Specifically, due to the Vm-to-spikes nonlinearity, the baseline elevation of membrane potential could have a reduced effect at the level of spiking activity when the response is weak (e.g., before stimulus

onset or at locations far from stimulus center), and an enhanced effect when the response is strong (e.g., near the peak of the stimulus evoked response). The possible effects of such nonlinearity on the predicted attentional modulations at the level of spiking activity are illustrated in Figure 7. This nonlinearity could lead to a small increase in the firing rates of V1 neurons in the absence of visual stimulation. Such an effect could be difficult to detect using single unit electrophysiology Lenvatinib price but may be more prominent in population responses. Consistent with this possibility, a recent study of attentional modulations at the level of single check details neurons in V1 found no baseline modulations in the absence of the stimulus or at very low stimulus contrasts (Thiele et al., 2009), while fMRI results in V1 show clear baseline modulations even when the stimulus is absent (e.g., Buracas and Boynton, 2007, Murray,

2008 and Pestilli et al., 2011). The attentional signals are initiated after fixation point dimming and shortly before the visually evoked responses ( Figure 6). This result implies that top-down modulations can be anticipatory in nature, consistent with previous studies (e.g., Ghose and Maunsell, 2002), and do not require extrastriate cortex to first process the visual stimulus. In fact, the attentional signal can be present even in the absence of the visual stimulus ( Figures 6B and 6E), consistent with some electrophysiological ( Luck et al., 1997, Reynolds et al., 2000 and Williford and Maunsell, 2006) and fMRI ( Kastner et al., 1999 and Ress et al., 2000) results. The attentional modulations in V1 operate at a larger spatial scale than the stimulus-evoked response. Because the baseline elevation extends beyond our imaged area, we cannot determine the exact spatial

extent of this top-down signal. However, Florfenicol since we observed no baseline elevation in attend-out trials, the top-down signal must have a limited spatial extent, at least in focal attention trials. In distributed attention trials, top-down signals could activate simultaneously four broad but separate V1 regions peaked at the representation of the possible stimulus locations, consistent with a recent finding in humans (Müller et al., 2003). Alternatively, a larger contiguous region could be activated, such as the V1 region corresponding to a ring at target eccentricity. The spatial extent and shape of the top-down attentional signal could be addressed in future VSDI experiments by systematically shifting the position of the stimuli relative to the position of the receptive fields in the imaged area.