Borders of multiple visual areas in humans revealed by functional magnetic resonance imaging

Spatial Selective Attention Affects Early Extrastriate But Not Striate Components of the Visual Evoked Potential

The effects of spatial selective attention on sensory processing in visual cortical areas were investigated by means of visual evoked potential (VEP) recordings and source localization techniques. Patterned stimuli were rapidly presented in random order to the left and right visual fields while subjects maintained central fixation and attended to one visual field at a time. Attended stimuli evoked enhanced P1 (100–130 msec) and N1 (120–200 msec) components of the VEP, whereas no effects of attention were observed on the C1 (50–100 msec) or P2 (200–240 msec) components. Spatiotemporal dipole modeling of the early VEP sources was carried out in relation to MRI-defined cortical anatomy. The dipolar generator of the C1 component was found to lie in calcarine cortex, the human homologue of area V1, whereas the attention-sensitive P1 generator was localized to ventral-lateral occipital cortex, within extrastriate area 19. These results support the hypothesis that spatial attention does not affect the initial activity evoked in area V1 but rather produces an enhancement within extrastriate visual areas of sensory signals arising from stimuli at attended locations.

Cortical magnification, scale invariance and visual ecology

The visual world of an organism can be idealized as a sphere. Locomotion towards the pole causes translation of retinal images that is proportional to the sine of eccentricity of each object. In order to estimate the human striate cortical magnification factor M, we assumed that the cortical translations, caused by retinal translations due to the locomotion, were independent of eccentricity. This estimate of M agrees with previous data on magnifications, visual thresholds and acuities across the visual field. It also results in scale invariance in which the resolution of objects anywhere in the visual field outside the fixated point is about the same for any viewing distance. Locomotion seems to be a possible determinant in the evolution of the visual system and the brain.

Detection of visual dysfunction in optic atrophy by functional magnetic resonance imaging during monocular visual stimulation

PURPOSE

To evaluate functional magnetic resonance imaging as an objective method for detecting visual dysfunction in various ophthalmologic disorders involving the optic nerve and the chiasm.

METHODS

We performed functional magnetic resonance imaging during monocular visual stimulation on seven patients with visual field loss caused by lesions of the optic nerve and the chiasm and on three normal control subjects with no visual field loss. We correlated static threshold perimetry in the seven patients with the results of functional magnetic resonance imaging.

RESULTS

In the three normal control subjects, we found good intrasubject similarity in areas of bilateral occipital lobe activation between monocular stimulation of the right and left eyes. In the patients with unilateral optic neuropathy, including glaucoma, stimulation of the affected eye induced no activation of the primary visual cortex in the portion corresponding to the central visual field defects and reduced activity of the associated visual cortex. In the patients with chiasmal compression, monocular stimulation resulted in a marked asymmetry of activation in the primary visual cortex, which corresponded to the visual field abnormality.

CONCLUSIONS

Functional magnetic resonance imaging appears to be useful in confirming the clinical diagnosis of optic atrophy because it can objectively disclose visual field loss, even a small defect such as central scotoma.

Information processing in the human brain: magnetoencephalographic approach

Rapid progress in effective methods to image brain functions has revolutionized neuroscience. It is now possible to study noninvasively in humans neural processes that were previously only accessible in experimental animals and in brain-injured patients. In this endeavor, positron emission tomography has been the leader, but the superconducting quantum interference device-based magnetoencephalography (MEG) is gaining a firm role, too. With the advent of instruments covering the whole scalp, MEG, typically with 5-mm spatial and 1-ms temporal resolution, allows neuroscientists to track cortical functions accurately in time and space. We present five representative examples of recent MEG studies in our laboratory that demonstrate the usefulness of whole-head magnetoencephalography in investigations of spatiotemporal dynamics of cortical signal processing.

Where in the brain does visual attention select the forest and the trees?

The perceptual world is organized hierarchically: the forest consists of trees, which in turn have leaves. Visual attention can emphasize the overall picture (global form) or the focal details of a scene (local components). Neuropsychological studies have indicated that the left hemisphere is biased towards local and the right towards global processing. The underlying attentional and perceptual mechanisms are maximally impaired by unilateral lesions to the temporal and parietal cortex. We measured brain activity of normal subjects during two experiments using 'hierarchically' organized figures. In a directed attention task, early visual processing (prestriate) areas were activated: attention to the global aspect of the figures activated the right lingual gyrus whereas locally directed attention activated the left inferior occipital cortex. In a subsequent divided attention task, the number of target switches from local to global (and vice versa) covaried with temporal-parietal activation. The findings provide direct evidence for hemispheric specialization in global and local perception; furthermore, they indicate that temporal-parietal areas exert attentional control over the neural transformations occurring in prestriate cortex.

Magnetic coil suppression of visual perception at an extracalcarine site

Perception of extrafoveal visual targets can be suppressed by magnetic stimulation over the occipital lobes, but the site of interference for this and similar phenomena has not been well defined. We modified a previously used technique to determine the locus of magnetic activation. Using butterfly stimulus coils of different sizes and electric field profiles, we determined a scalp position of minimum threshold and a level of stimulator output that produced 50% error rates for each coil. Intersection of the corresponding electric field profiles in air and in a saline model head was similar and identified superficial occipital cortex rather than the primary visual area as the site of perceptual suppression. Less direct analyses involving the distribution of induced electric fields produced the same conclusion. These results suggest specific hypotheses about the effects of magnetic stimulation on visual physiology.

Mapping striate and extrastriate visual areas in human cerebral cortex

Functional magnetic resonance imaging (fMRI) was used to identify and map the representation of the visual field in seven areas of human cerebral cortex and to identify at least two additional visually responsive regions. The cortical locations of neurons responding to stimulation along the vertical or horizontal visual field meridia were charted on three-dimensional models of the cortex and on unfolded maps of the cortical surface. These maps were used to identify the borders among areas that would be topographically homologous to areas V1, V2, V3, VP, and parts of V3A and V4 of the macaque monkey. Visually responsive areas homologous to the middle temporal/medial superior temporal area complex and unidentified parietal visual areas were also observed. The topography of the visual areas identified thus far is consistent with the organization in macaque monkeys. However, these and other findings suggest that human and simian cortical organization may begin to differ in extrastriate cortex at, or beyond, V3A and V4.

Selective attention to the color and direction of moving stimuli: electrophysiological correlates of hierarchical feature selection

Event-related brain potentials (ERPs) were recorded from subjects who attended to pairs of adjacent colored squares that were flashed sequentially to produce a perception of movement. The task was to attend selectively to stimuli in one visual field and to detect slower moving targets that contained the critical value of the attended feature, be it color or movement direction. Attention to location was reflected by a modulation of the early P1 and N1 components of the ERP, whereas selection of the relevant stimulus feature was associated with later selection negativity components. ERP indices of feature selection were elicited only by stimuli at the attended location and had distinctive scalp distributions for features mediated by "ventral" (color) and "dorsal" (motion) cortical areas. ERP indices of target selection were also contingent on the prior selection of location but initially did not depend on the selection of the relevant feature. These ERP data reveal the timing of sequential, parallel, and contingent stages of visual processing and support early-selection theories of attention that stipulate attentional control over the initial processing of stimulus features.

Imaging of brain iron by magnetic resonance: T2 relaxation at different field strengths

Soon after the advent of magnetic resonance imaging (MRI) as a diagnostic modality in the 1980s, it was recognized that some of the contrast found in brain imaging correlated with patterns of iron deposition. The presence of non-heme iron had previously been established by pathological studies on post-mortem brains. The iron concentration is highest in specific nuclei of the basal ganglia and some associated structures. It is low at birth and increases with age until a relatively constant level is reached at an age of 20-30 years. There is evidence for further increases in very elderly persons. Although iron is ubiquitous in human tissues, only in a few situations is the concentration large enough to affect MRI. Because MRI has the ability to detect, in a noninvasive fashion, the naturally occurring iron in the basal ganglia and related nuclei, it may be used to study the physiology and pathology of these important structures. Magnetic resonance imaging has confirmed the results of earlier post mortem studies of the anatomical localization and age-dependence of brain iron. Initial steps have been toward the use of MRI to study disorders of thought, movement, and behavior that are believed to be related to brain iron. However, additional understanding is required of the physical details of the contrast mechanism, the physiology of the iron accumulation, and the significance of abnormal patterns of iron deposition. In this report, data are presented on the normal variation in MRI parameters and their dependence on magnetic field strength. The potential clinical and basic science applications are briefly reviewed. Information from widely differing fields is relevant to the study of the physical and pathological significance of brain iron, and for this reason, extensive, although not exhaustive, literature references are included.

Functional brain imaging studies of cortical mechanisms for memory

Recent functional brain imaging studies in humans indicate that learning and memory involve many of the same regions of the cortex that process sensory information and control motor output. The forms of perceptual and motor learning that can occur without conscious recollection are mediated in part by contractions and expansions of representations in the sensory and motor cortex. The same regions are also engaged during the conscious storage and retrieval of facts and events, but these types of memory also bring into play structures involved in the active maintenance of memories "on line" and in the establishment of associative links between the information stored in different sensory areas. Although the picture of memory that is emerging from functional imaging studies is consistent with current physiological accounts, there are puzzles and surprises that will be solved only through a combination of human and animal studies.

Human visual cortex. Progress and puzzles

A wealth of data is now available on the functional organization of the human visual cortex. Caution is necessary in basing interpretations of such data on information gained from studies of the monkey visual cortex.

Object-related activity revealed by functional magnetic resonance imaging in human occipital cortex

The stages of integration leading from local feature analysis to object recognition were explored in human visual cortex by using the technique of functional magnetic resonance imaging. Here we report evidence for object-related activation. Such activation was located at the lateral-posterior aspect of the occipital lobe, just abutting the posterior aspect of the motion-sensitive area MT/V5, in a region termed the lateral occipital complex (LO). LO showed preferential activation to images of objects, compared to a wide range of texture patterns. This activation was not caused by a global difference in the Fourier spatial frequency content of objects versus texture images, since object images produced enhanced LO activation compared to textures matched in power spectra but randomized in phase. The preferential activation to objects also could not be explained by different patterns of eye movements: similar levels of activation were observed when subjects fixated on the objects and when they scanned the objects with their eyes. Additional manipulations such as spatial frequency filtering and a 4-fold change in visual size did not affect LO activation. These results suggest that the enhanced responses to objects were not a manifestation of low-level visual processing. A striking demonstration that activity in LO is uniquely correlated to object detectability was produced by the "Lincoln" illusion, in which blurring of objects digitized into large blocks paradoxically increases their recognizability. Such blurring led to significant enhancement of LO activation. Despite the preferential activation to objects, LO did not seem to be involved in the final, "semantic," stages of the recognition process. Thus, objects varying widely in their recognizability (e.g., famous faces, common objects, and unfamiliar three-dimensional abstract sculptures) activated it to a similar degree. These results are thus evidence for an intermediate link in the chain of processing stages leading to object recognition in human visual cortex.