Summation and segregation constitute two fundamental types of motion processing (Braddick,
1993; Burr & Thompson,
2011) and how the motion system implements their opposing requirements is not yet fully understood. Braddick (
1993) suggests that it could involve motion analysis at multiple spatial scales. Signals may be combined or differentiated within receptive fields that vary in size depending on the task at hand. Large motion-receptive fields could derive summation over extended areas to subserve perception of large uniform regions. Smaller receptive fields could generate strong signals at motion discontinuities and lead to the perception of segmented regions to assist with the detection of moving objects. Alternatively, the motion system may employ adaptive strategies to prevent summation over large, fixed areas so that the motion from two disparate objects does not inappropriately pool together. The motion system may adaptively switch between a mode that extensively pools motion signals and one that establishes segmentation, depending on the task (Allman, Miezin, & McGuinness,
1985a; Braddick,
1993; Maunsell & Van Essen,
1983; Movshon & Newsome,
1996; Raiguel, Hulle, Xiao, Marcar, & Orban,
1995; Smith, Singh, Williams, & Greenlee,
2001). Warren (
2004) suggested that the units in the visual system that are responsible for detection of optic flow motion (self-motion relative to the environment) are not involved in the segregation of moving objects because they fail to differentiate local object motion and optic flow, thus indicating a task-specific cortical mechanism that is responsible for the analysis and summation of large field motion. Indeed, there is evidence to suggest that medial superior temporal (MST) area possesses distinct functional regions. The dorsal part of MST in macaque is responsible for analysis of wide-field motion, whereas the ventral part is implicated in the analysis of small-field motion (Tanaka et al.,
1986; Tanaka, Sugita, Moriya, & Saito,
1993). This property may underlie how the visual system copes with summation and segregation, consistent with Braddick's (
1993) proposals. The spatial properties of the two regions support this idea as MSTd cells have relatively large receptive fields whereas MSTv cells have smaller receptive fields (Nelissen, Vanduffel, & Orban,
2006) and possess antagonistic surrounds (Tanaka et al.,
1986), which are thought to facilitate motion segregation (Allman et al.,
1985a), possibly by adaptively modulating the area of summation. There is also neurophysiological (Churan, Khawaja, Tsui, & Pack,
2008; Hunter & Born,
2011; Pack, Hunter, & Born,
2005) and behavioral evidence (Tadin, Lappin, Gilroy, & Blake,
2003) that the summation area of MT neurons is not fixed and can vary depending on stimulus conditions. Further, many neurons in area MT exhibit center-surround antagonism (Allman et al.,
1985a), suggesting that processing within MT also involves distinct mechanisms equipped for segregation, similar to those found in MST. More recently, Lui, Dobiecki, Bourne, and Rosa (
2012) have provided evidence consistent with separate systems in MT of marmoset monkeys, one which restricts summation over small areas and one which can extend integration over larger areas.