There is a substantial number of studies investigating how the primate brain performs visual search tasks, often focusing on cortical structures (e.g., Bichot, Rossi, & Desimone,
2005, Chelazzi, Miller, Duncan, & Desimone,
1993). While it was speculated for some time that only animals with large a neocortex may have mechanisms of visual search, it now seems clear that a structure like a neocortex is not necessary for pop-out sensitivity. But what then are the minimal requirements? Clearly, the responses within the classical receptive field of neurons are not enough; there must also be
interactions between cells beyond the classical receptive field. In fact, the saliency model of Li (
2002) proposes that horizontal connections between neurons in V1 provide enough contextual information to mediate the saliency of a stimulus. In birds, such substrates may be found in the the avian visual Wulst, which resembles in many respects the mammalian visual cortex (Nieder & Wagner,
1999; Pettigrew & Konishi,
1976; Wagner & Frost,
1993). While not much is known about lateral connections in the visual Wulst of barn owls, and the Wulst is not layered as the mammalian cortex, the experiments of Nieder and Wagner (
1999) on cognitive contours demonstrate a high level of connectivity which may also underlie pop-out, while its rich connectivity to other areas may also facilitate the interaction of bottom-up and top-down mechanisms for visual attention in general, (Connor, Egeth, & Yantis,
2004). However, no studies related to pop-out sensitivity have been conducted in the visual Wulst. Typical candidate areas for visual search are also those involved in saccade control and target selection, such as the cortical lateral interparietal cortex and frontal eye fields, or the midbrain superior colliculus (Bichot, Schall, & Thompson,
1996; Bisley & Goldberg,
2010; Fecteau & Munoz,
2006; Shen, Valero, Day, & Paré,
2011). The avian homologue of the superior colliculus is the optic tectum. Neurons in the optic tectum are sensitive to the intensity of stimuli in their receptive fields but less selective for individual features (Knudsen,
1982). Its involvement in stimulus competition by inhibition of weaker stimuli suggests a role in saliency computation, as well (Mysore, Asadollahi, & Knudsen,
2010,
2011). Indeed, some pop-out–like sensitivity has been observed in tectal cells of barn owls (Zahar et al.,
2012). That is, tectal cells were sensitive to contrasting motion stimuli. However, the data of Zahar et al. (
2012) did not show pop-out sensitivity in orientation as was found here. Therefore, it seems that more interactions than those present in the optic tectum are necessary to create pop-out sensitivity for orientation. More experiments are clearly necessary to find out what that substrate is and, perhaps more interestingly, what may be the minimal circuitry that can support pop-out sensitivity.