Visual Hemifield Research Articles

Interhemispheric visual competition after multisensory reversal of hemianopia

Unilateral lesions of visual cortex have the secondary consequence of suppressing visual circuits in the midbrain superior colliculus (SC), collectively producing blindness in contralesional space (“hemianopia”). Recent studies have demonstrated that SC visual responses and contralesional vision can be reinstated by a non‐invasive multisensory training procedure in which spatiotemporally concordant visual‐auditory pairs are repeatedly presented within the blind hemifield. Despite this recovery of visual responsiveness, the loss of visual cortex was expected to result in permanent deficits in that hemifield, especially when visual events in both hemifields compete for attention and access to the brain's visuomotor circuitry. This was evaluated in the present study in a visual choice paradigm in which the two visual hemifields of recovered cats were simultaneously stimulated with equally valent visual targets. Surprisingly, the expected disparity was not found, and some animals even preferred stimuli presented in the previously blind hemifield. This preference persisted across multiple stimulus intensity levels and there was no indication that animals were less aware of cues in the previously blind hemifield than in its spared counterpart. Furthermore, when auditory cues were combined with visual cues, the enhanced performance they produced on a visual task was no greater in the normal than in the previously blind hemifield. These observations suggest that the multisensory rehabilitation paradigm revealed greater inherent visual information processing potential in the previously blind hemifield than was believed possible given the loss of visual cortex.

Influence of cranial radiation by high-energy protons on the visuo-motor behavior of nonhuman primates

The visually driven instrumental conditioning on a one monkey (Macaca mulatta) was conducted after single-dose cranial irradiation with high-energy protons. The monkey executed saccades toward the visual stimuli and then performed the movement with ipsilateral hand correspondingly to their location in a right or left visual hemifield. The rate of correct responses with dominant right hand exceeded the rate of such responses with left hand and decreased briefly two months after irradiation. Then a month later, the rate of correct right-hand responses returned to the level before irradiation. No significant dynamics was revealed for the rate of correct left-hand responses. The proton irradiation effect on right-hand responses suggests possible short-term disturbances in the eye-hand coordination for right handedness while the visual perception remains resistant.

Low-Level Visual Information is Maintained Across Saccades by a 'Soft Hand-Off' between Visual Processing Areas

Subjective visual experience of a rich, detailed visual scene seems continuous despite us making several saccadic eye movements every second which radically change retinal input. Here, we investigated the time course of low-level visual information processing in the brain by decoding spatial frequency of an attended, stationary stimulus that changed from one visual hemifield to another due to a horizontal saccadic eye movement. Magneto-encephalography (MEG) was recorded while human participants made a large horizontal saccade and monitored the orientation of a grating stimulus, making spatial frequency task-irrelevant. Separate trials, in which participants maintained fixation, were used to train a classifier that was tested during saccade trials. Decoding performance showed that information about spatial frequency was present in the original retinotopic areas before, during and up to around 200 ms after the saccade. Post-saccadic information rapidly ramped up within a few tens of milliseconds, allowing for rapid decoding from the new retinotopic areas. There was an overlap of over 100 ms during which decoding was significant from both pre- and post-saccadic processing areas. These results suggest that the apparent richness of perception across saccades may be supported by the continuous availability of the low-level spatial frequency information that supports gist, object and ensemble perception, with a “soft hand-off” of this information during the initial visual processing sweep of the new fixation. Such a mechanism does not require remapping of low-level visual information before the saccade.

Population receptive field and connectivity properties of the early visual cortex in human albinism

In albinism, the pathological decussation of the temporal retinal afferents at the optic chiasm leads to superimposed representations of opposing hemifields in the visual cortex. Here, we assessed the equivalence of the two representations and the cortico-cortical connectivity of the early visual areas. Applying fMRI-based population receptive field (pRF)-mapping (both hemifield and bilateral mapping) and connective field (CF)-modeling, we investigated the early visual cortex in 6 albinotic participants and 4 controls. In albinism, superimposed retinotopic representations of the contra- and ipsilateral visual hemifield were observed on the hemisphere contralateral to the stimulated eye. This was confirmed by the observation of bilateral pRFs during bilateral mapping. Hemifield mapping revealed similar pRF-sizes for both hemifield representations throughout V1 to V3. The typical increase of V1-sampling extent for V3 compared to V2 was not found for the albinotic participants. The similarity of the pRF-sizes for opposing visual hemifield representations highlights the equivalence of the two maps in the early visual cortex. The altered V1-sampling extent in V3 might indicate the adaptation of cortico-cortical connections to visual pathway abnormalities in albinism. These findings thus suggest that conservative developmental mechanisms are complemented by alterations of the extrastriate cortico-cortical connectivity.

Two Spatially Distinct Posterior Alpha Sources Fulfill Different Functional Roles in Attention.

Directing attention helps to extract relevant information and suppress distracters. Alpha brain oscillations (8-12 Hz) are crucial for this process, with power decreases facilitating processing of important information and power increases inhibiting brain regions processing irrelevant information. Evidence for this phenomenon arises from visual attention studies (Worden et al., 2000); however, the effect also exists in other modalities, including the somatosensory system (Haegens et al., 2011) and intersensory attention tasks (Foxe and Snyder, 2011). We investigated in human participants (10 females, 10 males) the role of alpha oscillations in focused (0/100%) versus divided (40/60%) attention, both across modalities (visual/somatosensory; Experiment 1) and within the same modality (visual domain: across hemifields; Experiment 2) while recording EEG over 128 scalp electrodes. In Experiment 1, participants divided their attention between visual and somatosensory modality to determine the temporal/spatial frequency of a target stimulus (vibrotactile stimulus/Gabor grating). In Experiment 2, participants divided attention between two visual hemifields to identify the orientation of a Gabor grating. In both experiments, prestimulus alpha power in visual areas decreased linearly with increasing attention to visual stimuli. In contrast, prestimulus alpha power in parietal areas was lower when attention was divided between modalities/hemifields compared with focused attention. These results suggest there are two alpha sources, one of which reflects the "visual spotlight of attention" and the other reflects attentional effort. To our knowledge, this is the first study to show that attention recruits two spatially distinct alpha sources in occipital and parietal brain regions, acting simultaneously but serving different functions in attention.SIGNIFICANCE STATEMENT Attention to one spatial location/sensory modality leads to power changes of alpha oscillations (∼10 Hz) with decreased power over regions processing relevant information and power increases to actively inhibit areas processing "to-be-ignored" information. Here, we used detailed source modeling to investigate EEG data recorded during separate unimodal (visual) and multimodal (visual and somatosensory) attention tasks. Participants either focused their attention on one modality/spatial location or directed it to both. We show for the first time two distinct alpha sources are active simultaneously but play different roles. A sensory (visual) alpha source was linearly modulated by attention representing the "visual spotlight of attention." By contrast, a parietal alpha source was modulated by attentional effort, showing lowest alpha power when attention was divided.

Mimicry-dependent lateralization in the visual inspection of foreign eggs by American robins.

Brain lateralization, or the specialization of function in the left versus right brain hemispheres, has been found in a variety of lineages in contexts ranging from foraging to social and sexual behaviours, including the recognition of conspecific social partners. Here we studied whether the recognition and rejection of avian brood parasitic eggs, another context for species recognition, may also involve lateralized visual processing. We focused on American robins (Turdus migratorius), an egg-rejecter host to occasional brood parasitism by brown-headed cowbirds (Molothrus ater) and tested if robins preferentially used one visual hemifield over the other to inspect mimetic versus non-mimetic model eggs. At the population level, robins showed a significantly lateralized absolute eyedness index (EI) when viewing mimetic model eggs, but individuals varied in left versus right visual hemifield preference. By contrast, absolute EI was significantly lower when viewing non-mimetic eggs. We also found that robins with more lateralized eye usage rejected model eggs at higher rates. We suggest that the inspection and recognition of foreign eggs represent a specialized and lateralized context of species recognition in this and perhaps in other egg-rejecter hosts of brood parasites.

An enhanced role for right hV5/MT+ in the analysis of motion in the contra- and ipsi-lateral visual hemi-fields

Previous experiments have demonstrated that transcranial magnetic stimulation (TMS) of human V5/MT+, in either the left or right cerebral hemisphere, can induce deficits in visual motion perception in their respective contra- and ipsi-lateral visual hemi-fields. However, motion deficits in the ipsi-lateral hemi-field are greater when TMS is applied to V5/MT + in the right hemisphere relative to the left hemisphere. One possible explanation for this asymmetry might lie in differential stimulation of sub-divisions within V5/MT + across the two hemispheres. V5/MT + has two major sub-divisions; MT/TO-1 and MST/TO-2, the latter area contains neurons with large receptive fields (RFs) that extend up to 15° further into the ipsi-lateral hemi-field than the former. We wanted to examine whether applying TMS to MT/TO-1 and MST/TO-2 separately could explain the previously reported functional asymmetries for ipsi-lateral motion processing in V5/MT + across right and left cerebral hemispheres. MT/TO-1 and MST/TO-2 were identified in seven subjects using fMRI localisers. In psychophysical experiments subjects identified the translational direction (up/down) of coherently moving dots presented in either the left or right visual field whilst repetitive TMS (25 Hz; 70%) was applied synchronously with stimulus presentation. Application of TMS to MT/TO-1 and MST/TO-2 in the right hemisphere affected translational direction discrimination in both contra-lateral and ipsi-lateral visual fields. In contrast, deficits of motion perception following application of TMS to MT/TO-1 and MST/TO-2 in the left hemisphere were restricted to the contra-lateral visual field. This result suggests an enhanced role for the right hemisphere in processing translational motion across the full visual field.

Vertical and horizontal meridian modulations suggest areas with quadrantic representations as neural locus of the attentional repulsion effect.

The attentional repulsion effect (ARE) is a perceptual bias attributed to a covert shift of attention toward a peripheral cue, which, in turn, repulses the perceived position of a subsequently presented probe (Suzuki & Cavanagh, 1997). So far, probes were mainly presented around the vertical meridian. Other studies of perceptual biases reported disruptions when stimuli were presented across the vertical meridian. These disruptions were explained by separate representations of the left and right visual hemifields, projecting to opposite anatomical hemispheres. As the ARE is typically examined through two-alternative, forced-choice tasks in which the estimation of the probe's position is based on the cue's effectiveness to repulse the probe across the vertical meridian, no such asymmetry has been reported. To test for similar meridian disruptions in the ARE, we collected absolute estimations (computer mouse responses) of the perceived probe positions (Experiment 1a). As absolute estimations of memorized positions are associated with overestimated distances in reproduction, results had to be compared to a no-cue baseline condition (Experiment 1b). Through this new methodological approach, we found the ARE to be strongest when the attentional capturing cue and the subsequently presented probe were displayed in the same hemifield (Experiment 2a). In a further experiment (Experiment 2b), we observed that the ARE is not only disrupted at the vertical, but also at the horizontal meridian. These disruptions at both meridians suggest the involvement of visual neural areas with quadrantic representations, such as V2 and/or V3 in the generation of the ARE.

Visual Search for Letters in the Right Versus Left Visual Hemifields.

The current study investigated the relationships between attention, word processing, and visual field asymmetries. There is a discussion on whether each brain hemisphere possesses its own attentional resources and on how attention allocation depends on hemispheric lateralization of functions. We used stimuli with lateralized processing in an attentional task presented across the two visual hemifields. Three experiments investigated the visual search for a prespecified letter in displays containing words or nonwords, placed left and right to fixation, with a variable target letter position within the strings. In Experiment 1, two letter strings of the same type (words or nonwords) were presented to both visual hemifields. In Experiment 2, there was only one letter string presented right or left to fixation. In Experiment 3, two letter strings of different type were presented to both hemifields. Response times and accuracy data were collected. The results of Experiment 1 provide evidence for letter-by-letter search within a word in the left visual field (LVF), within a nonword in the right visual field (RVF), and for position-independent access to letters within a nonword in LVF and within a word in RVF. Experiment 3 produced similar results except for letter-by-letter search within words in RVF. In Experiment 2, for all types of letter strings in both hemifields, we observed the same letter-by-letter search. These results demonstrate that presence of stimuli in both one or two hemifields and the readiness to process a certain string type might contribute to the search for a letter within a letter string.

Hemispheric asymmetries in categorical facial expression perception.

Although many sensory phenomena vary continuously, humans tend to divide them into discrete categories with facial expressions being divided into categories such as happy, sad, anger, and fear. A critical effect of category use is categorical perception: same sized physical differences are better perceived if the difference occurs between two categories rather than within the same category. Here we investigate the lateralization of categorical perception of facial expressions. Categorical perception of facial expressions may be lateralized to the right hemisphere due to the intimacy of basic, possibly universal and innate, prototypical facial expression categories and face processing which is predominantly lateralized to the right hemisphere. Alternatively, facial expression categories may be a linguistic phenomenon and thus predominantly involve the left hemisphere. A visual half-field paradigm was used to assess categorical perception of morphed facial expressions running between happy-sad, and anger-fear. Across two experiments, the lateralization of categorical perception was found to be contingent upon previous exposure. Better between-than within-category discrimination of facial expressions was found in both visual half-fields when participants had previously been exposed to facial expression categories; but for naïve participants, categorical perception was restricted to the right visual hemifield, corresponding to the left cerebral hemisphere. Facial expressions presented to the right hemisphere are therefore not necessarily encoded in terms of their emotional expression category and such category-based encoding, when it does occur, may be due to the left hemisphere and so is likely to be subject to biases related to language. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Hemispheric Organization of Visual Word Recognition in Turkish Monolinguals

Recently obtained data from interdisciplinary research has expanded our knowledge on the relationship between language and the brain considerably. Numerous aspects of language have been the subject of research. Visual word recognition is a temporal process which starts with recognizing the physical features of words and matching them with potential candidates in the mental lexicon. Word frequency plays a significant role in this process. Other factors are the similarities in spelling and pronunciation, and whether words have meanings or are simply letter strings. The emotional load of the words is another factor that deserves a closer inspection as an overwhelming amount of evidence supports the privileged status of emotions both in verbal and nonverbal tasks. It is well-established that lexical processing is handled by the involvement of the brain hemispheres to varying degrees, and that the left hemisphere has greater involvement in verbal tasks as compared to the right hemisphere. Also, the emotional load of the verbal stimuli affects the specialized roles of the brain hemispheres in lexical processing.
 Despite the abundance of research on processing of words that belong to languages from a variety of language families, the number of studies that investigated Turkish, a language of Uralic-Altaic origin, is scarce. This study aims to fill the gap in the literature by reporting evidence on how Turkish words with and without emotional load are processed and represented in the brain. We employed a visual hemifield paradigm and a lexical decision task. The participants were instructed to decide if the letter strings presented either from the right or the left of the computer screen were real words or non-words. Their response times and accuracy of their answers were recorded. We obtained shorter response times and higher accuracy rates for real words than non-words as reported in the majority of studies in the literature. We also found that the emotional load modulated the recognition of words, supporting the results in the literature. Finally, our results are in line with the view of left hemispheric superiority in lexical processing in monolingual speakers.

Electrophysiological evidence of sustained spatial attention effects over anterior cortex: Possible contribution of the anterior insula.

Spatial attention can improve performance in terms of speed and accuracy; this advantage may be mediated by brain processes at both poststimulus (reactive) and prestimulus (proactive) stages. Here, we studied how visuospatial attention affects both proactive and reactive brain functions using event-related potentials (ERPs). At reactive stage, effects of attention on parietal-occipital components are well documented; little data are available on anterior components. Seventeen participants performed simple and discriminative response tasks, while voluntarily and steadily attending either the left or right visual hemifield throughout one block. Response speed was faster for the attended side. At ERP level, attending to one hemifield did not produce lateralization of proactive components-that is, the BP and the pN. As for poststimulus components, we confirmed the well-known amplitude effects on the P1, N1, and P3. More interesting are results for the prefrontal components previously neglected in tasks modulating spatial attention. Previous studies suggest that these components reflect perceptual and sensory-motor awareness (pN1 and pP1 components), and stimulus-response mapping (pP2 component) associated to anterior insular activity. Spatial attention enhanced the pN1 and the pP1 amplitude but had no effect on the pP2. Overall, results extend knowledge on spatial attention, showing that sustained spatial attention affects the activity of anterior areas, such as the anterior insula, in addition to the known influence on occipital-parietal areas. Top-down spatial attention is likely mediated by increased sensory and sensory-motor awareness for attended events; this effect is evident in reactive, not proactive, brain activity.

Impairments in Multisensory Integration after Stroke.

The integration of information from multiple senses leads to a plethora of behavioral benefits, most predominantly to faster and better detection, localization, and identification of events in the environment. Although previous studies of multisensory integration (MSI) in humans have provided insights into the neural underpinnings of MSI, studies of MSI at a behavioral level in individuals with brain damage are scarce. Here, a well-known psychophysical paradigm (the redundant target paradigm) was employed to quantify MSI in a group of stroke patients. The relation between MSI and lesion location was analyzed using lesion subtraction analysis. Twenty-one patients with ischemic infarctions and 14 healthy control participants responded to auditory, visual, and audiovisual targets in the left and right visual hemifield. Responses to audiovisual targets were faster than to unisensory targets. This could be due to MSI or statistical facilitation. Comparing the audiovisual RTs to the winner of a race between unisensory signals allowed us to determine whether participants could integrate auditory and visual information. The results indicated that (1) 33% of the patients showed an impairment in MSI; (2) patients with MSI impairment had left hemisphere and brainstem/cerebellar lesions; and (3) the left caudate, left pallidum, left putamen, left thalamus, left insula, left postcentral and precentral gyrus, left central opercular cortex, left amygdala, and left OFC were more often damaged in patients with MSI impairments. These results are the first to demonstrate the impact of brain damage on MSI in stroke patients using a well-established psychophysical paradigm.

Perceived shift of the centres of contracting and expanding optic flow fields: Different biases in the lower-right and upper-right visual quadrants.

We studied differences in localizing the centres of flow in radially expanding and contracting patterns in different regions of the visual field. Our results suggest that the perceived centre of a peripherally viewed expanding pattern is shifted towards the fovea relative to that of a contracting pattern, but only in the lower right and upper right visual quadrants and when a single speed gradient with appropriate overall speeds of the trajectories of the moving dots was used. The biases were not systematically related to differences of sensitivity to optic flow in different quadrants. Further experiments demonstrated that the biases were likely due to a combination of two effects: an advantage of global processing in favor of the lower visual hemifield and a hemispheric asymmetry in attentional allocation in favor of motion-induced spatial displacement in the right visual hemifield. The bias in the lower right visual quadrant was speed gradient-sensitive and could be reduced to a non-significant level with the usage of multiple speed gradients, possibly due to a special role of the lower visual hemifield in extracting global information from the multiple speed gradients. A holistic processing on multiple speed gradients, rather than a predominant processing on a single speed gradient, was likely adopted. In contrast, the perceived bias in the upper right visual quadrant was overall speed-sensitive and could be reduced to a non-significant level with the reduction of the overall speeds of the trajectories. The implications of these results for understanding motion-induced spatial illusions are discussed.

Relationship between Motor Laterality and Aggressive Behavior in Sheepdogs

Sheepdogs’ visuospatial abilities, their control of prey-driven behavior and their motor functions are essential characteristics for success in sheepdog trials. We investigated the influence of laterality on 15 sheepdogs’ (Canis familiaris) spontaneous turning motor pattern around a herd and on their behavior during the first encounter with sheep in a training session. The most relevant finding of this research was that the dogs displayed significantly more aggressive behavior toward the sheep when turning in a counterclockwise direction around the herd. Considering that in counterclockwise turns the sheep were in the dogs’ left visual hemifield, the high frequency of aggressions registered during counterclockwise turns suggests right hemisphere main activation. Overall, our results revealed the existence of a relationship between motor lateralization and aggressive behavior in dogs during sheepdog training and have practical implications for sheepdog training.

Split-Brain Babies? Differences in Representation of Bilaterally and Unilaterally Presented Visual Stimuli in Infancy.

Information needed for perception and action is often distributed across the two hemispheres of the human brain. During development, representations lateralized due to topographic sensory maps may be available independently before they can be integrated across hemispheres. These studies (total N = 211) investigate visual interhemispheric integration in two domains in infancy. In Experiment 1, infants (8–14 months) showed stronger evidence of representing the equality of two shapes when the shapes were presented in the same visual hemifield. In Experiments 2–4, infants (10–19 months) showed evidence of greater familiarization when shown 16 dots in one hemifield than when shown 8 dots in each hemifield. The possibility that interhemispheric integration poses an unusually late-resolved challenge in infant vision is discussed.

Top-Down Attention Is Limited Within but Not Between Feature Dimensions.

In natural vision, processing of spatial and nonspatial features occurs simultaneously; however, the two types of attention in charge of facilitating this processing have distinct mechanisms. Here, we tested the independence of spatial and feature-based attention at different stages of visual processing by examining color-based attentional selection while spatial attention was focused or divided. Human observers attended to one or two of four fields of randomly moving dots presented in both left and right visual hemifields. In the focused attention condition, the target stimulus was defined both by color and location, whereas in the divided attention condition stimuli of the target color had to be attended in both hemifields. Sustained attentional selection was measured by means of steady-state visual evoked potentials elicited by each of the frequency-tagged flickering dot fields. Additionally, target and distractor selection was assessed with ERPs to these stimuli. We found that spatial and color-based attention independently modulated the amplitude of steady-state visual evoked potentials, confirming independent top-down influences on early visual areas. In contrast, P3 amplitudes elicited only by targets and distractors of the attended color were subject to space-based enhancement, suggesting increasing integration of spatial and feature-based selection over the course of perceptual processing.

Attentional modulation of neural responses to illusory shapes: Evidence from steady-state and evoked visual potentials

Our perception of illusory shapes, such as the classic Kanizsa triangle, is thought to reflect the visual system’s capacity to fill in missing information associated with fragmented or partially occluded objects. Previous work has suggested that such ‘modal’ filling-in arises at relatively early stages of visual processing, prior to the allocation of focused attention, but few studies have examined whether attention influences neural responses to illusory shapes. Here we asked whether spatial attention affects neural activity evoked by illusory shapes by having participants focus on stimuli presented in one visual field (left or right), while ignoring those on the other side. In two separate experiments, we used electroencephalography (EEG) to measure steady-state visual evoked potentials (SSVEPs; Experiment 1) and event-related potentials (ERPs; Experiment 2), elicited by a Kanizsa square in one hemifield and a competing control stimulus, matched for all relevant physical features, in the other. In Experiment 1, the inducers used to construct the Kanizsa and control stimuli flickered at unique frequencies (10 and 13.33 Hz) to elicit SSVEPs. In Experiment 2, segments were removed briefly from static circular placeholders within the left and right visual hemifields to induce ERPs. In both experiments, participants were instructed to attend covertly to the stimulus on the left or right side to detect brief contrast changes of the inducers. SSVEP analyses revealed that illusory shapes that were ignored yielded a significantly larger neural response than those elicited by non-illusory control stimuli. By contrast, when the stimuli were attended, neural responses to the illusory and control stimuli were statistically indistinguishable. Consistent with these steady-state findings, results of the ERP analyses revealed that evoked responses to illusory shapes emerged earlier relative to control stimuli when they were ignored than when they were attended. Taken together the findings suggest that neural activity associated with modal completion is reliably modulated by spatial attention. Specifically, we show that neural responses to actively ignored illusory shapes are elicited earlier and are sustained at a higher amplitude than responses to control stimuli. This advantage for ignored shapes might reflect adaptive signalling of the presence of an object available for future goal-directed behaviour.

Slow Theta tACS of the Right Parietal Cortex Enhances Contralateral Visual Working Memory Capacity.

Recent research suggests alteration of visual working memory capacity by modulation of parietal theta frequency via transcranial alternating current stimulation (tACS). However, it remains to be clarified whether this effect is partly driven by co-stimulation of prefrontal cortex and subcortical structures. It was hypothesized that focal tACS over the parietal lobe without additional prefrontal or subcortical stimulation should lead to similar effects as reported in the literature. Healthy, young participants were tested on a visual working memory paradigm while receiving either focal parietal tACS at 4Hz, at 7Hz or sham stimulation. Focal right posterior 4Hz tACS led to increased working memory capacity strictly for the visual hemifield contralateral to stimulation. Exclusive stimulation of posterior cortex by 4Hz tACS replicates effects recently reported in literature, confirming that stimulation of the prefrontal cortex or subcortical structures are not a primary driver of these observations.

Embedded word priming elicits enhanced fMRI responses in the visual word form area.

Lexical embedding is common in all languages and elicits mutual orthographic interference between an embedded word and its carrier. The neural basis of such interference remains unknown. We employed a novel fMRI prime-target embedded word paradigm to test for involvement of a visual word form area (VWFA) in left ventral occipitotemporal cortex in co-activation of embedded words and their carriers. Based on the results of related fMRI studies we predicted either enhancement or suppression of fMRI responses to embedded words initially viewed as primes, and repeated in the context of target carrier words. Our results clearly showed enhancement of fMRI responses in the VWFA to embedded-carrier word pairs as compared to unrelated prime-target pairs. In contrast to non-visual language-related areas (e.g., left inferior frontal gyrus), enhanced fMRI responses did not occur in the VWFA when embedded-carrier word pairs were restricted to the left visual hemifield. Our finding of fMRI enhancement in the VWFA is novel evidence of its involvement in representational rivalry between orthographically similar words, and the co-activation of embedded words and their carriers.