Evoked currents in human visual cortex

Abstract

Distinguishing between close spaced sources is a difficult problem in dipole source analysis, especially between areas V1 and V2 (Ales et al., 2007, 2010). Utilizing retinotopic maps from Functional Magnetic Resonance Imaging (fMRI) has recently shown good success as an additional constraint to solve the close source problem (Ales et al., 2007, 2010; Hagler et al., 2008; Goh, 2008). This study sought to eliminate the fMRI step, and remove the time, cost, and reliability issues in fMRI retinotopy, while retaining the benefits of a retinotopic map in distinguishing between close sources and cancellation due to cortical folding. The Interactive Dipolefitting and Simultaneous Retinotopy (IDSR) technique developed here uses a four-minute Pattern Pulse multifocal Visual Evoked Potential (PPmfVEP) and a structural Magnetic Resonance Imaging (MRI) to create dipole source models. Good agreement was found between dipole models created with IDSR against fMRI based models with two participants. Four additional participants had dipole models created with IDSR, without the use of fMRI data, and their resulting current waveforms agree closely with the literature (Ales et al., 2007, 2010; Hagler et al., 2008; Goh, 2008). In addition, IDSR dipole models of all six participants were then evaluated with a repeat recording to verify the optimization step does not overfit the data, and that IDSR is generalizable. Dipole models created with the IDSR technique were used to measure stimulus effects on human V1 and V2 currents to achromatic and chromatic, luminance contrast (LC) and isoluminant (ISO) stimuli. Contrasts at 100% and 50% with all gray stimuli, and red-green ISO, were compared at 50cd/m{u00B2} and 20cd/m{u00B2} for two participants with PPmfVEP dartboard stimuli. The initial major peaks of V1 and V2 show different effects to contrast and luminance. Generally, V1's peak latency decreases with decreasing contrast (100% to 50% to ISO). V2's peak latency is similar for 100% and 50% contrast, but slower at ISO. For both V1 and V2 if the stimulus is bright the amplitudes to 100% and 50% contrast are similar, if the stimulus is dim then amplitude decreases with luminance. Chromatic effects were investigated to determine if ISO effects are wholly or partially effects of using red and green in ISO versus the achromatic 100% and 50% contrast stimuli. Three participants viewed different combinations of red, green, and gray PPmfVEP dartboard stimuli at ISO and 50% contrast, all at 20cd/m{u00B2} mean luminance. V1's initial major peak's latency kept the effect of slower to ISO vs LC regardless of color. V1's amplitude was affected more by the difference in red vs green in the stimuli compared to ISO vs LC. V2's initial major peak did not have a clear effect of ISO vs LC. Finally, a relationship between V1 and V2's components was identified. V1's first major peak (C1) and V2's second peak (C2) have similar responses to the same stimuli, as do V2's first major peak (C1) and V1's second peak (C2), suggesting an influence relationship of C1 on C2 between the visual areas.