Secondary Direction: Function, Tasks, Role & Diseases

Secondary directions are always oriented to a main direction (fixation). They differ from each other by respectively different spatial values and are significant for the emergence of the spatial sense. A rearrangement of the secondary directions always causes a change in the perception in space.

What is the secondary direction?

A secondary sense of direction is defined as a subjective sense of direction that deviates from the main sense of direction. A secondary direction of sight is defined as a subjective direction of sight that deviates from the main direction of sight. It forms a line between an object and a retinal location. It passes the approximate optical center of the eye, which all light rays cross. There are many secondary directions but only one principal direction. The image of a fixed object falls on the retinal center, the fovea centralis (also called foveola). This is the place of sharpest vision, because the resolving power is best here due to the high cone density. What is imaged on the fovea centralis subjectively conveys the feeling of direct viewing and forms the spatial value straight ahead. This is the main direction of perception. The perception of all other objects in the visual field is spatially relative to this main direction of perception. Extrafoveolar stimuli are set, which are perceived as secondary directions. The image of an object then occurs on a retinal location other than the fovea centralis. Visual acuity is noticeably lower at all these other locations. As a result, an object in the minor direction is seen out of focus and its spatial value is not straight on.

Function and task

The function of a collateral direction is to form spatial values by relating imaged objects to each other on the retina. Spatial values, in turn, determine the direction in which an object is perceived. Everything imaged on the foveola is perceived as straight ahead. Retinal locations to the right of the foveola have the spatial value to the left. Objects that irritate these locations are thus perceived as lying to the left. Retinal locations left / above / below the foveola have the spatial value right / below / above. Accordingly, objects that irritate these locations are perceived as lying to the right / below / above. The fact that the retina receives areal optical stimuli, and that these stimuli can be placed in spatial relation to each other, enables the emergence of spatial sense. The totality of all objects perceived in the visual field is assigned to what is directly looked at and thus to the main direction. This is called relative localization. It is independent of the direction of gaze. Relative localization, in turn, is the prerequisite for egocentric localization. With the help of this localization it is possible to assign where in the external space the object looked at is located in relation to the orientation of our body. The perception of secondary directions and their relation to the main direction is therefore important for the sense of space and for finding one’s way in space. The order of the external world or the physical space is reflected in the subjective visual space by the relative localization of the secondary directions. Foveolar fixation is the basic requirement for this normal order in space. For this to occur, anatomic and functional structures of the retina must be intact, physiologic development and maintenance of the principal direction of rotation with the foveola must be ensured, and the fovea centralis must be secured as the motor zero point of the eye.

Diseases and disorders

If foveolar fixation, as a basic requirement for the expression of spatial sense, is not present, disruption of orientation in space occurs. This is the case with pathologic changes in the retinal center. Macular diseases can cause an organic central scotoma, whereby fixation is only possible with a retinal site other than the foveola. Likewise, in the presence of a functional central scotoma underlying strabismus (strabismus), fixation is no longer possible with the site of sharpest vision. In order to see the object of interest at all then, it must be imaged at the scotomal rim. If the main direction of vision is further bound to the foveola, and the spatial values of the other retinal points remain oriented to it, it is no longer possible for the affected person to look at something directly, because the line of sight from the object to the center of the retina is disturbed. Subjectively, however, only this visual axis has the spatial value straight ahead.If this spatial value fails organically or functionally, this object is perceived only with a secondary direction. But the subjective sensation of looking past is connected with it. To be able to look at something at all, one has to look past it. This is then an eccentric attitude. This correlates with a noticeable decrease in visual acuity, since the resolving power decreases significantly away from the center of the retina. Thus, vision is blurred and egocentric localization is also disturbed. It therefore becomes difficult to judge where the perceived object is located in relation to one’s own body. In addition to eccentric fixation, there is also the case of eccentric fixation, in which the image of a viewed object also no longer falls on the foveola, but on an eccentric retinal point. This can occur in early childhood strabismus. The major direction of sight is then transferred to this retinal point and the relative localization is organized around the new major direction of sight. The secondary directions are oriented to it and are again related to it. This reorganization is again accompanied by a marked decrease in visual acuity and, in most cases, the entire visual field is no longer uniformly acquired.