Foveal Letter Research Articles

Upturn of the contour-interaction function at small flanking bar-to-target separations

Nearby flanking bars degrade letter identification and resolution, a phenomenon known as contour interaction. However, many previous studies found that the relationship between foveal letter identification and flanker separation is non-monotonic, with an upturn in performance at very small target-to-flanker separations. Here, we replicate this observation and show that a similar upturn occurs also for targets presented at 5 deg in the inferior field, if the target-to-flanker separation is sufficiently small. The presence and magnitude of the observed performance upturn depends on the flanking-bar width, being more evident for narrower compared to wider flanking bars. We interpret our results to indicate that neural interactions between nearby contours reduce performance when the target and flanking bars form discrete neural images. At sufficiently small separations, the images of the target and flanking bars can not be distinguished and performance is governed by the contrast of the target in the blended neural image.

Evaluation of a New Clinical Test of Fusion Status: A Pilot Study

Background Suppression is associated with binocular vision conditions such as amblyopia and strabismus. Commercial methods of testing fusion often only measure central fusion or suppression at near. The purpose of this pilot study was to assess a new iPad picture fusion test that assesses foveal and central fusion at near. Methods Participants aged 5 years and older presenting for eye examination at The Ohio State University College of Optometry were enrolled. Results from visual acuity, dry and wet refraction/retinoscopy, stereopsis and cover testing were recorded from the patient chart. The iPad picture fusion test, Worth four-dot, Worth type test with foveal letter targets, and Polarized four-dot were performed by one examiner in a randomized order at 40 cm. Testing was repeated with the anaglyphic filters reversed. Crosstabulation and McNemar chi-square analysis were used to compare the results between fusion testing devices. Results Of the fifty participants (mean age = 17.5), twelve reported suppression and one reported diplopia. Testability was excellent for all tests (98% to 100%). There were no significant differences between tests in reported results (P ≥ 0.22 for all comparisons). No difference in reported fusion or suppression status was observed with change in orientation of the anaglyphic filters. Six participants reported foveal suppression alone at near which was not identified with Worth four-dot at near. Conclusion The iPad picture fusion test provided excellent testability and agreement with commonly used tests of fusion and allowed testing of both central and foveal fusion at near. Nearly half (46%) of participants with suppression reported foveal suppression, supporting the importance of testing for foveal suppression.

An electrophysiological investigation of orthographic spatial integration in reading

During reading, word recognition speed is influenced by the amount of orthographic overlap with surrounding words. The nature of this phenomenon is not understood: some theories attribute it to low-level visual operations (i.e., parafoveal feature detectors influencing foveal letter detectors), whereas other theories assume that orthographic processing (i.e., letter position coding and word activation) occurs across multiple words in parallel. To arbitrate between these theories, we used electroencephalography to reveal the time course of orthographic spatial integration in a lexical decision task. Foveal target words were flanked on each side by parafoveal words, manipulated across three conditions: repetition flankers (e.g. rock rock rock), unrelated flankers (step rock step) and a no-flanker condition. Linear mixed-effect models were constructed to analyze EEG data on a trial-by-trial basis. Word recognition was worse in the unrelated flanker condition than in the repetition and no-flanker conditions. This behavioral pattern was accompanied by increased negativity in the N250 and N400 windows, associated with the activation of sub-lexical and lexico-semantic representations, respectively. Crucially, the absence of effects prior to 200 ms post-stimulus onset provides evidence against the involvement of low-level visual processes. We conclude that orthographic spatial integration is driven by parallel processing of multiple words, which leads to the activation of a larger set of sub-lexical nodes and more difficult processing at the lexical level when those words are orthographically unrelated.

Effects of flanker type and position on foveal letter recognition

Our ability to identify a foveally viewed letter at resolution threshold reduces when flankers are placed at a certain distance from it, compared to when it is presented on its own. In this study, we investigated how type and position of flankers influences foveal letter recognition. We measured participants’ performance to identify unflanked Sheridan-Gardiner letters (A, H, O, U, T, V or X), using a seven-alternative-forced-choice paradigm with the method of constant stimuli to obtain 80-90% correct response. Performance was measured again to identify a target letter in the presence of different flanker types. Flankers were bars and letters (placed either to the left and right of the target, above and below the target or on four sides of the target), and a surrounding box. Separation between the target letter and flankers varied between zero (abutting) and ten stroke widths (two letter sizes). For all flanker types, separation between the target ad the flankers significantly influenced performance (all p < 0.0001). Flankers abutting the target and at separation of one stroke width caused the largest reduction in performance, regardless of flanker type. For bar flankers, the largest drop in performance (up to 50 percent) occurred with bars placed on all four sides of the target. For letter flankers, flankers placed above and below the target reduced performance as much as four surrounding flankers. At separation of one stroke width, flanking letters and bars caused a further 10% reduction in performance than a surrounding box. Our results would have significant implications on the design of crowded visual acuity charts, especially those that are available on the market, which have different crowding features between them.

Foveal letter crowding: Is it due to contour interaction or gaze instability?

Foveal letter crowding: Is it due to contour interaction or gaze instability?

Effects of ocular transverse chromatic aberration on near foveal letter recognition

Transverse chromatic aberration (TCA) smears retinal images of peripheral stimuli. In reading, text information is extracted from both foveal and near fovea, where TCA magnitude is relatively small and variable. The present study investigated whether TCA significantly affects near foveal letter identification. Subjects were briefly presented a string of five letters centered one degree of visual angle to the left or right of fixation. They indicated whether the middle letter was the same as a comparison letter subsequently presented. Letter strings were rendered with a reddish fringe on the left edge of each letter and a bluish fringe on the right edge, consistent with expected left periphery TCA, or with the opposite fringe consistent with expected right periphery TCA. Effect of the color fringing on letter recognition was measured by comparing the response accuracy for fringed and non-fringed stimuli. Effects of lateral interference were examined by manipulating inter-letter spacing and similarity of neighboring letters. Results demonstrated significantly improved response accuracy with the color fringe opposite to the expected TCA, but decreased accuracy when consistent with it. Narrower letter spacing exacerbated the effect of the color fringe, whereas letter similarity did not. Our results suggest that TCA significantly reduces the ability to recognize letters in the near fovea by impeding recognition of individual letters and by enhancing lateral interference between letters.

A word-superiority effect in the presence of foveal load.

Foveal stimuli have been shown to disrupt visual information processing in the parafovea and periphery by their mere presence. In the present study, 6 subjects were presented letter triads 3.58 degrees to the right or left of the point of fixation. At the same time, a single letter was presented at the point of fixation that was either the same as the middle letter in the triad or different from any of the triad letters. On other trials, no letter was presented at the point of fixation. Analysis indicated a word superiority effect when a foveal letter was presented that was the same as the letter in the triad. Performance between words and nonwords did not differ significantly when the foveal letter was different or absent. It was concluded that the mere presence of foveal load alone is not disruptive to performance. Depending on the visual context of the target to be reported, the presence of a foveal stimulus may improve performance.

The effects of foveal load and visual context on peripheral letter recognition

Peripheral letter recognition was examined by presenting subjects with a letter triad centered 7.16 degrees to the right of fixation. At the same time, a single letter was presented at the point of fixation that was either the same as one of the letters in the triad or different from any of the triad letters. On other trials, no letter was presented at the point of fixation. Results supported and extended previous foveal load research. The disruptive effect of foveal load does not depend solely on the presence or absence of a foveal stimulus. Recognition of the first letter in non-words was disrupted when a foveal letter was presented that was different from the peripheral letter. Recognition of the middle letter for both words and non-words was disrupted when a foveal letter was presented that was the same as the middle letter. A significant interaction between foveal letter and triad type was also found. No evidence was found to suggest that recognition of a peripheral letter is significantly better when that same letter is also present at the point of fixation.

Size scaling and its effect on letter detection.

Two ways of scaling letter size across eccentricity were investigated in a choice reaction time (CRT) task. Experiments 1 and 2 tested cortical magnification theory (M-scaling), while Experiment 3 used scaled sizes drawn from Anstis's regression formula (A-scaling). Experiment 4 compared both scaling techniques, together with the effect of exposure duration and the absolute size of the foveal letter. Results showed that scaling effectiveness improved when large rather than small foveal letters were used. A-scaling with a large foveal letter provided a good fit for the data at parafoveal locations, but underestimated the letter sizes needed at large eccentricities. M-scaling with a large foveal letter size produced CRTs that were independent of eccentricity. Exposure duration did not substantially affect performance.

Letter Identification and Lateral Masking in Dyslexic and Average Readers

Lateral masking and letter identification in dyslexic and average readers were investigated with a methodology that corrected for some of the weaknesses of the Geiger and Lettvin studies (1986, 1987). Target letters were presented alone or embedded within three-letter arrays at retinal locations from 0 degrees to 15 degrees to the right or left of a fixation point. A foveal letter, which was the same as or different from a target letter, appeared as a distractor, and comparisons were made between scaled and unscaled stimuli. Dyslexic readers were found to be better than average readers at detecting scaled letters embedded in an array in some of the peripheral locations tested. Unlike the results of Geiger and Lettvin, however, this finding of better letter detection in the periphery by dyslexics was limited to selected conditions of the study. Reading groups were also found to differ in responding to the foveal distractor in the letter task. Detection of letters by average readers was affected by the type of distractor, but this variable was not found to influence the dyslexic readers. These findings suggest some differences between dyslexic and average readers in attention to stimuli presented at multiple locations in the visual field.

Sensory and cognitive determinants of reading speed

Fast readers and average readers were tested on four tasks. Neither peripheral letter identification nor susceptibility of foveal letter identification to patterned masking differed between the two groups. However, fast readers appear to pick up more information per fixation on structured textual material, as indexed by a forced-choice test. Furthermore, the average fast reader had a greater span of apprehension for unrelated elements. It appears that faster readers are able to encode more of the contents of each fixation, whether or not higher order linguistic structure is present. The results are inconsistent with the view that reading speed is dependent solely on the reader's ability to infer or fill in missing information.

Facilitation of foveal letter recognition by metacontrast

Foveal letter recognition is improved in the presence of a flanking letter context. Signal detection analysis is used to examine sensitivity changes attributable to specific contour interactions between target and context letters.