Andreas M. Rauschecker

Anatomy of the visual word form area: Adjacent cortical circuits and long-range white matter connections

Circuitry in ventral occipital-temporal cortex is essential for seeing words. We analyze the circuitry within a specific ventral-occipital region, the visual word form area (VWFA). The VWFA is immediately adjacent to the retinotopically organized VO-1 and VO-2 visual field maps and lies medial and inferior to visual field maps within motion selective human cortex. Three distinct white matter fascicles pass within close proximity to the VWFA: (1) the inferior longitudinal fasciculus, (2) the inferior frontal occipital fasciculus, and (3) the vertical occipital fasciculus. The vertical occipital fasciculus terminates in or adjacent to the functionally defined VWFA voxels in every individual. The vertical occipital fasciculus projects dorsally to language and reading related cortex. The combination of functional responses from cortex and anatomical measures in the white matter provides an overview of how the written word is encoded and communicated along the ventral occipital-temporal circuitry for seeing words.

Differential electrophysiological response during rest, self-referential, and non–self-referential tasks in human posteromedial cortex

The electrophysiological basis for higher brain activity during rest and internally directed cognition within the human default mode network (DMN) remains largely unknown. Here we use intracranial recordings in the human posteromedial cortex (PMC), a core node within the DMN, during conditions of cued rest, autobiographical judgments, and arithmetic processing. We found a heterogeneous profile of PMC responses in functional, spatial, and temporal domains. Although the majority of PMC sites showed increased broad gamma band activity (30–180 Hz) during rest, some PMC sites, proximal to the retrosplenial cortex, responded selectively to autobiographical stimuli. However, no site responded to both conditions, even though they were located within the boundaries of the DMN identified with resting-state functional imaging and similarly deactivated during arithmetic processing. These findings, which provide electrophysiological evidence for heterogeneity within the core of the DMN, will have important implications for neuroimaging studies of the DMN.

Learning to See Words

Skilled reading requires recognizing written words rapidly; functional neuroimaging research has clarified how the written word initiates a series of responses in visual cortex. These responses are communicated to circuits in ventral occipitotemporal (VOT) cortex that learn to identify words rapidly. Structural neuroimaging has further clarified aspects of the white matter pathways that communicate reading signals between VOT and language systems. We review this circuitry, its development, and its deficiencies in poor readers. This review emphasizes data that measure the cortical responses and white matter pathways in individual subjects rather than group differences. Such methods have the potential to clarify why a child has difficulty learning to read and to offer guidance about the interventions that may be useful for that child.

Changes in neural activity associated with learning to articulate novel auditory pseudowords by covert repetition

Learning to articulate novel combinations of phonemes that form new words through a small number of auditory exposures is crucial for development of language and our capacity for fluent speech, yet the underlying neural mechanisms are largely unknown. We used functional magnetic resonance imaging to reveal repetition–suppression effects accompanying such learning and reflecting discrete changes in brain activity due to stimulus‐specific fine‐tuning of neural representations. In an event‐related design, subjects were repeatedly exposed to auditory pseudowords, which they covertly repeated. Covert responses during scanning and postscanning overt responses showed evidence of learning. An extensive set of regions activated bilaterally when listening to and covertly repeating novel pseudoword stimuli. Activity decreased, with repeated exposures, in a subset of these areas mostly in the left hemisphere, including premotor cortex, supplementary motor area, inferior frontal gyrus, superior temporal cortex, and cerebellum. The changes most likely reflect more efficient representation of the articulation patterns of these novel words in two connected systems, one involved in the perception of pseudoword stimuli (in the left superior temporal cortex) and one for processing the output of speech (in the left frontal cortex). Both of these systems contribute to vocal learning. Hum Brain Mapp 2008.

Position sensitivity in the visual word form area

Seeing words involves the activity of neural circuitry within a small region in human ventral temporal cortex known as the visual word form area (VWFA). It is widely asserted that VWFA responses, which are essential for skilled reading, do not depend on the visual field position of the writing (position invariant). Such position invariance supports the hypothesis that the VWFA analyzes word forms at an abstract level, far removed from specific stimulus features. Using functional MRI pattern-classification techniques, we show that position information is encoded in the spatial pattern of VWFA responses. A right-hemisphere homolog (rVWFA) shows similarly position-sensitive responses. Furthermore, electrophysiological recordings in the human brain show position-sensitive VWFA response latencies. These findings show that position-sensitive information is present in the neural circuitry that conveys visual word form information to language areas. The presence of position sensitivity in the VWFA has implications for how word forms might be learned and stored within the reading circuitry.

Visual Feature-Tolerance in the Reading Network

A century of neurology and neuroscience shows that seeing words depends on ventral occipital-temporal (VOT) circuitry. Typically, reading is learned using high-contrast line-contour words. We explored whether a specific VOT region, the visual word form area (VWFA), learns to see only these words or recognizes words independent of the specific shape-defining visual features. Word forms were created using atypical features (motion-dots, luminance-dots) whose statistical properties control word-visibility. We measured fMRI responses as word form visibility varied, and we used TMS to interfere with neural processing in specific cortical circuits, while subjects performed a lexical decision task. For all features, VWFA responses increased with word-visibility and correlated with performance. TMS applied to motion-specialized area hMT+ disrupted reading performance for motion-dots, but not line-contours or luminance-dots. A quantitative model describes feature-convergence in the VWFA and relates VWFA responses to behavioral performance. These findings suggest how visual feature-tolerance in the reading network arises through signal convergence from feature-specialized cortical areas.

Phosphene threshold as a function of contrast of external visual stimuli

Transcranial magnetic stimulation (TMS) of the occipital lobe is frequently used to induce visual percepts by direct stimulation of visual cortex. The threshold magnetic field strength necessary to elicit a visual percept is often regarded as a measure of electrical excitability of visual cortex. Using single-pulse TMS during visual motion stimulus presentation, we investigated the relationship between different degrees of visual cortical preactivation and cortical phosphene threshold (PT). The two possible, mutually exclusive, predictions on the outcome of this experiment were that a) PT increases with stronger preactivation because of a decrease in the signal-to-noise ratio, or b) that PT decreases with increased preactivation because of the increase in neuronal response towards some threshold. PTs for single-pulse stimulation of the occipital lobe were determined for eight subjects while they passively viewed a horizontally drifting luminance-modulated sinewave grating. Gratings used were of four different luminance contrasts while the spatial and temporal frequencies remained constant. PTs were shown to increase significantly as the background grating increased in contrast. These results suggest that the neural activity underlying the perception of a phosphene can be considered a type of signal that can be partially masked by another signal, in this case the visual cortical activation produced by passive viewing of drifting gratings.

Human hippocampal increases in low-frequency power during associative prediction violations

Environmental cues often trigger memories of past events (associative retrieval), and these memories are a form of prediction about imminent experience. Learning is driven by the detection of prediction violations, when the past and present diverge. Using intracranial electroencephalography (iEEG), we show that associative prediction violations elicit increased low-frequency power (in the slow-theta range) in human hippocampus, that this low-frequency power increase is modulated by whether conditions allow predictions to be generated, that the increase rapidly onsets after the moment of violation, and that changes in low-frequency power are not present in adjacent perirhinal cortex. These data suggest that associative mismatch is computed within hippocampus when cues trigger predictions that are violated by imminent experience.

High-Resolution MR Imaging of the Orbit in Patients with Retinoblastoma

Retinoblastoma is the most common intraocular childhood malignancy, with a prevalence of one in 18,000 children younger than 5 years old in the United States. In 80% of patients, retinoblastoma is diagnosed before the age of three, and in 95% of patients, retinoblastoma is diagnosed before the age of five. Although reports exist of retinoblastoma in adults, onset beyond 6 years of age is rare. Broadly, retinoblastoma may be classified into two groups: sporadic and heritable. In either case, the origin of the tumor is a biallelic mutation in primitive neuroepithelial cells. Although their details vary, several staging schemes are used to describe the extent of retinoblastoma according to the following four general criteria: intraocular location, extraocular (extraorbital) location, central nervous system disease, and systemic metastases. In the past decade, substantial changes have taken place in terms of staging and monitoring treatment in patients with retinoblastoma. Diagnosis and treatment of retinoblastoma involve a multidisciplinary approach, for which imaging is a vital component. Increasing awareness and concerns about the effects of radiation in patients with retinoblastoma have led to a shift away from external-beam radiation therapy and toward chemotherapy and locoregional treatment, as well as the establishment of magnetic resonance imaging as the most important imaging modality for diagnosis, staging, and treatment monitoring.

Illusions of visual motion elicited by electrical stimulation of human MT complex.

Human cortical area MT+ (hMT+) is known to respond to visual motion stimuli, but its causal role in the conscious experience of motion remains largely unexplored. Studies in non-human primates demonstrate that altering activity in area MT can influence motion perception judgments, but animal studies are inherently limited in assessing subjective conscious experience. In the current study, we use functional magnetic resonance imaging (fMRI), intracranial electrocorticography (ECoG), and electrical brain stimulation (EBS) in three patients implanted with intracranial electrodes to address the role of area hMT+ in conscious visual motion perception. We show that in conscious human subjects, reproducible illusory motion can be elicited by electrical stimulation of hMT+. These visual motion percepts only occurred when the site of stimulation overlapped directly with the region of the brain that had increased fMRI and electrophysiological activity during moving compared to static visual stimuli in the same individual subjects. Electrical stimulation in neighboring regions failed to produce illusory motion. Our study provides evidence for the sufficient causal link between the hMT+ network and the human conscious experience of visual motion. It also suggests a clear spatial relationship between fMRI signal and ECoG activity in the human brain.