Preferred retinal loci and macular scotoma characteristics in patients

Document Sample
Preferred retinal loci and macular scotoma characteristics in patients Powered By Docstoc
					                     Preferred retinal loci and macular scotoma
                     characteristics in patients with age-related
                     macular degeneration
                     Ronald A. Schuchard, PhD

                     ABSTRACT • RÉSUMÉ
                     Many patients with macular scotomas due to age-related macular degeneration do not
                     perceive black spots in the visual field where the scotomas are located. Rather, they
                     describe objects as “vanishing,” “jumping out of nowhere” or “having blurry parts,” or
                     a combination of features. In addition, when the macular scotoma affects the fovea, the
                     visual system uses 1 or more preferred retinal loci (PRLs) as a “pseudofovea” to
                     perform visual tasks. Visual function testing with the scanning laser ophthalmoscope
                     has provided a wealth of information regarding how patients perceive the visual world
                     and how the oculomotor system directs eye movements. This article describes
                     2 specific functions of the oculomotor system, fixation stability and refixation
                     precision, with data collected from normally sighted people and patients with visual
                     field loss. The implications of the characteristics of PRLs and macular scotomas for
                     clinical testing are discussed.

                     Plusieurs patients qui ont des scotomes attribuables à une dégénérescence maculaire
                     liée à l’âge ne perçoivent pas de points noirs du champ visuel là où se situent les
                     scotomes. Ils décrivent des objets « qui disparaissent », « venus de nulle part » ou
                     « partiellement flous » ou ayant un mélange de ces caractéristiques. De plus, lorsque
                     les scotomes maculaires affectent la fovéa, l’appareil visuel utilise une ou plusieurs
                     zones de rétine préférentielles comme « pseudofovéa » pour accomplir des tâches
                     visuelles. L’examen de la fonction visuelle avec l’ophtalmoscope laser à balayage a
                     donné une abondance de renseignements sur la perception qu’ont les patients du
                     monde visuel et sur la façon dont le système oculomoteur dirige les mouvements de
                     l’œil. Cet article décrit 2 fonctions particulières du système oculomoteur, la stabilité
                     de la fixation et la précision de la refixation, avec des données recueillies chez des
                     personnes qui ont une vue normale et des patients qui ont subi une perte du champ
                     visuel. Il traite aussi des implications des caractéristiques des zones de rétine
                     préférentielles et des scotomes maculaires pour les tests cliniques.

T    he visual system with a central scotoma from age-
     related macular degeneration (AMD) chooses
(often without the conscious knowledge of the
                                                                             or more eccentric preferred retinal loci (PRLs) natu-
                                                                             rally and reliably develop to perform the foveal visual
                                                                             tasks such as recognizing objects and directing eye
patient) a preferred eccentric retinal area because the                      movements while reading or tracking objects.1–16 This
fovea can no longer perform visual tasks; that is, in an                     choosing by the visual system of an eccentric “pseu-
eye with a central scotoma affecting all of the fovea, 1                     dofovea” for visual-performance functions has been

From the Department of Veterans Affairs, Rehabilitation Research &           GA 30033-4004, USA; fax (404) 728-4837;
Development, Center of Excellence for Aging with Vision Loss, Atlanta, Ga.
                                                                             This article has been peer-reviewed.
Correspondence to: Dr. Ronald A. Schuchard, Atlanta VA Rehabilitation
Research & Development Center (151R), 1670 Clairmont Rd., Decatur,           Can J Ophthalmol 2005;40:303–12

Preferred retinal loci—Schuchard                                                                                                         303
Preferred retinal loci—Schuchard

Fig. 1—Characteristics of preferred retinal loci (PRLs) and scotomas in the right eye (left image) and left eye (right image) of a
patient with exudative age-related macular degeneration (AMD).The green solid line describes the border of the dense scotoma
(DS), the yellow letters describe the area of the relative scotoma (RS), the red circle describes the border of the PRL, and the blue
F indicates the best estimate of the location of the nonfunctioning fovea.

noted primarily by the use of the scanning laser oph-                mining where the eccentric PRL is located except by
thalmoscope (SLO). The SLO also allows accurate                      monitoring the location of visual-stimuli images on
characterization of the central visual field by macular              the retina. Fig. 1 shows how visual stimuli can be
perimetry with direct retinal observation of fixation                directly monitored with an SLO to determine scotoma
and perimetry targets.17,18 For example, fixation per-               and PRL characteristics. Generally, in the low-vision
formance has 2 operational definitions: refixation                   population with central scotomas, there is no consis-
precision is defined as the retinal area that a patient              tent retinal location relative to the scotoma for the
uses during repeated fixation (refixation); fixation sta-            PRLs. In a study of 825 patients, 84.4% of low-vision
bility is defined as the retinal area that a patient uses            eyes (1130 of 1339) demonstrated an established
while maintaining fixation. Refixation precision and                 PRL (foveal or eccentric) for fixation, which varied
fixation stability show variability, of course, and the              from 1.0° to 9.0° in diameter.14 Only 4.4% of the
distribution of retinal positions used during fixation               patients did not demonstrate a PRL in either eye.
performance is operationally defined as the retinal                  There was a dense scotoma within 2.5° of all of the
locus for fixation.                                                  eccentric PRLs, surprisingly, and it completely sur-
   The concept of using a retinal locus for visual tasks             rounded 17.4% of the PRLs (ring scotoma).
such as fixation is accepted in vision rehabilitation                   In an eye with a functioning fovea, fixation is the
but seldom used with normally sighted patients. The                  act of directing the eye toward the visual target of
term PRL has typically been reserved for patients                    regard, causing the image of the target to be within
whose visual system has chosen a preferred eccentric                 the fovea. The anatomic fovea is a retinal area about
retinal area for visual tasks because of a central                   5° (1500 mm) in diameter in which the central
scotoma. However, the visual system of patients with                 1.2°–1.7° diameter of photoreceptors (referred to as
                                                                     the anatomic foveola or clinical fovea) is composed
paracentral scotomas or even normal sight does
                                                                     entirely of cones.19,20 The traditional view is that
choose to use the fovea over other retinal areas; thus,
                                                                     within the centre of the fovea, where the cone density
the fovea can be referred to as their PRL.                           is highest, is a small and spatially invariant retinal area
RETINAL    CONSIDERATIONS                                            referred to as the optimal locus.21 The optimal locus
                                                                     has been hypothesized to direct the location of a fix-
  For patients with a functioning fovea, visual tasks                ation stimulus when fixation is initiated and to direct
are performed by aiming the eye such that the image                  drifts and microsaccades to maintain fixation.
of the visual target of regard is placed within the                     In an eye with a central scotoma affecting all of the
foveal area. For patients with a central scotoma from                fovea, eccentric fixation is the act of directing the eye
AMD involving all of the fovea, visual tasks are per-                toward the visual target of regard, causing the image
formed by aiming the eye such that the image of the                  of the target to be placed in 1 or more preferred
visual target of regard is placed within a PRL.                      eccentric retinal areas. Eccentric fixation refers to fix-
Unfortunately, there is no conclusive way of deter-                  ation performance in which the oculomotor system is

304     CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005
                                                                                  Preferred retinal loci—Schuchard

oriented to the eccentric retinal area, and the patient     are more likely to use a “secondary” PRL when fixat-
has the sensation of looking directly at the visual         ing on a target in a secondary position of gaze. The
target during fixation. Eccentric viewing, on the           use of multiple PRLs has also been observed for dif-
other hand, refers to viewing performance in which          ferent visual tasks13 and lighting conditions.15
the oculomotor system is not oriented to the target         Patients are often completely unaware of when or
and the patient has the sensation of looking above,         how they use multiple PRLs.
below or to either side of the target to see it. A shift
of the oculomotor reference from the fovea to a pre-        PRL   CHARACTERISTICS

ferred eccentric retinal area is possible in patients          The ability to make efficient and effective eye
with bilateral macular disease.7,8 However, even            movements with a PRL is often considered the most
though the saccades of patients with central scotomas
                                                            difficult and yet the most valuable component of
can consistently direct images to the PRL, they some-
                                                            visual tasks such as scanning and reading. The more
times have the latency and dynamic characteristics of
                                                            basic eye movements made by the visual system are
nonfoveal saccades.8 But the latency can be shortened
                                                            fixation, pursuit and saccadic movements. These
with practice, eventually tending asymptotically to
                                                            tasks are thought to possibly represent measurable
the normal saccadic duration.22 Eccentric viewing or
                                                            parameters of the PRL quality for visual performance;
fixation naturally and reliably occurs when the foveal
                                                            that is, to optimally function as a retinal locus for
areas in both eyes are no longer functional, such as
                                                            visual performance, the PRL needs to keep a visual
when both eyes have central scotomas.1–6,22
                                                            image in a discrete and stable retinal area (fixation
POSITION   OF GAZE CONSIDERATIONS                           stability), to track moving objects through space
                                                            (pursuit) and to move rapidly to objects of interest
   The primary position of gaze is defined as the posi-     appreciated in the visual field away from the PRL
tion of the eye when a patient is looking at a visual       (saccadic movements). These and other characteris-
target that is straight ahead. From this primary posi-      tics of the PRL are thought to be important in activ-
tion all ocular movements are initiated. More specifi-      ities of daily living for patients with AMD.
cally, the primary position of the eye is the position
against which all torsional, rotational and transla-        Refixation precision
tional movements are measured. The position of the
                                                               Subjects were asked to fixate for 30 s on a target
eye at the primary position of gaze is not necessarily
                                                            randomly placed in 1 of 25 positions determined by
identical to an anatomically straightforward position
                                                            a 5 × 5 grid within the 17° × 12° SLO field of view.
of the eye (as in an AMD patient with a central
scotoma). A secondary position of gaze is defined as        The centre of the grid was at the primary position of
any position of the eye represented by a vertical, hor-     gaze, and the remaining grid locations provided 24
izontal or oblique (a combination of vertical and hor-      secondary positions of gaze. Table 1 presents the char-
izontal) deviation from the primary position of gaze.       acteristics of the PRL for fixation described by a
It is expected that patients with functioning foveal        bivariate analysis that provides an elliptical fit of the
areas (e.g., patients with paracentral scotomas or          30-s retinal positions during fixation (unpublished
normal sight) will use the foveal area as their PRL for     data, 2003). The characteristics of the ellipsis are ori-
visual tasks when visual targets are in either the          entation of the major axis (± 90° from the positive
primary position of gaze or secondary positions of          horizontal axis), eccentricity (the ratio of the major to
gaze. In addition, it is expected that patients with        minor axes when a value of 1 indicates a circular
nonfunctioning foveal areas (owing to central sco-          shape), major axis length (in minutes of arc) and area
tomas) will use a preferred eccentric retinal area as the   (in minutes of arc squared).
PRL for fixation when targets are in either the                An analysis of variance in the characteristics of the
primary position of gaze or secondary positions of          PRL for fixation found by refixation precision was
gaze. However, patients with central scotomas use 1         first performed for 3 normally sighted subjects who
or more PRLs for visual tasks when visual targets are       had extensive experience fixating. Effects with p <
in the secondary positions of gaze and almost always        0.05 are reported as significant in this and all further
use a “primary” PRL for visual tasks when the targets       analyses of variance. The orientation of the major
are in the primary position of gaze;8 that is, patients     axes of the PRLs was not affected by the positions of

                                                                      CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005     305
Preferred retinal loci—Schuchard

 Table 1—Characteristics of ellipses representing preferred retinal loci for fixation found by refixation
 precision and studied by bivariate analysis
                                         Primary position of gaze                               Secondary positions of gaze
                                 Ma j o r a x i s                                          M a j o r a xi s

                                              Length                  Area (min                        Length                  Area (min
                             Angle            (min of                   of arc   Angle                 (min of                   of arc
                                     †                                                   †
 Subject*    Stimulus      (degrees)           arc)     Eccentricity‡ squared) (degrees)                arc)     Eccentricity‡ squared)
 EN-1        1° cross        –14.1             26.5        0.78            434          43.8            42.2         0.79          1 105
             12′ disc        –23.6             30.5        0.61            443         –63.9            44.2         0.78          1 190
             1° disc         –46.0             31.9        0.56            452         –86.6            58.1         0.66          4 042
 EN-2        1° cross        –13.6             28.8        0.55            392          24.0            82.1         0.40          2 056
             12′ disc         –3.0             28.8        0.60            389           1.1            44.4         0.72          1 120
             1° disc         –20.6             27.9        0.78            476         –71.4            47.8         0.65          1 176
 EN-3        1° cross         15.2             21.1        0.97            337          42.8            43.2         0.45            661
             12′ disc        –75.5             34.7        0.46            436          61.1            54.8         0.69          1 631
             1° disc          65.5             33.0        0.83            608          11.4            30.6         0.68          3 007
 IN-1        1° cross         42.8             59.8        0.77          2 172          14.2           100.5         0.56          4 443
 IN-2        1° cross        –38.6             57.9        0.88          2 317          37.7           199.2         0.44         13 769
 IN-3        1° cross         78.3             76.9        0.44          2 050         –16.9           150.5         0.73         13 028
 IN-4        1° cross         10.8             56.1        0.89          2 205         –81.2            88.5         0.61          3 741
 PS-1        1° cross         47.9             81.9        0.60          3 163          53.7            87.2         0.61          3 660
 PS-2        1° cross        –63.5             48.4        0.48            890         –23.0           172.3         0.50         11 667
 CS-1        1° cross         –2.2            212.9        0.55         19 759          13.9           549.2         0.36         84 770
 CS-2        1° cross        124.6            390.0        0.28         33 735         –64.4           555.0         0.64        155 790
 *Subjects were experienced (E) or inexperienced (I) in fixation and had normal (N) sight or had scotomas (S), either paracentral (P) with a
 functioning fovea or central (C). Subject PS-2 had a ring scotoma surrounding the fovea. All scotomas were due to age-related macular
   Plus or minus 90° from the positive horizontal axis.
   A value of 1 indicates a perfect circle.

gaze (primary or secondary), the different fixation                      position of gaze, additional studies were done with 4
stimuli or the different subjects. The eccentricity of                   normally sighted subjects who were inexperienced in
the PRLs also was not affected by the positions of                       fixation. These results and those for the experienced
gaze (primary or secondary), the different fixation                      subjects with the 1° cross were combined for analysis.
stimuli or the different subjects. However, the eccen-                   In all subjects, the major axis length and area of the
tricity values in Table 1 clearly indicate that the PRL                  PRLs were significantly larger with the primary posi-
for fixation is not a circle (eccentricity = 1) except in                tion of gaze than with the secondary positions of
a few cases (e.g., subject EN-3 with 1° cross at the                     gaze. However, all the inexperienced subjects had
primary position of gaze). The major axis length and                     larger fixation areas than the experienced subjects.
the area of the PRLs were not affected by different                      The same effect of experience on fixation perform-
fixation targets or different subjects. Therefore, the                   ance has been reported for fixation stability.23
orientation and shape of the PRLs for fixation change                       Patients with central visual field loss not affecting
in apparently random and nonsystematic ways. How-                        the fovea (e.g., patients with ring scotomas) would be
ever, the major axis length and area were significantly                  expected to have fixation performance similar to that
larger for secondary positions of gaze than for                          of normally sighted subjects. The results for 2
primary positions of gaze with all 3 fixation targets                    patients with paracentral scotomas shown in Table 1
and all 3 subjects (the interaction terms were not sig-                  are typical of these types of field loss: the PRLs for fix-
nificant).                                                               ation found with primary and secondary positions of
   To extend the finding that the major axis length                      gaze were similar to those of the normally sighted
and area of the PRL is affected by the fixation-target                   subjects. However, such patients sometimes place the

306     CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005
                                                                                   Preferred retinal loci—Schuchard

fixation target in an eccentric PRL for fixation. For        area of 68′ in diameter. In addition, the patients did
example, patient PS-2 twice placed the fixation target       not place the target in a small retinal area of 10.5′
in an eccentric retinal location outside the ring            most of the time and very seldom placed the fixation
scotoma when the target was at a secondary position          stimulus outside the small retinal area (a Gaussian
of gaze (these 2 eccentric fixation positions were not       distribution). Rather, patients placed the target in a
included in the bivariate analysis). The patients are        larger retinal area most of the time (a roughly flat dis-
often completely unaware of the change in the retinal        tribution of 40′ or 50′) and outside this larger area
fixation position. The last 2 patients listed in Table 1     very seldom.
had central scotomas affecting the fovea. The increase
in size of the PRL for fixation is typical of this type of   Comparison of results
field loss. Patient CS-1 used 2 eccentric retinal areas
                                                                The PRL sizes during a fixation-stability trial, at
to place the fixation target; patient CS-2 used only 1
                                                             both primary and secondary positions of gaze, were
PRL. Patients with central scotomas affecting the
                                                             very similar to the PRL sizes for the 24 repeated fixa-
fovea are also often completely unaware of using dif-
                                                             tions at the primary position of gaze during refixa-
ferent retinal positions for PRLs. It is not known why
                                                             tion-precision trials. Patients appear to use a larger
patients place fixation targets in locations less often
used for fixation.                                           retinal area for fixation when asked to fixate on a
                                                             target that is away from the primary position of gaze.
Fixation stability                                           However, when they have fixated on a target at a sec-
                                                             ondary position of gaze, they appear to be able to
   The retinal loci for fixation found by fixation sta-      maintain the image of the fixation target in a retinal
bility in 8 of the secondary positions of gaze (at the       locus for fixation that is the same as when they are
corners and the horizontal/vertical axes of the 17° ×        asked to fixate and maintain fixation on a target at
12° SLO field) were found along with 8 primary-              the primary position of gaze. Interestingly, even
position trials. An analysis of variance (repeated-          though the patients used a larger retinal area for sec-
measures design) was performed on the characteris-           ondary-position fixation, the diameters of the major
tics of the PRLs for fixation (again significance is         axes of the PRLs were almost always 1° or less. It may
reported for p < 0.05). Like refixation precision, the       not be a coincidence that the retinal positions for fix-
orientation of the major axis and the eccentricity of        ation are typically within an area of about 1.2° to 1.7°
the PRLs were not affected by the position of gaze           for most normally sighted patients. The central
(primary or secondary). Unlike refixation precision,         1.2°–1.7° area of the fovea, the foveola, is a retinal
however, major axis length was significantly affected        area with roughly equal resolution capability.
by position of gaze. Previous studies of fixation sta-
bility in normally sighted subjects had shown PRL            Binocular PRLs
and foveal areas for fixation of 31.6′ to 373′
squared.21–25 The PRL areas found with SLO were                 In previous investigations of PRLs each monocular
337′ to 443′ squared for small fixation targets with         PRL was studied separately, but individuals typically
experienced patients. The main difference between            perform activities of daily living with both eyes open.
the previously reported and the SLO values is the 3:1        Therefore, previous investigators have forced subjects
ratio between the 95% and 63% equal-frequency                to perform visual tasks monocularly without fully
PRLs, that is, the difference in the retinal areas in        understanding the consequences for the monocular
which one would expect to find 95% versus 63% of             PRL characteristics. In a recent study of binocular
the retinal positions of fixation.                           PRLs,26 67% of the patients saw stimuli with only 1
   To further investigate the concept of a larger retinal    eye (monocular perception) while performing simple
locus for fixation, the 400 samples (25 samples              free-viewing binocular fixation and perception tasks.
during 8 primary-position trials and 8 secondary-            No single visual factor (acuity, contrast, threshold
positions trials) for each patient performing fixation       sensitivity, fixation stability, saccadic ability, pursuit
stability in the SLO were analyzed to find the radius        ability or scotoma characteristics) had a statistically
from the centroid for each sample. The normalized            significant effect on perception of the stimulus,
cumulative frequency indicated that 95% of the time          monocular or binocular. However, if the subjects had
the patients kept the fixation stimulus within a retinal     a large difference in monocular visual function (e.g., a

                                                                        CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005     307
Preferred retinal loci—Schuchard

Fig. 2—In a patient with AMD and monocular perception, the binocular PRLs are not in the same direction or at relatively the same
distance from the nonfunctioning fovea in the right eye (left image) and the left eye (right image).

logMAR [logarithm of the minimum angle of resolu-                  movement tasks; that is, the fellow eye is actually fol-
tion] difference greater than 0.66, or more than                   lowing the lead of the “dominant” eye. Therefore,
6 lines of letters on the ETDRS [Early Treatment                   determining the dominant PRL can be critical in
Diabetic Retinopathy Study] chart), the dominant                   guiding the prescription of monocular-vision-
PRL was the PRL with better visual-function capa-                  enhancing equipment, testing monocular visual func-
bility. But, in the large group of subjects (82%) who              tion and other monocular issues.
did not have these large differences in visual-function
capability between the 2 eyes, the only way of                     IMPLICATIONS      OF   PRL AND    SCOTOMA
knowing which PRL was dominant was by a binocu-                    CHARACTERISTICS IN       AMD
lar perception test that directly evaluated which PRL
was dominant. The only other factor that seems to                     The idea of 1 or more PRLs instead of a single
contribute to monocular perception is noncorrespon-                retinal point being used for visual tasks is an impor-
dence of binocular PRLs. Fig. 2 shows an example of                tant concept for clinical vision testing and visual
binocular PRLs that are in different directions and at             tasks. Many clinical tests or treatments presume that
different distances from the nonfunctioning foveas.                the eye movement system is operating by referencing
Compare the PRLs in Fig. 2 with those in Fig. 1,                   a retinal point or at least a single retinal area. A few
where the binocular PRLs are in the same direction                 examples are presented to demonstrate the implica-
and at relatively the same distance from the nonfunc-              tions of PRLs and scotomas for clinical testing and
tioning foveas.                                                    visual tasks.
   The finding that 2 out of 3 patients with central               Reading
scotomas see visual stimuli at fixation with only 1 eye
does not mean that the patients are seeing the entire                 The ability of the eye to make movements so that
visual world with only 1 eye. Patients reported seeing             the visual-target image is placed or kept in the PRL
the world with both eyes, although not always appre-               has been shown to be correlated to reading rate and
ciating that when looking at a target in the central               reading errors.27–33 Previous reports on reading rate
visual field (e.g., a number, letter or word) they were            had shown that the existence of a central scotoma was
seeing the target with only 1 eye. Previous studies of             a much greater predictor of reading impairments than
PRL abilities in eye movements might be considered                 was reduced acuity.31,32 However, the ability of the
to have been testing similar types of tasks: typically             PRL to direct eye movements, both saccadic ability
the subject was asked to fixate, saccade to a small                (measured by the number of saccades and the saccade
target or follow a small target. It is possible that the           characteristics) and fixation stability (measured by
eye that actually is used by the visual system in binoc-           the retinal area of fixation), had a much higher corre-
ular tasks is a “dominant” eye that is better at eye-              lation with reading speed and correct reading rate

308     CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005
                                                                                  Preferred retinal loci—Schuchard

than did either acuity or scotoma existence.                Humphrey 10-2 test) could be the average of flashes
Furthermore, subjects with macular diseases who             actually presented at multiple retinal locations. The
were undergoing vision rehabilitation clustered into 2      instability inherent in eccentric PRLs for fixation also
groups, 1 showing significant improvement and the           makes accurate mapping of the scotoma boundaries
other showing no improvement in reading with                very difficult.
vision-rehabilitation training.33,34 That research also        Pericentral fixation targets are used when the
showed a strong association between saccade scores          purpose of the testing is to measure visual function in
and reading rate or reading errors. Patients with           the central macular retinal area (foveal area). These
AMD who present with low reading rates (fewer than          targets are intended to guide fixation by stimulating
10 words/min and more than 2 errors with a short            the perifovea (the outer limit of the macular area)
passage) and poor saccadic ability are less likely to       while providing no visual stimulation of the inner
show improvement in reading with vision rehabilita-         macular area, including the foveal area. When sub-
tion.                                                       jects are instructed to “fixate in the centre of the
                                                            target,” clinicians often assume that patients with
Visual search                                               central scotomas place the centre of the pericentral
                                                            fixation target within the PRL for fixation. They also
   Unlike reading, the ability of the eye to make           often assume that, when instructed to “look directly
movements so that the visual-target image is placed         at the centre of the target,” patients place the target
or kept in the PRL has been shown to not be highly          centre in the foveal area, even though the fovea is no
correlated to visual search efficiency or accuracy.         longer functioning owing to the central scotoma.
Saccades of patients with central scotomas were             However, most patients with central scotomas appear
found to consistently direct fixation targets to the        to orient images of targets to the PRL, and verbal
PRL even though the saccades sometimes had the              instructions do not change the location of the peri-
latency and dynamic characteristics of saccades not         central fixation target.7,8 Therefore, one cannot
directed by the fovea.12,22 The patients with AMD           assume that using pericentral fixation targets in clini-
also had, on average, more saccades of shorter length       cal tests (e.g., a big X in the tangent screen or Amsler
than normally sighted patients. However, the latency        grid), with or without verbal instructions, provides
of the first saccade and the number of saccades to          testing with the fovea directed at the centre of the test
move the search-target image into the PRL were not          area.
correlated to the search time (together or sepa-
rately).12 Rather, the time after the search-target         Patient awareness of macular scotomas
image was moved into the PRL was the greatest con-
tributor to the total visual search time; that is,             The presence of scotomas in the macula hinders the
patients with macular scotomas deliberated longer           performance of many daily activities, such as reading,
about the identity of the search target. Therefore, any     but most patients with AMD are unaware of even
patient with a macular scotoma, regardless of the           large defects in their central visual field. A valuable
retinal location of the PRL, will have impaired visual      application of the information gained from PRL
search ability beyond the inability to see targets          testing and macular perimetry is to educate the
located inside the scotoma.                                 patient about the presence of the scotomas, especially
                                                            when they occur relative to fixation. Macular perime-
Perimetry testing                                           try methods can be used to demonstrate to the
                                                            patient what a scotoma is and what compensatory eye
   The size and location of the PRL have obvious            movements can be used to “move the scotoma out of
implications for perimetric testing of patients with        the way.” These compensatory techniques are espe-
AMD. For example, perimetric results can be trans-          cially important for the nearly 1 out of 5 low-vision
lated in position, and if this translation is not recog-    patients who have fairly good visual function (as good
nized the location of the macular scotoma will be           as 20/40) but ring scotomas surrounding the PRL
inaccurately positioned in the visual field relative to     (Fig. 3). The patient who has gained an awareness of
the fovea. If the patient has a very large PRL for fixa-    the scotoma’s effects on visual tasks is more likely to
tion or multiple PRLs, the perimetry results reported       make accurate compensatory eye movements to
at 1 point of the visual field (e.g., the grid point in a   search for, find and identify pertinent visual informa-

                                                                      CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005     309
Preferred retinal loci—Schuchard

Fig. 3—A ring scotoma, a scotomatous area completely surrounding the PRL, is a typical stage of the progression of vision loss in
geographic atrophy.This patient has AMD due to geographic atrophy in the right eye (left image) and the left eye (right image).

                                                                   another example of when the concept of retinal locus
                                                                   for fixation is important. Calibrating eye trackers
                                                                   (e.g., the pupil eye tracker in the Humphrey Visual
                                                                   Field Analyzer) is done by having patients fixate on
                                                                   targets at known spatial locations. The recordings
                                                                   from the trackers are transformed to real-world
                                                                   dimensions by the relationships between these
                                                                   recordings and the known spatial locations of the fix-
                                                                   ation targets. The underlying assumption is that
                                                                   patients place the fixation targets in exactly the same
                                                                   retinal position each time they are instructed to look
                                                                   at the target; that is, one must assume that the PRL
                                                                   has no shift in retinal position when the fixation
                                                                   target is placed in different spatial locations, even
                                                                   though multiple PRLs occur. To make matters worse,
                                                                   the SLO results show that patients use a larger retinal
Fig. 4—PRL shift in the left eye of a patient with AMD. The
                                                                   locus for fixation when looking at fixation targets in
patient consistently used 2 PRLs in the same retinal locations     secondary positions of gaze (see Table 1). To develop
and at approximately the same light levels during 1 year of        the transformation algorithm for calibration, one
monitoring: the primary PRL (red circle) with typical bright       must have patients look at fixation targets in many
indoor light (about 100 cd/m2 or more) and the secondary           secondary positions of gaze.
PRL (purple circle) with less light (e.g., when watching televi-
sion).TS indicates the scotoma that exists at the threshold of     Eccentric-viewing training
the secondary PRL.
                                                                      The orientation of the oculocentric system (fovea
tion. The ability to be aware of the scotoma’s effects             or eccentric PRL) affects eccentric-viewing training.
on visual tasks is even more challenging when the                  Many vision rehabilitation practitioners believe that
patient is using multiple PRLs (Fig. 4), often without             patients with central scotomas benefit greatly from
conscious knowledge of changing retinal positions for              such training.35,36 A goal of this training is to enable
different lighting conditions or visual tasks.                     the patient to use a specific eccentric area of the retina
Calibration of eye-position instruments                            as if it were a fovea. To the extent that the PRL can
                                                                   play this role, this goal is reasonable. Results of fixa-
  The calibration of eye trackers for vision testing is            tion performance and visual-search performance,12 as

310     CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005
                                                                                            Preferred retinal loci—Schuchard

measured with an SLO, indicate that patients with                  6. Whittaker SG, Budd JM, Cummings RW. Eccentric fixation
central scotomas are able, and indeed likely, to use a                with macular scotoma. Invest Ophthalmol Vis Sci 1988;29:
PRL. The PRL appears to be developed by a patient
                                                                   7. White JM, Bedell HE. The oculomotor reference in humans
simply through experience of a central scotoma,                       with bilateral macular disease. Invest Ophthalmol Vis Sci
without formal training. However, it is certainly pos-                1990;31:1149–61.
sible that eccentric-viewing training could facilitate             8. Schuchard RA, Raasch TW. Retinal locus for fixation: peri-
this behaviour, making fixation more stable and accu-                 central fixation targets. Clin Vis Sci 1992;7:511–620.
rate or influencing the patient to use a more optimal              9. Schuchard RA. Validity and interpretation of Amsler grid
                                                                      reports. Arch Ophthalmol 1993;111:776–80.
eccentric retinal area for eccentric viewing. It is not           10. Guez JE, Le Gargasson JF, Rigaudiere F, O’Regan JK. Is there
known whether patients with central scotomas can be                   a systematic location for the pseudo-fovea in patients with
trained to develop a new, more optimal, PRL as the                    central scotoma? Vision Res 1993;33:1271–9.
reference for the oculocentric system. Further results            11. Culham LE, Fitzke FW, Timberlake GT, Marshall J.
are needed to determine what factors (e.g., resolution,               Assessment of fixation stability in normal subjects and
                                                                      patients using a scanning laser ophthalmoscope. Clin Vis Sci
field of view, proximity to the nonfunctioning fovea                  1993;8:551–61.
and binocular conditions) the visual system uses                  12. Schuchard RA, Fletcher DC. Preferred retinal locus: a review
when naturally choosing a retinal area as the PRL.                    with applications in low vision rehabilitation. Ophthalmol
                                                                      Clin North Am 1994;7:243–55.
CONCLUSION                                                        13. Sunness JS, Bressler NM, Maguire MG. Scanning laser oph-
                                                                      thalmoscopic analysis of the pattern of visual loss in age-
   Although there have been reports concerning some                   related geographic atrophy of the macula. Am J Ophthalmol
aspects of the PRL, the characteristics of the PRL are                1995;119:143–51.
still not well known and understood. For example,                 14. Fletcher DC, Schuchard RA. Preferred retinal loci relation-
more results need to be collected to determine                        ship to macular scotomas in a low-vision population.
whether single or multiple PRLs are influenced by                     Ophthalmology 1997;104:632-8.
                                                                  15. Lei H, Schuchard RA. Using two preferred retinal loci for dif-
retinal abnormalities. Also, what is known from previ-
                                                                      ferent lighting conditions in patients with central scotomas.
ous work needs to be reevaluated, since it was never                  Invest Ophthalmol Vis Sci 1997;38:1812–8.
determined that the PRLs that were studied were actu-             16. Sunness JS, Applegate CA, Haselwood D, Rubin GS. Fixation
ally used by the visual system in binocular (natural)                 patterns and reading rates in eyes with central scotomas from
viewing tasks. However, the characteristics that are                  advanced atrophic age-related macular degeneration and
known about the PRL have been shown to be impor-                      Stargardt disease. Ophthalmology 1996;103:1458–66.
tant in their relationship to activities of daily living for      17. Fletcher DC, Schuchard RA, Livingstone CL, Crane WG,
                                                                      Hu SY. Scanning laser ophthalmoscope macular perimetry
patients with AMD. Therefore, clinicians may want to
                                                                      and applications for low vision rehabilitation clinicians.
consider assessing the PRLs and scotomas of their                     Ophthalmol Clin North Am 1994;7:257–65.
patients as part of a complete evaluation.                        18. Sunness JS, Schuchard RA, Shen N, Rubin GS, Dagnelie G,
                                                                      Haselwood DM. Landmark-driven fundus perimetry using
REFERENCES                                                            the scanning laser ophthalmoscope. Invest Ophthalmol Vis Sci
1. von Noorden GK, Mackensen G. Phenomenology of eccen-
                                                                  19. McDonnell JM. Ocular embryology and anatomy. In: Ogden
   tric fixation. Am J Ophthalmol 1962;53:642–61.
                                                                      TE, editor. Retina. St. Louis: CV Mosby Company; 1989.
2. Dalgleish R, Naylor EJ. Bilateral eccentric fixation with no
                                                                  20. Bishop PO. Binocular vision. In: Moses RA, Hart WM,
   ocular deviation in a case of heredo-macular degeneration.         editors. Adler’s physiology of the eye. St. Louis: CV Mosby
   Br J Ophthalmol 1963;47:11–3.                                      Company; 1987.
3. Cummings RW, Whittaker SG, Watson GR, Budd JM.                 21. Kosnik W, Fikre J, Sekuler R. Visual fixation stability in older
   Scanning characters and reading with a central scotoma.            adults. Invest Ophthalmol Vis Sci 1986;27:1720–5.
   Am J Optom Physiol Opt 1985;62:833–43.                         22. Steinman RM. Effect of target size, luminance, and color on
4. Timberlake GT, Mainster MA, Peli E, Augliere RA, Essock            monocular fixation. J Opt Soc Am 1965;55:1158–65.
   EA, Arend LE. Reading with a macular scotoma. I. Retinal       23. Whittaker SG, Cummings RW, Swieson LR. Saccade control
   location of scotoma and fixation area. Invest Ophthalmol Vis       without a fovea. Vision Res 1991;31:2209–18.
   Sci 1986;27:1137–47.                                           24. Steinman RM, Cunitz GT. Fixation of targets near the
5. Timberlake GT, Peli E, Essock EA, Augliere RA. Reading             absolute foveal threshold. Vision Res 1968;8:277–86.
   with a macular scotoma. II. Retinal locus for scanning text.   25. Skavenski AA, Hansen RM, Steinman RM, Winterson BJ.
   Invest Ophthalmol Vis Sci 1987;28:1268–74.                         Quality of retinal image stabilization during small natural and

                                                                              CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005             311
Preferred retinal loci—Schuchard

    artificial body rotations in man. Vision Res 1979;19:675–83.        32. Legge GE, Ross JA, Isenberg LM, LaMay JM. Psycho-
26. Schuchard RA, Tekwani N, Hu SY. Binocular preferred                     physics of reading: clinical predictors of low-vision reading
    retinal loci: relationship of visual factors to binocular percep-       speed. Invest Ophthalmol Vis Sci 1992;33:677–87.
    tion. OSA Tech Dig Ser 1995;1:332–5.                                33. McMahon TT, Hansen M, Stelmack J, Oliver P, Viana MA.
27. Culham LE, Fitzke FW, Timberlake GT, Marshall J. Use of                 Saccadic eye movements as a measure of the effect of low
    scrolled text in a scanning laser ophthalmoscope to assess read-        vision rehabilitation on reading rate. Optom Vis Sci 1993;70:
    ing performance at different retinal locations. Ophthalmic Phy-         506–10.
    siol Opt 1992;12:281–6.                                             34. McMahon TT, Hansen M, Viana M. Fixation characteristics
28. Rubin GS, Turano K. Reading without saccadic eye move-                  in macular disease. Invest Ophthalmol Vis Sci 1991;32:
    ments. Vision Res 1992;32:895–902.                                      567–74.
29. Rubin GS, Turano K. Low vision reading with sequential              35. Holcomb JG, Goodrich GL. Eccentric viewing training.
    word presentation. Vision Res 1994;34:1723–33.                          J Am Optom Assoc 1976;47:1438–43.
30. Fletcher DC, Schuchard RA, Watson G. Relative locations of          36. Goodrich GL, Mehr EB. Eccentric viewing training and low
    macular scotomas near the PRL: effect on low vision reading.            vision aids: current practice and implications of peripheral
    J Rehabil Res Dev 1999;36:356–64.                                       retinal research. Am J Optom Physiol Opt 1986;63:119–26.
31. Legge GE, Rubin GS, Pelli DG, Schleske MM. Psycho-
    physics of reading. II. Low vision. Vision Res 1985;25:             Key words: age factor, macular degeneration, ocular fixation,
    253–65.                                                             scotoma

312      CAN J OPHTHALMOL—VOL. 40, NO. 3, 2005

Shared By: