what is form sense
It is the ability to discriminate between the shapes of the objects. Cones play a major role in this faculty. Therefore, form sense is most acute at the fovea,where there are maximum number of cones and decreases very rapidly towards the periphery .Visual acuity recorded by Snellen's test chart is a measure of the form sense.
Components of visual acuity: In clinical practice, measurement of the threshold of discrimination of two spatially-separated targets (a function of the fovea centralis) is termed visual acuity. However, in theory, visual acuity is a highly complex function that consists of the following components :
Minimum visible. It is the ability to determine whether an object is present or not.
Resolution (ordinary visual acuity). Discrimination of two
spatially separated targets is termed resolution. The minimum separation between the two points, which can be discriminated as two, is known as
minimum resolvable. Measurement of the threshold of discrimination is essentially an assessment of the function of the fovea centralis and is termed ordinary
visual acuity. Histologically, the diameter of a cone in the foveal region is 0.004 mm and this, therefore, represents the smallest distance between two cones.
It is reported that in order to produce an image of minimum size of 0.004mm (resolving power of the eye) the object must subtend a visual angle of 1 minute at the nodal point of the eye. It is called the minimum angle of resolution (MAR). The clinical tests determining visual acuity measure the form sense or reading ability of the eye. Thus, broadly, resolution refers to the ability to identify the spatial characteristics of a test figure. The test targets in these tests may either consist of letters (Snellen’s chart) or broken circle (Landolt’s ring). More complex
targets include gratings and checker board patterns. Recognition. It is that faculty by virtue of which an individual not only discriminates the spatial characteristics of the test pattern but also identifies the patterns with which he has had some experience. Recognition is thus a task involving cognitive components in addition to spatial resolution. For recognition, the individual should be familiar with the set of test figures employed in addition to being able to resolve them. The most common example of recognition phenomenon is identification of faces.
The average adult can recognize thousands of faces. Thus, the form sense is not purely a retinal function, as, the perception of its composite form (e.g., letters) is largely psychological.
Minimum discriminable refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity. The best example of minimum discriminable is vernier acuity, which refers to the ability to determine whether or not two parallel and straight lines are aligned in the frontal plane.
It is the ability to discriminate between the shapes of the objects. Cones play a major role in this faculty. Therefore, form sense is most acute at the fovea,where there are maximum number of cones and decreases very rapidly towards the periphery .Visual acuity recorded by Snellen's test chart is a measure of the form sense.
Components of visual acuity: In clinical practice, measurement of the threshold of discrimination of two spatially-separated targets (a function of the fovea centralis) is termed visual acuity. However, in theory, visual acuity is a highly complex function that consists of the following components :
Minimum visible. It is the ability to determine whether an object is present or not.
Resolution (ordinary visual acuity). Discrimination of two
spatially separated targets is termed resolution. The minimum separation between the two points, which can be discriminated as two, is known as
minimum resolvable. Measurement of the threshold of discrimination is essentially an assessment of the function of the fovea centralis and is termed ordinary
visual acuity. Histologically, the diameter of a cone in the foveal region is 0.004 mm and this, therefore, represents the smallest distance between two cones.
It is reported that in order to produce an image of minimum size of 0.004mm (resolving power of the eye) the object must subtend a visual angle of 1 minute at the nodal point of the eye. It is called the minimum angle of resolution (MAR). The clinical tests determining visual acuity measure the form sense or reading ability of the eye. Thus, broadly, resolution refers to the ability to identify the spatial characteristics of a test figure. The test targets in these tests may either consist of letters (Snellen’s chart) or broken circle (Landolt’s ring). More complex
targets include gratings and checker board patterns. Recognition. It is that faculty by virtue of which an individual not only discriminates the spatial characteristics of the test pattern but also identifies the patterns with which he has had some experience. Recognition is thus a task involving cognitive components in addition to spatial resolution. For recognition, the individual should be familiar with the set of test figures employed in addition to being able to resolve them. The most common example of recognition phenomenon is identification of faces.
The average adult can recognize thousands of faces. Thus, the form sense is not purely a retinal function, as, the perception of its composite form (e.g., letters) is largely psychological.
Minimum discriminable refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity. The best example of minimum discriminable is vernier acuity, which refers to the ability to determine whether or not two parallel and straight lines are aligned in the frontal plane.
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