Biomedical Engineering Reference
In-Depth Information
established featuring new visual properties. We cannot expect this new system
to behave in a manner fully analogous to the natural system, nor can we assume
that a technical device in a biological environment will perform as predicted on
technical requirement specification sheets. For this reason it is indispensable to
develop a specialized set of tests for evaluating visual functions in patients with
an AVD.
In this chapter, we will briefly review standard methods to assess visual
functions and their limitations for testing patients with AVDs. Next we will
describe the theoretical concept of a new test design based on three main pillars.
These pillars will then be discussed in detail and actual implementations for each
subtest will be presented.
Testing Visual Functions in Normal Individuals
and its Limitations in AVD
Tests used today for assessing natural vision are designed to measure
psychophysical thresholds for specific subfunctions like light sensitivity and
resolution, or they qualitatively assess functions like stereopsis or certain color
vision tests. The most widely used method to describe visual function is visual
acuity testing. This test gives a fast first impression about the integrity of the
entire system, as it depends on the proper function of optic, sensory, and neuronal
systems. Visual acuity is the measurement of the ability to discriminate two
stimuli separated in space at high contrast relative to the background. Different
charts and characters can be used, and the test result can be expressed with
various types of measurements like Snellen notation, logMAR, decimal acuity,
or log (decimal acuity)
logMAR [1, 2]. Some specialized tests for the
assessment of very low vision have also been developed (e.g. LoVE [3]).
The visual field can be examined using either static or kinetic perimetry. The
first of these presents small light stimuli on a hemisphere with various degrees
of eccentricity from the center. By changing light intensity the threshold of
perception for each individual spot can be determined. Kinetic perimetry uses
a moveable light point that is gradually shifted from the far periphery into
the center. The eccentricity is recorded at which the patient can first see this
target. This procedure is repeated for several directions, light intensities, and
target sizes. Thus rings of equal light sensitivities (“isopters”) are determined.
Prerequisite for useful test results are good compliance of the patient and stable
fixation of gaze on a central target.
Electrophysiological examinations are capable of detecting fluctuations in
locally measured electrical potentials generated by neural activity. Depending on
the positioning of electrodes and test procedures, different functional elements
can be tested. The electroretinogram (ERG) detects light-evoked potentials
generated in the outer retina (a-wave) and inner retina (b-wave) [4]. Inner retina
function can be assessed by the pattern ERG [5].
Visually evoked cortical potentials (VEP) can be detected as responses of
the visual cortex to a visual stimulus [6]. Area V1 of the brain is responsible
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