g., Batty and Taylor, 2002). Most interestingly the largest P1 is elicited for targets requiring a conjunction search as compared to targets Copanlisib order that are characterized by single features, including color pop-out features (e.g. Taylor and Khan, 2000). Furthermore, the latency of the P1 for a conjunction search tends to be longer than those for single features (cf. Taylor et al., 1999 and Taylor et al., 2001a). Most surprisingly, however, is the finding that array size increases P1 amplitude but decreases latency. In a search paradigm in which array size was varied between 5, 9 and 17 items, the largest P1 and the shortest P1 latencies were found for the largest array with 17

items (Taylor et al., 2001b). Although the C1 component is primarily associated with perceptual encoding, research by Herrmann seems to imply that the P1 component also is a sensory component (Busch et al., 2004, Busch et al., 2006a, Busch et al., 2006b and Fründ

et al., 2007). We try to show here that the P1 is not modulated by physical properties per se but only if they are relevant for early categorization or if they elicit reflexive selleck monoclonal antibody attention. One important physical property that affects sensory processes is stimulus size. Due to the retinotopic architecture of V1, large stimuli are processed in large cortical areas and small stimuli in small areas. If an electrophysiological parameter is directly affected by stimulus size, it appears save to conclude that it is directly modulated by physical stimulus properties. As an example, let us consider the study by Busch et al. (2006b) who used abstract stimuli that consisted of two areas, a small circular center and a large

surrounding. In keeping global stimulus size identical (the center area plus the surrounding area is of the same size for all stimuli), two experimental conditions with a small target (consisting of Tenofovir clinical trial the center area) and a large target (consisting of the surround area) were compared. In different blocks of trials either the center or the surround area was the target stimulus. In both conditions targets were defined by the orientation of one of two possible gratings. Thus, in both conditions, targets were defined by the same physical property. Busch et al. (2006b) observed that target size did not affect P1 amplitude size. Large and small targets elicited P1 amplitudes of identical magnitude. In a simple object (square vs. circle) discrimination task, Busch et al. (2004), however, found that P1 amplitude increases with increasing stimulus size, decreases with increasing eccentricity (stimuli presented more laterally elicit smaller P1 components) but is unaffected by exposure duration. The respective findings – as depicted in Fig. 4 – show in addition that for eccentric stimuli, P1 amplitudes are much larger over ipsilateral recording sites. Is this unequivocal evidence that the P1 is a sensory ERP component? Let us first consider the effect of stimulus size. In contrast to Busch et al.