Biomedical Engineering Reference
In-Depth Information
Table 5.3. Elements of psychophysical testing.
Basic Light and Motion (BaLM)
Test
Evaluation of four basic qualities of
vision: light, temporal resolution,
light localization, and motion
Grating acuity (BaGA)
Definition of the theoretical
capacity of resolution of the device
Freiburg Visual Acuity and
Contrast Test (FrACT)
Computerized measurement of
visual acuity
still not able to recognize it. Distortion due to various reasons might be one
obstacle. Another possible situation that must be considered in which resolution
and recognition do not match is the generation of a patchy visual impression.
AVDs do not necessarily create a visual image that covers one contiguous area.
Rather a kind of patchwork of seeing and not-seeing areas may be present within
a certain area with restituted functions. Nevertheless, it is important even in
these situations to gain valid information about the resolution and visual acuity
an AVD could provide under optimal conditions.
For this reason we suggest a grating acuity test as a second level of
psychophysical testing. These tests are well known for determining optical
transfer functions of optical devices and use patterns with black and white lines
with different spatial frequencies. Certain limitations of this test method in the
human visual system are well known and will be discussed later on in detail.
Psychophysical testing can be completed by a standardized test for measuring
visual acuity, as in the Freiburg visual acuity and contrast test (FrACT) (see
<http://www.michaelbach.de/fract.html>. This is a fully computerized test to
estimate the psychometric threshold.
All three of these tests can be performed sequentially, using one common
platform, i.e. a computer with a customized display.
Electrophysiological tests in clinical trials of AVDs are of limited value. Their
principal application is in experimental animal testing. Measuring the ERG may
demonstrate the ability of an AVD to generate electrical potentials at a retinal
level or prove function and integrity of the device by recording electrical pulses
delivered by an AVD under biological conditions. This is suitable only for retinal
prostheses, and even in this case the conclusions from these results may be of
limited value.
The VEP can measure activation of the visual cortex after stimulation by
an AVD. However, this examination is not necessarily correlated with a useful
visual impression. For this reason we expect electrophysiological tests to be of
low validity and relevance in clinical trials and would not suggest using them as
a tool to determine the efficacy of such devices.
Technical Setup for Psychophysical Testing
All three psychophysical tests mentioned above are computerized tests. Herein
we describe a technical implementation as it has been used for evaluating Basic
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