THE PROCESSING OF FIRST-ORDER AND 2ND-ORDER MOTION IN HUMAN VISUAL-CORTEX ASSESSED BY FUNCTIONAL MAGNETIC-RESONANCE-IMAGING (FMRI)

We have examined the activity levels produced in various areas of the human occipital cortex in response to various motion stimuli using fun ctional magnetic resonance imaging (fMRI) methods. In addition to stan dard luminance-defined (first-order) motion, three types of second-ord er motion were used. The areas examined were the motion area V5 (MT) a nd the following areas that were delineated using retinotopic mapping procedures: V1, V2, V3, VP, V3A, and a new area that we refer to as V3 B. Area V5 is strongly activated by second-order as well as by first-o rder motion. This activation is highly motion-specific. Areas V1 and V 2 give good responses to all motion stimuli, but the activity seems to be related primarily to the local spatial and temporal structure in t he image rather than to motion processing. Area V3 and its ventral cou nterpart VP also respond well to all our stimuli and show a slightly g reater degree of motion specificity than do V1 and V2. Unlike V1 and V 2, the response in V3 and VP is significantly greater for second-order motion than for first-order motion. This trend is evident, but less m arked, in V3A and V3B and absent in V5. The results are consistent wit h the hypothesis that first-order motion sensitivity arises in V1, tha t second-order motion is first represented explicitly in V3 and VP, an d that V5 land perhaps also V3A and V3B) is involved in further proces sing of motion information, including the integration of motion signal s of the two types.