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Earliest Cortical Activity Evoked by Visual Stimuli of Increasing Size: A Neuromagnetic Study

Neuromagnetic studies using small visual stimuli have localized 3 to 5 active regions during the first 160 ms after the stimulus presentation [1, 6]. They demonstrated retinotopical organization of both striate [1] and extra-striate regions [6] by using small stimuli of increasing size to account for the cortical magnification for foveal and peripheral locations in the lower right visual field (LRVF). The goal of the present study is to take advantage of the retinotopical organization of the striate cortex and known cortical magnification factors for the human visual cortex [4] to examine the effects of the expected increases of the cortical activations related to the stimulus size increases. Six female subjects (age range 23–35 years) participated in the study. Two cycles, circular sinusoids (Target) stimuli were presented at the center of the visual field and at foveal (3 degrees) and peripheral (12 degrees) locations in the LRVF as well as peripheral (12 degrees) location in the upper left visual field (ULVF) with a duration of 300 ms. The sizes of only central and foveal stimuli were changed and ranged in diameter 0.3, 0.9, and 2 degrees, respectively. Stimulus presentation rate was 1 Hz with an ISI jitter of 500 ms. Measurements were conducted at the MEG laboratory of the Friedrich Schiller University in Jena using a 306-channel whole-head Elekta Neuromag system. Spatio-temporal source analysis was conducted using Calibrated Start Spatio-Temporal (CSST) algorithm within MRIVIEW package [2, 3] which uses a multi-start downhill simplex minimizer. Multi-dipole models of increasing model order [5] were evaluated for each data set while thousands of starting points were randomly selected from cortical and close-to-cortical pre-selected brain voxels in the volumetric MRI data that were available for each subject. Spatio-temporal analysis of early visual activity up to 160 ms after the stimulus onset demonstrated, for all subjects, activity in occipital, temporal and parietal cortex – total of 4 to 6 bilateral sources for centrally presented stimuli, independent of the stimulus size, compared to 3 to 5 sources evoked by foveal stimuli in the LRVF. The effects of stimulus size increases were examined in this preliminary analysis only on the earliest localized activity in the occipital cortex during the time interval of 60 – 100 ms, presumably reflecting only striate cortex activity. Amplitude modulations and latency changes both of the measured waveforms and estimated cortical source dynamics were evident. For both centrally and foveally located stimuli the increasing size resulted in shorter latencies of the evoked responses. For all stimulus sizes, however, latencies were shorter for the LRVF stimulation compared to the centrally presented stimuli. Amplitude increase for the earliest evoked activity correlated with the stimulus size increase for all subjects. Identified earliest occipital source locations for stimuli presented at the same eccentricity, but of different sizes, were different. Extended stimuli resulted in deeper sources. Depth difference between the earliest occipital sources evoked by the smallest and the two more extended stimuli was prominent (about 2 cm) compared to only few millimeters of difference between the sources evoked by the two extended stimuli. Visual modality offers a unique possibility of a controlled increase in the cortical area activation related to the increases in stimulus size. While neither extra-cranial MEG measurements nor any other noninvasive functional brain imaging modality can give information about the size of the cortical activity our MEG study demonstrated that effects of an increase of the size of the cortical activation do result in measurable alterations of the latency as well as amplitude modulations both in measurement and functional source space. The size of cortical activation does affect source depth estimation as well. Distributed source modeling approaches will be applied to this data set in the future to examine their effectiveness in localizing extended cortical activations. References: Aine, C. (1996), 'Retionotopic organization of human visual cortex: Departures from the classical model', Cerebral Cortex, vol. 6, pp. 354-361. Huang, M. (1998), 'Multi-start downhill simplex method for spatio-temporal source localization in magnetoencephalography', Electroencephalography and Clinical Neurophysiology, vol. 108, pp. 32-44. Ranken , D. (2002), 'MEG/EEG forward and inverse modeling using MRIVIEW', Proceedings of Biomag2002, pp. 785-787. Sereno, M. (1995), 'Borders of multiple visual areas in human revealed by functional

magnetoencephalography; cortical magnification; focal vs. extended cortical activations

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