This study

The results from this study show that the decline in log contrast sensitivity in the central visual field is non-linear and is best fit by a bilinear function, where the initial slope is approximately twice as steep as the subsequent shallower decline. The scale invariance found by previous investigators (e.g. Pointer & Hess, 1989) is confirmed, as are the two visual field anisotropies (both the horizontal-vertical and superior-inferior anisotropies from Abrams et al., 2012). The results show that the stimulus orientation effects, if present at all, are small enough to not be a necessary feature in a model of the central nine degrees of the visual field. This allows a simple two-dimensional attenuation surface to be developed via radial interpolation of the bilinear functions that are fitted to the data, providing a generalised map of contrast sensitivity to stimuli of arbitrary orientation in the central visual field.

4.3 Methods

4.3.1 Equipment

Three experimental set-ups were used. In each case, stimuli were stored in a CRS ViSaGe and presented on a gamma-corrected CRT monitor (Nokia Multigraph 445X, Philips MGD403, or Eizo Flexscan T68). All monitors had a refresh rate of 120 Hz, and mean luminances varied from 60 to 85 cd/m2 _{between the monitors. The stimuli had 12 pixels per carrier cycle for}

spatial frequencies of 2 to 4 c/deg (sufficient to avoid luminance artefact problems that might arise from adjacent pixel non-linearity, see García-Pérez & Peli, 2001). The viewing distance for the 4 c/deg stimuli was 1.19 metres. At this distance, 48 pixels on the screen subtended 1 degree of visual angle. The viewing distance was adjusted to scale the retinal image to the desired spatial frequency (59.5 - 119 cm for the range 2 - 4 c/deg). For stimuli with a spatial

frequency below 2 c/deg, the stimulus was first doubled in size on the screen (24 pixels per car- rier cycle) and the viewing distance was adjusted appropriately (41.7 - 83.3 cm for the range 0.7 - 1.4 c/deg).

In Experiment 1, the two principal observers (ASB and DHB) used different equipment setups. To ensure that this was not responsible for any differences in their results both observers ran a subset of the Experiment 1 conditions on the other equipment to that on which they collected their Experiment 1 data. The results from each laboratory were found to be in agreement within observer, rather than being dependent on which equipment was used.

4.3.2 Stimuli Horizontal Left oblique Right oblique Vertical

Figure 4.1: Cartesian-separable log-Gabor stimuli generated in cosine-phase with ori- entations of (left to right): 90◦_{, 135}◦_{, 45}◦_{and 0}◦_.

The stimuli for this experiment were luminance-modulated cosine-phase log-Gabors patches (see General Methods chapter, Section 3.4). For the main experiments here, the stimuli used had a spatial frequency bandwidth of 1.6 octaves (full width at half height) and an orientation bandwidth of ±25◦ _{(half widths at half height). These bandwidths were chosen in order to}

attempt to match the stimuli to the receptive fields found in V1 (see Section 3.4.5). The log- Gabors used in this study were presented at four different orientations (horizontal: 90◦_{, left}

oblique: 135◦_{, right oblique: 45}◦_{and vertical: 0}◦_{; see Figure 4.1) and six spatial frequencies}

(0.7, 1, 1.4, 2, 2.8 and 4 c/deg). The stimulus duration was 100 ms. Stimulus contrasts were calculated as delta-contrast and expressed in dB re 1% (see Section 3.2).

4.3.3 Observers

Data were collected from four observers: ASB, DHB, SAW and TSM. The observers were 22, 28, 44 and 46 years old respectively, and wore optical correction appropriate for the viewing distances tested when required. Experiments were performed binocularly with natural pupils.

4.3.4 Procedures

In Experiment 1, 4 c/deg log-Gabor stimuli of all four orientations were presented at four ec- centricities (0, 6, 12 and 18 cycles; or 0, 1.5, 3 and 4.5 degrees of visual angle) along eight hemi-

0° 45° 90° 135° 180° 225° 270° 315°

F

Figure 4.2: Diagram showing the four meridians tested in these experiments, and the ec- centricities (in carrier cycles) along those meridians that were used in Experiment 1. “F” marks the fixation circle. “+ve” and “-ve” labels refer to the direction along the meridian that is plotted in the graphs below.

meridians (0◦_{, 45}◦_{, 90}◦_{, 135}◦_{, 180}◦_{, 225}◦_{, 270}◦_{and 315}◦_{) radiating from the centre of the vi-}

sual field. A diagram of these locations is shown in Figure 4.2. Stimuli were blocked such that in each session thresholds were only being determined for a stimulus of a single orientation at a single position in the visual field (i.e. this design does not feature any extrinsic uncertainty). This gave 100 blocks (the 4 patch orientations in Figure 4.1 at each of the 25 locations in Fig- ure 4.2), which were all repeated in a randomised order four times by observers ASB and DHB. Two more observers (SAW and TSM) provided additional data for a subset of the conditions. Thresholds were measured using a two-interval forced-choice (2IFC) three-down, one-up stair- case procedure (see General Methods chapter, Section 3.5). Feedback was provided. Each condition was repeated four times by each observer. Contrast detection thresholds for each repetition were calculated using a probit fit to the staircase data (see General Methods, Sec- tion 3.6), allowing the mean and standard error to be calculated across repetitions.

To reduce extrinsic uncertainty, there was a continuously visible circle (diameter of 3 carrier cycles, line width of 1 pixel, with a contrast of 25%) placed to surround the location where the target would appear. An identical circle was also used for fixation, such that in the condition where the target was at fixation the two circles were coincident. In Experiment 2, the circles were replaced with pairs of dots as described in the results section for that experiment. For the majority of the experiments the observer fixated in the centre of the monitor, with the stimuli appearing at a location on the screen relative to this central fixation. This differs from

the more conventional method where the stimuli are located in the centre of the display in or- der to avoid potential problems that might arise from inhomogeneities in the monitor (such as variations in luminance across the display, see García-Pérez & Peli, 2001). A subset of condi- tions from Experiment 1 were retested where the target appeared in the centre of the screen and the fixation point was placed at various positions on the monitor to control where in the visual field the stimulus was presented. The results from the two different experiments were generally in agreement, with no systematic differences between them.

The methods for the other experiments were similar to those for Experiment 1, with the dif- ferences described in the relevant part of the Results section.