Technical specification of the Saccadometer system

Principle of Operation

Saccadometer is the medical research instrument, allowing to evoke the strictly controlled saccadic responses, measure it and automatically analyse their latencies and durations. Saccades results from observation, following with the eyes, the step-wise jumping of the visual target. Targets are provided by the laser spot projectors which are mounted directly on the fore-head support plate, which carries also the eye movement sensor assembly.

Eye movement measurement

Saccadometer measures the Eye Movements (EM) by utilizing the method of Direct Infra Red Oculography, which is embedded in the Cyclop ODS sensor. Its design is optimized for easy set-up and minimal intrusiveness, which are crucial for monitoring the oculomotor functions in the non-laboratory settings.

Unlike other IR-Oculography systems, which require to place the infrared emitters/sensors around the eye in its direct vicinity, the Cyclops-ODS opto-electronic transducers, are located between the eyes, thus hiding the sensor assembly behind the „shadow” of the nose. Such ergonomically optimised sensor arrangement, does not limit the visual field and the measuring system can work without interfering with the subject’s visual functions. It does not require any kind of mechanical adjustments of the sensor position in relation to the eye. All necessary adjustments are performed automatically by the Saccadometer control system.

The Cyclops-ODS sensor measures the resultant rotations of the left and the right eye taken together. Due to the conjugacy and synchronicity of the saccadic eye movements, the rotation of left and right eyes, can be added and averaged. The inner canthi of the left and right eyes are illuminated with the low intensity (see the safety considerations) IR light. The difference between the amounts of IR reflected back from the eye surfaces, carries the information about the eye position changes. (relative eye position measurement requires to define the initial fixation position). In the Cyclop ODS, both eyes contribute equally to the generation of the EM signal. The measurements of the left and right eye rotations average, are used as the standard EM measurement procedure in the clinical electro-nystagmography. The values acquired by this method are the sum of electro-oculographic potentials, received from the temple parts of the left and the right eye (Oculus Dexter Sinister - ODS). The Cyclop-ODS is equipped with the Automatic Gain Control (AGC) system, which compensates the differences of anatomical shapes of the nose and eye cavities. It allows to measure the eye movements with the same physical gain, equal for all subjects, without influencing the signal to noise ratio (constant gain of the photo-detecting circuitry).

Eye movement signal processing

The voltage generated by the EM sensor is digitised into the 12 bit data with the sampling rate 1kHz and afterwards differentiated into the eye velocity signal. The saccades are identified as the instances when the eye velocity exceeds the velocity threshold equal 5 deg/s. Saccadic latency is measured as the time difference between the switching ON the target on its new fixation position and the onset of saccade. Saccadic duration is defined as the time difference between the saccade onset and the instant when the eye velocity returns to the value below the 5 deg/s threshold.

Eye movement measurement quality

Measuring range

• along the horizontal axes ±35 deg

Bandwidth

• 500 Hz

Noise level

• Reference range: 20 degrees
• Peak to peak value: 0.51% of the reference range, what gives 0.1 degree
• Signal to noise ratio (SNR): 45.8 dB

Linearity

• The linearity of the Saccadometer system has been measured on the four subjects in ±10 deg range which is the operating range of the device. The subjects have been told to look at the fixations spots equally spaced 2 deg apart, thus for each subject 10 samples has been acquired. The calculated value of non-linearity is 2% in average and 4.5% in the worst case what gives 0.4 degree and 0.9 degree respectively.

Safety Considerations

Saccadometer, was designed following the strict rules of safety, as specified for medical instrumentation.

Electrical Hazard

The system is battery powered and the voltage in the system does not exceeds 3V. The mechanical arrangement of terminal sockets on the SaccadoMeter Control & Display Unit (CDU), excludes the possibility to connect erroneously another plugs-in. Ones the sensor, which is attached to the patient, has been connected to the CDU, the only socket left free, is the one provided for accommodating the optical fibber plug It makes the connection, between the patient and the computer (used optionally for saving on-line the test results), galvanically isolated by 1.5m plastic optical fibber.

Laser Radiation

The SaccadoMeter is using three laser spot projectors taken from the commercially available Laser Pointers (LP). The manufacturer provides following CAUTION information on the LP package:

LASER RADIATION DO NOT STARE INTO BEAM
Laser Diode Wavelength 630 – 680 nm
Max output < 1mw
CLASS LASER PRODUCT

The brightness of the laser spots don’t need to be that high, as provided by the original LP. As the first step, the laser output power was reduced by factor 0.6, by means of decreasing the laser supply voltage from 4.5V to 3V, without changing the current limiting resistor (31 Ohms provided by the LP manufacturer). The further reduction of the laser spot brightness,was achieved by powering the laser diode with 50% duty cycle, what reduced the average laser output by factor 0.5. Additionally the user can reduce the laser duty cycle further, down to 0.15 (in four steps) and depending on the room illumination, adjusts the laser spot brightness to the minimum acceptable level. The appropriate CAUTION – Do Not Stare Into Beam, is placed on the front side, of the forehead laser assembly mounting plate.

Level of IR irradiance of the eye surface

According the calculations and direct measurement the IR irradiance does not exceeds 0.6 mW/cm2 and is well below the level accepted as the safe for the eyes (10 mW/cm2) Please refer to the measurement protocol (Saccadometer eye surface irradiation report).

The level of irradiance depends on the output of IR illuminators, as well as on the distance between the IR source and the illuminated surface (eye cavity). The irradiance level,was calculated assuming the average distance of 10 mm. This distance can vary depending on the anatomical shapes of the nose and the eye cavity. Normally it cannot be excluded that the sensor will be placed closer to the eye, what will cause the increase of the eye IR irradiance. In the Saccadometer due to implementation of the active adjustment of the IR illumination,such situations are excluded. The Saccadometer control system automatically compensates the varying distance by adjusting the intensity of IR source. When the IR illuminator comes closer to the eye surface, its intensity is automatically reduced, thus the IR irradiance can be held constant on the calculated level.

Certification

At this stage of development, the SaccadoMeter can be considered as the custom made-experimental device. It was designed and manufactured to fulfill the highest standards of SAFETY, which are required for the medical diagnostic device.