Motion sensitivity of human V6: a magnetoencephalography study.

Recent studies suggest the presence of a human homologue of monkey V6 in the dorsal posterior bank of the parieto-occipital sulcus. Monkey V6 comprises a retinotopic representation with relative peripheral visual field emphasis and is sensitive to visual motion. We studied sensitivity to visual motion in human parieto-occipital sulcus. Our upper peripheral visual field stimulus enabled us to distinguish V6 from neighbouring areas, whose upper VF representation is located far from V6. We recorded neuromagnetic signals while the subjects (N=10) fixated and a grating first appeared and then started to drift. The most prominent sustained activation for motion was at the posterior bank of the dorsal parieto-occipital sulcus; that is at the known location of the human V6. This finding suggests that human V6 is a motion-sensitive area. The responses in V6 occurred early, with about the same latency as in V1, in line with known connections in the monkey brain. In addition, on the medial surface of the hemisphere we observed a fast sequence of activations following V6: first precuneus and later an area at the dorsal end of the cingulate sulcus. On the lateral side, both temporo-occipital area and intraparietal sulcus were active, but with delayed onset compared to V6. This rapid flow of visual information along the medial dorsal visual pathway supports the view that in humans, as in monkeys, the V6 and the connected areas could be involved in online control of visually guided actions.

Topography of attention in the primary visual cortex.

Previous research suggests that feedback circuits mediate the effect of attention to the primary visual cortex (V1). This inference is mainly based on temporal information of the responses, where late modulation is associated with feedback signals. However, temporal data alone are inconclusive because the anatomical hierarchy between cortical areas differs significantly from the temporal sequence of activation. In the current work, we relied on recent physiological and computational models of V1 network architecture, which have shown that the thalamic feedforward, local horizontal and feedback contribution are reflected in the spatial spread of responses. We used multifocal functional localizer and quantitative analysis in functional magnetic resonance imaging to determine the spatial scales of attention and sensory responses. Representations of 60 visual field regions in V1 were functionally localized and four of these regions were targets in a subsequent attention experiment, where human volunteers fixated centrally and performed a visual discrimination task at the attended location. Attention enhanced the peak amplitudes significantly more in the lower than in the upper visual field. This enhancement by attention spread with a 2.4 times larger radius (approximately 10 mm, assuming an average magnification factor) compared with the unattended response. The corresponding target region of interest was on average 20% stronger than that caused by the afferent sensory stimulation alone. This modulation could not be attributed to eye movements. Given the contemporary view of primate V1 connections, the activation spread along the cortex provides further evidence that the signal enhancement by spatial attention is dependent on feedback circuits.

fMRI of peripheral visual field representation.

OBJECTIVE: Despite mapping tools for central visual field, delineation of peripheral visual field representations in the human cortex has remained a challenge. Access to large visual field and differentiation of retinotopic areas with robust mapping procedures and automated analysis are beneficial in basic research and could accelerate development of clinical applications. METHODS: We constructed a simple optical near view system for wide visual field stimulation, and examined the topology of retinotopic areas. We used multifocal (mf) design, which enables analysis with general linear model and standard fMRI softwares and is easily automated. RESULTS: Our stimulation method enabled individual mapping of visual field up to 50 degrees of eccentricity and showed that retinotopic visual areas extended through posterior cerebrum. In addition, we located a separate peripheral upper visual field representation in parieto-occipital (PO) sulcus. CONCLUSIONS: These functional results are in line with earlier histological data, and support recent findings on human V6, a retinotopic area in the medial PO sulcus with an apparent emphasis on peripheral visual field. SIGNIFICANCE: Our projection system and mf-design together enable efficient and robust retinotopic mapping of wide visual field, which can at low cost be adapted to any clinical environment with visual back-projection system.

Central luminance flicker can activate peripheral retinotopic representation.

We aimed to study cortical responses to uniform luminance stimulus in different conditions. We stimulated the central visual field with luminance flicker and reversal of checkerboard pattern contrast and mapped the visual field representation up to 50 degrees of eccentricity. Our results show spreading of cortical BOLD responses when visual stimulus contains mean luminance change in dark surround and no spreading when the stimulus surround has bright illumination. No cortical region was more sensitive to luminance flicker than to pattern reversal during both stimulation setups. We suggest that the spread of luminance responses in retinotopic cortical areas results from intraocular scattering of light. Light scattered inside the eye spreads visual stimulation on the retina, and the contrast of the scattered light is strongest when the surround of the stimulus is dark. The stray light is potential and often neglected source of an artefact in visual experiments, and the responses due to stray light can erroneously be interpreted as indicators for local cortical sensitivity to luminance.

Comparison of minimum current estimate and dipole modeling in the analysis of simulated activity in the human visual cortices.

Magnetoencephalographic(MEG) data are typically interpreted using source models because of the nonunique inverse problem. Although single current dipoles, adequately representing local active areas, can be identified accurately, multiple and overlapping sources form a challenge for MEG modeling. We tested the performances of multidipole modeling and minimum current estimate (MCE) in the analysis of complicated source configurations. Simulated current sources were placed to physiologically meaningful areas of the human visual cortices. Ten volunteers from the laboratory staff analyzed four different simulations with both dipole modeling and MCE without prior information of the sources. In general, the same sources were found using both modeling methods. The subjects tended to report more false sources with MCE than with dipole model, in part due to their inexperience with the method. Dipole model was more accurate than MCE both in time and space for nonsimultaneous sources but both methods performed similarly when sources overlapped in time. For all source configurations, considerably smaller source amplitudes were reported with MCE than with dipole model.