Computer based studies

2.1.1 CRT monitors

Cathode ray tube (CRT) monitors have traditionally been considered to be the gold standard monitor for use in the psychophysical and physiological studies of vision. The image on a CRT monitor is generated by the excitation of a phosphor layer which emits light when struck by an electron beam. The electron beam scans from the top left to the bottom right corner of the monitor. The decay in the fluorescence of the phosphor in combination with the scanning of the electron beam results in a flicker at 60 to 120 hertz. Whilst invisible to the eye, this can make exact calculation of the stimulus duration difficult (Mulholland et al., 2015). There is also a lack of independence between neighbouring pixels such that interactions between adjacent pixels can affect luminance values (Pelli, 1997). CRTs are no longer commercially available, having been superseded by lighter

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and flatter liquid crystal display (LCD) monitors. However, whilst the limitations of CRT monitors are recognised, the critical shortcomings of current LCD displays mean that these screens are not widely used in vision research.

LCD screens consist of a layer of liquid crystals sandwiched between two polarising filters with a light source at the back of the monitor. As voltage is applied to the layer of liquid crystals, they align to block the light from passing through, the intensity of which is controlled by the voltage applied. Whilst these monitors do demonstrate pixel independence, the sluggish change induced in the liquid crystals means they are slower in response compared to CRT monitors with greater variability in response times over repeated measurements (Elze and Tanner, 2012). The resulting image persistence causes problems such as motion blur in rapidly changing stimuli (although some newer models of LCD screen overcome this using compensatory image processing) compared to the shorter- persistence phosphors used in CRT monitors (Wang and Nikolic, 2011, Ghodrati et al., 2015). In addition, the luminance of LCDs can depend strongly on the viewing angle, a problem that persists even with newer models (Wang and Nikolic, 2011, Ghodrati et al., 2015). This is particularly relevant for experiments reported in this thesis in which stimuli are presented in off-centre locations whilst the observer maintains central fixation.

The computer-based studies for this research were thus conducted on one of two gamma (γ) corrected high resolution (1280 x 1024 pixels) CRT monitors; Dell Ultrascan P991 CRT monitor (Dell Corp. Ltd, Brackness, Berkshire, UK) for Aim 1 and Dell Trinitron P992 CRT monitor (Dell Corp. Ltd, Brackness, Berkshire, UK) for Aims 2 - 5 and 8. A warming up period for the screens of at least half an hour

was allowed before any experiments were conducted in order to ensure that stable luminance outputs were achieved.

2.1.2 Visual stimuli

The high-pass stimuli (Figure 2.1) and conventional letters, both of a 5 x 5 unit grid design, were generated using MATLAB (version 7.6, Mathworks, Inc., Natick, MA, USA) and were presented using an Apple Macintosh computer (Apple, Inc., Cupertino, CA). True 14-bit contrast resolution was achieved in hardware using a Bits++ video processor (Cambridge Research Systems, Ltd., Rochester, UK).

Figure 2.1: The high-pass filtered letter set

The VO stimuli of a pseudo high-pass design were constructed with an inner black core flanked by a white border half the width of the central section. These stimuli were presented on a grey background such that the mean luminance of the letter was the same as the background. A ‘stroke width’ in these stimuli was considered to be the dark centre with the two white flanks. Screen luminance was measured using the Konica Minolta Chroma Luminance Meter (CS-100, Tokyo, Japan).

2.1.3 Experimental methods

In psychophysical experiments, four main methods which are described below are employed for determining end point thresholds. For the studies reported in this research, the method of limits or adjustment and maximum likelihood adaptive procedures were used.

Method of limits (MOL) or adjustment – The subject is asked to control the

level of the property of interest of the stimulus (‘stimulus level’) until a particular criterion is satisfied.

Staircase procedures – The stimulus level starts at a high level and is reduced

until a mistake is made by the subject at which point the staircase may reverse and is increased until a correct response is attained, whereupon another reversal takes place and so forth. The values of the reversals are typically averaged. The behaviour of the staircase can be altered using different step sizes and termination rules.

Maximum likelihood adaptive procedures – These are staircase procedures in

which the next presentation level of the stimulus is positioned each time at the maximum likelihood estimate of threshold using available information; specifically the subject’s previous responses and prior knowledge of their likely performance. QUEST (Watson and Pelli, 1983), an example of such an adaptive staircase procedure uses prior knowledge (represented by the prior probability density function) combined with information from previous trials (represented by the likelihood function) to form the posterior probability density function, to

decide the placement of the subsequent trial. The final estimate of threshold is based however only on the modal value of the likelihood function, in order to avoid bias induced by prior experience. QUEST is used in a number of studies in this thesis with termination rules specified as a fixed number of trials.

Method of constant stimuli – The stimulus level is presented at a range of

predetermined values in an interleaved random order which prevents prediction or expectation but can result in a large number of trials. The percent correct for each level is then calculated and a frequency-of-seeing curve plotted.

2.1.4 Stimulus presentation times

Seiple et al., (2001) demonstrated that thresholds for letter identification require a stimulus duration far longer than that needed for letter detection. The computer based test letter stimuli in the studies in this thesis were presented for 500 ms, based on the reported critical duration time for foveal visual acuity based on letter identification for background luminances ranging from 10 to 200 cd/m2

(Niwa and Tokoro, 1997). Whilst critical duration times for letter identification are longer in the periphery than the fovea (Seiple et al., 2001), stimuli durations of 500 ms were also used for peripheral resolution experiments in order to prevent thresholds being affected by eye movements (voluntary or involuntary) to the stimulus location.

2.1.5 Refractive error determination and correction

Refractive error for foveal viewing was determined initially using retinoscopy. The appropriate lenses were placed in a trial frame in front of the test eye in line

with the foveal stimulus with the fellow eye occluded. This was subjectively refined by maximising the contrast of a high SF target displayed on the CRT monitor at the relevant working distance (Thibos et al., 1996). A similar procedure was used for refractive error correction at 10 degree eccentricity in the nasal field (temporal retina) used for a number of the studies with the lenses positioned in front of the test eye in line with the peripheral stimulus.