Little is known about the contribution of early components of the motion pathway to the analysis of direction and speed of motion. These issues will be examined by studying lesion effects on motion perception. Cats will be used in these studies since their retinal and cortical organization will allow us to eliminate individual components of the motion pathway and because properties of motion mechanisms in cat parallel those of man. We will examine the role of cortical areas 17 and 18 in the processing of direction and speed of motion, since differences in the anatomical connections and receptive field properties of these two areas suggest different processing of visual information, with area 18 playing a more important role in motion perception. Restricted ibotenic acid lesions will be made in physiologically identified portions of area 17 or 18 and contrast sensitivity measured for detecting moving gratings, and for discriminating their direction and speed of motion. These results will allow us to determine whether at these early stages of cortical processing, motion and pattern signals are processed separately and whether speed and direction signal are precessed by the same cortical area. We will also examine the contribution of the lateral suprasylvian (LS) cortex to motion perception. Neurons in this extrastriate cortical area are directionally selective, receive inputs from areas 17 and 18, and are therefore likely to represent a second state of motion processing. We recently showed with ibotenic acid lesions of LS that it is involved in the analysis of stimulus speed, but is not required to signal opposite directions of motion. We will determine which of the two major subdivisions (PMLS or PLLS) of LS cortex is critical for speed discrimination by testing cats with lesions restricted to one of these subdivisions. We will also examine the contribution of LS to higher level motion analysis by asking cats to detect coherent motion of dynamic random dot targets. The detection of coherence can only be accomplished by pooling motion information over space and time. These studies will provide important information concerning the role of this extrastriate cortical area, considered by many to be analogous to the primate MT cortex, in processing direction and speed of motion. Finally, we will examine the contribution of the Y-pathway to motion perception. Physiological properties of the Y-cells and their projections to cortical areas implicated in motion analysis (area 18 and LS cortex) suggest that they may play and important role in coding of stimulus motion. We will selectively eliminate Y-fibers by unilateral application of a controlled pressure block to the optic nerve of cats. The selectivity of the block will be evaluated physiologically by recording evoked potentials from the optic chiasm, and anatomically by examining HRP labelled normal and treated retinas. We will measure contrast sensitivity for detecting moving gratings, and discriminating their direction and speed, following selective degeneration of T-cell fibers. These studies will provide the first insights into the contribution of this likely analog of the M-pathways in primates to motion perception.
