(Donald Shankweiler and Isabelle Y. Liberman)
Because speech is universal and reading is not, we may suppose that the latter is more difficult and less natural. Indeed, we know that a large part of the early education of the school child must be devoted to instruction in reading and that the instruction often fails, even in the most favorable circumstances. [judging from the long history of debate concerning the proper methods of teaching children to read [Mathews 1966], the problem has always been with us. Nor do we appear to have come closer to a solution: we are still a long way from understanding how children learn to read and what has gone wrong when they fail.
Since the child already speaks and understands his language at the time that reading instruction begins, the problem is to discover the major barriers in learning to perceive language by eye. It is clear that the first requirement for reading is that the child be able to segregate the letter segments and identify them with accuracy and speed. Some children undoubtedly do fail to learn to recognize letters and are unable to pass on to succeeding stages of learning to read; but, as we shall see, there are strong reasons for believing that the principal barriers for most children are not at the point of visual identification of letter shapes. There is no general agreement, however, about the succeeding stages of learning to read, their time course, and the nature of their special difficulties. In order to understand reading and compare it with speech, we need to look closely at the kinds of difficulties the child has when he starts to read, that is, his misreadings, and ask how these differ from errors in repeating speech perceived by ear. In this way, we may begin to grasp why the link between alphabet and speech is difficult.
In the extensive literature about reading since the 1890s there have been sporadic surges of interest in the examination of oral reading errors as a means of studying the process of reading acquisition. The history of this topic has been well summarized by Weber  (...).
We are, in addition, curious to know whether the difficulties in reading are to be found at a visual stage or at a subsequent linguistic stage of the process This requires us to consider the special case of reversal errors, in which optical considerations are, on the face of it, primary. Our inquiry into linguistic aspects of reading errors then leads us to ask which constituents of words tend to be misread, and whether the same ones tend to be misheard. We examine errors with regard to the position of the constituent segments within the word and the linguistic status of the segments in an attempt to produce a coherent account of the possible causes of the error pattern in reading.
The Word as the Locus of Difficulty in Beginning Reading
One often encounters the claim that there are many children who can read individual words yet do not seem able to comprehend connected text [Anderson and Dearborn 1952; Goodman 1968]. The existence of such children is taken to support the view that methods of instruction that stress spelling-to-sound correspondences and other aspects of decoding are sufficient and many even produce mechanical readers who are expert at decoding but fail to comprehend sentences. (...)
The Contribution of Visual Factors to the Error Pattern in Beginning
Reading: The Problem of Reversals
We have seen that a number of converging results support the belief that the primary locus of difficulty in beginning reading is the word. But within the word, what is the nature of the difficulty? To what extent are the problems visual and to what extent linguistic?
In considering this question, we asked first whether the problem is in the perception of individual letters. There is considerable agreement that after the first grade, even those children who have made little further progress in learning to read do not have significant difficulty in visual identification of individual letters [Vernon 1960; Shankweiler 1964; Doehring 1968].
REVERSALS AND OPTICAL SHAPE PERCEPTION
The occurrence in the alphabet of reversible letters may present special problems, however. The tendency for young children to confuse letters of similar shape that differ in orientation (such as "b, d, p, q") is well known. Gibson and her colleagues [1962, 1965] have isolated a number of component abilities in letter identification and studied their developmental course by the use of letter-like forms that incorporate basic features of the alphabet. They find that children do not readily distinguish pairs of shapes that are 180-degree transformations (i.e., reversals) of each other at age 5 or 6, but by age 7 or 8, orientation has become a distinctive property of the optical character. It is of interest, therefore, to investigate how much reversible letters contribute to the error pattern of 8-year-old children who are having reading difficulties.
Reversal of the direction of letter sequences (e.g., reading "from" for "form") is another phenomenon that is usually considered to be intrinsically related to orientation reversal. Both types of reversals are often thought to be indicative of a disturbance in the visual directional scan of print in children with reading disability (see Benton  for a comprehensive review of the relevant research). One early investigator considered reversal phenomena to be so central to the problems in reading that he used the term "strephosymbolia" to designate specific reading disability [Orton 1925]. We should ask, then, whether reversals of letter orientation and sequence loom large as obstacles to learning to read. Do they covary in their occurrence, and what is the relative significance of the optical and linguistic components of the problem?
RELATIONSHIPS BETWEEN REVERSALS AND OTHER TYPES OF ERRORS
It was found that, even among these poor readers, reversals accounted for only a small proportion of the total error, through the list was constructed to provide maximum opportunity for reversals to occur. Separating the two types, we found that sequences reversals accounted for 15 percent of the total errors made, and orientation errors only 10 percent, whereas other consonant errors accounted for 32 percent of the total and vowel errors 43 percent. Moreover, individual differences in reversal tendency were large (rates of sequence reversal ranged from 4 to 19 percent; rates for orientation reversal ranged from 3 to 31 percent). Viewed in terms of opportunities for error, orientation errors occurred less frequently than other consonant errors. Test-retest comparisons showed that whereas other reading errors were rather stable, reversals -- and particularly orientation reversals -- were unstable.
ORIENTATION REVERSALS AND REVERSALS OF SEQUENCES:
NO COMMON CAUSE?
Having considered the two types of reversals separately, we find no support for assuming that they have a common cause in children with reading problems. Among the poor third-grade readers, sequence reversal and orientation reversal were found to be wholly uncorrelated with each other, whereas vowel and consonant errors correlated 0.73. (...)
ORIENTATION ERRORS: VISUAL OR PHONETIC?
In further pursuing the orientation errors, we examined the nature of the substitutions among the reversible letters "b, d, p, g." Tabulation of these showed that the possibility of generating another letter by a simple 180-degree transformation is indeed a relevant factor in producing the confusions among these letters. This is, of course, in agreement with the conclusion reached by Gibson and her colleagues .
At the same time, other observations [I. Y. Liberman, Shankweiler et al. 1971] indicate that letter reversals may be a symptom and not a cause of reading difficulty. (...)
Linguistic Aspects of the Error Pattern in Reading and Speech
"In reading research, the deep interest in words as visual displays stands in contrast to the relative neglect of written words as linguistic units represented graphically." [Weber 1968, p.113]
The findings we have discussed in the preceding section suggest that the chief problems the young child encounters in reading words are beyond the stage of visual identification of letters. It therefore seemed profitable to study the error pattern from a linguistic point of view.
(...) the substantially greater error rate for final consonants than for initial ones is certainly contrary to what would be expected by an analysis of the reading process in terms of sequential probabilities. If the child at the early stages of learning to read were able to utilize the constraints that are built into the language, he would make fewer errors at the end than at the beginning, not more. In fact, what we often see is that the child breaks down after he has gotten the first letter correct and can go no further. We will suggest later why this may happen.
MISHEARING DIFFERS FROM MISREADING
In order to understand the error pattern in reading, it should be instructive to compare it with the pattern of errors generated when isolated monosyllables are presented by ear for oral repetition. (...)
The error pattern for oral repetition shows some striking differences from that in reading. With auditory presentation, errors in oral repetition averaged 7 percent when tabulated by phoneme, as compared with 24 percent in reading, and were about equally distributed between initial and final position, rather than being markedly different. Moreover, contrary to what occurred when the list was read, fewer errors occurred
on vowels than on consonants.
It is clear from the figure that the perception of speech by reading has problems which are separate and distinct from the problems of perceiving speech by ear. We cannot predict the error rate for a given phoneme in reading from its error rate in listening. If a phoneme were exactly as difficult to read as to hear, the point would fall on the diagonal line that has been dotted in. Vertical distance from the diagonal to any point below it is a measure of the specific difficulty of reading the phoneme as distinguished from listening to it. Although the reliability of the individual points in the array has not been assessed, the trends are unmistakable. The points are very widely scattered for the consonants. As for the vowels, they are seldom misheard but often misread (suggesting, incidentally, that the high error rate on vowels in reading cannot be an artificial transcription difficulties).
The following analysis illustrates how vowel errors may be analyzed to discover whether, in fact, the error pattern is nonrandom and, if it is, to discover what the major substitutions are. Figure 2 shows a confusion matrix for vowels based on the responses of 11 children at the end of the third grade (Group C2 in Table 6) who are somewhat retarded in reading. Each row in the matrix refers to a vowel phoneme represented in the words (of List 2) and each column contains entries of the transcriptions of the responses given in oral reading. Thus the rows give the frequency distribution for each vowel percentaged against the number of occurrences, which is approximately 25 per vowel per subject.
It may be seen that the errors are not distributed randomly. (...) Hence we may conclude that the error rate on vowels in our list is related to the number of orthographic representations of each vowel.
The data thus support the idea that differences in error rate among vowels reflect differences in their orthographic complexity. Moreover, as we have said, the fact that vowels, in general, map onto sound more complexly than consonants is one reason they tend to be misread more frequently than consonants.
It may be, however, that these orthographic differences among segments are themselves partly rooted in speech. Many data from speech research indicate that vowels are often processed differently from consonants when perceived by ear. A number of experiments have shown that the tendency to categorical perception is greater in the encoded stop consonants than in the unencoded vowels [A. M. Liberman, Cooper et al. 1967; A. M. Liberman 1970]. It may be argued that as a consequence of the continuous nature of their perception, vowels tend to be somewhat indefinite as phonologic entities, as illustrated by the major part they play in variation among dialects and the persistence of allophones within the same geographical locality. By the same reasoning, it could be that the continuous nature of vowel perception is one cause of complex orthography, suggesting that one reason that multiple representations are tolerated may lie very close to speech.
We should also consider the possibility that the error pattern of the vowels reflects not just the complex relation between letter and sound but also confusions that arise as the reader recodes phonetically. There is now a great deal of evidence [Conrad 1964, this volume] that normal readers do, in fact, recode the letters into phonetic units for storage and use in short-term memory. If so, we should expect that vowel errors would represent displacements from the correct vowels to those that are phonetically adjacent and similar, the more so because, as we have just noted, vowel perception is more nearly continuous than categorical. That such displacements did in general occur is indicated in Figure 2 by the fact that the errors tend to lie near the diagonal. More data and, in particular, a more complete selection of items will be required to determine the contribution to vowel errors of orthographic complexity and the confusions of phonetic receding.
Summary and Conclusions
In an attempt to understand the problems encountered by the beginning reader and children who fail to learn, we have investigated the child's misreadings and how they relate to speech. The first question we asked was whether the major barrier to achieving fluency in reading is at the level of connected text or in dealing with individual words. Having concluded from our own findings and the research of others that the word and its components are of primary importance, we then looked more closely at the error patterns in reading words.
Since reading is the perception of language by eye, it seemed important to ask whether the principal difficulties within the word are to be found at a visual stage of the process or at a subsequent linguistic stage. We considered the special case of reversals of letter sequence and orientation in which the properties of visual confusability are, on the face of it, primary. We found that although optical reversibility contributes to the error rate, for the children we have studied it is of secondary importance to linguistic factors. Our investigation of the reversal tendency then led us to consider whether individual differences in reading ability might reflect differences in the degree and kind of functional asymmetries of the cerebral hemisphere. Although the evidence is at this time not clearly supportive of a relation between cerebral ambilaterality and reading disability, it was suggested that new techniques offer an opportunity to explore this relationship more fully in the future.
When we turned to the linguistic aspects of the error pattern in words, we found, as others have, that medial and final segments in the word are more often misread than initial ones and vowels more often than consonants. We then considered why the error pattern in mishearing differed from misreading in both these respects. In regard to segment position, we concluded that children in the early stages of learning to read tend to get the initial segment correct and fail on subsequent ones because the do not have the conscious awareness of phonemic segmentation needed specially in reading but not in speaking and listening.
As for vowels in speech, we suggested, first of all, that they may tend to be heard correctly because they are carried by the strongest portion of the acoustic signal. In reading, the situation is different: alphabetic representation of the vowels possess no such special distinctiveness. Moreover, their embedded placement within the syllable and their orthographic complexity combine to create difficulties in reading.
Evidence for the importance of orthographic complexity was seen in our data by the fact that the differences among vowels in error rate in reading were predictable from the number of orthographic representations of each vowel. However, we also considered the possibility that phonetic confusions may account for a significant portion of vowel errors, and we suggested how this hypothesis might be tested.
We believe that the comparative study of reading and speech is of great importance for understanding how the problems of perceiving language by eye differ from the problems of perceiving it by ear, and for discovering why learning to read, unlike speaking and listening, is a difficult accomplishment.