Dataset Information

Parallelism in the brain's visual form system.

ABSTRACT: We used magnetoencephalography (MEG) to determine whether increasingly complex forms constituted from the same elements (lines) activate visual cortex with the same or different latencies. Twenty right-handed healthy adult volunteers viewed two different forms, lines and rhomboids, representing two levels of complexity. Our results showed that the earliest responses produced by lines and rhomboids in both striate and prestriate cortex had similar peak latencies (40 ms) although lines produced stronger responses than rhomboids. Dynamic causal modeling (DCM) showed that a parallel multiple input model to striate and prestriate cortex accounts best for the MEG response data. These results lead us to conclude that the perceptual hierarchy between lines and rhomboids is not mirrored by a temporal hierarchy in latency of activation and thus that a strategy of parallel processing appears to be used to construct forms, without implying that a hierarchical strategy may not be used in separate visual areas, in parallel.

SUBMITTER: Shigihara Y

PROVIDER: S-EPMC3995019 | biostudies-literature | 2013 Dec

REPOSITORIES: biostudies-literature

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Parallelism in the brain's visual form system.

Shigihara Yoshihito Y Zeki Semir S

The European journal of neuroscience 20131007 12

We used magnetoencephalography (MEG) to determine whether increasingly complex forms constituted from the same elements (lines) activate visual cortex with the same or different latencies. Twenty right-handed healthy adult volunteers viewed two different forms, lines and rhomboids, representing two levels of complexity. Our results showed that the earliest responses produced by lines and rhomboids in both striate and prestriate cortex had similar peak latencies (40 ms) although lines produced st ...[more]

PMID: 24118503

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Project description:The visual word form area (VWFA) is a region in the left occipitotemporal sulcus of literate individuals that is purportedly specialized for visual word recognition. However, there is considerable controversy about its functional specificity and connectivity, with some arguing that it serves as a domain-general, rather than word-specific, visual processor. The VWFA is a critical region for testing hypotheses about the nature of cortical organization, because it is known to develop only through experience (i.e., reading acquisition), and widespread literacy is too recent to have influenced genetic determinants of brain organization. Using a combination of advanced fMRI analysis techniques, including individual functional localization, multivoxel pattern analysis, and high-resolution resting-state functional connectivity (RSFC) analyses, with data from 33 healthy adult human participants, we demonstrate that (1) the VWFA can discriminate words from nonword letter strings (pseudowords); (2) the VWFA has preferential RSFC with Wernicke's area and other core regions of the language system; and (3) the strength of the RSFC between the VWFA and Wernicke's area predicts performance on a semantic classification task with words but not other categories of visual stimuli. Our results are consistent with the hypothesis that the VWFA is specialized for lexical processing of real words because of its functional connectivity with Wernicke's area.SIGNIFICANCE STATEMENT The visual word form area (VWFA) is critical for determining the nature of category-related organization of the ventral visual system. However, its functional specificity and connectivity are fiercely debated. Recent work concluded that the VWFA is a domain-general, rather than word-specific, visual processor with no preferential functional connectivity with the language system. Using more advanced techniques, our results stand in stark contrast to these earlier findings. We demonstrate that the VWFA is highly specialized for lexical processing of real words, and that a fundamental factor driving this specialization is its preferential intrinsic functional connectivity with core regions of the language system. Our results support the hypothesis that intrinsic functional connectivity contributes to category-related specialization within the human ventral visual system.

Project description:Progressive alexia is an acquired reading deficit caused by degeneration of brain regions that are essential for written word processing. Functional imaging studies have shown that early processing of the visual word form depends on a hierarchical posterior-to-anterior processing stream in occipito-temporal cortex, whereby successive areas code increasingly larger and more complex perceptual attributes of the letter string. A region located in the left lateral occipito-temporal sulcus and adjacent fusiform gyrus shows maximal selectivity for words and has been dubbed the 'visual word form area'. We studied two patients with progressive alexia in order to determine whether their reading deficits were associated with structural and/or functional abnormalities in this visual word form system. Voxel-based morphometry showed left-lateralized occipito-temporal atrophy in both patients, very mild in one, but moderate to severe in the other. The two patients, along with 10 control subjects, were scanned with functional magnetic resonance imaging as they viewed rapidly presented words, false font strings, or a fixation crosshair. This paradigm was optimized to reliably map brain regions involved in orthographic processing in individual subjects. All 10 control subjects showed a posterior-to-anterior gradient of selectivity for words, and all 10 showed a functionally defined visual word form area in the left hemisphere that was activated for words relative to false font strings. In contrast, neither of the two patients with progressive alexia showed any evidence for a selectivity gradient or for word-specific activation of the visual word form area. The patient with mild atrophy showed normal responses to both words and false font strings in the posterior part of the visual word form system, but a failure to develop selectivity for words in the more anterior part of the system. In contrast, the patient with moderate to severe atrophy showed minimal activation of any part of the visual word form system for either words or false font strings. Our results suggest that progressive alexia is associated with a dysfunctional visual word form system, with or without substantial cortical atrophy. Furthermore, these findings demonstrate that functional MRI has the potential to reveal the neural bases of cognitive deficits in neurodegenerative patients at very early stages, in some cases before the development of extensive atrophy.

Dataset Information

Parallelism in the brain's visual form system.

Publications

Parallelism in the brain's visual form system.

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