Determinación de carga sísmica

In this pilot study the ability of normal and low vision subjects to fixate targets at different retinal locations was assessed. A variety of stimuli was presented, some target arrays were stationary and others were moving text. In the case o f the latter, the stimuli were presented within a small portion of the raster, termed the

"window". The method of using a window to control the extent of text presented was similar to the technique devised by Legge et al (1985a), where a TV monitor was

masked to provide windows of different sizes. In this study, the window was 4.3 degrees horizontally and the vertical dimension varied with the size o f text. The size of the window limited the area o f retina used and was important in discriminating function at specific locations.

To quantify fixation stability, images of the subjects’ fundus were acquired throughout a period of fixation of 7.5 seconds. In all subjects there was sufficient contrast in these images that retinal features such as blood vessels could be used as landmarks for comparison between images. Images were stored on video tape (sVHS) and sample images were acquired by means of the Wild Vision frame grabber and digitised by the Archimedes computer. The number o f frames and the speed of acquisition of images was limited by the capacity of the computer. Forty four images were digitised in sequence at a rate of 6 per second. Of these the first 32 frames of good quality, e.g. unaffected by blinks, were used. Thus 5.5 seconds of fixation were sampled. One image of the sequence, usually the first, was taken as a "master", to which the other images were manually aligned by the operator using the computer mouse to superimpose prominent features. This alignment procedure was the same as the "flicker method" described in the previous pilot study (section 2.3.1). Alignment of the digitised images resulted in measurements of displacement on both the x and y axes which were recorded in pixels.

The X, y values provided a measurement of eye movements and these were used to calculate the Bivariate Contour Ellipse Area (BCEA) (Steinman, 1965;

Tatsuoka, 1971; Ditchbum, 1973a; Timberlake et al, 1986), which allowed comparison of our data with existing literature. The BCEA is a two dimensional ellipse which describes the retinal area within which the centre of the target was imaged 68% of the time, and it is expressed in min arc^ The standard deviations of the eye positions in the horizontal and vertical meridians were also calculated.

The shape and orientation of the bivariate contour ellipse, as shown on the graphs in the results section, depended upon the type of eye movements that

movements were identical in size and frequency then the ellipse would appear circular. However, if the observer made large horizontal scanning movements when reading the scrolling text then the ellipse would be extended horizontally. Similarly, if vertical eye movements were predominant, e.g. during letter recognition o f a large target, then the ellipse would be vertically orientated.

A computer programme was designed to handle raw data, to undertake the calculations and to draw the ellipses. Dummy data, generated by the computer, were used to provide evidence that the programme was functioning correctly.

The BCEAs were analysed using the Student’s t test and analysis o f variance (ANOVA) with multifactorial structure. As the data set was unbalanced the ANOVA required adjustment (Montgomery, 1984).

Subjects:

Twelve observers with no known ocular problems and seven patients with macular disease were recruited to the study.

Procedure:

The subject was positioned in a headrest specifically designed to minimise head movements. Dental bites would have been preferable method for minimising head movements, but some patients would have found this unacceptable.

With low contrast fixation targets it was often difficult for the operator to discriminate the stimuli against the background image of the fundus and as a result we developed a method of locating the stimuli by means of a calibrated grid on the computer monitor.

EXPERIMENT 1 : The effect of fixation target form and size.

The fixation stability of four normal and seven low vision subjects were studied using the following visual stimuli:

(i) "basic" Snellen E. For normal observers the letter size was 20 min arc, for low vision patients the basic size (i.e. minimum resolvable) varied up to 80 min arc.

or 8 and was used to study the effect of a larger stimulus on the fixation stability. (iii) fixation cross embedded in letters, e.g. TH +L H . This target was used to investigate the effect of contrast and texture around the fixation point.

For each individual subject the size and contrast (both positive and negative) o f the basic Snellen E and the embedded cross were selected such that the targets were easily recognised.

The preferred retinal location (PRL) was determined by asking each subject to fixate the stimulus. The operator then observed the location o f the stimulus on the fundus image displayed on the TV monitor.

This experiment was completed on two separate occasions on subject CJ.

A further eight normal observers were studied using targets of negative contrast only:

(i)-(iii) as above

(iv) a single cross, i.e. + , sized 20 min arc.

(v) a grid pattern of the letter H extending 100x100 min arc with a fixation cross in the centre.

All subjects were directed to "watch the target carefully and steadily". In addition, one observer (AM) was instructed to "hold the eye still on one specific point on the target".

EXPERIMENT 2: Alternative retinal locations (ARLs) for fixation in low vision subjects.

In four patients (JW, AA, DB and KA) a series of further observations were made, using targets (i) or (iii) above, in that they were encouraged to fixate with ARLs (i.e. not their primary PRL), either of their own choice or determined by the investigator.

EXPERIMENT 3: The effect of scrolled text on fixation stability.

window and four retinal locations were examined. Subjects were directed to hold fixation on a stationary fixation target (extending 10 min arc) within the SLO raster while sequences of random letters were scrolled at the fovea, and 2, 4 and 6 degrees superiorly. In practice, those patients with gross central scotomas were unable to locate the fixation target. This group were instructed to fix their gaze on an alternative fixation target which was the smallest they could see.

In the first set of measurements both positive and negative contrast text was used and the velocities were 0.6, 1.1 and 1.7 degrees/sec (these are referred to as speed

1, 2 and 3 respectively). After analysis of these data, a further group o f observers were examined with negative stimuli with velocities of 0.6 (speed 1), 1.7 (speed 3) and 4.4 (speed 4) degrees/sec.

The subjects were asked to identify the letters as they were scrolled across the fixation point or at the eccentric retinal locations. In order to preserve undisturbed fixation, subjects were not required to read the letters aloud. In a single

presentation some 30 letters might be viewed and it was unreasonable to expect the subject to recite such sequences. In this study our prime aim was to assess fixation stability but in section 2.3.3 below the accuracy of letter recognition was addressed in detail.