Illustration of<unk>  Axis of the principal meridian *Regular<unk>  – principal meridians are perpendicular. (The steepest and flattest meridians of the eye are called principal meridians.) *With-the-rule<unk>  – the vertical meridian is steepest (a rugby ball or American football lying on its side). *Against-the-rule<unk>  – the horizontal meridian is steepest (a rugby ball or American football standing on its end). *Oblique<unk>  – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees. *Irregular<unk>  – principal meridians are not perpendicular. In with-the-rule<unk> , the eye has too much "plus" cylinder in the horizontal axis relative to the vertical axis (i.e., the eye is too "steep" along the vertical meridian relative to the horizontal meridian). Vertical beams of light focus in front (anterior) to horizontal beams of light, in the eye. This problem may be corrected using spectacles which have a "minus" cylinder placed on this horizontal axis. The effect of this will be that when a vertical beam of light in the distance travels towards the eye, the "minus" cylinder (which is placed with its axis lying horizontally – meaning in line with the patient's horizontal meridian relative to the excessively steep vertical meridian) will cause this vertical beam of light to slightly "diverge", or "spread out vertically", before it reaches the eye. This compensates for the fact that the patient's eye converges light more powerfully in the vertical meridian than the horizontal meridian. Hopefully, after this, the eye will focus all light on the same location at the retina, and the patient's vision will be less blurred. In against-the-rule<unk> , a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis). Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. Both 0 and 180 degrees lie on a horizontal line at the level of the center of the pupil, and as seen by an observer, 0 lies on the right of both the eyes. Irregular<unk> , which is often associated with prior ocular surgery or trauma, is also a common naturally occurring condition. The two steep hemimeridians of the cornea, 180° apart in regular<unk> , may be separated by less than 180° in irregular<unk>  (called nonorthogonal irregular<unk> ; and/or the two steep hemimeridians may be asymmetrically steep—that is, one may be significantly steeper than the other (called asymmetric irregular<unk> . Irregular<unk>  is quantified by a vector calculation called topographic disparity. Focus of the principal meridian With accommodation relaxed: *Simple<unk>  *Simple hyperopic<unk>  – first focal line is on the retina, while the second is located behind the retina. *Simple myopic<unk>  – first focal line is in front of the retina, while the second is on the retina. *Compound<unk>  *Compound hyperopic<unk>  – both focal lines are located behind the retina. *Compound myopic<unk>  – both focal lines are located in front of the retina. *Mixed<unk>  – focal lines are on both sides of the retina (straddling the retina). Throughout the eye<unk> , whether it is regular or irregular, is caused by some combination of external (corneal surface) and internal (posterior corneal surface, human lens, fluids, retina, and eye-brain interface) optical properties. In some people, the external optics may have the greater influence, and in other people, the internal optics may predominate. Importantly, the axes and magnitudes of external and internal<unk>  do not necessarily coincide, but it is the combination of the two that by definition determines the overall optics of the eye. The overall optics of the eye are typically expressed by a person's refraction; the contribution of the external (anterior corneal)<unk>  is measured through the use of techniques such as keratometry and corneal topography. One method analyzes vectors for planning refractive surgery such that the surgery is apportioned optimally between both the refractive and topographic components.
Astigmatism
Paragrammatism
Apophlegmatism
Agrammatism
Social-emotional agnosia
Boanthropy
Integrative agnosia
Visuospatial dysgnosia