Processing in most visual cortical areas, including PFC, is strongly biased toward the contralateral visual hemifield (Funahashi et al., 1990; Hagler and Sereno, 2006; Kastner et al., 2007; Medendorp et al., 2007; Pasternak et al., 2015; Rainer et al., 1998; Voytek and Knight, 2010; Wimmer et al., 2016). Prefrontal spiking activity (Buschman et al., 2011) and gamma power (Kornblith et al., 2015) increase with WM load—the number of items held in memory at one time—but only for items in the contralateral visual hemifield. In contrast, beta power shows increasing suppression for increasing numbers of items in either visual hemifield (Kornblith et al., 2015; Medendorp et al., 2007). Our results confirm these findings. This distinction might reflect the fact that beta oscillations are thought to correlate with broadly selective inhibitory processes (Engel and Fries, 2010; Jensen and Mazaheri, 2010; Lundqvist et al., 2016). It might also reflect beta’s having a stronger influence from more bilateral top-down or recurrent signals, while spiking and theta/gamma oscillations are dominated by feedforward signals from strongly lateralized visual cortex (Bastos et al., 2015; van Kerkoerle et al., 2014).
