The P & p illusion

Identical letters can appear to differ in size and shape depending on whether they are interpreted as uppercase or lowercase. The effect is most dramatic with the letter p. Examination of subjects’ estimates of the magnitude of the effect for different stimuli suggests that two factors are involved in the illusion. One factor depends only on whether a letter is interpreted as uppercase or lowercase. Thisfactor can be manipulated by changes in the size or case ofthe surrounding letters. The second factor, which depends on the possibility of interpreting a vertical line alternatively as an ascender or a descender(as in p or y), seems to involve a change in the perceived size of the letters’ loops.     

The P&p illusion

Identical letters can appear to differ in size and shape depending on whether they are interpreted as uppercase or lowercase. The effect is most-dramatic with the letter p. Examination of subjects' estimates of the magnitude of the effect for different stimuli suggests that two factors are involved in the illusion. One factor depends only on whether a letter is interpreted as uppercase or lowercase. This factor can be manipulated by changes in the size or case of the surrounding letters. The second factor, which depends on the possibility of interpreting a vertical line alternatively as an ascender or a descender (as in p or y), seems to involve a change in the perceived size of the letters' loops.     

Flanker and negative flanker effects in letter identification

In a speeded choice reaction time task, responses to centrally presented letter targets can be altered by the identity of surrounding task-irrelevant letters (flankers). In the standard flanker effect, flankers associated with the same response as the target lead to faster and more accurate responses, whereas flankers associated with a different response lead to slower and more error-prone responses. B. A. Eriksen and C. W. Eriksen (1974, pp. 143-149) have argued that these flanker effects occur through response competition. We present data from a novel version of the Eriksen task, in which some targets and flankers consist of letter forms that are morphed versions of target letters. In this paradigm, flankers induce classic flanker effects on well-formed targets. But flankers induce an opposite effect, termed a negative flanker effect on morphed letter targets. For example, targets that are morphs between the letters "A" and "H" are more likely to be identified as an "A" when flanked by an "H." The interpretation advanced here is that there are two distinct kinds of flanker effects contrast enhancement in perceptual processes and response competition in response selection processes.     

Perception of the Ebbinghaus illusion in 5- to 8-month-old infants

The Ebbinghaus illusion is a geometric illusion based on a size-contrast between a central circle and surrounding circles. A central circle surrounded by small inducing circles is perceived as being larger than a central circle surrounded by large inducing circles. In the present study we investigated 5- to 8-month-old infants' perception of the Ebbinghaus illusion using a preferential-looking paradigm. We measured the preference between a central circle surrounded by small inducing circles (overestimated figure) and a central circle surrounded by large inducing circles (underestimated figure). Infants showed a significant preference for the overestimated figure when the central circle was flashing, but not when it was static. Furthermore, there was no preference between the two figures when the central circles were removed. These results suggest that infants' preference reflects their perception of the size illusion of the central circle. There is a possibility that 5- to 8-month-old infants perceive the Ebbinghaus illusion.     

Displacement effects caused by converging lines and dots

Changes induced in orientation were examined under conditions of a dot or full-line version of a stimulus consisting of a test and inducing line. 44 subjects visually extended the test line to the surrounding circle on 11 trials and indicated their response by a mark on the circle. Magnitude of illusion was reduced by 54% for the dot version compared with the full-line form, but both produced an illusion significantly greater than zero. A significant practice effect was obtained with full lines but not with the dot form. Results are discussed in terms of lateral inhibition theory and related research.     

Attending to an object's color entails attending to its location: support for location-special views of visual attention

Van der Heijden, Kurvink, de Lange, de Leeuw, and van der Geest (1996) argued that the results supporting the location-special view obtained by Tsal and Lavie (1988) were due to uncontrollable shifts of fixation, rather than reflecting the properties of the attentional system. In the present study, we present an improved variation of the Tsal and Lavie (1988) paradigm and reassert our claim that location is a special dimension. Subjects were presented with circular arrays of six letters of different colors. Three of the letters were enclosed by (Experiment 1) or superimposed on (Experiments 2, 3, and 4) different colored shapes. The subjects were instructed to report the (target) shape with a given color (e.g., report whether the red shape was a square, a circle, or a triangle) and then either freely report letters from the array (Experiments 1, 2, and 4) or identify a prespecified target letter (Experiment 3). In all four experiments, performance was substantially better for the letter that appeared in the location of the to-be-reported shape (location letter) than for the letter that shared its color (color letter). We conclude that attending to the stimulus color entails directing attention to its location.     

Attending to an object’s color entails Attending to its location: Support for location-special views of visual attention

Van der Heijden, Kurvink, de Lange, de Leeuw, and van der Geest (1996) argued that the results supporting the location-special view obtained by Tsal and Lavie (1988) were due to uncontrollable shifts of fixation, rather than reflecting the properties of the attentional system. In the present study, we present an improved variation of the Tsal and Lavie (1988) paradigm and reassert our claim that location is a special dimension. Subjects were presented with circular arrays of six letters of different colors. Three of the letters were enclosed by (Experiment 1) or superimposed on (Experiments 2,3, and 4) different colored shapes. The subjects were instructed to report the (target) shape with a given color (e.g., report whether the red shape was a square, a circle, or a triangle) and then either freely report letters from the array (Experiments 1, 2, and 4) or identify a prespecified target letter (Experiment 3). In all four experiments, performance was substantially better for the letter that appeared in the location of the to-be-reported shape (location letter) than for the letter that shared its color (color letter). We conclude that attending to the stimulus color entails directing attention to its location.     

Ebbinghaus illusions: Size contrast or contour interaction phenomena?

Forty-eight graduate and undergraduate students between 17 and 30 years of age judged the size of the central circle in Ebbinghaus figures presented under either simultaneous or successive viewing conditions. Successive presentation eliminated the illusion produced by the normally overestimated figure and diminished that produced by the underestimated figure. Under simultaneous viewing conditions, increasing the number of surrounding circles reduced the magnitude of the reversed illusion produced by the normally overestimated figure but had no effect on the illusion produced by the underestimated figure. Under successive viewing conditions, increasing the number of surrounding circles had no effect on the magnitude of illusion produced by the normally overestimated figure but increased the illusion produced by the underestimated figure. These data were interpreted as inconsistent with a size contrast explanation of the Ebbinghaus illusions and suggested that these illusions may be produced by a contour interaction process.     

Recognition vs recall of visually vs acoustically confusable letter matrices

In an attempt to separate auditory and visual components in short-term memory, five subjects were exposed to letter matrices composed of six visually confusable letters, six acoustically confusable letters, or a mixture of the two, under two response conditions: recognition and recall. A 50-msec stimulus presentation was followed by a variable dark interval of 1, 250, 1,000, or 3,000 msec. In the recall condition, the interval was followed by a buzzer which signaled the subject to recall, in any order, as many letters as possible. In the recognition condition, the variable interval was followed by a second letter matrix which was either identical to the first matrix or differed from is by one letter. Subjects responded either "same" or "different." The results support the notion that the auditory component plays a major role in recall, whereas the visual component dominates in recognition.     

Sex differences in the reading process revealed by a letter-detection task

Four experiments used a task in which males and females had to work through a passage and circle instances of a target letter. Coltheart, Hull and Slater (1975) have previously used this task to show that females have greater difficulty than males in detecting target letters, especially letters in the and other silent letters. The four experiments failed to replicate Coltheart et al.'s findings and, in fact, frequently found significant sex differences in the opposite direction. This discrepancy in results could not be accounted for in terms of procedural differences between the two studies. It was concluded that Coltheart et al.'s (1975) results with the letter detection task may not be reliable.     

Effect of contrast level and temporal order on the Ebbinghaus circles illusion

Forty-eight undergraduates between 17 and 24 years of age judged the size of the central circle in Ebbinghaus figures presented under either simultaneous or successive viewing conditions. Successive presentation diminished the magnitude of illusion produced by the underestimated Ebbinghaus figure and totally eliminated the illusion normally produced by the overestimated figure. Variations in lightness contrast of the central and surrounding circles significantly altered the magnitude of illusion produced by both simultaneous Ebbinghaus figures as well as that produced by the successive underestimated figure. These data suggest the presence of a contour interaction process in both simultaneous Ebbinghaus illusions.     

Parafoveal letter-position coding in reading

The masked-priming lexical decision task has been the paradigm of choice for investigating how readers code for letter identity and position. Insight into the temporal integration of information between prime and target words has pointed out, among other things, that readers do not code for the absolute position of letters. This conception has spurred various accounts of the word recognition process, but the results at present do not favor one account in particular. Thus, employing a new strategy, the present study moves out of the arena of temporal- and into the arena of spatial information integration. We present two lexical decision experiments that tested how the processing of six-letter target words is influenced by simultaneously presented flanking stimuli (each stimulus was presented for 150 ms). We manipulated the orthographic relatedness between the targets and flankers, in terms of both letter identity (same/different letters based on the target’s outer/inner letters) and letter position (intact/reversed order of letters and of flankers, contiguous/noncontiguous flankers). Target processing was strongly facilitated by same-letter flankers, and this facilitatory effect was modulated by both letter/flanker order and contiguity. However, when the flankers consisted of the target’s inner-positioned letters alone, letter order no longer mattered. These findings suggest that readers may code for the relative position of letters using words’ edges as spatial points of reference. We conclude that the flanker paradigm provides a fruitful means to investigate letter-position coding in the fovea and parafovea.     

Effects of spatial separation between stimuli in whole report from brief visual displays

Direct measurements of the effects of spatial separation between stimuli in whole report from brief visual displays are reported. The stimuli were presented on the periphery of an imaginary circle centered on fixation. In Experiment 1, each display showed two capital letters (letter height approximately equal 1.3 degrees, width approximately equal 0.9 degree, eccentricity approximately equal 5.5 degrees). The proportion of correctly reported letters was a strictly increasing, decelerating function of the spatial separation between the letters for center-to-center separations ranging from less than 2 degrees to more than 10 degrees of visual angle. Experiment 2 yielded similar results with triples of letters. Experiment 3 showed that accuracy increased with spatial separation for report of two short words, and Experiment 4 showed the same result for words presented upside down. The results are explained by a model of lateral masking (crowding) based on competitive interactions within receptive fields of cortical neurons.     

Intrusion and location errors in the naming of letters in words and non-words

Summary In two experiments subjects were asked to report the identity of a position-cued critical letter in a linear array of letters. The identification errors made were classified either as intrusion (i.e., a report of a letter not in the array), or as location (i.e., a report of a noncritical or context letter from the array), and the arrays used were either unpronounceable non-words or words. In the first experiment all stimuli were four letters long and were presented in two parts: a leading array in which the information from two quadrants of a vertical by horizontal division of each letter was presented, and, after intervals of 40 and 160 ms, a trailing array of the complementary letter parts. The cue for the critical letter appeared either 80 ms before the initiation of the leading array or 320 ms after. The results showed a word advantage in all conditions, and an analysis of mislocations as proportions of total errors did not show that they differed significantly for non-words and for words. In the second experiment the stimuli were eight letters long and they were presented intact, followed after 140 ms by a patterned mask. The critical letter was again pre- and post-cued, and arrays subtended either 1.6° or 5° of visual angle. The results again showed a general word advantage and no difference in the proportions of mislocations for non-words and words. The results of both studies failed to provide support for the notion that the non-words probably required more attention than the words, and since this differential-attention hypothesis questioned the equality of encoding assumption of the Interactive Activation model of word perception, its rejection supported the latter proposition. It was concluded, therefore, that the encoding of the strings of letters in these studies was independent of their linguistic properties.     

Letter detection: A window to unitization and other cognitive processes in reading text

Experiments are reviewed that use the letter-detection task, in which subjects read text and circle target letters. Evidence is provided that the letter-detection task reveals the processing units used in reading text and is influenced as well by visual, phonetic, and a combination of semantic and syntactic factors. Specifically, it is shown that circling a target letter in a word depends on the familiarity of the word’s visual configuration, the location of the word in the reader’s visual field, the phonetic representation of the letter in the word, and a combination of the word’s meaning and its grammatical function.     

The acquisition of abstract letter codes

A modification of Posner's letter-matching paradigm proposed by Beller (1971) was employed to study the development of abstract visual and name codes for letters. Second-, fourth-, and sixth-grade students (ages 8, 10, and 12 years, respectively) were presented in advance one of a pair of to-be-matched letters. Case agreement (upper- and lowercase) between the prime letter and the letter pair was manipulated to differentiate facilitation based on concrete features of the stimulus letter from that based on the letter's abstract visual code. For physical matches second graders showed greater facilitation from same-case primes than from different-case primes, whereas sixth graders benefited equally from both types of prime. For same-name and different conditions, no qualitative developmental differences were revealed as all subjects benefited from letter primes irrespective of case agreement. The acquisition and utilization of abstract visual codes for letters are a later acquisition than name codes, which were clearly operative at the lowest grade level in the present experiment.     

Application of geometric models to letter recognition: distance and density

This article reviews studies in which a single letter is visually presented under adverse conditions and the subject's task is to identify the letter. The typical results for such studies are (a) certain pairs of letters are more often confused than other pairs of letters; (b) certain letters are more easily recognized than others; and (c) confusion errors for a letter pair are often asymmetric, the number of errors differing depending on which letter of the pair is presented as the stimulus. A geometric model incorporating the properties of distance and spatial density (after Krumhansl) is presented to account for these results. The present application of the distance-density model assumes that each letter is constructed in a typical 5 X 7 dot matrix. Each letter is represented in 35-dimensional space based on its constituent dots. A central idea behind the model, embodied in the property of spatial density, is that an explanation of typical results must take into account the relationship of the entire stimulus set to both the presented letter and the responded letter. Specifically, according to the model, (a) pairs of letters that are close in geometric space are more often confused than pairs of letters that are distant; (b) letters that are in less spatially dense regions are more easily recognized than letters that are in more spatially dense regions; and (c) asymmetric confusion errors result when one member of a letter pair is in a denser region than the other member of the letter pair. The distance-density model is applied to published and unpublished results of the authors as well as published results from two other laboratories. Alternative explanations of the three typical letter recognition results are also considered. The most successful alternative explanations are (a) confusions are an increasing function of the number of dots that two letters share; (b) letters constructed from fewer dots are easier to recognize; and (c) asymmetries arise when one member of a letter pair is more easily recognized, since that letter then has fewer confusion errors to give to the other letter of the pair. The model is discussed in terms of the distinction between template matching and feature analysis. An alternative classification of letter recognition models is proposed based on the global versus local qualities of features and the spatial information associated with each feature. The model is extended to explain reaction time study results. It is suggested that the distance-density model can be used to create optimal letter fonts by minimizing interletter confusions and maximizing letter recognizability.     

Frequency discrimination: assessing global-level and element-level units in memory

Subjects' knowledge of how often various events occur was used to assess the retention of memory units for word-like strings of letters. A series of strings was presented at one of three exposure durations. Within the series, the frequencies of occurrence of different strings and of the letters composing the strings were varied orthogonally. At relatively long exposure durations, subjects could discriminate the frequency of occurrence for both strings and their constituent letters. The formation of global-level (string) memory units was indicated by judgments of string frequency being unaffected by either the frequencies of their component letters or experimental conditions (brief exposures) that prohibited accurate judgment of letter frequency. Although judgments of letter frequency were sometimes biased by the frequency of the strings containing the letters, the success with which the judgments discriminated different levels of letter frequency did not depend on the activation of string-level memory units. Furthermore, subjects' frequency judgments for letters were not predictable from their recall of the strings containing the letters. These results, which could not be explained by Tversky and Kahneman's (1973) "availability heuristic," provided evidence for the formation of element-level (letter) memory units. A converging experiment established that element-level frequency information could be abstracted from words as well as nonwords, and further, that this information was stored in long-term memory.     

The meaning of the mask matters: evidence of conceptual interference in the attentional blink

The rapid serial visual presentation (RSVP) experiment reported here investigated the role of conceptual interference in the attentional blink (AB). Subjects were presented with RSVP streams that contained five stimuli: Target 1, a distractor, Target 2, a second distractor, and a symbol mask. Target 1 was a green letter, Target 2 was a red letter, and the distractors were either white letters or white digits. The stimuli were presented in a font typically seen on the face of a digital watch. Thus, "S" and "O" were identical to "5" and "0," respectively. This allowed us to present streams that were conceptually different even though featurally identical: The two letter targets were followed by distractors that were recognized either as "5" and "0" or as "S" and "O." The AB was substantially attenuated when subjects were told the distractors were digits rather than letters. This result indicates that conceptual interference plays a role in the AB.