Functionally Separable Font-invariant and Font-sensitive Neural Populations in Occipitotemporal Cortex.

Reading relies on the rapid visual recognition of words viewed in a wide variety of fonts. We used fMRI to identify neural populations showing reduced fMRI responses to repeated words displayed in different fonts ("font-invariant" repetition suppression). We also identified neural populations showing greater fMRI responses to words repeated in a changing font as compared with words repeated in the same font ("font-sensitive" release from repetition suppression).

An fMRI study of visual hemifield integration and cerebral lateralization.

The human brain integrates hemifield-split visual information via interhemispheric transfer. The degree to which neural circuits involved in this process behave differently during word recognition as compared to object recognition is not known. Evidence from neuroimaging (fMRI) suggests that interhemispheric transfer during word viewing converges in the left hemisphere, in two distinct brain areas, an "occipital word form area" (OWFA) and a more anterior occipitotemporal "visual word form area" (VWFA).

Visual Cortical Representation of Whole Words and Hemifield-split Word Parts.

Reading requires the neural integration of visual word form information that is split between our retinal hemifields. We examined multiple visual cortical areas involved in this process by measuring fMRI responses while observers viewed words that changed or repeated in one or both hemifields. We were specifically interested in identifying brain areas that exhibit decreased fMRI responses as a result of repeated versus changing visual word form information in each visual hemifield.

Double representation of the wrist and elbow in human motor cortex.

Movements of the fingers, hand and arm involve overlapping neural representations in primary motor cortex (M1). Monkey M1 exhibits a core-surround organisation in which cortical representation of the hand and fingers is surrounded by representations of the wrist, elbow and shoulder. A potentially homologous organisation in human M1 has only been observed in a single study, a functional MRI (fMRI) study by [J.

Figure-ground representation and its decay in primary visual cortex.

We used fMRI to study figure-ground representation and its decay in primary visual cortex (V1). Human observers viewed a motion-defined figure that gradually became camouflaged by a cluttered background after it stopped moving. V1 showed positive fMRI responses corresponding to the moving figure and negative fMRI responses corresponding to the static background.

Face inversion reduces the persistence of global form and its neural correlates.

Face inversion produces a detrimental effect on face recognition. The extent to which the inversion of faces and other kinds of objects influences the perceptual binding of visual information into global forms is not known. We used a behavioral method and functional MRI (fMRI) to measure the effect of face inversion on visual persistence, a type of perceptual memory that reflects sustained awareness of global form.

Equal degrees of object selectivity for upper and lower visual field stimuli.

Functional MRI (fMRI) studies of the human object recognition system commonly identify object-selective cortical regions by comparing blood oxygen level-dependent (BOLD) responses to objects versus those to scrambled objects. Object selectivity distinguishes human lateral occipital cortex (LO) from earlier visual areas. Recent studies suggest that, in addition to being object selective, LO is retinotopically organized; LO represents both object and location information.

The role of temporal synchrony as a binding cue for visual persistence in early visual areas: an fMRI study.

We examined the role of temporal synchrony-the simultaneous appearance of visual features-in the perceptual and neural processes underlying object persistence. When a binding cue (such as color or motion) momentarily exposes an object from a background of similar elements, viewers remain aware of the object for several seconds before it perceptually fades into the background, a phenomenon known as object persistence. We showed that persistence from temporal stimulus synchrony, like that arising from motion and color, is associated with activation in the lateral occipital (LO) area, as measured by functional magnetic resonance imaging.

fMRI reveals greater within- than between-hemifield integration in the human lateral occipital cortex.

Early visual areas within each hemisphere (V1, V2, V3/VP, V4v) contain distinct representations of the upper and lower quadrants of the contralateral hemifield. As receptive field size increases, the retinotopy in higher-tier visual areas becomes progressively less distinct. Using functional magnetic resonance imaging (fMRI) to map the visual fields, we found that an intermediate level visual area, the lateral occipital region (LO), contains retinotopic maps with a contralateral bias, but with a combined representation of the upper and lower visual field.

The neural correlates of change detection in the face perception network.

A common view is that visual processing within the ventral visual stream is modulated by attention and awareness. We used fMRI adaptation to investigate whether activation in a network of brain regions involved with face recognition--namely the fusiform face area (FFA), occipital face area (OFA) and right superior temporal sulcus (rSTS)--was modulated by physical changes to face stimuli or by observers' awareness of the changes. We sequentially presented two matrices of four faces.

Isolation of saccade inhibition processes: rapid event-related fMRI of saccades and nogo trials.

Previous functional magnetic resonance imaging (fMRI) studies have compared saccade trials and nogo trials, which required subjects to look at peripheral visual stimuli and to inhibit automatic saccades evoked by peripheral stimuli, respectively. These studies surprisingly reported no activation differences in cortical saccade regions between the two tasks, despite their opposite response requirements. Here, we re-examined this issue by comparing saccades and nogo trials using a rapid event-related fMRI design in which saccade trials were presented twice as frequently as nogo trials to make the saccade response more prepotent.

Frontoparietal activation with preparation for antisaccades.

Several current models hold that frontoparietal areas exert cognitive control by biasing task-relevant processing in other brain areas. Previous event-related functional magnetic resonance imaging (fMRI) studies have compared prosaccades and antisaccades, which require subjects to look toward or away from a flashed peripheral stimulus, respectively. These studies found greater activation for antisaccades in frontal and parietal regions at the ends of long (>or=6 s) preparatory periods preceding peripheral stimulus presentation.

Human parietal "reach region" primarily encodes intrinsic visual direction, not extrinsic movement direction, in a visual motor dissociation task.

Posterior parietal cortex (PPC) participates in the planning of visuospatial behaviors, including reach movements, in gaze-centered coordinates. It is not known if these representations encode the visual goal in retinal coordinates, or the movement direction relative to gaze. Here, by dissociating the intrinsic retinal stimulus from the extrinsic direction of movement, we show that PPC employs a visual code.

Task-related laterality effects in the lateral occipital complex.

Using functional imaging, we investigated the effects of two different tasks on activation in the lateral occipital complex (LOC). Alternating blocks of intact and scrambled objects were presented. In one task, subjects responded when an object repeated (matching task).

Inhibition and generation of saccades: rapid event-related fMRI of prosaccades, antisaccades, and nogo trials.

Flexible, adaptive behavior often requires the inhibition of automatic responses in favor of voluntary response generation. The antisaccade task requires active inhibition of the automatic saccade to a peripheral stimulus followed by generation of a voluntary antisaccade to the opposite location. Previous studies demonstrated greater functional magnetic resonance imaging (fMRI) activation for antisaccades than prosaccades in cortical saccade areas but did not distinguish the relative contributions of saccadic inhibition and generation.

Representation of head-centric flow in the human motion complex.

Recent neuroimaging studies have identified putative homologs of macaque middle temporal area (area MT) and medial superior temporal area (area MST) in humans. Little is known about the integration of visual and nonvisual signals in human motion areas compared with monkeys. Through extra-retinal signals, the brain can factor out the components of visual flow on the retina that are induced by eye-in-head and head-in-space rotations and achieve a representation of flow relative to the head (head-centric flow) or body (body-centric flow).

Directional selectivity of BOLD activity in human posterior parietal cortex for memory-guided double-step saccades.

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in storing target locations for delayed double-step saccades. To do so, we exploited the laterality of a subregion of PPC that preferentially responds to the memory of a target location presented in the contralateral visual field. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the locations of two sequentially flashed targets, presented in either the same or different visual hemifields, and then saccaded to them in sequence.

Remapping the remembered target location for anti-saccades in human posterior parietal cortex.

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in anti-saccades. To do so, we exploited the laterality of a subregion of the PPC for remembered target location. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the location of a flashed target, then were instructed to plan either an anti- or pro-saccade to that location, and finally were instructed to execute the movement.

Perceptual continuity and the emergence of perceptual persistence in the ventral visual pathway.

Perceptual continuity is an important aspect of our experience of the visual world. In this study, we focus on an example of perceptual continuity involving the maintenance of figure-ground segregation despite the removal of binding cues that initiated the segregation. Fragmented line drawings of objects were superimposed on a background of randomly oriented lines.

Segregation and persistence of form in the lateral occipital complex.

While the lateral occipital complex (LOC) has been shown to be implicated in object recognition, it is unclear whether this brain area is responsive to low-level stimulus-driven features or high-level representational processes. We used scrambled shape-from-motion displays to disambiguate the presence of contours from figure-ground segregation and to measure the strength of the binding process for shapes without contours. We found persisting brain activation in the LOC for scrambled displays after the motion stopped indicating that this brain area subserves and maintains figure-ground segregation processes, a low-level function in the object processing hierarchy.

Integration of target and effector information in human posterior parietal cortex for the planning of action.

Recently, using event-related functional MRI (fMRI), we located a bilateral region in the human posterior parietal cortex (retIPS) that topographically represents and updates targets for saccades and pointing movements in eye-centered coordinates. To generate movements, this spatial information must be integrated with the selected effector. We now tested whether the activation in retIPS is dependent on the hand selected.

A contralateral preference in the lateral occipital area: sensory and attentional mechanisms.

Here we examined the level of the lateral occipital (LO) area within the processing stream of the ventral visual cortex. An important determinant of an area's level of processing is whether it codes visual elements on both sides of the visual field, as do higher visual areas, or prefers those in the contralateral visual field, as do early visual areas. The former would suggest that LO, on one side, combines bilateral visual elements into a whole, while the latter suggests that it codes only the parts of forms.

Static ocular counterroll is implemented through the 3-D neural integrator.

Static head roll about the naso-occipital axis is known to produce an opposite ocular counterroll with a gain of approximately 10%, but the purpose and neural mechanism of this response remain obscure. In theory counterroll could be maintained either by direct tonic vestibular inputs to motoneurons, or by a neurally integrated pulse, as observed in the saccade generator and vestibulo-ocular reflex. When simulated together with ocular drift related to torsional integrator failure, the direct tonic input model predicted that the pattern of drift would shift torsionally as in ordinary counterroll, but the integrated pulse model predicted that the equilibrium position of torsional drift would be unaffected by head roll.

Interaction of retinal image and eye velocity in motion perception.

When we move our eyes, why does the world look stable even as its image flows across our retinas, and why do afterimages, which are stationary on the retinas, appear to move? Current theories say this is because we perceive motion by summation: if an object slips across the retina at r degrees/s while the eye turns at e degrees/s, the object's perceived velocity in space should be r + e. We show that activity in MT+, the visual-motion complex in human cortex, does reflect a mix of r and e rather than r alone. But we show also that, for optimal perception, r and e should not summate; rather, the signals coding e interact multiplicatively with the spatial gradient of illumination.

Gaze-centered updating of visual space in human parietal cortex.

Single-unit recordings have identified a region in the posterior parietal cortex (PPC) of the monkey that represents and updates visual space in a gaze-centered frame. Here, using event-related functional magnetic resonance imaging, we identified an analogous bilateral region in the human PPC that shows contralateral topography for memory-guided eye movements and arm movements. Furthermore, when eye movements reversed the remembered horizontal target location relative to the gaze fixation point, this PPC region exchanged activity across the two cortical lobules.