Peripheral vision

In the central visibility field objects can be identified, while the peripheral visibility field is used to perceive objects and their localization. If the human eye takes a stimulus in the periphery, it is brought to the Fovea Centralis for a clear identifica-tion [50]. The Fovea Centralis can only occupy 1.5^◦ around the visual axis which

2.2. DETECTION OF VISUAL TARGETS 35 corresponds to a circle with a diameter of 1.0 cm at viewing distance of 40.0 cm.

The entire visual procedure is divided into three parts: First, a peripheral detection of an object occurs, followed by a saccade and a fixation [50] [5].

To make a statement about what kind of object is involved, it must be depicted in the Fovea Centralis. The Fovea Centralis is indicated as the place of the sharpest sight on the retina. The eye constantly carries out very small movements. In order to capture objects in the whole visual field the rest of the retina has been provided with other preferred features which allow to quickly detect rapidly moving objects, i.e. possible dangerous obstacles. A movement of the object in the peripheral visual field provokes a jerky movement (reflective saccade). In order to be able to track the object, the eye makes slow follow-up movements. During the actual identification, the image of the visual object is held for 150.0 ms (reading) to 500.0 ms during the search of an object [48].

Extrafoveal perception is also important for the perception of large complex scenes (e.g. crossroads). Fundamental information like large contrast gradients along the object contours, motion, etc. need to be detected. Several interdependencies have an influence on the mapping of the visual space and perception of transient happen-ings. Besides the extrafoveal perception the performance of realistic visual objects is of importance. This can be explained by the fact that the decrease of visual capa-bility starting from the fovea is not abrupt but gradual, so that stimuli of sufficient intensity can still be perceived up to large eccentricities in periphery.

The whole procedure can be expressed on the basis of a control circuit for infor-mation acquisition processing, that is illustrated in Figure 2.8. The question is to

current visual field

Saccade programming Afferent visual system

Ocular muscles Environment of the

Figure 2.8: Control circuit for information acquisition processing. The visual impression of the current environment is transported to the brain via the afferent visual system. In the brain area that is responsible for the saccade programming, the next saccade target is selected and the next saccade aim point is calculated to reach the next point. Appropriate neural signals are sent to the brain stem, which move the outer eye muscles in motion and the eyes to the new aim point [1].

what extent the significance of this functional classification (foveal and extrafoveal perception) on the retina for road traffic is. As a rule, it is assumed that objects which are relevant to the driver are not randomly located at their fixation point, but appear at some point in the paracentric or peripheral visual field. The peripheral vi-sion function must perceive the object. The gaze movement, which is responsible for

the resolution of the viewing application, must signal whether the object is relevant or not. If this is the case, a gaze movement is performed which transports the object to the fovea to receive more object details. A vehicle driver therefore continually uses this functional antagonism between the fovea and periphery to transfer objects, which are relevant, for an exact analysis in the retina centre. A safe participation in road traffic is only possible if both components of the visual perception are intact.

The visual impression of the current environment is transported to the brain via the afferent visual system. In the brain area that is responsible for the saccade programming the next saccade target is selected and the next saccade aim point is calculated to reach the next point of interest. Appropriate neural signals are sent to the brain stem, which move the outer eye muscles in motion and the eyes to the new aim point.

Real field studies with monitoring eye tracking devices have shown that the gaze behaviour is also dependent on the traffic situation [56] [21].

of relevant object

Foveal fixation Release of saccade

Mental processing, Peripheral perception

Motoric reaction decision

Figure 2.9: Object appears at some point in the paracentric or peripheral visual field. After re-leasing saccades the object is fix-ated in the fovea. The informa-tion is processed and followed by a motoric reaction of the driver (e.g. a braking action) [1].

Much more gaze movements have to be carried out in a city traffic situation in order to be able to capture all relevant objects compared to driv-ing on a rural road. In this case, the saccades are restricted to a relatively small central visual angle area in which the roadway section far away from the driver is located. The position of the points of view depends also on the course of the roadway. In a right bend, the target points on the right edge of the road are piled up, in a left bend the tar-get points are on the left, i.e. in direction of the bend [5].

Moreover, it has been shown that the strategy by which the driver triggers the gaze behaviour is influ-enced by his experience. While a new driver is still relatively undirected and aimless gazing, an expe-rienced driver has already learned to unconsciously direct his points of view to the places where the oc-currence of dangerous objects is to be expected in an increased degree [56]. The driver’s vision strat-egy therefore aims to capture a maximum of rele-vant information with as few as possible eye move-ments. A maximum of three saccades per second can be performed as a larger number is not possible due to the required processing time [57]. An illus-tration of the procedure is shown in Figure 2.9.

It has been shown that the predominant number of saccades is in an eccentricity range up to a maxi-mum of 10.0^◦. Saccades with larger amplitude are rarely triggered. If peripherally located objects greater than 10.0^◦ need to be de-tected, the driver takes a head turn to support. It often also leads to a combination of saccades and head rotations in order to keep the partial movements of both

com-2.2. DETECTION OF VISUAL TARGETS 37 ponents as low as possible. The triggering of saccades and head rotations and thus the selection of the fixation points is usually unconscious [58].