Task and stimulus demands influence letter-case-specific priming in the right cerebral hemisphere

A greater tendency to complete single-completion word stems (e.g. "BEY") to form previously read whole words (e.g. "BEYOND") was found when test stems were presented in the same letter case as their previously encoded words, compared with the different letter case, but only when stems were presented directly to the right hemisphere (i.e. in the left visual field) and not when they were presented directly to the left hemisphere (i.e. in the right visual field). This finding with single-completion stems was robust (i.e. observed for both lowercase and uppercase stems) when the initial encoding task was perceptually demanding, but it was test-case dependent (i.e. observed for uppercase but not lowercase stems) when the initial encoding task was not perceptually demanding. Results and theory help to explain why letter-case-specific priming in right-hemisphere test presentations is typically test-case dependent when priming is measured using perceptual identification at test, but is consistently robust when priming is measured using multiple-completion word stems (e.g."BEA") at test. Demands from both the stimuli and tasks affect the relative contributions of abstract and specific subsystems to the processing of visual forms.     

The neural substrates of visual implicit memory: do the two hemispheres play different roles?

Identification of visually presented words is facilitated by implicit memory, or visual priming, for past visual experiences with those words. There is disagreement over the neuro-anatomical substrates of this form of implicit memory. Several studies have suggested that this form of priming relies on a visual word-form system localized in the right occipital lobe, whereas other studies have indicated that both hemispheres are equally involved. The discrepancies may be related to the types of priming tasks that have been used because the former studies have relied primarily on word-stem completion tasks and the latter on tasks like word-fragment completion. The present experiments compared word-fragment and word-stem measurements of visual implicit memory in patients with right occipital lobe lesions and patients with complete callosotomies. The patients showed normal visual implicit memory on fragment completion tests, but essentially no visual priming on standard stem completion tests. However, when we used a set of word stems that had only one correct solution for each test item, as was true of the items in the fragment completion tests, the patients showed normal priming effects. The results indicate that visual implicit memory for words is not solely dependent upon the right hemisphere, rather it reflects changes in processing efficiency in bilateral visual regions involved in the initial processing of the items. However, under conditions of high lexical competition (i.e., multiple completion word stems), the lexical processes, which are dominant in the left hemisphere, overshadow the visual priming supported by the left hemisphere.     

Effects of form familiarity on perception of words,pseudowords, and nonwords in the two cerebral hemispheres

Previous investigations of hemispheric processes of word perception provide a mixed picture of the sensitivity of each hemisphere to the familiarity of the visual form of lateralized displays. We investigated this issue by presenting words, pseudowords, and nonwords briefly to either the left (LH) or right (RH) hemisphere in lowercase, uppercase, and a matched, unfamiliar mixed-case form, and used an eye tracker to ensure central fixation and the Reicher-Wheeler task to suppress influences of stimulus asymmetry. Familiarity of form exerted a substantial effect on perception. In particular, perception of LH and RH displays of words, pseudowords, and nonwords was least accurate for mixed case, intermediate for upper case, and most accurate for lowercase. However, form had no effect on the LH advantage observed for words, pseudowords, and nonwords, indicating that form affected processing in both hemispheres to a similar extent. Moreover, LH and RH displays both showed that mixed case disrupted performance most for words, and more for pseudowords than for nonwords, indicating the sensitivity to form shown by each hemisphere reflected more than a general perceptual process. Implications for the role of form familiarity in hemispheric processing of words are discussed.     

Mechanisms of Pure Alexia: Spatially Based Impairment,Letter Identification Deficit,or Both?

We studied reading performance for words and for isolated letters in a pure alexic patient. She performed reasonably well when naming isolated letters but was slower in reading letters than a control subject when reaction times (RTs) were recorded. When the patient read isolated letters, RTs were slower for a subset of letters that cannot be recognized from their left part alone (e.g. “b”, an ambiguous letter, could be read “b” “h” “l” or “k” whereas “a” has no predictable confounders). We observed a significant positive correlation between the RTs for reading a word and the mean RTs for reading each of its composing letters before its uniqueness point (i.e. the point, when reading from the left to the right, where a word cannot be a word other than the one it is). This result suggests that, in our patient, the letter identification deficit can account for the slow, letter-by-letter reading behaviour, insofar as each letter represents a perceptual problem. Our findings can be accounted for by a deficit in the parallel processing of the left and right parts of each letter, compounded with a bias to process first the left part of the letter, and may thus reconcile the hypotheses of spatiallybased deficit (Rapp and Caramazza, 1991) and of a perceptual deficit occurring at the letter identification level (Behrmann and Shallice, 1995; Perri et al., 1996).     

Top‐down and bottom‐up influences on the left ventral occipito‐temporal cortex during visual word recognition: An analysis of effective connectivity

The functional role of the left ventral occipito‐temporal cortex (vOT) in visual word processing has been studied extensively. A prominent observation is higher activation for unfamiliar but pronounceable letter strings compared to regular words in this region. Some functional accounts have interpreted this finding as driven by top‐down influences (e.g., Dehaene and Cohen [ 2011 ]: Trends Cogn Sci 15:254–262; Price and Devlin [ 2011 ]: Trends Cogn Sci 15:246–253), while others have suggested a difference in bottom‐up processing (e.g., Glezer et al. [ 2009 ]: Neuron 62:199–204; Kronbichler et al. [ 2007 ]: J Cogn Neurosci 19:1584–1594). We used dynamic causal modeling for fMRI data to test bottom‐up and top‐down influences on the left vOT during visual processing of regular words and unfamiliar letter strings. Regular words (e.g., taxi) and unfamiliar letter strings of pseudohomophones (e.g., taksi) were presented in the context of a phonological lexical decision task (i.e., “Does the item sound like a word?”). We found no differences in top‐down signaling, but a strong increase in bottom‐up signaling from the occipital cortex to the left vOT for pseudohomophones compared to words. This finding can be linked to functional accounts which assume that the left vOT contains neurons tuned to complex orthographic features such as morphemes or words [e.g., Dehaene and Cohen [ 2011 ]: Trends Cogn Sci 15:254‐262; Kronbichler et al. [ 2007 ]: J Cogn Neurosci 19:1584–1594]: For words, bottom‐up signals converge onto a matching orthographic representation in the left vOT. For pseudohomophones, the propagated signals do not converge, but (partially) activate multiple orthographic word representations, reflected in increased effective connectivity. Hum Brain Mapp 35:1668–1680, 2014. © 2013 Wiley Periodicals, Inc.     

Interference and priming within and across visual fields in a lexical decision task

A lower-case target word or non-word was laterally presented by itself or simultaneously with an upper-case distractor word in the same or in the opposite visual field. The distractor words were either semantically associated or unrelated to the target. Subjects performed a lexical decision task on the target. Two kinds of selection within and across visual fields were investigated. Selection of an external target relevant for the response (filtering) and selection of an internal target by automatic priming. No lateral asymmetry was found for automatic priming. Filtering was equally efficient within hemispheres, but strongly asymmetrical across hemispheres. Unrelated distractor words presented to the right hemisphere interfered less with target processing in the left hemisphere than vice versa. Thus, all distractor words received equivalent semantic processing; those presented to the right hemisphere, however, were least costly to exclude from task-relevant processing in the left hemisphere.     

Presentation of words to separate hemispheres prevents interword illusory conjunctions

We tested the hypothesis that division of inputs between the hemispheres could prevent interword letter migrations in the form of illusory conjunctions. The task was to decide whether a centrally-presented consonant-vowel-consonant (CVC) target word matched one of four CVC words presented to a single hemisphere or divided between the hemispheres in a subsequent test display. During half of the target-absent trials, known as conjunction trials, letters from two separate words (e.g., "tag" and "cop") in the test display could be mistaken for a target word (e.g., "top"). For the other half of the target-absent trails, the test display did not match any target consonants (Experiment 1, N = 16) or it matched one target consonant (Experiment 2, N = 29), the latter constituting true "feature" trials. Bi- as compared to unihemispheric presentation significantly reduced the number of conjunction, but not feature, errors. Illusory conjunctions did not occur when the words were presented to separate hemispheres.     

Evaluating a split fovea model of visual word recognition: effects of case alternation in the two visual fields and in the left and right halves of words presented at the fovea

Two experiments are reported exploring the effect of cAsE aLtErNaTiOn on lexical decisions to words and nonwords presented laterally or centrally. In line with previous research, Experiment 1 found that case alternation slowed lexical decision responses to words more in the right visual field (RVF) than in the left visual field (LVF). In Experiment 2, the words and nonwords were all presented centrally. There were three conditions, a condition in which the word and nonwords were presented in lower case letters, a condition in which the letters to the left of the central fixation were case alternated (e.g., aMbItion, mOdLants) and a condition in which the letters to the right of fixation were case alternated (e.g., collApSe, pireNtOl). Alternating the case of letters to the right of fixation slowed lexical decision responses more than alternating letter case to the left of fixation. The results provide further support for a split fovea account of visual word recognition according to which those letters of a centrally-fixated word that fall to the left of fixation are processed initially by the right cerebral hemisphere while those letters that fall to the right of fixation are processed initially by the left cerebral hemisphere, with the characteristics of the left and right hemispheres being revealed in the processing of initial and final letters in centrally presented words.     

Hemispheric asymmetries in memory processes as measured in a false recognition paradigm

Although memory differs in important ways between the left and right cerebral hemispheres, the nature of these differences remains controversial. We examined this issue in two experiments using a false memory paradigm that allowed novel tests of two theories that have not been assessed in a common paradigm previously. Lists of semantically related words (e.g., bed, rest, wake…), all highly associated to one “critical” word (e.g., sleep), were presented auditorily during a study phase. Memory for both the related words and the critical words was measured in a subsequent old/new recognition test using divided-visual- field word presentations. The most important results were that the ability to correctly reject previously unpresented words was greater when test items were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH) and that participants were more confident in correctly rejecting unpresented words when test items were presented to the RVF/LH than to the LVF/RH. Results were in line with the theory that associative activation of semantic information is restricted in the left hemisphere but diffuse in the right; however, these results contrasted with the theory that memory traces are interpretive in the left hemisphere but veridical in the right. A potential resolution to the seemingly contradictory theories of asymmetries in memory processing is briefly discussed.     

Lateralised covert attention in word identification

The right visual field superiority in word recognition has been attributed to an attentional advantage by the left brain hemisphere. We investigated whether such advantage involves lateralised covert attention, in the absence of overt fixations on prime words. In a lexical decision task target words were preceded by an identical or an unrelated prime word. Eye movements were monitored. In Experiment 1 lateralised (to the left or right of fixation) prime words were parafoveally visible but foveally masked, thus allowing for covert attention but preventing overt attention. In Experiment 2 prime words were presented at fixation, thus allowing for both overt and covert attention. Results revealed positive priming in the absence of fixations on the primes when these were presented in the right visual field. The effects of covertly attended primes were nevertheless significantly reduced in comparison with those of overtly attended primes. It is concluded that word identification can be accomplished to a significant extent by lateralised covert attention alone, with right visual field advantage.     

Interhemispheric cooperation and activation in integration of verbal information

Subjects had to judge the semantic relation between two tachistoscopically presented German adjectives. In Exp. I, the two words were projected simultaneously, either both to the left visual field (i.e. to the right hemisphere, LVF-RH), or both to the right visual field (i.e. to the left hemisphere, RVF-LH), or one word to each visual field (i.e. to both hemispheres, EVF-BH). The results yielded a significantly higher performance in the condition EVF-BH than in the two unilateral conditions, without a significant difference between the latter two conditions. In Exp. II, the two words were shown successively. With bilateral presentation, a distinction was made as to whether the left visual field (EVF-BH-L) or the right visual field (EVF-BH-R) was stimulated first. The significantly highest performance was shown in conditions RVF-LH and EVF-BH-R compared with the condition EVF-BH-L, and the significantly poorest performance in the condition LVF-RH compared with the condition EVF-BH-L. The results provide evidence for (1) an intrahemispheric interference with simultaneous unilateral presentation, (2) a cooperative interhemispheric interaction with simultaneous bilateral presentation, and (3) an interhemispheric priming effect with successive bilateral presentation when the left hemisphere was stimulated first.     

Meaningful physical changes mediate lexical-semantic integration: top-down and form-based bottom-up information sources interact in the N400

Models of how the human brain reconstructs an intended meaning from a linguistic input often draw upon the N400 event-related potential (ERP) component as evidence. Current accounts of the N400 emphasise either the role of contextually induced lexical preactivation of a critical word (Lau, Phillips, & Poeppel, 2008) or the ease of integration into the overall discourse context including a wide variety of influencing factors (Hagoort & van Berkum, 2007). The present ERP study challenges both types of accounts by demonstrating a contextually independent and purely form-based bottom-up influence on the N400: the N400 effect for implausible sentence-endings was attenuated when the critical sentence-final word was capitalised (following a lowercase sentence context). By contrast, no N400 modulation occurred when the critical word involved a change from uppercase (sentence context) to lowercase. Thus, the N400 was only affected by a change to uppercase letters, as is often employed in computer-mediated communication as a sign of emphasis. This result indicates that N400 amplitude is reduced for unexpected words when a bottom-up (orthographic) cue signals that the word is likely to be highly informative. The lexical-semantic N400 thereby reflects the degree to which the semantic informativity of a critical word matches expectations, as determined by an interplay between top-down and bottom-up information sources, including purely form-based bottom-up information.     

Developmental mirror-writing is paralleled by orientation recognition errors

The writing attempts of children often feature mirror-reversals of individual letters. These reversals are thought to arise from an adaptive tendency to mirror-generalize. However, it is unclear whether mirror-writing is driven by mirror-generalisation of the visual letter forms, or of the actions for writing them. We report two studies of the relationship between mirror-writing and the ability to recognize whether a visually presented letter is in the correct orientation, amongst primary and preschool children learning to read and write in English. Children who produced more mirror-writing also made more orientation recognition errors, for uppercase (Study 1, n?=?44) and lowercase letters (Study 2, n?=?98), and these relationships remained significant when controlling for age. In both studies, the letters more often reversed in writing were also more prone to orientation recognition errors. Moreover, the rates of mirror-writing of different uppercase letters were closely similar between the dominant and non-dominant hands (Study 1). We also note that, in the recognition tasks, children were more likely to accept reversed letters as correct, than to reject correctly oriented letters, consistent with a tendency to mirror-generalize the visual letter forms. In every aspect, these results support a major role for visual representations in developmental mirror-writing.     

Priming effects in a letter-by-letter reader depend upon access to the word form system

Several types of cognitive and neuropsychological evidence suggest that priming effects on such implicit memory tests as word identification are mediated by a pre-semantic visual word form system that can operate independently of episodic memory. We investigate priming in a letter-by-letter reader, P.T., whose pattern of performance on neuropsychological tests indicates preservation of the word form system. Experiment 1 revealed robust priming on a word identification test following letter-by-letter study of target words, despite P.T.'s great difficulty in identifying non-studied words. Experiment 2 showed that the priming effect was modality specific whereas Experiment 3 indicated that recall of previously studied words was not modality specific, thus indicating that the observed priming could not be attributed to explicit memory strategies. Experiment 4 revealed no priming of illegal nonwords on a letter identification test. The results support the notion that priming on the word identification test depends on access to the word form system.     

Intact implicit memory in patients with frontal lobe lesions.

Patients with frontal lobe lesions and control subjects were administered tests of word-stem completion priming. In this implicit memory test, subjects are first presented words (e.g. MOTEL, PARADE) in an incidental learning paradigm. Following word presentation, subjects are shown word stems (e.g. MOT, PAR) and asked to produce the first word that comes to mind. Patients with frontal lobe lesions exhibited normal levels of word-stem completion. These findings indicate that implicit memory can operate normally despite damage to the prefrontal cortex. The present results substantiate previous neuropsychological and positron emission tomography findings which indicate that word priming depends critically on posterior cortical areas.     

Evaluating a split processing model of visual word recognition: effects of word length

A new theory of visual word recognition is based on the fact that the fovea is split in humans. When a reader fixates the center of a written word, the initial letters of the word that are to the left of fixation are projected first to the right cerebral hemisphere (RH) while the final letters are projected to the left cerebral hemisphere (LH). This paper explores the possibility that this has consequences for the early processing of the beginning and ends of centrally fixated words: specifically that lexical decision RTs are affected by the number of letters to the left of fixation but not by the number of letters to the right of fixation. For centrally presented five- and eight-letter words, we manipulated number of letters presented to the right or to the left of a fixation point (Experiment 1). We found that longer latencies to longer letter strings characterised the processing of the initial letters of words while LH word recognition features characterised the ends of words. Experiment 2 was a lateralized version of Experiment 1, and revealed the well established visual field and word length interaction. The results supported the split fovea theory.     

Rightward bisection errors for letter lines: the role of semantic information

When bisecting words in their middle, people reveal leftward bisection errors. This tendency might emerge from an attentional bias towards the beginning of the word. However, when longer meaningless letter strings are presented, people reveal a rightward bisection bias. To test the role of semantic information on leftward or rightward bisection biases, we tested letter line bisection performance in healthy right-handed students in four independent experiments. A third of the letter lines contained an embedded four-letter word to the left of true centre, another third contained an embedded four-letter word to the right of true centre, while the remaining lines contained no words. Half of these words were emotional words, the other half were neutral words. Results across experiments revealed a stronger rightward bisection bias: (i) for letter lines containing emotional as compared to neutral words, (ii) for letter lines containing words in the left as compared to right half of the lines, and (iii) for those experiments in which the spatial position of letter lines remained within a narrow body-centred space. Findings from this study suggest that letter line bisection performance might be only minimally determined by visuo-spatial attention. Rather, letter line perception might activate the left hemisphere more than the right hemisphere, shifting the subjective midpoint to the right of true centre. Leftward bisection biases for words only, as had been described in the literature, may thus have resulted from automated reading strategies rather than from attentional biases towards the left hemispace.     

Semantically convergent and semantically divergent priming in the cerebral hemispheres: lexical decision and semantic judgment

The effects of semantically divergent and semantically convergent priming on the processing of lexical ambiguity by the two cerebral hemispheres were examined in two visual hemifield experiments. The experiments investigated the ability of the right hemisphere (RH) and the left hemisphere (LH) to summate activation from two single word primes followed by a laterally presented ambiguous target word. In a lexical decision task (experiment 1), the two priming words were either both related to the dominant meaning of the target (new, fresh-NOVEL), or to the subordinate meaning (story, book-NOVEL), or to one dominant and one subordinate meaning (new, story-NOVEL). Results indicated that the LH benefited most from semantically convergent primes that converged onto the dominant meaning of the ambiguous target word, whereas the RH benefited most from semantically mixed (divergent) primes, that diverge on alternate meanings of the ambiguous target word. We used the same stimuli in a semantic relatedness judgment task (experiment 2), and found that the facilitation in the RH was significantly larger when the primes were mixed than when both primes converged on a single (i.e. either dominant or subordinate) meaning of the to-be-presented target word. In contrast, the only facilitation found in the LH was when the two primes were associated with a single meaning (either dominant or subordinate) of the to-be-presented target word. When the primes were mixed, there was no facilitation in the LH. These results support previous findings indicating that during word recognition, the RH activates a broader range of related meanings than the LH, including alternate meanings of ambiguous words. Thus, by summating activation for seemingly incongruous elements, the RH may be critically involved in at least one important component of verbal creativity.