The effects of forced serial processing on identification of words and random letter strings

Skilled readers identified words and random letter strings displayed one letter at a time on the screen of a computer-controlled oscilloscope. Letters were either displayed in a single position, or painted left-to-right in adjacent positions. Adjacent displays were either unmasked, masked by a letter-like pattern following each letter (backward masking) or masked by a pattern preceding each letter (forward masking). Unmasked words were identified with a high degree of accuracy, and accuracy was independent of the exposure duration of the letters over the range 50–200 msec. Masked adjacent word displays and one-position word displays were identified much less accurately, and accuracy was highly dependent on letter exposure duration. In contrast to these results, identification of random strings, or of letters within random strings, was almost unaffected by the presence of the mask and was dependent on letter exposure duration under all display conditions. The results are interpreted as evidence that (a) masking forces subjects to process serial displays one letter at a time, and (b) words, or large segments of words, are habitually processed in parallel, while random strings are processed as a series of individual letters or small chunks.     

Forced serial processing of words and letter strings: A reexamination

Words and nonword strings, three and seven letters long, were displayed serially (i.e., one letter at a time) or simultaneously, with or without a backward mask following display of each letter or string. Recognition of words, and of individual letters within words, was markedly impaired in the masked serial condition relative to the unmasked serial, unmasked simultaneous, and masked simultaneous conditions. Analogous differences were smaller or nonexistent for seven-letter nonwords; however, three-letter nonwords produced relatively “wordlike” data. Implications for the issue of spatially serial vs. parallel processing in word recognition are discussed.     

The role of lateral masking and orthographic structure in letter and word recognition.

The present experiments evaluated the contribution of orthographic structure and lateral masking in the perception of letter, word, and nonword test displays. Performance was tested in a backward recognition masking experiment in which a masking stimulus followed the test display after a variable blank interstimulus interval. In agreement with previous findings across different experiments, words were recognized better than single letterd at short interstimulus intervals, but the opposite was the case at long intervals. Performance on the nonwords resembled performance on letters at short masking intervals and performance on words at long masking intervals. The quantitative results were described by a processing model that incorporates the effects of lateral masking and orthographic structure in the dynamic processing of letter strings. Lateral masking tends to lower the potential perceptibility of letters whereas orthographic structure can reduce the uncertainty of the candidate letters in the letter sequence. The present model predicts that the quantitative contribution of each of these processes to performance is critically dependent upon the processing time available before the onset of the masking stimulus.     

Processing of sequentially presented letters

The effect on the number of letters S can report of the duration of each sequentially presented letter was compared with that of processing time, defined as the time from the onset of a letter to the onset of the next letter. Four Ss were each shown 1250 common English words, from four to eight letters long, one letter at a time. Each letter acted as a visual noise field for the preceding letter. The duration of each letter and the interval between letters was varied independently. The S reported the letters he saw after each word was displayed. It was found that the processing time (onset to onset) predicted the number of letters correctly reported, regardless of the partition between on time and off time. A calculation was made of the number of milliseconds of on plus off time that are needed to ensure correct report of each letter. This time was independent of the duration of the processing time, but was positively correlated with the number of letters in the word. This correlation is probably in part artifactual, so that no claim can be made that it takes longer to process a letter of a long as compared to a short word.     

Influence of parafoveal processing on the missing-letter effect

According to the parafoveal-processing hypothesis, letters are more often missed in function words than in content words because the former are more likely to be identified in the parafovea, where letter processing is not available. Contrary to previous demonstrations, more omissions occurred in function words than in content words when parafoveal processing was not available because words were displayed in column format, text was read through a 5-letter window, or words were presented 1 at a time on a computer screen. In all experiments, impeding parafoveal processing decreased omission rates for function but not for content words. In the last experiment, direct monitoring of eye movements revealed that, for both fixated and skipped words, letters in function words are missed more often than content words. These results are best interpreted within a model including the structural precedence hypothesis and stressing the importance of visual factors.     

The role of configuration in the identification of visually degraded words

Two experiments were carried out in which reaction time for identification of words in sets of four was measured. The words were of three letters, with dichotomous variation of the first and third letters, in either upper- or lowercase, with letters chosen to give maximum variation in word configuration in lowercase. Four types of display were used, differing in size and masking. In Experiment 1, the words were easy to identify in uppercase as well as in lowercase, because the letters were easily discriminable in either case, and the results showed no advantage to lowercase words at any level of degradation. In Experiment 2, the letters were more difficult to discriminate in uppercase, and with the most severe degradation, the words in lowercase were identified more rapidly than those in uppercase. Thus the reaction time results showed an advantage of the configural properties available in lowercase only with words difficult to identify in uppercase and with extreme amounts of degradation. Error analyses of both experiments showed that errors were made to individual letters, not on the basis of the configuration, for words in both cases. It is concluded that letter processing occurs even when there is evidence for the use of configural information, thus implying a predominant role of letter perception in word identification. Some reservation is expressed whether whole-word configuration is the only pertinent configural property and whether the results from such simple identification tasks are appropriate to an understanding of natural reading.     

Vibrotactile letter recognition: The effects of a ma king stimulus

A series of experiments examined the effect of masking stimuli on the ability of observers to recognize letters of the alphabet through their fingertips. The letters were generated on the 6 × 24 vibrotactile array of the Optacon, a reading aid for the blind. Letter recognition was interfered with by the presence of masking stimuli occurring at the same site on the skin either before (forward masking) or after (backward masking) the target letter had been presented. In general, backward masking interfered with letter recognition more than did forward masking. Backward masking was particularly effective for letters in which the information critical for identification is located on the right side of the letter. Presenting the letters reversed resulted in more forward masking for those letters with critical information now located on the left side. Increasing the time between the target letter and the masking stimuli resulted in improved letter recognition. The implications of the results for tactile reading are discussed.     

Are all letters really processed equally and in parallel? Further evidence of a robust first letter advantage

This present study examined accuracy and response latency of letter processing as a function of position within a horizontal array. In a series of 4 Experiments, target-strings were briefly (33 ms for Experiments 1 to 3, 83 ms for Experiment 4) displayed and both forward and backward masked. Participants then made a two alternative forced choice. The two alternative responses differed just in one element of the string, and position of mismatch was systematically manipulated. In Experiment 1, words of different lengths (from 3 to 6 letters) were presented in separate blocks. Across different lengths, there was a robust advantage in performance when the alternative response was different for the letter occurring at the first position, compared to when the difference occurred at any other position. Experiment 2 replicated this finding with the same materials used in Experiment 1, but with words of different lengths randomly intermixed within blocks. Experiment 3 provided evidence of the first position advantage with legal nonwords and strings of consonants, but did not provide any first position advantage for non-alphabetic symbols. The lack of a first position advantage for symbols was replicated in Experiment 4, where target-strings were displayed for a longer duration (83 ms). Taken together these results suggest that the first position advantage is a phenomenon that occurs specifically and selectively for letters, independent of lexical constraints. We argue that the results are consistent with models that assume a processing advantage for coding letters in the first position, and are inconsistent with the commonly held assumption in visual word recognition models that letters are equally processed in parallel independent of letter position.     

The interfering effect of word perception on letter identification

Subjects identified a single lowercase letter in a visual display by pressing one of two buttons. Two letters were assigned to each response. Groups received one of three context conditions: word, nonword, or single-letter displays. In words and nonwords, the flanking letters adjacent to the target varied as to whether they were response compatible or incompatible with the target. Single letters produced faster response latencies than either multi-letter condition, and words yielded slower latencies than did nonwords. Items Containing an incompatible-response flanking letter produced longer latencies than items containing a compatible flanking letter. Subgroups of subjects with different characteristic processing patterns were identified with a separate test. These subgroups were differentially affected by the context conditions in the letter-identification task. A greater subgroup difference was found in nonwords than in words.     

Reading Ability and the Use of Context in Orthographic Information Processing

Reading ability and context use in orthographic processing during silent reading were investigated. Poor readers and reading-level matched controls were presented target words containing a letter substitution in two contrasting context conditions. The hypothesis was that presenting a word in a highly predictable context would induce readers to proceed through the text without completely processing orthographic units at lower processing levels in the hierarchy (e.g., constituent letters). This general context effect was found. Normal and poor readers did not differ in context dependency. Poor readers more often missed substitutions, regardless of context. Poor readers also processed orthographic information less accurately. Target letters located in final word position were missed most often. Substitutions located in high bigram-frequency letter clusters were more often missed, and this effect was independent of intraword location. The implications of these results for understanding poor readers' basic reading problems are discussed.     

Direct assessments of the processing time hypothesis for the missing-letter effect

When participants search for a target letter while reading, they make more omissions if the target letter is embedded in frequently used words or in the most frequent meaning of a polysemic word. According to the processing time hypothesis, this occurs because familiar words and meanings are identified faster, leaving less time for letter identification. Contrary to the predictions of the processing time hypothesis, with a rapid serial visual presentation procedure, participants were slower at detecting target letters for more frequent words or the most frequent meaning of a word (Experiments 1 and 2) or at detecting the word itself instead of a target letter (Experiment 3). In Experiments 4 and 5, participants self-initiated the presentation of each word, and the same pattern of results was observed as in Experiments 1 and 3. Positive correlations were also found between omission rate and response latencies.     

Differential backward masking of words and letters by masks of varying orthographic structure

The effects of structural relationships between targets and masks were investigated using a backward-masking paradigm. Specifically, the masking of single letters, common fiveletter words, and five-letter pseudowords masked by a blank flash, strings of overlapped letters, pseudowords, and words was investigated. Target duration was varied from 2 to 32 msec, with mask duration held constant at 25 msec. The dependent measure was the critical interstimulus interval for correct target identification. Letters were more effectively masked than words and pseudowords. A blank mask caused the least amount of masking, followed by the overlapped letter strings, and then the word and pseudoword masks. In addition to the overall greater masking effectiveness for the three patterned masks, overlapped letter strings masked letters more effectively than they did words. The implications of current theories of masking for these results and the implications of these results for theories of word recognition were discussed.     

Word superiority over isolated letters: The neglected case of forward masking

Previous research shows that when briefly presented alphabetic stimuli are followed by pattern masks, letters in words are reported more accurately than are isolated letters (the “Word-Letter Phenomenon,” or WLP); however, when these masks are replaced by blank fields, the WLP disappears. These findings have led to the popular notion that the WLP reflects selective masking of ongoing stimulus processing and so critically depends on the use of poststimulus masks. Here we report three experiments which re-examine the role of masking in the WLP by contrasting the effects of postmasked displays with the effects of premasked displays in which words and isolated letters werepreceded by a pattern mask and followedby a completely blank field. Despite the critical role generally assigned to poststimulus pattern masks, similar WLPs were obtained with pre- and postmasked displays. Implications for theories of word and letter recognition are discussed.     

Selective attention and target search with brief visual displays

In a series of four experiments, subjects were presented with eight item arrays tachistoscopically. In two tasks, the subject was required to recall a digit presented among seven letters or a letter among seven digits. The remaining tasks required the subject to detect the presence of a specific letter either in the context of seven random letters or seven digits. An examination of the effects of a masking stimulus for various target positions suggested that subjects could attend selectively when the target was a different category than its context but had to process the entire array when the target was the same category as the context. The effects of selective attention and masking are explained in terms of parallel and sequential identification processes.     

Processing letters in words at different levels

Subjects indicated whether two letters, two words, or a letter and the first letter of a word were the same. Letter targets were matched more quickly than word targets when the stimuli were presented simultaneously. When the target and comparison stimuli were separated by a 3-sec interval, word targets were matched more quickly than a letter and a letter in a word. It was also shown that the physical similarity of the targets and comparison stimuli had a greater effect in the simultaneous matching conditions. These findings are consistent with a model of word processing in which letters are individually compared prior to word identification at a physical level of processing. At a higher level of processing, words may be encoded as a unit, and the identification of the letters within the word may require a decoding of the word unit.     

Effects of increased letter spacing on word identification and eye guidance during reading

The effect of increasing the space between the letters in words on eye movements during reading was investigated under various word-spacing conditions. Participants read sentences that included a high- or low-frequency target word, letters were displayed normally or with an additional space between adjacent letters, and one, two, or three spaces were present between each word. The spacing manipulations were found to modulate the effect of word frequency on the number and duration of fixations on target words, indicating, more specifically, that letter spacing affected actual word identification under various word-spacing conditions. In addition, whereas initial fixations landed at the preferred viewing position (i.e., to the left of a word’s center) for sentences presented normally, landing positions were nearer the beginnings of words when letter spacing was increased, and even nearer the beginnings of words when word boundary information was lacking. Findings are discussed in terms of the influence of textual spacing on eye movement control.     

Identification and pronunciation effects in a verbal reaction time task for words,pseudowords, and letters

The question addressed in this investigation was whether faster reading and pronunciation of words than orthographically regular pseudowords is due to faster identification or to faster programming and execution of the motor response. In Experiment I, three different response conditions (naming, threechoice signaled responding, and one-choice signaled responding) were employed to separate the identification and articulation processes in a verbal reaction time task. It was found that, for all intents and purposes, single, isolated letters are processed as if they were very short words. Words are read and pronounced 72 msec faster than pseudowords. Words are also pronounced 30 msec faster than pseudowords even if the reader has longer than 1 sec to identify the stimulus (three-choice condition) or to both identify the stimulus and preprogram the response (one-choice condition). The data indicate that words are identified about 52 msec faster and articulated about 30 msec faster than pseudowords. Since the number of response alternatives (one or three) does not interact with stimulus type (letter, word, or pseudoword) in the signaled response control condition, the 30-msec difference is due to response execution and not to differential response programming. Response programming takes in the neighborhood of 236 msec. Experiment 2 investigated the effect of local orthographic context upon the identification of the first letter of a string of letters. No difference was found in identifying the initial letter of words and pseudowords, but the initial letter of these orthographically regular letter strings was identified and named 10 msec faster than the initial letter of orthographically anomalous strings of letters (anagrams). The data from the two experiments are supportive of theories of reading that assume (1) that the letters of visually presented words are processed simultaneously, in parallel, (2) that there is a relatively direct access and retrieval of the phonological memory codes for the names of words, and (3) that orthographically regular pseudowords having no representation in the phonological lexicon undergo a grapheme-to-phoneme transformation that takes longer to finish than the direct spelling-to-sound process used for words.     

How does interhemispheric communication in visual word recognition work? Deciding between early and late integration accounts of the split fovea theory

It has recently been shown that interhemispheric communication is needed for the processing of foveally presented words. In this study, we examine whether the integration of information happens at an early stage, before word recognition proper starts, or whether the integration is part of the recognition process itself. Two lexical decision experiments are reported in which words were presented at different fixation positions. In Experiment 1, a masked form priming task was used with primes that had two adjacent letters transposed. The results showed that although the fixation position had a substantial influence on the transposed letter priming effect, the priming was not smaller when the transposed letters were sent to different hemispheres than when they were projected to the same hemisphere. In Experiment 2, stimuli were presented that either had high frequency hemifield competitors or could be identified unambiguously on the basis of the information in one hemifield. Again, the lexical decision times did not vary as a function of hemifield competitors. These results are consistent with the early integration account, as presented in the SERIOL model of visual word recognition.     

The Delay in Visual Reorientation following Exposure to a Change in Direction of Resultant Force on a Human Centrifuge

Errors in reporting a cued target letter appearing among a string of letters more often reflect the mislocation of a letter appearing elsewhere in the string than the intrusion of one not in the string. The current experiment was conducted to determine the representation of letters at the stage at which errors occur. Four letters (from a set of 12 chosen to contain counterpart pairs that were similar physically, phonemically, or both physically and phonemically) appeared in each exposure, with the target letter indicated by a cue in the postexposure mask. Letter strings presented to one group of subjects were flanked on either side by a pound sign (#) to assess the effect of lateral masking on the terminal letter in the string. Physical similarity dictated the pattern of mislocations between counterparts, suggesting a physical rather than phonemic or abstract representation. Lateral masking played no significant role in the difference between intrusions and mislocations.     

Representation of letters when mislocation errors occur

Errors in reporting a cued target letter appearing among a string of letters more often reflect the mislocation of a letter appearing elsewhere in the string than the intrusion of one not in the string. The current experiment was conducted to determine the representation of letters at the stage at which errors occur. Four letters (from a set of 12 chosen to contain counterpart pairs that were similar physically, phonemically, or both physically and phonemically) appeared in each exposure, with the target letter indicated by a cue in the postexposure mask. Letter strings presented to one group of subjects were flanked on either side by a pound sign (#) to assess the effect of lateral masking on the terminal letter in the string. Physical similarity dictated the pattern of mislocations between counterparts, suggesting a physical rather than phonemic or abstract representation. Lateral masking played no significant role in the difference between intrusions and mislocations.