Antigona Martinez

Involvement of striate and extrastriate visual cortical areas in spatial attention.

We investigated the cortical mechanisms of visual-spatial attention while subjects discriminated patterned targets within distractor arrays. Functional magnetic resonance imaging (fMRI) was used to map the boundaries of retinotopic visual areas and to localize attention-related changes in neural activity within several of those areas, including primary visual (striate) cortex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50–55 milliseconds after the stimulus was not modulated by attention. The earliest facilitation of attended signals was observed in extrastriate visual areas, at 70–75 milliseconds. We hypothesize that the striate cortex modulation found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during attention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.

Cortical sources of the early components of the visual evoked potential

This study aimed to characterize the neural generators of the early components of the visual evoked potential (VEP) to isoluminant checkerboard stimuli. Multichannel scalp recordings, retinotopic mapping and dipole modeling techniques were used to estimate the locations of the cortical sources giving rise to the early C1, P1, and N1 components. Dipole locations were matched to anatomical brain regions visualized in structural magnetic resonance imaging (MRI) and to functional MRI (fMRI) activations elicited by the same stimuli. These converging methods confirmed previous reports that the C1 component (onset latency 55 msec; peak latency 90–92 msec) was generated in the primary visual area (striate cortex; area 17). The early phase of the P1 component (onset latency 72–80 msec; peak latency 98–110 msec) was localized to sources in dorsal extrastriate cortex of the middle occipital gyrus, while the late phase of the P1 component (onset latency 110–120 msec; peak latency 136–146 msec) was localized to ventral extrastriate cortex of the fusiform gyrus. Among the N1 subcomponents, the posterior N150 could be accounted for by the same dipolar source as the early P1, while the anterior N155 was localized to a deep source in the parietal lobe. These findings clarify the anatomical origin of these VEP components, which have been studied extensively in relation to visual‐perceptual processes. Hum. Brain Mapping 15:95–111, 2001.

Putting spatial attention on the map: timing and localization of stimulus selection processes in striate and extrastriate visual areas

This study investigated the cortical mechanisms of visual-spatial attention in a task where subjects discriminated patterned targets in one visual field at a time. Functional magnetic imaging (fMRI) was used to localize attention-related changes in neural activity within specific retinotopic visual areas, while recordings of event-related brain potentials (ERPs) traced the time course of these changes. The earliest ERP components enhanced by attention occurred in the time range 70-130 ms post-stimulus onset, and their neural generators were estimated to lie in the dorsal and ventral extrastriate visual cortex. The anatomical areas activated by attention corresponded closely to those showing increased neural activity during passive visual stimulation. Enhanced neural activity was also observed in the primary visual cortex (area V1) with fMRI, but ERP recordings indicated that the initial sensory response at 50-90 ms that was localized to V1 was not modulated by attention. Modeling of ERP sources over an extended time range showed that attended stimuli elicited a long-latency (160-260 ms) negativity that was attributed to the dipolar source in area V1. This finding is in line with hypotheses that V1 activity may be modulated by delayed, reentrant feedback from higher visual areas.

Parametric manipulation of conflict and response competition using rapid mixed-trial event-related fMRI

In the current study we examined the influence of preceding context on attentional conflict and response competition using a flanker paradigm. Nine healthy right-handed adults participated in a rapid mixed trial event-related functional magnetic resonance imaging (fMRI) study, in which increasing numbers of either compatible or incompatible trials preceded an incompatible trial. Behaviorally, reaction times on incompatible trials increased as a function of the number of preceding compatible trials. Several brain regions showed monotonic changes to the preceding context manipulation. The most common pattern was observed in anterior cingulate, dorsolateral prefrontal, and superior parietal regions. These areas showed an increase in activity for incompatible trials as the number of preceding compatible trials increased and a decrease in activity for incompatible trials as the number of preceding incompatible trials increased. Post hoc analysis showed that while the MR signal in the anterior cingulate and dorsolateral prefrontal regions peaked before the superior parietal region, the dorsolateral prefrontal MR signal peaked early and remained at this level. These findings are consistent with the conflict monitoring theory that postulates that the anterior cingulate cortex detects or monitors conflict, while PFC is involved in control adjustments that may then lead to modulation of superior parietal cortex in top-down biasing of attention.

Nonlinear temporal dynamics of the cerebral blood flow response.

The linearity of the cerebral perfusion response relative to stimulus duration is an important consideration in the characterization of the relationship between regional cerebral blood flow (CBF), cerebral metabolism, and the blood oxygenation level dependent (BOLD) signal. It is also a critical component in the design and analysis of functional neuroimaging studies. To study the linearity of the CBF response to different duration stimuli, the perfusion response in primary motor and visual cortices was measured during stimulation using an arterial spin labeling technique with magnetic resonance imaging (MRI) that allows simultaneous measurement of CBF and BOLD changes. In each study, the perfusion response was measured for stimuli lasting 2, 6, and 18 sec. The CBF response was found in general to be nonlinearly related to stimulus duration, although the strength of nonlinearity varied between the motor and visual cortices. In contrast, the BOLD response was found to be strongly nonlinear in both regions studied, in agreement with previous findings. The observed nonlinearities are consistent with a model with a nonlinear step from stimulus to neural activity, a linear step from neural activity to CBF change, and a nonlinear step from CBF change to BOLD signal change. Hum. Brain Mapping 13:1–12, 2001.

Neural correlates of refixation saccades and antisaccades in normal and Schizophrenia subjects

BACKGROUND Schizophrenia subjects demonstrate difficulties on tasks requiring saccadic inhibition, despite normal refixation saccade performance. Saccadic inhibition is ostensibly mediated via prefrontal cortex and associated cortical/subcortical circuitry. The current study tests hypotheses about the neural substrates of normal and abnormal saccadic performance among subjects with schizophrenia. METHODS Using functional magnetic resonance imaging, blood oxygenation level-dependent (BOLD) data were recorded while 13 normal and 14 schizophrenia subjects were engaged in refixation and antisaccade tasks. RESULTS Schizophrenia subjects did not demonstrate the increased prefrontal cortex BOLD contrast during antisaccade performance that was apparent in the normal subjects. Schizophrenia subjects did, however, demonstrate normal BOLD contrast associated with refixation saccade performance in the frontal and supplementary eye fields, and posterior parietal cortex. CONCLUSIONS Results from the current study support hypotheses of dysfunctional prefrontal cortex circuitry among schizophrenia subjects. Furthermore, this abnormality existed despite normal BOLD contrast observed during refixation saccade generation in the schizophrenia group.

Early Cross-Modal Interactions in Auditory and Visual Cortex Underlie a Sound-Induced Visual Illusion

When a single flash of light is presented interposed between two brief auditory stimuli separated by 60–100 ms, subjects typically report perceiving two flashes (Shams et al., 2000, 2002). We investigated the timing and localization of the cortical processes that underlie this illusory flash effect in 34 subjects by means of 64-channel recordings of event-related potentials (ERPs). A difference ERP calculated to isolate neural activity associated with the illusory second flash revealed an early modulation of visual cortex activity at 30–60 ms after the second sound, which was larger in amplitude in subjects who saw the illusory flash more frequently. These subjects also showed this early modulation in response to other combinations of auditory and visual stimuli, thus pointing to consistent individual differences in the neural connectivity that underlies cross-modal integration. The overall pattern of cortical activity associated with the cross-modally induced illusory flash, however, differed markedly from that evoked by a real second flash. A trial-by-trial analysis showed that short-latency ERP activity localized to auditory cortex and polymodal cortex of the temporal lobe, concurrent with gamma bursts in visual cortex, were associated with perception of the double-flash illusion. These results provide evidence that perception of the illusory second flash is based on a very rapid dynamic interplay between auditory and visual cortical areas that is triggered by the second sound.

Magnocellular pathway impairment in schizophrenia: evidence from functional magnetic resonance imaging.

Sensory processing deficits in schizophrenia have been documented for several decades, but their underlying neurophysiological substrates are still poorly understood. In the visual system, the pattern of pathophysiology reported in several studies is suggestive of dysfunction within the magnocellular visual pathway beginning in early sensory cortex or even subcortically. The present study used functional magnetic resonance imaging to investigate further the neurophysiological bases of visual processing deficits in schizophrenia and in particular the potential role of magnocellular stream dysfunction. Sinusoidal gratings systematically varying in spatial frequency content were presented to subjects at low and high levels of contrast to differentially bias activity in magnocellular and parvocellular pathways based on well established differences in neuronal response profiles. Hemodynamic responses elicited by different spatial frequencies were mapped over the occipital lobe and then over the entire brain. Retinotopic mapping was used to localize the occipital activations with respect to the boundaries of visual areas V1 and V2, which were demarcated in each subject. Relative to control subjects, schizophrenia patients showed markedly reduced activations to low, but not high, spatial frequencies in multiple regions of the occipital, parietal, and temporal lobes. These findings support the hypothesis that schizophrenia is associated with impaired functioning of the magnocellular visual pathway and further suggest that these sensory processing deficits may contribute to higher-order cognitive deficits in working memory, executive functioning, and attention.

Objects Are Highlighted by Spatial Attention

Selective attention may be focused upon a region of interest within the visual surroundings, thereby improving the perceptual quality of stimuli at that location. It has been debated whether this spatially selective mechanism plays a role in the attentive selection of whole objects in a visual scene. The relationship between spatial and object-selective attention was investigated here through recordings of event-related brain potentials (ERPs) supplemented with functional magnetic brain imaging (fMRI). Subjects viewed a display consisting of two bar-shaped objects and directed attention to sequences of stimuli (brief corner offsets) at one end of one of the bars. Unattended stimuli belonging to the same object as the attended stimuli elicited spatiotemporal patterns of neural activity in the visual cortex closely resembling those elicited by the attended stimuli themselves, albeit smaller in amplitude. This enhanced neural activity associated with object-selective attention was localized by use of ERP dipole modeling and fMRI to the lateral occipital extrastriate cortex. We conclude that object-selective attention shares a common neural mechanism with spatial attention that entails the facilitation of sensory processing of stimuli within the boundaries of an attended object.

Neural basis of the ventriloquist illusion

The ventriloquist creates the illusion that his or her voice emerges from the visibly moving mouth of the puppet [1]. This well-known illusion exemplifies a basic principle of how auditory and visual information is integrated in the brain to form a unified multimodal percept. When auditory and visual stimuli occur simultaneously at different locations, the more spatially precise visual information dominates the perceived location of the multimodal event. Previous studies have examined neural interactions between spatially disparate auditory and visual stimuli [2-5], but none has found evidence for a visual influence on the auditory cortex that could be directly linked to the illusion of a shifted auditory percept. Here we utilized event-related brain potentials combined with event-related functional magnetic resonance imaging to demonstrate on a trial-by-trial basis that a precisely timed biasing of the left-right balance of auditory cortex activity by the discrepant visual input underlies the ventriloquist illusion. This cortical biasing may reflect a fundamental mechanism for integrating the auditory and visual components of environmental events, which ensures that the sounds are adaptively localized to the more reliable position provided by the visual input.