Biomedical Engineering Reference
In-Depth Information
and chessboard as well as the grey table more rapidly. This shows that different tasks
may benefit differently from various modes. A significant learning effect was evident as
times and mistakes would decrease with repeated testing, probably leading to an even-
tual plateau point where times do not get much faster. It is apparent that as people keep
repeating a task they are unfamiliar with, they will improve at it. There should be no
reason why it is not the same when using a visual prosthesis simulator, or even a patient
with a visual prosthesis implant itself.
The Ball Interception Test results show a clear upward trend in successfully inter-
cepted balls as the frame rate is increased. This was exactly as expected, as a lower
frame rate would give a user less time to react and would not allow for smooth track-
ing of the ball's movement. The differences between the control mode and non-control
modes, as well as between the consecutive control modes themselves all showed sig-
nificant differences in results. It was noticed however, that a considerable number of
'misses' that occurred in the tests were due to misjudgement of where the user thought
they had actually moved the paddle to (the paddle would be offset from the balls final
location by only a small margin). This is clearly to do with the ability of the participant
to coordinate themselves with the paddle placement without actually looking at it. Al-
though, we did allow them some time to 'calibrate' with a simple hand-eye coordination
task before each test, the subject's still seemed to have some trouble.
5.5
Limitations of the System
While our system uses a physiologically based model for mapping of phosphenes, it
does not represent the gaze-locked nature of a cortical implant. In the case of a real
cortical visual prosthesis, the patient will not be able to focus on different points of
the visual field with eye movements. In our system however, the user is able to scan
the presented pattern voluntarily. To overcome this limitation, an eye-tracker would
be required to allow the system to move the pattern along with the movement of the
user's eyes, therefore 'locking' the gaze at a specific point (usually at the center) in the
presented pattern.
6
Conclusions and Future Work
This paper has presented a simulator for a cortical visual prosthesis. By addressing
fundamental limitations in current simulator systems through its portability, and phys-
iologically based phosphene mapping, the system has met expectations and makes a
good platform for investigation, improvement and tuning of algorithms for use with a
visual prosthesis. The completion of preliminary psychophysical testing has shown that
the number of greyscale intensities has a significant effect on results for certain tasks. It
was also shown that reducing the frame rate can have a significant effect on the ability of
the user to observe and interact with moving objects/things in the environment. Finally,
a learning effect was found to be present with repeated trials and will need to be ad-
dressed in future work with broader and more rigorous sets of psychophysical testing. It
is hoped that, through the use of the HatPack simulator and through further psychophys-
ical testing, valuable insight can be gained and used to improve the implementation of
future visual prosthesis devices.
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