Responses to continuously changing optic flow in area MST
M. Paolini, C. Distler, F. Bremmer, M. Lappe & K.-P. Hoffmann
Journal of Neurophysiology, 84:730-743, 2000
- We studied the temporal behavior and tuning properties of MST neurons in response to con- stant fow feld stimulation
and continuously changing fow feld stimulation (transitions), obtained by morphing one fow feld into another. During
transitions, the fow felds resembled the motion pattern seen by an observer during changing ego-motion. Our aim
was to explore the behavior of MST cells in response to changes in the fow feld pattern and to establish if the
responses of MST cells are temporally independent or if they are affected by contextual information from preceding
stimulation. We frst tested whether the responses obtained during transitions were linear with respect to the two
stimuli defning the transition. In over half of the transitions, the cell response was non-linear: the response
during the transition could not be predicted by the linear interpolation between the stimulus before and after
the transition. Non-linearities in the responses could arise from a dependence on temporal context or from non-linearities
in the tuning to fow feld patterns. To distinguish between these two hypotheses, we ft the responses during transitions
and during continuous stimuli to the predictions of a temporally independent model (temporal-independence test)
and we compared the responses during transitions to the responses elicited by inverse transitions (temporal-symmetry
test). The effect of temporal context was signifcant in only 7.2% and 5.5% of cells in the temporal-independence
test and in the temporal-symmetry test, respectively. Most of the non-linearities in the cell responses could be
accounted for by non-linearities in the tuning to fow feld stimuli (i.e., the responses to a restricted set of
fow felds did not predict the responses to other fow felds). Tuning non-linearities indicate that a complete characterization
of the tuning properties of MST neurons cannot be obtained by testing only a small number of fow felds. Because
the cells' responses do not depend on temporal context, continuously changing stimulation can be used to characterize
the receptive feld properties of cells more effciently than constant stimulation. Temporal independence in the
responses to transitions indicates that MST cells do not code for second-order temporal properties of fow feld
stimuli, i.e. for changes in the fow feld through time which can be construed as paths through the environment.
Information about ego-motion 3D paths through the environment may either be processed at the population level in
MST or in other cortical areas.
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