IV.
THE
BABY
EYE
PAD
TOOL
1. The whys and the wherefores
Since a battery of tests for early evaluation of vision in children is now available, research is engaged in developing tools which could be both more adequate to the particular characteristics of the little patients and practical for the clinical use. On account of the previously treated arguments, we can assert that it is necessary to find the best agreement between the experimental accuracy and the flexibility needed when we reach a behavioural response in neonates. Furthermore, it is necessary that the assessment tools be the handiest possible ones, to fulfill the ambition to use them in a very early period of life, for example in preterm children in a Neonatal Intensive Care Unit (NICU).
The Roman research group of Ricci, Romeo, Mercuri et al. [22-24, 74, 87] described their experiences with a portable kit containing a battery of nine tests, which can be administered to 31 weeks of gestational age preterm newborns. These tests can detect visual abnormalities also at this early age and the authors describe the kit as handy and portable, notwithstanding the several different elements in which it is composed.
The team of the 0-2 Neurology Section of the Stella Maris Institute had the idea to use the new portable technology features of a tablet to present visual stimuli to children. Thanks to the work of Raffaele Mazziotti, psychologist engaged in research of the plasticity of the visual system, and Bruno Zanchettin, Java programmer, an application called ViEye, for AndroidTM platforms, has been realized.
ViEye allows the administration of some visual behavioural tests, commonly utilized in the clinical practice. In the complete project the application will permit the execution of several different visual tasks: Fix Shift (competitive and not competitive); Contrast Sensibility; Fix and Follow tasks (in a vertical and horizontal way); Static and Drifting Visual Acuity. A further development of the project, which could improve the application, would be the addition of other tests to assess different proprieties of the visual system, in order to have a diagnostic battery adaptable to any clinical or research necessity.
36 In order to evaluate the usability and the reliability of the EyePad tool, a population of low risk term-newborns has been submitted to a static visual acuity evaluation with a Teller Acuity Cards-like procedure. For this work it has been used an AsusTM Transformer Prime TF201 tablet, running AndroidTM 4.1.1 system.
The goals of this work were: to describe the clinical experience; to demonstrate de external validity of the test’s results, comparing them with the normative data of healthy newborns described in literature; to demonstrate the internal validity of the test’s results by a test/re-test analysis.
1.1 The weighted up-down method
The embedded algorithm of the application may generate the function that describes the examined visual competence (psychometric function), and it converges into a resulting value calculated with the method described by Christian Kaernbach [72]. Criteria described in this paper established the law ruling the direction and the step-size between a grating stimulus width and the following one. The forward or backward variation is defined by the subject’s response to the stimulus.
In the simple 1-up 1-down method, the stimulus difficulty is incremented by 1 for each correct response. In case of incorrect response, it is decreased by 1. This method will converge into the half point of the psychometric function (X50). In case of tasks in which the chance performance is high, for instance the possible operator error between two alternative responses (chance performance 50%), the converging point in X50 is not correct. It is necessary to switch the converging result to a point of the function depending on the weight we attributed to the chance influence: X75 would be the halfway point and the most natural choice for a threshold estimate in this kind of tasks. Similar criteria were adopted in the FPL experiment designs of the ‘70s, in which the 75% of the correct trials were to be achieved to define a positive response. The weighted up-down method can converge to any desired X point on the psychometric function, allowing the application to obtain the optimal step size for X75.
The rule is quite similar to the simple up-down rule: each correct response leads to an increase in signal level, each incorrect response to a decrease of it. But the step size, Sup, for upward steps may now differ from the step size, Sdown, for downward steps. The equilibrium condition for convergence point Xp is:
The weighted up-down method is not restricted to forced-choice tasks. For instance, a series of yes/no tasks with 50% noise presentations will allow a chance performance (a probability of correct answer) of 50%.
The application runs this procedure automatically. Therefore the tester is masked from the spatial frequency in each trial, and the psychometric function is derived rigorously from a pseudorandom rather than the sequential presentation of the grating width made in Teller Acuity Card assessment.
1.2 The Static Acuity test in ViEye
The visual acuity test in ViEye reproduces the FPL paradigm but, in this instance, it is not necessary to repeat a fixed number of trials, for each grating width, to achieve 75% of the positive responses. The ViEye algorithm extrapolates the acuity value in the X75 point of the psychometric function by the method described above.
The test battery frequencies are previously inserted by the operator. The battery can contain several values, compatibly with the maximum resolution of the tablet screen.
The operator holds the EyePad showing the screen to the baby, and he can only see the backside of the instrument.
Therefore, the operator is blinded to the grating width and to its position, on the left or on the right side of the screen. In this manner the trial response is not biased by tester’s expectations and the objectivity of the rigorous FPL method is preserved.
38 The tester observes the directional visual response of the baby, and transfers the answer to the EyePad simply touching the screen on the side in which he judges that the stimulus is.
The EyePad notifies with a short vibration that it received the response, so the operator has a feedback from the instrument. The grating presentation is followed by a neutral stimulus which has the function of recalling the infant’s attention on the screen, and consists of a schematic moving human face. When the operator judge that the baby’s attention is focused on the EyePad, he touches the screen again, and a long vibration signal alerts him that a new grating stimulus has been presented.
- Settings.
Stimuli are presented at a distance of 25-30 cm from the EyePad screen, a parameter which can be modified in the application settings.
The brightness of the screen has been set at its 25%, corresponding to 31cnd/m2. This value have been chosen after some preliminary trials, which evaluated the responses of babies with the same characteristics of the experimental population. The gray scale in the application was
Fig.1: Two examples of the display appearance during the visual acuity test, with two possible grating widths
gamma corrected for this value of brightness to obtain the best resolution of grating stimuli and grey background.
A testing session that contained a set of eleven different grating frequencies in cyc/cm has been created. The frequencies are transformed in cyc/deg automatically by the instrument, on the basis of the distance from the screen. The value of 1.4 cyc/deg has been described in literature as the mean visual acuity value for babies in the first week of life [73, 74], and this has been chosen as midpoint value of the frequencies of the test battery. Proceeding with a ½ octave step between a frequency and the next one, it has been inserted four higher frequency stimuli and six lower frequency stimuli in the battery.
To assess older infants it is possible to modify the grating width values, according with the infant’s age-related acuity characteristics. Moreover, for older infants who can be bored by the monotonous presentation of gratings, it is possible to vary between five different neutral stimuli (pointer faces), which appear after a grating, in order to maintain the infant interest during all the testing time.
