Gaze-centered first-order analysis of optic flow
based on spatiotemporal filtering of cortical flow fields:
A novel view of MST functional properties.

Silvio P. Sabatini, Fabio Solari, Roberto Carmeli, Paolo Cavalleri and Giacomo M. Bisio
DIBE - University of Genoa, Via Opera Pia 11a, 16145 Genova, ITALY

Medial superior temporal (MST) cells are probably optimally suited to the analysis
of visual motion in terms of first-order elementary flow components
(EFCs).  To investigate the neurophysiological basis of this perceptual property,
we propose a functional model of the motion pathway from the middle temporal
(MT) cells to MST cells. At MT level, a wave-like spatiotemporal activation
function (cortical flow) is introduced as a physicalist representation of the optic
flow which preserves its qualitative properties.  At MST level spatiotemporal
filtering operations on this activation function approximate a template matching
between the optic flow stimulus and the cell preferences to EFCs.  We adopt a
gaze-centered representation, in which the optic flow is analyzed in terms of
how it is related to the fixation point.  Each MST model cell is selective to an
EFC referred to the fixation point and is characterized by a specific tuning to the
mean speed of motion in optic flow stimuli.  For each MST unit the best stimulus
is centered on the fovea, though, different stimulus configurations can trigger a
cell response provided that the subpattern of the optic flow locally matches the
cell's preference for motion component and speed. Observed physiological
properties relating MST cell response to the placement of optic flow stimuli in
the visual field are discussed from this new (gaze-centered) perspective.