Visual Field Analyzer with Physiological Switch

Abstract
The project’s goal is to offer an improvement to an existing Visual Field test that is done nowadays. The test is not very credible because it’s based on the patient response to random blinking LEDs. The patient should tell by pressing a push button if he saw or didn’t see the LED. In order to neutralize the patient impact on the test, we checked the possibility of the pupil size conditioning . The conditioning assumption is based on the pupil size changes caused by external illumination changes . We want to see if we can use this pupil property as a “”physiological switch””.

The problem
The Visual Field test is performed using a device in shape of a half sphere with dozens of LEDs spread all over it’s surface . At the center of the sphere there is pedestal which the patient puts his head on , so his eye is equally distant (more or less) from every LED. During the test one of the LEDs is randomly turn on and the patient should press a push button to indicate that he saw the light. The patient not pressing the button after a blinking led is interpreted by the device as vision fault at the same spatial angle .
The problem is that the test is not very credible . The problem is ,among other things , that the test is based on the patient response which can be affected from many factors as tiredness , patient concentration extent , mental condition , etc .A new test results can be different even if taken few hours after the last test.

The basic approach
The basic solution of the problem is to create a situation in which the examination is taking place without any voluntary or objective response of the patient. In general – using physiological processes that taking pace in different conditions to get vision faults indications. The first idea that came up was to use skin conductivity changes caused by different environmental simulations as a physiological switch . We abandoned this idea because the external stimulus that can cause a measurable change in skin conductivity is very aggressive and unpleasant. The second idea , which we implemented and tested in this project , is using the pupil size sensitivity to external illumination changes . The way we wanted to exploit this pupil property is by creating a conditioning between the pupil size and external stimulus – LED and peripheral light . This is kind of “”physiological switch”” – the conditioning , the non voluntary change in the pupil size , that created after a learning process, will give an indication to vision faults.

Figure 1 – schematic description of the setup

Tools
As the basis of our setup we used the device that serves to do the examinations today OPTOPOL-PTS-900 . To this device we added a red LED at the center , infra red LED ,and peripheral light.

Figure 2 – picture of the device and a “”patient”” under our experiment

A camera is installed inside and being used to take serial snapshots of the eye.

Figure 3 – series of eye images taken by the installed camera

We controlled the light and the LEDs using a National Instruments data acquisition card of type PCI-6023 from with connection board BNC-2120 using Labview software. The image processing was also programmed in Labview.

Conclusions
1. Our experiments results don’t show the existence of conditioning response. 2. Using better equipment (camera , higher sampling rate ) may give different results.
3. After turning on the peripheral light – significant reduction of pupil size can be seen .
4. Under the assumption of this equipment usage – the conditioning method can’t be used as a solution to the given problem .

Acknowledgment
We are grateful to our project supervisor Dr. Danny Lange for his help and guidance throughout this work . We also would like to thank Johannan Erez for his support and for his technical guidance and to the other lab staff – Ina, Danny and Aharon . Thanks to Eli Shushan for his time and effort guiding helping us building control devices .