Biomedical Engineering Reference
In-Depth Information
Fig. 4.
Resultant visual field of implemented visuotopic map
w
=
klog
(
Z
w
+
a
)
∈
C
(1)
Visual field
Z
w
can be represented as a complex exponential where
r
represents the
eccentricity and
θ
represents polar angle.
Z
w
=
re
iθ
∈
C
(2)
Rearranging the Monopole equation describes visual field
Z
w
as a function of visual
cortex
w
.
Z
w
=
e
(
k
)
−
a
∈
C
(3)
The electrode array of the implant was assumed to be a linear array placed on the visual
cortex closer to the foveal region. The visuotopic map was created using MATLAB
and ported over to the FPGA for use as a large lookup table. Approximate values were
used for the Monopole equation parameters, which are reasonably consistent with the
various values used in the literature:
k
=15,
a
=0.7 [12,17,18]. The exact dimensions and
intended locations of the implant are still not known, the eccentricity and polar angle
were limited to an
18
×
18
linear array on the visual cortex that cover the following
values on:
r
=[10,40],
θ
=[-0.8
π
2
]. This only represents the left visual cortex,
corresponding with the right visual hemifield. The
18
,0.8
π
2
18
array was duplicated for the
right visual cortex, creating another array on the left visual hemifield. This produces a
total electrode count of 648. These assumptions were taken to make better use of the
limited screen resolution of the head-mounted display while remaining realistic to the
'log-polar' mapping of the visual cortex. However, new maps can be simply regenerated
on MATLAB to accommodate any changes to this and implemented into our system.
The resultant visual field of our implemented map is shown in Figure 4.
×
3.2
Averaging Sampler
Figure 5 outlines our averaging sampler implementation. After NTSC decoding, the
image stream from the camera is made available one pixel at a time in a sequential
fashion. As each pixel arrives at the sampling section of the system, its X & Y pixel
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