While obtaining an accurate estimate of neural conduction velocities in vivo in the human central nervous system is problematic without the use of invasive intra-cranial electrodes, several previous studies have used electroencephalography (EEG) to estimate inter-hemispheric transmission time (IHTT) from the event-related potentials (ERPs) evoked by unilaterally-presented visual stimuli (Brown and Jeeves, 1993; Moes et al., 2007; Nowicka et al., 1996; Saron and Davidson, 1989). IHTT has typically been calculated as the latency of an early ERP component (e.g., P1 or N1) measured at a posterior electrode site over the cerebral hemisphere contralateral to the visual stimulus (e.g., PO8 for a left-visual field stimulus) subtracted from the latency of the component measured at the homologous electrode site over the hemisphere ipsilateral to the stimulus (i.e., PO7 in this example). IHTT is an attractive measure for investigating the integrity of fiber tracts in vivo because its anatomical substrate, the visual fibers of the corpus callosum, is well-characterized. Visual stimuli are more commonly used than auditory stimuli in IHTT research because of the fact that in the auditory system, in contrast to the visual system, much of the inter-hemispheric transfer occurs prior to reaching primary sensory cortex (Kandel et al., 2000).
