Size matters: MEG empirical and simulation study on source localization of the earliest visual activity in the occipital cortex

Abstract

While the relationship between sensory stimulation and tasks and the size of the cortical activations is generally unknown, the visual modality offers a unique possibility of an experimental manipulation of stimulus size-related increases of the spatial extent of cortical activation even during the earliest activity in the retinotopically organized primary visual cortex. We used magnetoecephalography (MEG), visual stimuli of increasing size, and numerical simulations on realistic cortical surfaces to explore the effects of increasing spatial extent of the activated cortical sources on the neuromagnetic fields, location estimation biases, and source resolution. Source localization was performed assuming multiple dipoles in a sphere model using an efficient, automatically restarted multi-start simplex minimizer within the Calibrated Start Spatio-Temporal (CSST) algorithm. We found size-related effects on amplitude and latencies and differences in relative locations of the earliest occipital sources evoked by stimuli of increasing size presented at the same eccentricity. This finding was confirmed by single patch simulations. Additionally, simulations of multiple extended sources demonstrated size-related increase in limits in source resolution for bilaterally simulated sources, biases in location estimates for a given separation of sources, and limits in source resolution due to source multiplicity within a hemisphere.

Modulatory role of the prefrontal generator within the auditory M50 network

“…While earlier studies suggesting frontal lobe activation (Garcia-Rill et al, 2008; Weiland et al, 2008, Korzyukova et al, 2007) used minimum norm approaches and detected only a diffuse activity within the frontal lobe this is, to our knowledge, the first report of both the localization of the M50 source in the medial PF cortex and characterization of its dynamics. The sensitivity of the MEG signals to changes in depth, cortical extent, cortical morphology and noise level, as well as the use of multi-dipole source modeling have been examined in numerous studies (Aine et al, 2012; Josef Golubic et al, 2011; Ahlfors et al, 2010; Stephen et al 2003, 2005; Huang et al, 1998; Supek and Aine, 1993, 1997; Mosher et al, 1992,). Numerical simulation results strongly indicate that source depth and its cortical extent are the main factors that compromise the sensitivity of MEG to neural activity in the human cortex (Josef Golubic et al, 2011; Hillebrand and Barnes, 2002).…”

Face activated neurodynamic cortical networks

“…As MEG measures mainly activity from the cortical fissures [17], different folding of the cortical surface yields different magnetic fields on the head surface. The large inter-individual differences found in this study are likely to be related both to differences in individual functional geometries well documented for V1 [2,5,46] and the central presentation of the visual stimuli that cause cancellation of the magnetic fields on the head surface as shown by simulation [20] and empirical studies [25], which prevents localization of all active sources. Some MEG studies have already discussed the origin of the interindividual differences in visual MEG data (e.g., [1,2]).…”

Effects of reconstructed magnetic field from sparse noisy boundary measurements on localization of active neural source

“…Magnetoencephalography (MEG) is a useful noninvasive clinical tool for investigating human neurological functions, in both pathology and applications such as localization of the epileptogenic zone [18], cortex function [5,9], and brain-machine interface [13]. As MEG is generally free from distortion caused by distinct electric conductivity differences between the brain, skull, and scalp, its measurement is a direct reflection of the primary neuronal currents, from which neural source activities with high temporal and spatial resolution can be detected.…”