Determination of an animal's heading is an important function of the extrastriate cortex and is vital for useful interpretation of the world around them. Models of heading determination have suggested that analysis of the optic flow field caused by an animal's motion can be used to derive heading using coarse population codes, in a population of neurons each of which integrates optic flow information over an extended area (Graziano, Andersen, & Snowden,
1994; Lappe & Rauschcker,
1993; Page & Duffy,
2003; Perrone & Stone,
1998). Area MST has been widely proposed as the site for such analysis of optic flow, and microstimulation of MST has been shown to affect heading judgments (Britten & van Wezel,
1998). However, in many situations heading corresponds to the center of the expanding motion of the optic flow field, and in these situations there may be relatively local operations that can determine heading more simply and more accurately by locating this center of motion. When heading does not correspond to the center of motion (i.e. when the animal is not looking in the direction of their heading), it can only be discriminated with an accuracy of 1.5–3° (Britten & Van Wezel,
2002).
In the present study we examine observers' ability to locate the center of an optic flow pattern, compared to their sensitivity for detecting the presence of a globally coherent flow pattern. The centers of motion of rotating, expanding and contracting motions are perceptually very obvious, and our data show that their locations can be discriminated very accurately when fixated.
Neurons in macaque MSTd respond to rotating, expanding and contracting patterns of motion (Graziano et al.,
1994), have very large receptive fields (Duffy & Wurtz,
1991a) reflecting considerable spatial summation, and many give position-invariant responses to optic flow stimuli (Duffy & Wurtz,
1991b; Graziano et al.,
1994). Human psychophysics of global motion processing shows considerable spatial summation for radial and rotational patterns (Burr, Morrone, & Vaina,
1998), and fMRI has shown these patterns to activate the human MT complex (MT+) (Morrone et al.,
2000), so it is likely that human MT+ performs a similar role to macaque MSTd. Indeed, there is fMRI evidence for an optic flow-specific region of human MT+, perhaps a human homologue of MST (Huk, Dougherty, & Heeger,
2002; Morrone et al.,
2000; Smith, Wall, Williams, & Singh,
2006).
The detection of globally coherent patterns, and the localization of their centers, also arises for static equivalents of optic flow. Sensitivity to global form patterns has been found in macaque V4, whose neurons respond preferentially to radial and concentric gratings (Gallant, Braun, & Van Essen,
1993; Gallant, Connor, Rakshit, Lewis, & Van Essen,
1996), show significant position invariance (Gallant et al.,
1996), and have large receptive fields (Desimone & Schein,
1987). Human psychophysics of global form processing also shows large-scale spatial summation of concentric (Wilson, Wilkinson, & Asaad,
1997) and radial (Wilson & Wilkinson,
1998) patterns, while fMRI shows these patterns to activate an area of human cortex which may be homologous with V4 (Wilkinson et al.,
2000).
In this study we investigate a series of tasks involving the localization (position discrimination) of the centers of radial and circular motion and form patterns within the visual field, and compare these to tasks involving detection of the same patterns. The use of coherence thresholds as a common metric in both tasks allows a direct comparison of the variables affecting performance. The results show that many of the properties associated with detection tasks, particularly summation over an extensive area of the visual field, are not seen for fine position discrimination tasks. Rather, fine position discrimination appears to require very local processing.