Do the major retinal parallel processing streams play different roles in visual search

This research examined the role that the two major classes of retinal ganglion cells in primates, magnocellular-projecting and parvocellular-projecting cells, play in the prominent visuo-motor behavior that best characterizes primate vision, visual search. These ganglion cell classes provide the major input to the dorsal and ventral cortical processing streams, respectively, which are thought to play distinctive roles in eye movements. The project explored the hypothesis that the role of these early visual pathways in search can be predicted from the capabilities of the cortical streams to which they contribute. The dorsal cortical system, dominated by input through the magnocellular pathway, helps mediate the ability of primates to code and then recall the location of objects in extrapersonal space. The ventral cortical pathway, dominated by parvocellular cell input, is critical to the spatial allocation of attention, object recognition and visual processing in the central visual field.;The study found that magnocellular lesions severely disrupted visually-guided and memory-guided eye movements, including both search and non-search saccades. The most dramatic effects of magnocellular lesions involved substantial disruption of the accuracy of both search and non-search eye movements. One major finding, hypermetric saccades when targets were surrounded by distractors was specific to visual search, with no alteration found when no distractors were present. The study also found that magnocellular lesions severely disrupted both search and non-search eye movements when the task tested short-term memory for target location. These results support the hypothesis that the magnocellular pathway plays an important role in both search and non-search eye movements due to its intimate connection with the dorsal cortical pathway.;On the other hand, no changes in eye movements were found following parvocellular lesions. The parvocellular lesions were located in relatively central vision, from about 10 to 20 degrees eccentricity but no changes in either search or non-search eye movements were produced by these lesions. This result suggests that, unlike the important role the parvocellular pathway plays in visual perception, it plays little or no role in guiding eye movements.