varying vision impairment levels for decades. From contact lenses to prescription sunglasses to LASIK (laser-assisted in situ keratomileusis) and IOLs (intraocular lens implants), there are many ways to fix poor vision and find prescription glasses online based on budget and severity……..Continue reading….
By: Deanna Ritchie
Source: Due
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Critics:
In humans and a number of other mammals, light enters the eye through the cornea and is focused by the lens onto the retina, a light-sensitive membrane at the back of the eye. The retina serves as a transducer for the conversion of light into neuronal signals. This transduction is achieved by specialized photoreceptive cells of the retina, also known as the rods and cones, which detect the photons of light and respond by producing neural impulses.
These signals are transmitted by the optic nerve, from the retina upstream to central ganglia in the brain. The lateral geniculate nucleus, which transmits the information to the visual cortex. Signals from the retina also travel directly from the retina to the superior colliculus. The lateral geniculate nucleus sends signals to the primary visual cortex, also called striate cortex. Extrastriate cortex, also called visual association cortex is a set of cortical structures, that receive information from striate cortex, as well as each other.
Recent descriptions of visual association cortex describe a division into two functional pathways, a ventral and a dorsal pathway. This conjecture is known as the two streams hypothesis. The human visual system is generally believed to be sensitive to visible light in the range of wavelengths between 370 and 730 nanometers of the electromagnetic spectrum. However, some research suggests that humans can perceive light in wavelengths down to 340 nanometers (UV-A), especially the young.
Under optimal conditions these limits of human perception can extend to 310 nm (UV) to 1100 nm (NIR). Hermann von Helmholtz is often credited with the first modern study of visual perception. Helmholtz examined the human eye and concluded that it was incapable of producing a high-quality image. Insufficient information seemed to make vision impossible. He, therefore, concluded that vision could only be the result of some form of “unconscious inference”, coining that term in 1867.
He proposed the brain was making assumptions and conclusions from incomplete data, based on previous experiences. Inference requires prior experience of the world.
Examples of well-known assumptions, based on visual experience, are:
- light comes from above;
- objects are normally not viewed from below;
- faces are seen (and recognized) upright;
- closer objects can block the view of more distant objects, but not vice versa; and
- figures (i.e., foreground objects) tend to have convex borders.
The study of visual illusions (cases when the inference process goes wrong) has yielded much insight into what sort of assumptions the visual system makes. Another type of unconscious inference hypothesis (based on probabilities) has recently been revived in so-called Bayesian studies of visual perception. Proponents of this approach consider that the visual system performs some form of Bayesian inference to derive a perception from sensory data.
However, it is not clear how proponents of this view derive, in principle, the relevant probabilities required by the Bayesian equation. Models based on this idea have been used to describe various visual perceptual functions, such as the perception of motion, the perception of depth, and figure-ground perception. The “wholly empirical theory of perception” is a related and newer approach that rationalizes visual perception without explicitly invoking Bayesian formalisms.
It can also be noted that there are different types of eye movements: fixational eye movements (microsaccades, ocular drift, and tremor), vergence movements, saccadic movements and pursuit movements. Fixations are comparably static points where the eye rests. However, the eye is never completely still, and gaze position will drift. These drifts are in turn corrected by microsaccades, very small fixational eye movements. Vergence movements involve the cooperation of both eyes to allow for an image to fall on the same area of both retinas.
This results in a single focused image. Saccadic movements is the type of eye movement that makes jumps from one position to another position and is used to rapidly scan a particular scene/image. Lastly, pursuit movement is smooth eye movement and is used to follow objects in motion.
There is considerable evidence that face and object recognition are accomplished by distinct systems. For example, prosopagnosic patients show deficits in face, but not object processing, while object agnosic patients (most notably, patient C.K.) show deficits in object processing with spared face processing. Behaviorally, it has been shown that faces, but not objects, are subject to inversion effects, leading to the claim that faces are “special”.
Further, face and object processing recruit distinct neural systems. Notably, some have argued that the apparent specialization of the human brain for face processing does not reflect true domain specificity, but rather a more general process of expert-level discrimination within a given class of stimulus, though this latter claim is the subject of substantial debate. Using fMRI and electrophysiology Doris Tsao and colleagues described brain regions and a mechanism for face recognition in macaque monkeys.
The inferotemporal cortex has a key role in the task of recognition and differentiation of different objects. A study by MIT shows that subset regions of the IT cortex are in charge of different objects. By selectively shutting off neural activity of many small areas of the cortex, the animal gets alternately unable to distinguish between certain particular pairments of objects. This shows that the IT cortex is divided into regions that respond to different and particular visual features. In a similar way, certain particular patches and regions of the cortex are more involved in face recognition than other object recognition.
Some studies tend to show that rather than the uniform global image, some particular features and regions of interest of the objects are key elements when the brain needs to recognise an object in an image.[30][31] In this way, the human vision is vulnerable to small particular changes to the image, such as disrupting the edges of the object, modifying texture or any small change in a crucial region of the image.
Studies of people whose sight has been restored after a long blindness reveal that they cannot necessarily recognize objects and faces (as opposed to color, motion, and simple geometric shapes). Some hypothesize that being blind during childhood prevents some part of the visual system necessary for these higher-level tasks from developing properly. The general belief that a critical period lasts until age 5 or 6 was challenged by a 2007 study that found that older patients could improve these abilities with years of exposure.
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