Performance for identifying luminance-defined letters in peripheral vision improves with training. The purpose of the present study was to examine whether performance for identifying contrast-defined letters also improves with training in peripheral vision, and whether any improvement transfers to luminance-defined letters. Eight observers were trained to identify contrast-defined letters presented singly at 10° eccentricity in the inferior visual field. Before and after training, we measured observers’ thresholds for identifying luminance-defined and contrast-defined letters, embedded within a field of white luminance noise (maximum luminance contrast = 0, 0.25, and 0.5), at the same eccentric location. Each training session consisted of 10 blocks (100 trials per block) of identifying contrast-defined letters at a background noise contrast of 0.5. Letters (x-height = 4.2°) were the 26 lowercase letters of the Times-Roman alphabet. Luminance-defined letters were generated by introducing a luminance difference between the stimulus letter and its mid-gray background. The background noise covered both the letter and its background. Contrast-defined letters were generated by introducing a differential noise contrast between the group of pixels that made up the stimulus letter and the group of pixels that made up the background. Following training, observers showed a significant reduction in threshold for identifying contrast-defined letters (p < 0.0001). Averaged across observers and background noise contrasts, the reduction was 25.8%, with the greatest reduction (32%) occurring at the trained background noise contrast. There was virtually no transfer of improvement to luminance-defined letters, or to an untrained letter size (2× original), or an untrained retinal location (10° superior field). In contrast, learning transferred completely to the untrained contralateral eye. Our results show that training improves performance for identifying contrast-defined letters in peripheral vision. This perceptual learning effect seems to be stimulus-specific, as it shows no transfer to the identification of luminance-defined letters. The complete interocular transfer, and the retinotopic (retinal location) and size specificity of the learning effect are consistent with the properties of neurons in early visual area V2.
Letter recognition; Peripheral vision; Perceptual learning; Second-order
The purpose of this study was to define the quantitative relationship between the temporal characteristics of additive luminance noise and the properties of the spatial contrast sensitivity function (CSF). CSFs were obtained from two observers using Gabor patch targets of short duration that were added to white luminance noise with a range of root-mean-square contrasts (crms). The noise was either dynamic or static and was either of the same duration as the test target (synchronous) or of longer duration (asynchronous). For targets presented in asynchronous dynamic, synchronous dynamic, and synchronous static noise, the CSFs became increasingly band-pass with increasing crms, whereas the CSFs were low-pass at all levels of crms for targets presented in asynchronous static noise. For all noise types, the properties of the CSFs were well-predicted by the linear amplifier model (LAM), in which the signal energy at threshold (Et) is related linearly to noise spectral density (N). The fundamentally different characteristics of CSFs obtained in asynchronous static noise can be accounted for by a previous proposal that this noise type biases contrast sensitivity toward transient (inferred magnocellular) mechanisms. The other three modes of noise presentation appear to emphasize detection by sustained (inferred parvocellular) mechanisms.
contrast sensitivity; spatial vision; visual noise; sustained/transient; magnocellular/parvocellular
In alphabetic orthographies, letter identification is a critical process during the recognition of visually presented words. In the present experiment, we examined whether and when visual form influences letter processing in two very distinct alphabets (Roman and Arabic). Disentangling visual versus abstract letter representations was possible because letters in the Roman alphabet may look visually similar/dissimilar in lowercase and uppercase forms (e.g., c-C vs. r-R) and letters in the Arabic alphabet may look visually similar/dissimilar, depending on their position within a word (e.g., vs. -). We employed a masked priming same–different matching task while ERPs were measured from individuals who had learned the two alphabets at an early age. Results revealed a prime–target relatedness effect dependent on visual form in early components (P/N150) and a more abstract relatedness effect in a later component (P300). Importantly, the pattern of data was remarkably similar in the two alphabets. Thus, these data offer empirical support for a universal (i.e., across alphabets) hierarchical account of letter processing in which the time course of letter processing in different scripts follows a similar trajectory from visual features to visual form independent of abstract representations.
This study investigated how the temporal properties of additive luminance noise influence threshold contrast and affect estimates of equivalent noise and sampling efficiency. Threshold contrast was obtained from four visually normal observers for a 2-cycle-per-degree Gabor patch across a range of target durations in the absence and presence of additive luminance noise that was either static or dynamic. In addition, the temporal relationship between target and noise was either synchronous (simultaneous presentation of both) or asynchronous (noise duration longer than target duration). For both synchronous and asynchronous presentation modes, the extent of temporal integration differed for targets presented in dynamic vs. static noise. Furthermore, for a fixed-duration target, increasing the degree of temporal asynchrony between target and noise monotonically increased threshold contrast in dynamic noise, but had a non-monotonic effect on threshold contrast in static noise. For both dynamic and static noise, estimates of equivalent noise and sampling efficiency were dependent on the degree of temporal asynchrony between target and noise, presumably due to temporal integration of the noise. The observed differences between the effects of dynamic and static noise are consistent with a previous proposal that target detection in the presence of these two noise types tends to be governed by sustained-like and transient-like visual mechanisms, respectively.
Visual noise; temporal vision; contrast sensitivity; equivalent noise; sampling efficiency
The Gestalt psychologists reported a set of laws describing how vision groups elements to recognize objects. The Gestalt laws “prescribe for us what we are to recognize ‘as one thing’.” (Köhler, 1920). Were they right? Does object recognition involve grouping? Tests of the laws of grouping have been favorable, but mostly assessed only detection, not identification, of the compound object. The grouping of elements seen in the detection experiments with lattices and ‘snakes in the grass’ is compelling, but falls far short of the vivid everyday experience of recognizing a familiar, meaningful, named thing, which mediates the ordinary identification of an object. Thus, after nearly a century, there is hardly any evidence that grouping plays a role in ordinary object recognition. To assess grouping in object recognition, we made letters out of grating patches and measured threshold contrast for identifying these letters in visual noise as a function of perturbation of grating orientation, phase, and offset. We define a new measure, “wiggle,” to characterize the degree to which these various perturbations violate the Gestalt law of good continuation. We find that efficiency for letter identification is inversely proportional to wiggle, and is wholly determined by wiggle, independent of how the wiggle was produced. Thus the effects of three different kinds of shape perturbation on letter identifiability are predicted by a single measure of goodness of continuation. This shows that letter identification obeys the Gestalt law of good continuation, and may be the first confirmation of the original Gestalt claim that object recognition involves grouping.
Gestalt; grouping; contour integration; good continuation; letter identification; object recognition; features; snake in the grass; snake letters; dot lattice
The Gestalt psychologists reported a set of laws describing how vision groups elements to recognize objects. The Gestalt laws “prescribe for us what we are to recognize ‘as one thing’” (Köhler, 1920). Were they right? Does object recognition involve grouping? Tests of the laws of grouping have been favourable, but mostly assessed only detection, not identification, of the compound object. The grouping of elements seen in the detection experiments with lattices and “snakes in the grass” is compelling, but falls far short of the vivid everyday experience of recognizing a familiar, meaningful, named thing, which mediates the ordinary identification of an object. Thus, after nearly a century, there is hardly any evidence that grouping plays a role in ordinary object recognition. To assess grouping in object recognition, we made letters out of grating patches and measured threshold contrast for identifying these letters in visual noise as a function of perturbation of grating orientation, phase, and offset. We define a new measure, “wiggle”, to characterize the degree to which these various perturbations violate the Gestalt law of good continuation. We find that efficiency for letter identification is inversely proportional to wiggle and is wholly determined by wiggle, independent of how the wiggle was produced. Thus the effects of three different kinds of shape perturbation on letter identifiability are predicted by a single measure of goodness of continuation. This shows that letter identification obeys the Gestalt law of good continuation and may be the first confirmation of the original Gestalt claim that object recognition involves grouping.
Gestalt; Grouping; Contour integration; Good continuation; Letter identification; Object recognition; Features; Snake in the grass; Snake letters; Dot lattice
A classification image (CI) technique has shown that static luminance noise near visually completed contours affects the discrimination of fat and thin Kanizsa shapes. These influential noise regions were proposed to reveal “behavioral receptive fields” of completed contours–the same regions to which early cortical cells respond in neurophysiological studies of contour completion. Here, we hypothesized that 1) influential noise regions correspond to the surfaces that distinguish fat and thin shapes (hereafter, key regions); and 2) key region noise biases a “fat” response to the extent that its contrast polarity (lighter or darker than background) matches the shape's filled-in surface color.
To test our hypothesis, we had observers discriminate fat and thin noise-embedded rectangles that were defined by either illusory or luminance-defined contours (Experiment 1). Surrounding elements (“inducers”) caused the shapes to appear either lighter or darker than the background–a process sometimes referred to as lightness induction. For both illusory and luminance-defined rectangles, key region noise biased a fat response to the extent that its contrast polarity (light or dark) matched the induced surface color. When lightness induction was minimized, luminance noise had no consistent influence on shape discrimination. This pattern arose when pixels immediately adjacent to the discriminated boundaries were excluded from the analysis (Experiment 2) and also when the noise was restricted to the key regions so that the noise never overlapped with the physically visible edges (Experiment 3). The lightness effects did not occur in the absence of enclosing boundaries (Experiment 4).
Under noisy conditions, lightness induction alters visually completed shape. Moreover, behavioral receptive fields derived in CI studies do not correspond to contours per se but to filled-in surface regions contained by those contours. The relevance of lightness to two-dimensional shape completion supplies a new constraint for models of object perception.
We assessed whether or not the sensitivity for identifying luminance-defined and contrast-defined letters improved with training in a group of amblyopic observers who have passed the critical period of development. In Experiment 1, we tracked the contrast threshold for identifying luminance-defined letters with training in a group of 11 amblyopic observers. Following training, six observers showed a reduction in thresholds, averaging 20%, for identifying luminance-defined letters. This improvement transferred extremely well to the untrained task of identifying contrast-defined letters (average improvement = 38%) but did not transfer to an acuity measurement. Seven of the 11 observers were subsequently trained on identifying contrast-defined letters in Experiment 2. Following training, five of these seven observers demonstrated a further improvement, averaging 17%, for identifying contrast-defined letters. This improvement did not transfer to the untrained task of identifying luminance-defined letters. Our findings are consistent with predictions based on the locus of learning for first- and second-order stimuli according to the filter-rectifier-filter model for second-order visual processing.
Amblyopia; Perceptual learning; Training; First-order; Second-order; Letter recognition
Amblyopia is a much-studied but poorly understood developmental visual disorder that reduces acuity, profoundly reducing contrast sensitivity for small targets. Here we use visual noise to probe the letter identification process and characterize its impairment by amblyopia. We apply five levels of analysis — threshold, threshold in noise, equivalent noise, optical MTF, and noise modeling — to obtain a two-factor model of the amblyopic deficit: substantially reduced efficiency for small letters and negligibly increased cortical noise. Cortical noise, expressed as an equivalent input noise, varies among amblyopes but is roughly 1.4× normal, as though only 1/1.4 the normal number of cortical spikes are devoted to the amblyopic eye. This raises threshold contrast for large letters by a factor of √1.4 = 1.2×, a negligible effect. All 16 amblyopic observers showed near-normal efficiency for large letters (> 4× acuity) and greatly reduced efficiency for small letters: 1/4 normal at 2× acuity and approaching 1/16 normal at acuity. Finding that the acuity loss represents a loss of efficiency rules out all models of amblyopia except those that predict the same sensitivity loss on blank and noisy backgrounds. One such model is the last-channel hypothesis, which supposes that the highest-spatial-frequency channels are missing, leaving the remaining highest-frequency channel struggling to identify the smallest letters. However, this hypothesis is rejected by critical band masking of letter identification, which shows that the channels used by the amblyopic eye have normal tuning for even the smallest letters. Finally, based on these results, we introduce a new “Dual Acuity” chart that promises to be a quick diagnostic test for amblyopia.
amblyopia; noise; efficiency; cortical noise; Pelli-Levi Dual Acuity Chart
Using the external noise plus training paradigm, we have consistently found that two independent mechanisms, stimulus enhancement and external noise exclusion, support perceptual learning in a range of tasks. Here, we show that re-weighting of stable early sensory representations through Hebbian learning (Petrov et al., 2005, 2006) can generate performance patterns that parallel a large range of empirical data: (1) perceptual learning reduced contrast thresholds at all levels of external noise in peripheral orientation identification (Dosher & Lu, 1998, 1999), (2) training with low noise exemplars transferred to performance in high noise, while training with exemplars embedded in high external noise transferred little to performance in low noise (Dosher & Lu, 2005), and (3) pre-training in high external noise only reduced subsequent learning in high external noise, whereas pre-training in zero external noise left very little additional learning in all the external noise conditions (Lu et al., 2006). In the augmented Hebbian re-weighting model (AHRM), perceptual learning strengthens or maintains the connections between the most closely tuned visual channels and a learned categorization structure, while it prunes or reduces inputs from task-irrelevant channels. Reducing the weights on irrelevant channels reduces the contributions of external noise and additive internal noise. Manifestation of stimulus enhancement or external noise exclusion depends on the initial state of internal noise and connection weights in the beginning of a learning task. Both mechanisms reflect re-weighting of stable early sensory representations.
Re-weighting; Hebbian learning; Stimulus enhancement; External noise exclusion; Mechanisms of perceptual learning
Letters are broad-band visual stimuli with information useful for discrimination over a wide range of spatial frequencies. Yet, recent evidence suggests that observers use only a single, fixed spatial-frequency channel to identify letters, and that the scale of that channel, in units of letter size, is determined by the size of the letter (scale dependence). We report two letter-identification experiments using critical-band masking. With sufficiently high-amplitude, low- or high-pass masking noise, observers switched to a different range of spatial frequencies for the task. Thus, letter channels are not fixed for a given letter size. When an additional white-noise masker was added to the stimulus to flatten the contrast-sensitivity function, the letter channel used by the observer still depended on letter size, further supporting the hypothesis that letter identification is scale dependent.
letter identification; letter channels; critical-band masking; scale invariance; channel switching; equivalent input noise; ideal observer
Crowding, the difficulty in identifying a letter embedded in other letters, has been suggested as an explanation for slow reading in peripheral vision. In this study, we asked whether crowding in peripheral vision can be reduced through training on identifying crowded letters, and if so, whether these changes will lead to improved peripheral reading speed. We measured the spatial extent of crowding, and reading speeds for a range of print sizes at 10° inferior visual field before and after training. Following training, averaged letter identification performance improved by 88% at the trained (the closest) letter separation. The improvement transferred to other untrained separations such that the spatial extent of crowding decreased by 38%. However, averaged maximum reading speed improved by a mere 7.2%. These findings demonstrated that crowding in peripheral vision could be reduced through training. Unfortunately, the reduction in the crowding effect did not lead to improved peripheral reading speed.
crowding; perceptual learning; training; reading
Amblyopes show specific deficits in processing second-order spatial
information (e.g. Wong, Levi, & McGraw
(2001). Is second-order spatial loss in amblyopia explained by the
loss of first-order spatial input? Vision Research, 41,
2951–2960). Recent work suggests there is a significant degree of
plasticity in the visual pathway that processes first-order spatial information
in adults with amblyopia. In this study, we asked whether or not there is
similar plasticity in the ability to process second-order spatial information in
adults with amblyopia. Ten adult observers with amblyopia (five strabismic, four
anisometropic and one mixed) were trained to identify contrast-defined
(second-order) letters using their amblyopic eyes. Before and after training, we
determined observers’ contrast thresholds for identifying
luminance-defined (first-order) and contrast-defined letters, separately for the
non-amblyopic and amblyopic eyes. Following training, eight of the 10 observers
showed a significant reduction in contrast thresholds for identifying
contrast-defined letters with the amblyopic eye. Five of these observers also
showed a partial transfer of improvement to their fellow untrained non-amblyopic
eye for identifying contrast-defined letters. There was a small but
statistically significant transfer to the untrained task of identifying
luminance-defined letters in the same trained eye. Similar to first-order
spatial tasks, adults with amblyopia benefit from perceptual learning for
identifying contrast-defined letters in their amblyopic eyes, suggesting a
sizeable degree of plasticity in the visual pathway for processing second-order
Amblyopia; Letter recognition; Perceptual learning; Second-order
Performance for discriminating single mirror -image letters in peripheral vision can be as good as that in central vision, provided that letter size is scaled appropriately (Higgins, Arditi & Knoblauch, 1996 Vision Research). In this study, we asked whether or not there is a reduction in performance for discriminating mirror -image letters when the letters are flanked closely by other letters, compared with unflanked (single) letters; and if so, whether or not this effect is greater in peripheral than in central vision. We compared contrast thresholds for detecting and identifying mirror -image letters “b” and “d” for a range of letter separations, at the fovea and 10° eccentricity, for letters that were scaled in size. For comparison, thresholds were also determined for a pair of non - mirror-image letters “o” and “x”. Our principal finding is that there is an additional loss in sensitivity for identifying mirror -image letters (“bd”), compared with non - mirror-image letters (“ox”), when the letters are flanked closely by other letters. The effect is greater in peripheral than central vision. An auxiliary experiment comparing thresholds for letters “d” and “q” vs. “b” and “d” shows that the additional loss in sensitivity for identifying crowded mirror -image letters cannot be attributed to the similarity in letter features between the two letters, but instead, is specific to the axis of symmetry. Our results suggest that in the presence of proximal objects, there is a specific loss in sensitivity for processing broad -band left-right mirror images in peripheral vision.
Crowding; letter identification; mirror images
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° in the lower visual field of four normally sighted observers. Two observers were also tested at 10° 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.
crowding; first order; second order; peripheral vision; letter identification
Amblyopia is a developmental abnormality that results in deficits for a wide range of visual tasks, most notably, the reduced ability to see fine details, the loss in contrast sensitivity especially for small objects and the difficulty in seeing objects in clutter (crowding). The primary goal of this study was to evaluate whether crowding can be ameliorated in adults with amblyopia through perceptual learning using a flanked letter identification task that was designed to reduce crowding, and if so, whether the improvements transfer to untrained visual functions: visual acuity, contrast sensitivity and the size of visual span (the amount of information obtained in one fixation). To evaluate whether the improvements following this training task were specific to training with flankers, we also trained another group of adult observers with amblyopia using a single letter identification task that was designed to improve letter contrast sensitivity, not crowding. Following 10,000 trials of training, both groups of observers showed improvements in the respective training task. The improvements generalized to improved visual acuity, letter contrast sensitivity, size of the visual span, and reduced crowding. The magnitude of the improvement for each of these measurements was similar in the two training groups. Perceptual learning regimens aimed at reducing crowding or improving letter contrast sensitivity are both effective in improving visual acuity, contrast sensitivity for near-acuity objects and reducing the crowding effect, and could be useful as a clinical treatment for amblyopia.
What underlies contrast sensitivity improvements in adults with anisometropic amblyopia following perceptual learning in grating contrast detection? In this paper, we adopted the external noise approach (Z.-L. Lu & B. A. Dosher, 1998) to identify the mechanisms underlying perceptual learning in adults with anisometropic amblyopia. By measuring contrast thresholds in a range of external noise conditions at two performance levels (79.3% and 70.7%), we found that a mixture of internal additive noise reduction and external noise exclusion underlay training induced contrast sensitivity improvements in adults with anisometropic amblyopia. In comparison, normal adults exhibited only small amount of external noise exclusion under the same training conditions. The results suggest that neural plasticity may be more robust in amblyopia, lending further support of perceptual learning as a potential treatment for adult amblyopia.
perceptual learning; internal noise reduction; external noise exclusion; contrast detection; contrast sensitivity
We evaluated computerized training and testing programs with children who were having difficulties learning prereading skills. The programs were derived from equivalence research and were written in authoring software designed for educators. After learning to match uppercase and lowercase printed letters to the corresponding letter names (Tasks 1 and 2), the children matched the letters to one another (Tasks 4 and 5). Then, after learning to match uppercase letters to sounds (Task 3), they also matched lowercase letters to sounds (Task 6) and matched printed to spoken words (Tasks 7 and 8). The results recommend equivalence-based protocols and user-friendly software in further development of prereading instruction.
We report a new phenomenon in cross-modal cuing of visual spatial attention - simultaneous auditory peripheral cues are “hyper-effective” - more effective than auditory peripheral (AP), visual central (VC), or visual peripheral (VP) cues that preceded the target with ample time for preparatory orienting. The time courses and mechanisms of visual spatial attention were measured in a four-location Gabor orientation identification task for targets embedded in systematically varying amounts of external noise [Lu and Dosher, Vision Research, 1998] and for cue-target asynchronies (CTOAs) between 0 and 240 ms. Large CTOA pre-cuing improvements in contrast thresholds occurred in high external noise conditions for AP, VC, and VP cues. In low external noise conditions, pre-cuing advantages occurred for visual peripheral (VP) cues, but not for visual central cues (VC), replicating Lu and Dosher [Journal of Experimental Psychology: Human Perception and Performance, 2000]. Auditory peripheral pre-cues (AP) were similar to VP cues at large cue-target delays, but demonstrated a large cross-modal cuing advantage for simultaneous auditory peripheral cues (AP) in both high and low external noise. We conclude that endogenous attention (visual central pre-cuing) excludes external noise, while exogenous attention (both visual and auditory peripheral pre-cuing) enhances the stimulus but also excludes external noise if informative peripheral cues are used.
Intra-modal cuing; Cross-modal cuing; Time course of attention; endogenous attention; exogenous attention; stimulus enhancement; external noise exclusion; mechanisms of attention
Objects in natural scenes are spatially broadband; in contrast, feature detectors in the early stages of visual processing are narrowly tuned in spatial frequency. Earlier studies of feature integration using gratings suggested that integration across spatial frequencies is suboptimal. Here we re-examined this conclusion using a letter identification task at the fovea and at 10 deg in the lower visual field. We found that integration across narrow-band (1-octave) spatial frequency components of letter stimuli is optimal in the fovea. Surprisingly, this optimality is preserved in the periphery, even though feature integration is known to be deficient in the periphery from studies of other form-vision tasks such as crowding. A model that is otherwise a white-noise ideal observer except for a limited spatial resolution defined by the human contrast sensitivity function and using internal templates slightly wider in bandwidth than the stimuli is able to account for the human data. Our findings suggest that deficiency in feature integration found in peripheral vision is not across spatial frequencies.
spatial frequency channels; summation; letter identification; fovea; periphery
In the present study we examined the use of perceptual learning to improve motion processing in older and younger individuals. Using the Perceptual Template Model (Lu & Dosher, 1998; 1999), age-related differences in baseline perceptual inefficiencies and changes due to training were assessed for additive internal noise, tolerance to external noise, and internal multiplicative noise. In Experiments 1 and 2 we trained participants by manipulating contrast in noise embedded sine-wave gratings and Random Dot Cinematograms (RDCs). The results indicate that older observers have higher additive internal noise and lower tolerance to external noise compared to younger observers. The rate of perceptual learning in older observers was found to be similar to that of younger observers suggesting that plasticity of motion processing mechanisms is well preserved in advancing age. Transfer of learning between sine-wave gratings and RDCs for both older and younger observers was examined in an analysis of pre/post-test measurements. The results indicate that transfer of learning occurred for both age groups. This suggests that older individuals maintain a sufficient degree of plasticity to allow generalization between sine-wave gratings and RDCs. In addition, training with RDCs was found to produce greater perceptual learning than training with sine-wave gratings. These experiments provide important findings regarding changes in perceptual efficiency for motion perception in older adults and suggest that perceptual learning is an effective approach for recovering from age-related declines in visual processing.
aging; perceptual learning; motion; modeling; perceptual efficiency
The present study employed Dynamic Causal Modeling to investigate the effective functional connectivity between regions of the neural network involved in top-down letter processing. We used an illusory letter detection paradigm in which participants detected letters while viewing pure noise images. When participants detected letters, the response of the right middle occipital gyrus (MOG) in the visual cortex was enhanced by increased feed-backward connectivity from the left inferior frontal gyrus (IFG). In addition, illusory letter detection increased feed-forward connectivity from the right MOG to the left inferior parietal lobules. Originating in the left IFG, this top-down letter processing network may facilitate the detection of letters by activating letter processing areas within the visual cortex. This activation in turns may highlight the visual features of letters and send letter information to activate the associated phonological representations in the identified parietal region.
letter processing; word processing; top-down processing; fMRI; dynamic causal modeling
Crowding is a prominent phenomenon in peripheral vision where nearby objects impede one’s ability to identify a target of interest. The precise mechanism of crowding is not known. We used ideal observer analysis and a noise-masking paradigm to identify the functional mechanism of crowding. We tested letter identification in the periphery with and without flanking letters and found that crowding increases equivalent input noise and decreases sampling efficiency. Crowding effectively causes the signal from the target to be noisier and at the same time reduces the visual system’s ability to make use of a noisy signal. After practicing identification of flanked letters without noise in the periphery for 6 days, subjects’ performance for identifying flanked letters improved (reduction of crowding). Across subjects, the improvement was attributable to either a decrease in crowding-induced equivalent input noise or an increase in sampling efficiency, but seldom both. This pattern of results is consistent with a simple model whereby learning reduces crowding by adjusting the spatial extent of a perceptual window used to gather relevant input features. Following learning, subjects with inappropriately large windows reduced their window sizes; while subjects with inappropriately small windows increased their window sizes. The improvement in equivalent input noise and sampling efficiency persists for at least 6 months.
peripheral vision; crowding; perceptual learning; ideal observer analysis
The decoding of visually presented line segments into letters, and letters into words, is critical to fluent reading abilities. Here we investigate the temporal dynamics of visual orthographic processes, focusing specifically on right hemisphere contributions and interactions between the hemispheres involved in the implicit processing of visually presented words, consonants, false fonts, and symbolic strings. High-density EEG was recorded while participants detected infrequent, simple, perceptual targets (dot strings) embedded amongst a of character strings. Beginning at 130 ms, orthographic and non-orthographic stimuli were distinguished by a sequence of ERP effects over occipital recording sites. These early latency occipital effects were dominated by enhanced right-sided negative-polarity activation for non-orthographic stimuli that peaked at around 180 ms. This right-sided effect was followed by bilateral positive occipital activity for false-fonts, but not symbol strings. Moreover the size of components of this later positive occipital wave was inversely correlated with the right-sided ROcc180 wave, suggesting that subjects who had larger early right-sided activation for non-orthographic stimuli had less need for more extended bilateral (e.g., interhemispheric) processing of those stimuli shortly later. Additional early (130–150 ms) negative-polarity activity over left occipital cortex and longer-latency centrally distributed responses (>300 ms) were present, likely reflecting implicit activation of the previously reported ‘visual-word-form’ area and N400-related responses, respectively. Collectively, these results provide a close look at some relatively unexplored portions of the temporal flow of information processing in the brain related to the implicit processing of potentially linguistic information and provide valuable information about the interactions between hemispheres supporting visual orthographic processing.
word reading; ERPs; visual cortex; visual orthography
To test whether first- and second-order stimuli are processed independently in amblyopic vision, we measured thresholds for identifying a target letter flanked by two letters for all combinations of first- and second-order targets and flankers. We found that (1) the magnitude of crowding is greater for second- than for first-order letters for target and flankers of the same order type; (2) substantial but asymmetric cross-over crowding occurs such that stronger crowding is found for a second-order letter flanked by first-order letters than for the converse; (3) the spatial extent of crowding is independent of the order type of the letters. Our findings are consistent with the hypothesis that crowding results from an abnormal integration of target and flankers beyond the stage of feature detection, which takes place over a large distance in amblyopic vision.
amblyopia; crowding; first order; second order; letter identification