The Biology

Neurophysiological studies have found that MST receives strong projection from the MT [46], [47], [48], and most of the neurons in the dorsal part of the the MST area (MSTd) respond selectively to different types of global optic flow stimuli. Listed below are the major response properties found by several neurophysiological studies ([49], [50], [51], [52], [53]).

• MST receives strong projection from MT, and the receptive fields (ranging from 10 to 100 degrees) of MSTd cells are much larger than that of the MT cells;
• MSTd cells respond selectively to different types of motion stimuli, (also called motion components): unidirectional planar motions (translations), clockwise and counter-clockwise circular motions (rotations), outward and inward radial motions (expansions and contractions), and clockwise or counter-clockwise, inward or outward circuloradial (spiral) motions. These optic flow patterns are illustrated below:

  

• There are three different types of MSTd neurons, called single, double and triple-component cells, which respond to one, two, or three motion components, respectively. The double-component cells can be planocircular, planoradial or circuloradial (spiral cells). The three types of neurons form a continuum of response characteristics. There are also neurons that do not respond to any of the motion components.

  

  

  

  

  

• Many MST cells show a sloping response profile (their responses are stronger for certain stimulus locations than at others), indicating that they are position dependent. Position independent response selectivity is most prominent in single-component cells, while position dependent response selectivity is most prominent in multiple-component cells.

• MSTd neurons respond selectively to radial and circular motion patterns with different locations of center of motion (COM). The distribution of their preferred COMs covers all areas of the visual field. Some neurons prefer COMs located in the central area of the visual field, some prefer COMs in the peripheral area. The responses of multi-component cells have different preferred areas in the receptive field for different types of motions. These preferred areas usually do not coincide.

• MSTd neurons responding most strongly to one of the 8 types of motions listed in the periodical spectrum in the second row of figure above also respond, though less strongly, to the neighboring motion patterns. The responses of these MSTd neurons to the 8 types of motions form a tuning curve which can be fitted with a Gaussian function.