Crowding between first- and second-order letter stimuli in normal foveal and peripheral vision

Abstract

Evidence that the detection of first- and second-order visual stimuli is processed by separate pathways abounds. This study asked whether first- and second-order stimuli remain independent at the stage of processing where crowding occurs. We measured thresholds for identifying a first-order (luminance defined) or second-order (contrast defined) target letter in the presence of two second- or first-order flanking letters. For comparison, we also measured thresholds when the target and flanking letters were all first or second order. Contrast of the flankers was 1.6 times their respective contrast thresholds. Measurements were obtained at the fovea and 10 degrees in the lower visual field of four normally sighted observers. Two observers were also tested at 10 degrees nasal visual field. As expected, in both the fovea and periphery, the magnitude of crowding (threshold elevation) was maximal at the closest letter separation and decreased as letter separation increased. The magnitude of crowding was greater for second- than for first-order target letters, independent of the order type of flankers; however, the critical distance for crowding was similar for first- and second-order letters. Substantial crossover crowding occurred when the target and flanking letters were of different order type. Our finding of substantial interaction between first- and second-order stimuli suggests that the processing of these stimuli is not independent at the stage of processing at which crowding occurs.

Figure 1

The trigram “skp” illustrated for the four testing conditions. The nickname by which we refer to each of the condition is given below each trigram: (top left) a first-order target (middle) letter flanked by two first-order letters, “111”; (top right) a second-order target letter flanked by second-order flankers, “222”; (bottom left) a first-order target letter flanked by second-order letters, “212”; (bottom right), a second-order target letter flanked by first-order letters, “121”. Regardless of conditions, the task of the observer was to identify the middle letter of each trigram. During testing (not in this figure), the visibility of the flanking letters was matched between first- and second-order letters (see text for details).

Figure 2

Threshold elevation—ratio of contrast threshold for identifying flanked and unflanked (single) letters—is plotted as a function of letter separation in degrees for the four trigram conditions. In each panel, data from individual observers are plotted as unfilled symbols (black: fovea; red: 10° lower field; blue: 10° nasal field). Solid lines represent the best fit curves (see text for details) through the foveal (black) or the combined peripheral (magenta) data. The black and magenta arrows represent the letter sizes (averaged across the observers) used at the fovea (1°) and at 10° eccentricity (both lower and nasal fields: 2.7°), respectively. At each retinal location (fovea or 10° eccentricity), three letter separations were tested, to which we refer as 1×, 1.2×, and 2× in the main text. Dashed lines represent the null effect (absence of threshold elevation). Error bars represent ±1 SEM.

Figure 3

Crowding obtained at the smallest letter separation (1×) is compared for the four trigram conditions (111, 222, 212, and 121) at the fovea (left panel) and at 10° eccentricity (right panel: red, lower visual field [LVF]; blue, nasal visual field [NVF]). Individual observers’ values are plotted as unfilled symbols of different shapes. Group-averaged values are represented by black (fovea) and magenta (periphery) filled circles, with the error bars representing the 95% CIs. Dashed lines represent the null effect (absence of threshold elevation).

Figure 4

The spatial extent of crowding (2× the critical distance, in degrees) is plotted as a function of trigram condition for the fovea (black circles) and 10° eccentricity (magenta circles). Dashed lines represent the receptive field sizes in V4 as reported by Smith, Singh, et al. (2001) at these two retinal locations (black: fovea; magenta: 10° eccentricity).