Scotoma UP206

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 mammalian eye has a scotoma in its field of vision, usually termed its blind spot. This is a location with no photoreceptor cells, where the retinal ganglion cell axons that compose the optic nerve exit the retina. This location is called the optic disc. There is no direct conscious awareness of visual scotomas. They are simply regions of reduced information within the visual field. Rather than recognizing an incomplete image, patients with scotomas report that things "disappear" on them.

The presence of the blind spot scotoma can be demonstrated subjectively by covering one eye, carefully holding fixation with the open eye, and placing an object (such as one's thumb) in the lateral and horizontal visual field, about 15 degrees from fixation (see the blind spot article). The size of the monocular scotoma is 5×7 degrees of visual angle.

Beyond its literal sense concerning the visual system, the term scotoma is also used metaphorically in several fields, including neurology, neuropsychology, psychology, philosophy, and politics. The common theme of all the figurative senses is of a gap not in visual function but in the mind's perception, cognition, or world view. Their concrete connection to the literal sense, however, is by the connection between the nervous system and the mind, via the chain of links from sensory input, to nerve conduction, to the brain, to perception (the processing and interpreting of that input) via the brain-mind correlation, to psychological function. Thus there is not only (or not necessarily) a visual inability to see an aspect of reality but also (or instead) a mental inability to conceive even the possibility of seeing that aspect, due to a cognitive schema that lacks any provision for it.