It has long been recognized that the visual system can integrate information over space and over time. In fact, such integration is among the most-studied attributes of vision (Blake & Sekuler,
2006). Although early studies focused on spatio-temporal integration of low-level attributes such as luminance, integration extends also to higher level stimulus attributes, including motion direction (Watamaniuk, Sekuler, & Williams,
1989) and speed (Watamaniuk & Duchon,
1992), line length (Morgan et al.,
2000), size (Albrecht & Scholl,
in press; Brady & Alvarez,
2010; Chong & Treisman,
2003), and emotion of facial expressions (Haberman et al.,
2009; Haberman & Whitney,
2007). When multiple stimuli are presented within the receptive fields of different neurons, spatio-temporal activation patterns sometimes exhibit what can be described as “neural averaging,” with the resulting activation approximating an average of the neural responses evoked by the stimuli individually (Chelazzi, Duncan, Miller, & Desimone,
1998; Kastner et al.,
2001; Zoccolan, Cox, & DiCarlo,
2005). The influence of a Non-Target stimulus in our experiments can be thought of perceptual averaging, a behavioral correlate of neural averaging, on each trial, of responses evoked by the spatial frequency of the Target Gabor and by the spatial frequency of the Non-Target Gabor. In fact, perceptual averaging has been demonstrated with a range of stimuli and tasks (e.g., Albrecht & Scholl,
in press; Chong & Treisman,
2005; Parkes et al.,
2001). Sweeny, Grabowecky, Paller, and Suzuki (
2009) proposed that perceptual averaging depends upon the relationships among the receptive fields within which stimuli fall. In the case at hand, if Target and Non-Target Gabors were widely separated in space and therefore fell within distinct receptive fields, visual averaging would be foreclosed. As a result, the Non-Target stimulus would exert no influence on the Target stimulus. Some preliminary data from our laboratory are consistent with this hypothesis (Huang & Sekuler,
2010). Those data demonstrate that the influence of the Non-Target stimulus disappears when the Target and Non-Target are presented simultaneously but within different hemifields (∼5° apart).