Foveal contour interactions and crowding effects at the resolution limit of the visual system

Abstract

We describe several experiments on contour interactions and crowding effects at the resolution limit of the visual system. As test stimuli we used characters that are often employed in optometric practice for testing visual acuity: Landolt C's, Snellen E's, and rectangular gratings. We tested several hypotheses that have been put forward to explain contour interaction and crowding effects. In Experiment 1 and Experiment 2, Landolt C's were the test stimuli, and bars, or Landolt C's, or gratings served as distractors. In Experiment 1, we showed that neither scale invariance nor spatial frequency selectivity is a characteristic of foveal crowding effects. These results allowed us to conclude that mechanisms other than lateral masking contribute to observers' performance in 'crowded' tasks. R. F. Hess, S. C. Dakin, and N. Kappor (2000) suggested that the spatial frequency band most appropriate for target recognition is shifted by the surrounding bars to higher spatial frequencies that cannot be resolved by observers. Our Experiment 2 rejects this hypothesis as the experimental data do not follow theoretical predictions. In Experiment 3, we employed Snellen E's, both as test stimuli and as distractors. The masking functions were similar to those measured in Experiment 1 when the test Landolt C was surrounded by Landolt C's. In Experiment 4, we extended the range of test stimuli to rectangular gratings; same-frequency or high-frequency gratings were distractors. In this case, if the distracting gratings had random orientation from trial to trial, the critical spacing was twice larger than in the first three experiments. If the orientation of the distractors was fixed during the whole experiment, the critical spacing was similar to that measured in the first three experiments. We suggest that the visual system can use different mechanisms for the discrimination of different test stimuli in the presence of particular surround. Different receptive fields with different spatial characteristics can be employed. To explain why crowding effects at the resolution limit of the visual system are not scale invariant, we suggest that a range of stimuli, slightly varying in size, may all be processed by the same neural channel--the channel with the smallest receptive fields of the visual system.

Figures

Figure 1

Examples of the stimuli used in the experiments.

Figure 2

Percent of correct responses is plotted against spatial separation between the test Landolt C and four surrounding bars (see Fig.1, a) for five observers. The legends on each graph give the sizes of the test Landolt Cs.

Figure 3

Percent of correct responses is plotted against spatial separation between the test Landolt and surrounding bars (Fig.1, a), or surrounding Landolt Cs (Fig.1, b), or rectangular gratings of different frequency (Fig.1, c, d, e).

Figure 4

Amplitude difference spectra for a test Landolt C surrounded by four tangential bars, and one of the possible layouts of surrounding Landolt C's (all letters are rotated to the left). Separation between the test and the distractors increases from the top to the bottom from 0 separation (upper curves) to 5 bar widths (bottom curves). Black lines denote amplitude difference spectrum for the isolated Landolt C, red lines show amplitude difference spectrum for the same Landolt C in the presence of the surround.

Figure 5

Percent of correct responses plotted against spatial separation for three observers (MD, NT and KT). The Landolt C was surrounded either by four tangential bars (black lines and black symbols), or by four Landolt C's rotated outwards (red lines and red symbols), or by four Landolt C's rotated inwards (green lines and green symbols), or by four Landolt C's facing left (blue lines and blue symbols). The vertical lines correspond to one, three and five gap widths. Error bars represent SEM.

Figure 6

Percent of correct responses is plotted against spatial separation between the test Snellen E and four surrounding Snellen E's (see Fig.1, h). The dashed vertical line shows the critical spacing for observer IR; the solid vertical line shows the critical spacing for observers TA and EL.

Figure 7

Percent of correct responses is plotted against spatial separation between the test rectangular grating and four surrounding gratings of different spatial frequency having random orientation, Solid vertical lines denote the critical spacing when surrounding gratings had the same spatial frequency as the test grating; dashed lines show the critical spacing when surrounding gratings were of high spatial frequency.

Figure 8

Same as Fig.7, but in this experimental series the surrounding gratings had constant orientation from trial to trial.

Fig.9

Top panel: Weighting functions of bar-detectors (left) and edge-detectors (right). The test Landolt C is superposed so that its centre coincides with the centre of the weighting functions; the diameter is equal to the size of excitatory area of the line detector. Bottom panel: Theoretical masking functions calculated using bar- and edge detectors.