A Real-Time and Portable Bionic Eye Simulator - Biomedical Engineering Systems and Technologies - page 34

All processing performed on the image stream from the camera is implemented us-
ing Verilog hardware description language. Unlike conventional code that is written for
execution on a processor that runs at a specific clock speed, Verilog describes the way
logic gates are to be arranged and connected and so is compiled into a synthesisable
logic solution that can be either synchronous (operate with reference to a clock), asyn-
chronous (without reference to a clock) or a mixture of the two. A Verilog solution was
chosen due to the ability to create functions that can run in parallel, resulting in a low
latency real-time system.

3.1 Visuotopic Mapping

Early physiological research [21,25] proved that 'points' in the visual field correspond
to specific locations on the visual cortex, inferring a 'map' or transfer function between
visual field points and the visual cortex. Furthermore, that map is mostly continuous
in that neighbouring points in the visual field correspond with neighbouring points on
the visual cortex. The map or transfer function which describes the translation of points
between the visual cortex to its corresponding points on the visual field is known as the
visuotopic map.
Due to the physiological non-linear properties of the visual cortex, the visuotopic
map is also non-linear and 'distorted'. In humans, the phenomenon known as cortical
magnification describes how a small region at the centre of the visual field, known as
the fovea, corresponds with a much larger area of the visual cortex [11,13]. Early work
by Schwartz [21] indicated an approximation to the mapping by a 'log-polar' represen-
tation, where linear points on the visual cortex correspond to eccentrically logarithmic
and angularly linear points in the visual field. The foveal region is represented this way
as a dense packing of points in the centre of the visual field which corresponds to a
disproportionately larger region on the visual cortex. Also important to note is that the
visual cortex is spread over both halves of the brain with the left visual cortex corre-
sponding with the right visual hemifield and vice versa, due to cross-over of the optic
nerves [3].
Mathematical models that came from this include the Monopole model (defined from
the 'log-polar' observations) [17,19,21], the Wedge-Dipole model (adds a second pa-
rameter to Monopole model to account for curvature in the periphery region of the vi-
sual cortex) [2,17] and more recently the Double-Sech model (adds a shear function to
the Wedge-Dipole model to account for changing local isotrophy as well as increasing
accuracy of mapping at higher levels of visual cortex V2, V3) [18,19].
As the implant is anticipated to consist of a linear array of electrodes, the resulting
phosphene pattern would not be linear but rather follow this log-polar mapping. It would
be useful and more accurate to model the output visualisation based off a mathematical
model of the visuotopic mapping. Since the implant is expected to be placed in the
primary visual cortex V1 and closer to the foveal side of the visual cortex, the Monopole
model was chosen to model the output visualisation as it was mathematically simpler
and still provides reasonable accuracy.
The Monopole equation (1) describes the left visual cortex ' w ' as a complex function
of the right visual hemifield ' Z w '. ' C ' is the set of complex numbers, and ' k 'isa

dilation factor constant.