Fig. 19.1 Visual acuity is dependent on eccentricity. With increasing eccentricity visual acuity decreases rapidly. The range of reading ability is limited on the one hand by the minimum size of the reading visual field (2° each, right and left of fixation), on the other hand by the resolution (for newspaper print in 25 cm approximately 0.4 is required). This acuity is found at the margin of the fovea. It becomes clear that measuring foveal visual acuity is not sufficient for gauging reading ability, because this assesses single optotype recognition (Modified after [6, 7])

extent is required: 2° each to the right and left of fixation, and 1° each above and below [8]. This minimum reading visual field corresponds approximately to the ‘visual span’ [9] and the ‘word identification span’ [10]. Its extent is more or less in accordance with the size of the fovea. Therefore, visual acuity alone is an insufficient measure for reading ability, because it tests only one optotype at a time (Fig. 19.1). The range of reading ability is limited on the one hand by the resolution of the retinal area used and on the other hand by its minimum extent, the minimum reading visual field. The total perceptual span during one fixation, in the following called ‘reading visual field’, can exceed this minimum up to 5° (or 15 letters) in reading direction [11].

Figure 19.2 shows a fundus image which was produced by a Scanning Laser Ophthalmoscope (SLO), with different morphological and functional data: The proportion of foveola and fovea, visual acuity and cone-density depend on eccentricity. The blue oval displays the minimum reading visual field.

In order to see the next group of letters clearly, the eye has to make a saccade. The eye movements during reading are characterized by a regular sequence of saccades and fixations, which results in a typical staircase pattern (Fig. 19.3).

Fig. 19.2 Relationship between morphological and functional data, drawn on an SLO-fundus image: The proportions of foveola and fovea, the minimum size of the reading visual field (blue oval), visual acuity (yellow curve) and cone density (black curve) dependent on eccentricity (Modified after [6])

Saccade

Return sweep

Fixation

Fig. 19.3 Eye movements during reading (schematic): regular sequence of saccades and fixations, return sweeps – resulting in a typical staircase pattern

19.3Reading with a Central Scotoma

19.3.1The Reading Visual Field Related to Other Parameters

(Figs. 19.4 and 19.5)

19.3.1.1The Reading Visual Field Related to the 30° Visual Field (Fig. 19.4a)

On the left, the minimum reading visual field (required for newspaper print) is displayed. It is obvious that visual field defects in the centre disturb the reading process significantly.

In the middle, a patient with an absolute central scotoma is shown. With central fixation, the reading visual field is completely covered by the scotoma and is functionless. Most of these patients develop a valuable adaptive strategy (Fig. 19.4 right): The patient uses a

Text

c

Fig. 19.4 The reading visual field related to other parameters:

Related to the 30° visual field (a): Left: minimum size under normal conditions. Middle: In absolute central scotoma and central fixation, the reading visual field is covered by the scotoma and functionless. Right: In absolute central scotoma and eccentric fixation, the scotoma and the blind spot are shifted. The new reading visual field is located on a healthy retinal area with a lower resolution. Related to the fundus (b): Left: the normal situation. Middle: macular degeneration and central fixation. Right:

eccentric fixation above the lesion corresponds to the fixation below the scotoma. Related to the text (c): Left: the normal situation: only within the marked area (within the central 4°) is the text perceived clearly. Middle: in absolute central scotoma and central fixation there is no reading ability. Right: absolute central scotoma and eccentric fixation: the new reading visual field does not have sufficient resolution. When the text is magnified, reading ability is regained (Modified after [12])

new, healthy visual field area on the margin of the scotoma. This new reading visual field becomes the centre of the visual field. Therefore, the scotoma is shifted, as well as the blind spot. The blind spot serves as a reference scotoma and shows the extent of the shift.

19.3.1.2 The Reading Visual Field Related to the Fundus (Fig. 19.4b)

On the left the normal situation, in the middle and on the right the macular degeneration is displayed. Fixation below the scotoma means fixation above the lesion. The eccentric retinal fixation area, also called ‘preferred retinal locus (PRL)’, has a lower resolution than a healthy fovea. (The term ‘eccentric fixation’ is here

used for any non-foveolar fixation, independent of the subjective gaze direction of the patient).

19.3.1.3 The Reading Visual Field Related to the Text (Fig. 19.4c)

On the left the normal situation: due to the visual acuity curve, only in the marked area ( the central 4°) the text is perceived clearly. Middle: In absolute central scotoma and central fixation, there is no reading ability.

Right: In eccentric fixation, a healthy visual field area at the margin of the scotoma is used. Therefore, the scotoma is shifted. The new retinal fixation locus at the edge of the scotoma does not have a sufficient solution for reading newspaper print. When the text is

Fig. 19.5 Demonstration of central (a) eccentric (b) fixation related to the eye globe position. In central fixation, the object falls directly into the scotoma. In eccentric fixation above the

lesion, i.e. below the scotoma, gaze direction is shifted upwards as well as the scotoma, the object becomes free and is seen with an eccentric retinal locus

magnified, reading ability is regained. This is the basis for the efficiency of magnifying visual aids.

19.3.1.4 Eccentric Fixation Related to the Globe (Fig. 19.5)

Figure 19.5 demonstrates eccentric fixation related to the globe. Fixation above the lesion, i.e. below the scotoma, causes a shift of the scotoma and of the gaze position upwards.

19.3.2The Significance of Fixation Behaviour

Patients with eccentric fixation show preferred directions of scotoma shift (Fig. 19.6): In own studies using SLO and Tübingen Manual Perimetry, approximately 80% of the patients shifted the scotoma upwards, 14% to the right [12, 15]. Similar results were reported by Aulhorn [16] based on perimetric investigations. In contrast, other authors described a shift to the right

to occur much more frequently – up to 63% [13, 14]. The corresponding retinal fixation loci are therefore located mostly above or left of the lesion, i.e. in the upper left quadrant in both eyes. This should be considered before retinal surgery. A shift of the scotoma into the upper visual field seems to be the best condition for reading, because then the line is free and the lower visual field is not impaired in orientation on the page. The choice of a PRL-location is not only dependent on the best local retinal resolution at the margin of the scotoma, but is also influenced by focal visual attention [17, 18].

Figure 19.7 shows the SLO-fundus image of a patient with AMD: She reads the text with an eccentric PRL above the lesion. She is fixating the ‘i’. The patient sees the text upright.

In case of an incomplete absolute central scotoma – a ring scotoma – central fixation persists, but the central island is too small for reading. This explains a discrepancy between visual acuity and reading ability. Often, reading ability can be regained in a later