Biomedical Engineering Reference
In-Depth Information
Hit rates and stimulation parameters (hex size, contrast, brightness, speed of
movement, number of trials, and number of choices) are recorded.
The four tests can be adjusted toward increasing demands for visual functions.
The software is not designed to automatically find an individual threshold,
it rather tests for an implicit threshold. That is, thresholds are predefined by
choosing certain stimulation parameters, the test results are then simply “above
threshold,” or “below threshold”. Changes during a therapeutical trial can be
monitored by hit rates and reaction times.
Individual thresholds can be found by repeating these tests with different
settings (e.g. different brightness), finding thresholds is described later in the
Section “Analysis of test results”.
Software Solutions: 2. BaGA Test to Asses Grating Acuity
Gratings are patterns of lines whose brightness changes in a sinusoidal or rectan-
gular pattern. The density of these lines is described by the spatial frequency.
Figure 5.5 shows a sinusoidal grating. Gratings have some limitations for testing
visual performance in natural systems due to the specific image-processing
properties of the visual system [16, 17]. It was demonstrated that the visibility
of orientation of the lines does not correlate under all circumstances with visual
acuity as commonly measured.
In amblyopic patients (who suffered deprivation of adequate visual stimulation
during a sensitive period in childhood) grating acuity can be considerably higher
than visual acuity measured with common optotypes. Hess et al. [18] gave a
good explanation for this phenomenon. They asked amblyopic patients to draw
exactly the pattern they can see when watching a grating. These drawings are
shown in Figure 5.6.
As can be seen, the gratings as seen by these patients show a high degree of
distortion. This is thought to be due to inadequate neuronal wiring at the visual
Figure 5.5. Example image of a high-contrast sinusoidal grating as used in the BaGA test.
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