Word recognition and face recognition following posterior cerebral artery stroke: Overlapping networks and selective contributions

It has been suggested that word- and face recognition rely on a common cerebral network for perceptual processing. To further explore this hypothesis, face- and word recognition were assessed in seven patients with focal lesions after stroke of the posterior cerebral artery in either hemisphere. The aim was to investigate if problems in face- and word recognition would co-occur, if testing was sensitive and patients were not pre-selected based on behavioural symptoms. Patients and matched controls were given a reading test and a delayed matching paradigm with faces, objects, and words. Interestingly, all patients with word recognition difficulties had problems in face recognition, independently of the affected hemisphere, supporting the existence of a bilateral perceptual network for faces and words. However, two patients showed selective face recognition problems after unilateral damage to either hemisphere, suggesting that parts of this network may be more critical for face than for word recognition.     

The Modulation of Visual and Task Characteristics of a Writing System on Hemispheric Lateralization in Visual Word Recognition—A Computational Exploration

Through computational modeling, here we examine whether visual and task characteristics of writing systems alone can account for lateralization differences in visual word recognition between different languages without assuming influence from left hemisphere (LH) lateralized language processes. We apply a hemispheric processing model of face recognition to visual word recognition; the model implements a theory of hemispheric asymmetry in perception that posits low spatial frequency biases in the right hemisphere and high spatial frequency (HSF) biases in the LH. We show two factors that can influence lateralization: (a) Visual similarity among words: The more similar the words in the lexicon look visually, the more HSF/LH processing is required to distinguish them, and (b) Requirement to decompose words into graphemes for grapheme‐phoneme mapping: Alphabetic reading (involving grapheme‐phoneme conversion) requires more HSF/LH processing than logographic reading (no grapheme‐phoneme mapping). These factors may explain the difference in lateralization between English and Chinese orthographic processing.     

The relationship between reading ability and lateralized lexical decision

Although lexical decision remains one of the most extensively studied cognitive tasks, very little is known about its relationship to broader linguistic performance such as reading ability. In a correlational study, several aspects of lateralized lexical decision performance were related to vocabulary and reading comprehension measures, as assessed using the Nelson-Denny Reading Test. This lateralized lexical decision task has been previously shown to demonstrate (1) independent contributions from both hemispheres, as well as (2) interhemispheric interactions during word recognition. Lexical decision performance showed strong relationships with both reading measures. Specifically, vocabulary performance correlated significantly with left visual field (LVF) word accuracy and LVF non-word latency, both measures of right hemisphere performance. There were also significant, though somewhat weaker, correlations between reading comprehension and RVF non-word latency. Lexicality priming, a measure of interhemispheric communication during lexical decision, was also correlated with reading comprehension. These results suggest that hemispheric interaction during word recognition is common, and that lexical processing contribution from the right hemisphere, something commonly taken as minor and inconsequential, can lead to significant performance benefits and to individual differences in reading.     

Hemispheric asymmetry in cross-race face recognition

This study examines two phenomena related to face perception, both of which depend on experience and holistic processing: perceivers process faces more efficiently in the right hemisphere of the brain (a hemispheric asymmetry), and they typically show greater recognition accuracy for members of their racial ingroup (a cross-race recognition deficit). The current study tests the possibility that these two effects are related. If asymmetry depends on experience, it should be particularly evident with (more familiar) ingroup faces; if cross-race recognition relies on holistic processing, it should be particularly evident for faces presented to the right hemisphere. Black and White participants viewed Black and White faces presented to either the left or right visual field. As predicted, participants showed a more pronounced asymmetry for ingroup (rather than outgroup) faces, and cross-race recognition deficits were more pronounced for stimuli presented to the left (rather than the right) visual field.     

Dissociation of short- and long-term face memory: evidence from long-term recency effects in temporal lobe epilepsy

We tested whether memory deficits in temporal lobe epilepsy (TLE) are better described by a single- or dual-store memory model. To this aim, we analyzed the influence of TLE and proactive interference (PI) on immediate and 24-h long-term recency effects during face recognition in 16 healthy participants and 18 right and 21 left non-surgical TLE patients. PI in healthy participants or TLE erased the long-term recency effect, but left the immediate recency effect unaffected. Although the immediate recency effect was still visible in right TLE patients, the number of detected recency items during immediate recognition was decreased in right TLE compared to left TLE. Right TLE was also related to decreased detection of pre-recency items during delayed recognition compared to left TLE, and decreased detection of pre-recency items during immediate recognition under PI. The results show that the temporal lobes are necessary for the long-term recency effect, but not for the immediate recency effect, and thus speak for a dissociation of short- and long-term memory for faces. Right TLE is related to more severe long-term memory deficits than left TLE and is also related to additional short-term memory deficits for faces.     

Hemisphere differences in conscious and unconscious word reading

Hemisphere differences in word reading were examined using explicit and implicit processing measures. In an inclusion task, which indexes both conscious (explicit) and unconscious (implicit) word reading processes, participants were briefly presented with a word in either the right or the left visual field and were asked to use this word to complete a three-letter word stem. In an exclusion task, which estimates unconscious word reading, participants completed the word stem with any word other than the prime word. Experiment 1 showed that words presented to either visual field were processed in very similar ways in both tasks, with the exception that words in the right visual field (left hemisphere) were more readily accessible for conscious report. Experiment 2 indicated that unconsciously processed words are shared between the hemispheres, as similar results were obtained when either the same or the opposite visual field received the word stem. Experiment 3 demonstrated that this sharing between hemispheres is cortically mediated by testing a split-brain patient. These results suggest that the left hemisphere advantage for word reading holds only for explicit measures; unconscious word reading is much more balanced between the hemispheres.     

Developmental dyslexia: Electrophysiological evidence of clinical subgroups

Event-related potentials (ERPs) to word and to musical-chord stimuli were recorded in 13 dyslexic boys and 13 age-matched normal readers. Normal readers and dyslexics whose reading handicaps involved visual-spatial processing deficits had greater word versus musical-chord ERP waveform differences over the left as compared to the right hemisphere. Dyslexics whose reading difficulties were related to auditory-verbal processing deficits did not exhibit this asymmetry. These results are interpreted as supportive of the hypothesis that the latter group of dyslexics has failed to develop normal left hemisphere specialization for processing of auditory-linguistic material.     

Can we learn from the clinically significant face processing deficits,prosopagnosia and capgras delusion?

This review describes two clinically significant face processing deficits, prosopagnosia and Capgras delusion, and provides new knowledge about the face recognition process by a convergence of empirical findings. These empirical findings are structured around two questions that are viewed from the perspectives of the two deficits. First is the question of hemispheric specificity, which inquires into the degree of each hemisphere's contribution to the face recognition process. Second is the question of dual neural pathways, which addresses the possibility that the face recognition process proceeds along two parallel pathways in the brain. Findings from the hemispheric specificity studies reinforce the current view that right hemisphere involvement is necessary for face recognition while left hemisphere involvement is minimal. Findings from the dual neural pathways studies reinforce the plausible but yet unproven hypothesis that two neural pathways pass information from the visual association cortex in the occipital lobe toward the temporal lobes and limbic system when faces are seen and recognized. These findings, which also indicate that each of the dual neural pathways carries different, nonredundant information, could be instrumental in showing that the pathways play different roles in the manifestations of the clinically significant face processing deficits, prosopagnosia and Capgras delusion.     

Evidence for right hemisphere phonology in a backward masking task

The extent to which orthographic and phonological processes are available during the initial moments of word recognition within each hemisphere is under specified, particularly for the right hemisphere. Few studies have investigated whether each hemisphere uses orthography and phonology under constraints that restrict the viewing time of words and reduce overt phonological demands. The current study used backward masking in the divided visual field paradigm to explore hemisphere differences in the availability of orthographic and phonological word recognition processes. A 20 ms and 60 ms SOA were used to track the time course of how these processes develop during pre-lexical moments of word recognition. Nonword masks varied in similarity to the target words such that there were four types: orthographically and phonologically similar, orthographically but not phonologically similar, phonologically but not orthographically similar and unrelated. The results showed the left hemisphere has access to both orthography and phonology early in the word recognition process. With more time to process the stimulus, the left hemisphere is able to use phonology which benefits word recognition to a larger extent than orthography. The right hemisphere also demonstrates access to both orthography and phonology in the initial moments of word recognition, however, orthographic similarity improves word recognition to a greater extent than phonological similarity.     

Hemisphere differences in the mood state-dependent effect for recognition of emotional faces

Forty subjects viewed 10 pictures of facial expressions of emotion while they experienced a happy mood and 10 pictures while they experienced a sad mood. Later, while re-experiencing either a happy or sad mood, they were tested for recognition of these 20 target pictures intermixed with 20 distractors. Recognition of the 10 pictures seen earlier in a disparate mood was impaired significantly when they were presented at testing to the right hemisphere, but not when presented to the left. The right hemisphere appears to store the subject's mood as an integral part of a memory representation for an emotionally expressive face. When that face is presented at testing to the right hemisphere, recognition depends on whether the subject's test mood matches the mood stored in the representation. In contrast, the left hemisphere appears to store the subject's mood separately from encoded visual information about a face, and so recognition of a face presented at testing to the left hemisphere is unaffected by changes in mood.     

Hemispheric differences in the perception of words and faces in deaf and hearing children

The purpose of this study was to investigate hemispheric functional asymmetry in 18 normal hearing children and 18 congenitally deaf children aged 13-14 years. The task was identification of a visual stimulus (3-letter word or photograph of a face) presented in either the left or right visual field. The children responded by pointing to the target stimulus on a response card which contained four different words or three different faces. The percentage of errors for presentations to the two visual fields were analysed to determine hemispheric dominance. The pattern of hemispheric differences for the hearing children was consistent with that from previous investigations. The results for the deaf children differed from those of the normals. In word perception we observed a right hemisphere advantage and in the face recognition a lack of hemispheric differences. These results point to a lack of auditory experiences which is affecting the functional organization of the two hemispheres. It is suggested that the necessity to make use of visuo-spatial information in the process of communication causes right hemisphere dominance in verbal tasks. This may influence the perception of other visuo-spatial stimuli which may yield a lack of hemispheric asymmetry in face recognition.     

The importance of interhemispheric transfer for foveal vision: a factor that has been overlooked in theories of visual word recognition and object perception

In this special issue of Brain and Language, we examine what implications the division between the left and the right brain half has for the recognition of words presented in the center of the visual field. The different articles are a first indication that taking into account the split between the left and the right cerebral hemisphere need not be an inescapable nuisance in models of visual word recognition but may in fact form the clue to the solution of a longstanding problem within this literature. Also, the fact that interhemispheric transfer has implications for foveal word recognition should interest laterality researchers, as it makes their findings more central to normal reading. In this introductory article, I first present a rough picture of the current (lack of) evidence for a bilateral representation of the fovea and the absence of a callosal delay. I then briefly discuss the suggestions made by the different authors on how to integrate the foveal split within current models of visual word recognition.     

Early development of letter specialization in left fusiform is associated with better word reading and smaller fusiform face area

A functional region of left fusiform gyrus termed “the visual word form area” (VWFA) develops during reading acquisition to respond more strongly to printed words than to other visual stimuli. Here, we examined responses to letters among 5‐ and 6‐year‐old early kindergarten children (N = 48) with little or no school‐based reading instruction who varied in their reading ability. We used functional magnetic resonance imaging (fMRI) to measure responses to individual letters, false fonts, and faces in left and right fusiform gyri. We then evaluated whether signal change and size (spatial extent) of letter‐sensitive cortex (greater activation for letters versus faces) and letter‐specific cortex (greater activation for letters versus false fonts) in these regions related to (a) standardized measures of word‐reading ability and (b) signal change and size of face‐sensitive cortex (fusiform face area or FFA; greater activation for faces versus letters). Greater letter specificity, but not letter sensitivity, in left fusiform gyrus correlated positively with word reading scores. Across children, in the left fusiform gyrus, greater size of letter‐sensitive cortex correlated with lesser size of FFA. These findings are the first to suggest that in beginning readers, development of letter responsivity in left fusiform cortex is associated with both better reading ability and also a reduction of the size of left FFA that may result in right‐hemisphere dominance for face perception.     

Bilateral capacity for speech sound processing in auditory comprehension: evidence from Wada procedures

Data from lesion studies suggest that the ability to perceive speech sounds, as measured by auditory comprehension tasks, is supported by temporal lobe systems in both the left and right hemisphere. For example, patients with left temporal lobe damage and auditory comprehension deficits (i.e., Wernicke's aphasics), nonetheless comprehend isolated words better than one would expect if their speech perception system had been largely destroyed (70-80% accuracy). Further, when comprehension fails in such patients their errors are more often semantically-based, than-phonemically based. The question addressed by the present study is whether this ability of the right hemisphere to process speech sounds is a result of plastic reorganization following chronic left hemisphere damage, or whether the ability exists in undamaged language systems. We sought to test these possibilities by studying auditory comprehension in acute left versus right hemisphere deactivation during Wada procedures. A series of 20 patients undergoing clinically indicated Wada procedures were asked to listen to an auditorily presented stimulus word, and then point to its matching picture on a card that contained the target picture, a semantic foil, a phonemic foil, and an unrelated foil. This task was performed under three conditions, baseline, during left carotid injection of sodium amytal, and during right carotid injection of sodium amytal. Overall, left hemisphere injection led to a significantly higher error rate than right hemisphere injection. However, consistent with lesion work, the majority (75%) of these errors were semantic in nature. These findings suggest that auditory comprehension deficits are predominantly semantic in nature, even following acute left hemisphere disruption. This, in turn, supports the hypothesis that the right hemisphere is capable of speech sound processing in the intact brain.     

Effects of visual complexity and sublexical information in the occipitotemporal cortex in the reading of Chinese phonograms: a single-trial analysis with MEG

We employ a linear mixed-effects model to estimate the effects of visual form and the linguistic properties of Chinese characters on M100 and M170 MEG responses from single-trial data of Chinese and English speakers in a Chinese lexical decision task. Cortically constrained minimum-norm estimation is used to compute the activation of M100 and M170 responses in functionally defined regions of interest. Both Chinese and English participants’ M100 responses tend to increase in response to characters with a high numbers of strokes. English participants’ M170 responses show a posterior distribution and only reflect the effect of the visual complexity of characters. On the other hand, the Chinese participants’ left hemisphere M170 is increased when reading characters with high number of strokes, and their right hemisphere M170 is increased when reading characters with small combinability of semantic radicals. Our results suggest that expertise with words and the decomposition of word forms underlies processing in the left and right occipitotemporal regions in the reading of Chinese characters by Chinese speakers.     

In Your Right Mind: Right Hemisphere Contributions to Language Processing and Production

The verbal/nonverbal account of left and right hemisphere functionality is the prevailing dichotomy describing the cerebral lateralization of function. Yet the fact that the left hemisphere is the superior language processor does not necessarily imply that the right hemisphere is completely lacking linguistic ability. This paper reviews the growing body of research demonstrating that, far from being nonverbal, the right hemisphere has significant language processing strength. From prosodic and paralinguistic aspects of speech production, reception, and interpretation, to prelexical, lexical and postlexical components of visual word recognition; strong involvement of the right hemisphere is implicated. The evidence reviewed challenges the notion that language is solely a function of the “verbal” left hemisphere, indicating that the right cerebral hemisphere makes significant and meaningful contributions to normal language processing as well.     

Functional neuroimaging studies of reading and reading disability (developmental dyslexia)

Converging evidence from a number of neuroimaging studies, including our own, suggest that fluent word identification in reading is related to the functional integrity of two consolidated left hemisphere (LH) posterior systems: a dorsal (temporo-parietal) circuit and a ventral (occipito-temporal) circuit. This posterior system is functionally disrupted in developmental dyslexia. Reading disabled readers, relative to nonimpaired readers, demonstrate heightened reliance on both inferior frontal and right hemisphere posterior regions, presumably in compensation for the LH posterior difficulties. We propose a neurobiological account suggesting that for normally developing readers the dorsal circuit predominates at first, and is associated with analytic processing necessary for learning to integrate orthographic features with phonological and lexical-semantic features of printed words. The ventral circuit constitutes a fast, late-developing, word identification system which underlies fluent word recognition in skilled readers.     

Hemispheric specialization and independence for word recognition: a comparison of three computational models

Two findings serve as the hallmark for hemispheric specialization during lateralized lexical decision. First is an overall word advantage, with words being recognized more quickly and accurately than non-words (the effect being stronger in response latency). Second, a right visual field advantage is observed for words, with little or no hemispheric differences in the ability to identify non-words. Several theories have been proposed to account for this difference in word and non-word recognition, some by suggesting dual routes of lexical access and others by incorporating separate, and potentially independent, word and non-word detection mechanisms. We compare three previously proposed cognitive theories of hemispheric interactions (callosal relay, direct access, and cooperative hemispheres) through neural network modeling, with each network incorporating different means of interhemispheric communication. When parameters were varied to simulate left hemisphere specialization for lexical decision, only the cooperative hemispheres model showed both a consistent left hemisphere advantage for word recognition but not non-word recognition, as well as an overall word advantage. These results support the theory that neural representations of words are more strongly established in the left hemisphere through prior learning, despite open communication between the hemispheres during both learning and recall.     

Alexia without agraphia: An experimental case study

A 51-year-old right-handed male experienced reading difficulty and an upper right visual field cut after a head injury. The patient's letter naming was impaired and word naming was painstakingly slow. Words were decoded in a letter by letter sequence and performance varied directly with word length. Apart from his reading difficulties the patient was linguistically competent. He was sensitive to manipulations of word frequency and orthographic regularity. He wrote spontaneously and could identify the written counterpart to a spoken word with relative ease. Computerized axial tomography revealed damage to the posterior temporal-parietal region of the left hemisphere.