The adverse impact of adjacent contours on letter visual acuity is known as crowding but there is conflicting evidence that foveal crowding may be reduced or disappears under low contrast conditions. Potential differences in foveal crowding with contrast on clinical measurements of visual acuity, including test-retest repeatability, were assessed. Visual acuity was measured at the fovea on adult participants with normal vision under three different contrast levels (- 90, - 10 and - 5%). Three rows of 5 letters, each row differing in size by 0.05 logarithm of the minimum angle of resolution (logMAR) from largest to smallest were displayed at the center of a monitor. Crowding was varied by varying the separation between horizontally adjacent letters from 100% optotype size to 50%, 20% and 10% optotype size. Inter-row spacing was proportional to optotype size. Observers read the letters on the middle row only. Measurements continued by reducing the size of the letters until 3 or more errors were made and were repeated on two separate days. Visual acuity worsened as both letter contrast decreased and inter-optotype separation reduced (expressed as a percentage of letter width). When expressed in minutes of arc of separation the impact of crowding was the same across all contrasts. Crowding occurs for both high and low contrast charts and should be considered when assessing low contrast visual acuity. Test-retest repeatability showed little or no dependence on either contrast or inter-optotype separation.

• MeSH
• dospělí MeSH
• fovea centralis * MeSH
• lidé MeSH
• zraková ostrost MeSH
• Check Tag
• dospělí MeSH
• lidé MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

Nearby flanking objects degrade visual resolution. If the flankers are similar to the acuity target, this influence is called crowding (CW), whereas if the flanking stimuli are simple bars then the phenomenon is known as contour interaction (CI). The aim of this study was to compare the influence of the number and position of flankers on foveal CW and CI to investigate possible differences in mechanism of these two effects. Five normal observers viewed single, foveally presented Sloan letters surrounded by 1, 2 or 4 flankers (either a Sloan letter or one-stroke-width bars), presented at several edge-to-edge separations. Single flankers were presented in the right, left, top or bottom position, 2 flankers were placed equally to the right and left or top and bottom of the central target, and 4 flankers were equally spaced in all four directions. Percent correct letter identification was determined for each type, number, position and separation of flankers and confusion matrices were constructed for separations equal to 20% and 100% letter width. Increasing the number of flankers caused an increase in the magnitude of both phenomena. CW showed a greater magnitude than CI for higher numbers of flankers. Analysis of confusion matrices suggests that in addition to the edge-to-edge interaction that appears to mediate CI, letter substitution and feature pooling contribute significantly to CW when higher numbers of flankers are presented. Foveal CW is more strongly influenced by an increase in the number of flankers than CI, which can be explained by the presence of additional interaction effects.

• MeSH
• fovea centralis * MeSH
• lidé MeSH
• nahuštění v prostoru MeSH
• rozpoznávání obrazu MeSH
• vnímání tvaru * MeSH
• zraková ostrost MeSH
• Check Tag
• lidé MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

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.

• MeSH
• analýza rozptylu MeSH
• citlivost na kontrast fyziologie   MeSH
• dospělí MeSH
• lidé středního věku MeSH
• lidé MeSH
• nahuštění v prostoru MeSH
• senioři MeSH
• světelná stimulace MeSH
• vnímání tvaru fyziologie   MeSH
• zraková ostrost fyziologie   MeSH
• Check Tag
• dospělí MeSH
• lidé středního věku MeSH
• lidé MeSH
• mužské pohlaví MeSH
• senioři MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

SIGNIFICANCE: Both foveal and peripheral contour interactions are based on, as yet, unexplained neural mechanisms. Our results show that, unlike foveal contour interaction, peripheral contour interaction cannot be explained on the basis of the antagonistic structure of neural receptive fields. PURPOSE: Foveal contour interaction is markedly reduced for mesopic compared with photopic targets. This finding is consistent with an explanation based on the antagonistic structure of neural receptive fields. However, no reduction was found for low-luminance targets in the periphery, possibly because the luminances used previously remained substantially above peripheral scotopic detection thresholds. In this study, we compared foveal and peripheral contour interactions for long-wavelength photopic and mesopic targets, which would be expected to significantly elevate the peripheral retinal detection threshold. METHODS: Five normal observers viewed a randomly selected Sloan letter surrounded by four flanking bars at several edge-to-edge separations (min arc). Photopic and mesopic stimuli were viewed foveally and at 6° peripherally through a selective red filter that ensured that mesopic targets were within 1 log unit of detection threshold at both retinal locations. RESULTS: Whereas the magnitude of foveal contour interaction was substantially less at mesopic compared with photopic luminance (20 vs. 46% reduction of percent correct, on average), no significant difference was observed in peripheral contour interaction, which had average mesopic and photopic magnitudes of 38 and 40%. Moreover, confusion matrices representing photopic and mesopic contour interaction differed in the fovea but not in the periphery. The extent of contour interaction did not change with luminance at either retinal location. CONCLUSIONS: Our results indicate that, although the characteristics of foveal contour interaction can be accounted for by the antagonistic structure of neural receptive fields, the same mechanism is not compatible with the characteristics of peripheral contour interaction.

• MeSH
• dospělí MeSH
• fovea centralis fyziologie   MeSH
• lidé MeSH
• senzorické prahy fyziologie   MeSH
• světlo MeSH
• vidění barevné fyziologie   MeSH
• vnímání tvaru fyziologie   MeSH
• zraková pole fyziologie   MeSH
• Check Tag
• dospělí MeSH
• lidé MeSH
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• srovnávací studie MeSH

SIGNIFICANCE: Contour interaction describes an impairment of visual acuity produced by nearby flanking features, which exerts a significant impact in many clinical tests of visual acuity. Our results indicate that the magnitude of interaction depends either on the flanker contrast energy (i.e., the product of flanker contrast and width) or the flanker contrast alone, depending on the contrast energy of the flankers. PURPOSE: The discrimination of acuity targets is impaired by the presence of nearby flanking contours, a phenomenon known as contour interaction. METHODS: In this study, we measured percent correct identification for threshold size, high-contrast Sloan letters at the fovea and at 5° in the inferior visual field for different combinations of flanking-bar width, and Weber contrast corresponding to specific fixed values of contrast energy (width × contrast, in %-min arc). RESULTS: For flanking bars with low-contrast energy, contour interaction exhibited no systematic dependence on the flanking-bar width. However, when the flanking bars had higher contrast energy, narrower high-contrast bars produced significantly greater contour interaction than wider bars of lower contrast. CONCLUSIONS: The results are consistent with the interpretation that contour interaction depends primarily on the contrast energy of flanking contours when their contrast energy is low. As the contrast energy of the flanking contours increases, the magnitude of contour interaction depends on the flanker contrast. For high-contrast flanking contours, the magnitude of contour interaction saturates when the width of the flanking contours is approximately 20% of letter size.

• MeSH
• citlivost na kontrast fyziologie   MeSH
• dospělí MeSH
• fovea centralis MeSH
• lidé středního věku MeSH
• lidé MeSH
• senioři MeSH
• vnímání tvaru fyziologie   MeSH
• zraková ostrost fyziologie   MeSH
• zraková pole fyziologie   MeSH
• Check Tag
• dospělí MeSH
• lidé středního věku MeSH
• lidé MeSH
• mužské pohlaví MeSH
• senioři MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

In the present study, we asked whether contour interaction undergoes significant changes for different luminance levels in the central and peripheral visual field. This study included nine normal observers at two laboratories (five at Palacky University Olomouc, Czech Republic and four at the University of Houston, USA). Observers viewed a randomly selected Sloan letter surrounded by four equally spaced bars for several separations measured edge-to-edge in min arc. Stimuli were viewed foveally under photopic and mesopic luminances and between 5° and 12° peripherally for four different background luminances of the display monitors, corresponding to photopic, mesopic, scotopic, and dim scotopic levels. The extent of the contour interaction in the fovea is approximately 20 times smaller than in the periphery. Whereas the magnitude of foveal contour interaction markedly decreases with decreasing luminance, no consistent luminance-induced change occurs in peripheral contour interaction. The extent of contour interaction does not scale with the size of the target letter, either in the fovea or peripherally. The results support a neural origin of contour interaction consistent with the properties of center-surround antagonism.

• MeSH
• dospělí MeSH
• fovea centralis MeSH
• lidé středního věku MeSH
• lidé MeSH
• mladý dospělý MeSH
• noční vidění fyziologie   MeSH
• rozpoznávání obrazu fyziologie   MeSH
• senioři MeSH
• světelná stimulace MeSH
• vidění barevné fyziologie   MeSH
• zraková pole MeSH
• Check Tag
• dospělí MeSH
• lidé středního věku MeSH
• lidé MeSH
• mladý dospělý MeSH
• mužské pohlaví MeSH
• senioři MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• multicentrická studie MeSH
• práce podpořená grantem MeSH
• Research Support, N.I.H., Extramural MeSH

PURPOSE: The decrease in visual acuity under low luminance conditions is well known. Recent laboratory evidence showed that crowding under low luminance (mesopic) light levels is less robust than under photopic conditions. The present study examines whether such differences in crowding influence clinical measurements of mesopic visual acuity, including test-retest repeatability. METHODS: Twenty adult subjects with normal or corrected to normal visual acuity were recruited for the study. Monocular visual acuity was measured under photopic (228 cd/m2) and mesopic (0.164 cd/m2) luminance conditions using a letter chart, similar in principle to the ETDRS logMAR chart, presented on a computer monitor. Three rows of five letters, each row differing in size by 0.05 logMAR from largest to smallest were displayed at the center of the monitor. The level of crowding was varied by varying the separation between horizontally adjacent letters from 100% optotype size to 50, 20, and 10% optotype size. Inter-row spacing was proportional to optotype size. Observers read the letters on the middle row only. Measurements continued by reducing the size of the letters, until three or more errors on the middle row were made. Each correctly identified letter contributed 0.01 to the recorded logMAR score. All measurements were repeated for each subject on two separate days. RESULTS: Visual acuity (logMAR) was significantly better under photopic than mesopic luminance conditions with a mean difference of 0.48 logMAR. Visual acuity also decreased with decreasing letter separation (i.e. increase in crowding). However, the decrease in visual acuity for the smallest letter separation was less under the mesopic luminance condition, even after accounting for the increased size of threshold acuity letters. Test-retest repeatability for mesopic and photopic conditions was not significantly different. CONCLUSIONS: Crowding under mesopic luminance conditions has less impact on visual acuity than under photopic luminance.

• MeSH
• citlivost na kontrast MeSH
• dospělí MeSH
• lidé MeSH
• mladiství MeSH
• mladý dospělý MeSH
• referenční hodnoty MeSH
• světelná stimulace MeSH
• vidění barevné fyziologie   MeSH
• vidění mezopické fyziologie   MeSH
• zraková ostrost * MeSH
• zrakové testy metody   MeSH
• Check Tag
• dospělí MeSH
• lidé MeSH
• mladiství MeSH
• mladý dospělý MeSH
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH