Third International Visual Field Symposium - Imaging and Perimetry ...

Bebie et al. (1976), Koerner et al. (1977) and Spahr et al. (1978).
It is well known that the main source of errors stems, in careful determi-
nations of the static visual field, from the statistical nature of patient re-
sponses. The threshold luminance can only be statistically defined, namely
as that luminance which is perceived with a 50% probability. In order to
raise the probability of a ‘seen’ response from 16% to 84% the stimulus
luminance has to be raised by a factor of approximately two to four. Conse-
quently, the accuracy of thresholds derived from five to seven stimuli,
which is routine in static procedures, will necessarily be poor.
Apart from these procedural imperfections, the individual spread u
which characterises the ability of a subject to reproduce the same threshold
result is slightly increased by reversible long-term fluctuations of the true
threshold. Though, by definition, u includes all noise sources, we shall use
the term ‘measurement error’ for brevity throughout this paper.
Table 1 gives the mean fluctuations as derived from repeated determina-
tions of the visual field of 45 mostly elderly experienced patients with
various pathological disturbances. Independent examinations were separated
by an interval of several days. For kinetic perimetry, u has been obtained by
transforming isopter displays to profile sections and comparing these for
several independent examinations.
These figures may strike one as surprisingly high. They reflect, neverthe-
less, reliable values, provided that experienced subjects with pathological
disturbances of the visual field are involved. The conclusions to be drawn
for the evaluation of visual fields will be worked out in the following sec-
tion.
DETECTABILITY OF VISUAL FIELD DEFECTS
The detection of defects, which are either small in their spatial dimension,
or which exhibit only minor sensitivity reductions, is an extremely difficult
task. A critical discussion is intimately connected with the measuremept
error together with geometrical considerations concerning the examination
grid.
TubZe I. Measurement error ofstandardprocedures.Derivedfromrepeateddetermination
of 45 pathological visual fields. IJ: mean of individual error (and standard error of the
mean 0). d: population distribution of individual error. Numerical values are in dB
(1 dB = 0.1 log unit). Spahr et al. (1978).
Automated static perimetry
(Octopus)
Conventional static perimetry
(Goldmann perimeter)
Kinetic perimetry
(indepenknt examiners)
296
0 d
3.0 + 0.1 3.0 f 0.8
3.8 + 0.2 3.8 f 1.3
3.4 3.4 +_ 1.8