g., clicking noise, scalp and neck muscle twitches), our reliance on empirically based determination of optimal positioning of the TMS coil increased

confidence in the results. To interpret our findings regarding the temporal processing of filtered and unfiltered faces (i.e., the SF by forward/backward masking interaction effect), it would be useful to view these findings within the basic vision framework of the dual-channel model of retino-cortical dynamics (Breitmeyer 1984; Ogmen 1993; Ogmen et al. 2003). An early formulation of this model has postulated that a feedforward mechanism, involving the afferent, unidirectional flow of information from the retina to and through the #learn more keyword# visual cortex, was sufficient to account for early visual processing (Breitmeyer and Ganz 1976; Breitmeyer 1984). However, data have been accumulating to suggest that the activity of cortical neurons is not determined by this feedforward sweep alone (Enns and Di Lollo 2000; Lamme and Roelfsema Inhibitors,research,lifescience,medical 2000; Lamme et al. 2000; Wokke et al. 2013). Instead, conscious visual processing appears to require iterative feedforward–feedback reentrant exchanges of neural signals Inhibitors,research,lifescience,medical among levels (Hupe et al. 1998; Di Lollo et al. 2000; Pascual-Leone and Walsh 2001;

Rassovsky et al. 2005). Reentrant processes, which have become a major focus in cognitive science, are thought to occur as ascending Inhibitors,research,lifescience,medical and descending pathways form an iterative loop, so that ascending stimuli would be influenced by descending top-down activity through this process (Di Lollo et al. 2000; Lamme and Roelfsema 2000; Breitmeyer et al. 2004). Studies examining Inhibitors,research,lifescience,medical visual suppression through single-pulse TMS suggest that forward masking reflects the suppression of the early responses in V1 activating the cortical feedforward sweep, whereas backward masking reflects mostly the later V1 responses due to reentrant activation from post-V1 levels (Corthout et al. 1999; Lamme and Methisazone Roelfsema

2000; Breitmeyer et al. 2004; Wokke et al. 2013). Consistent with other TMS studies of early visual information processing (Corthout et al. 1999), in this study BSF face stimuli were suppressed more with forward than backward TMS masking, suggesting greater reliance on the feedforward process. The filtered HSF faces, on the other hand, were most strongly suppressed in the backward masking components, potentially demonstrating the increasing involvement of reentrant activation from post-V1 levels (Corthout et al. 1999; Breitmeyer et al. 2004). It should also be noted that the TMS pulse delivered to the visual cortex primarily affects visual processing of M and P neurons at cortical levels (Amassian et al. 1989; Pascual-Leone and Walsh 2001; Antal et al. 2002).