The specific goal of the proposed research is to quantify postnatal development of spatial orientation selectivity in the human visual system. Selective preference for oriented features is a unique characteristic of visual cortical neurons and possibly represents an important mechanism for increasing sensitivity to 2- dimensional images. Orientation selectivity is very broad or absent at birth in many species and its maturation probably involves increases in tonic inhibition between cortical cells (Tsumato & Sato, 1985). Very little is known about its developmental course in humans. A prolonged development is likely, based upon recent reports that response to gross alterations in orientation is not present until 6 postnatal weeks (Braddick et al, 1986). Orientation bandwidth, or range of selectivity, estimated through psychophysical means is similar to the average orientation bandwidth of cells in striate cortex. The orientation bandwidth metric may thus provide a relatively specific index of maturation in the virual cortex. The posposed studies will measure the developmental changes in the range of orientation specificity in infants between the ages of 6 and 36 weeks. Orientation bandwidth will be estimated using the adaptation/threshold-elevation method with the equivalent contrast transformation. Bandwidth estimates obtained with this procedure are independent of sensitivity differences (Movshon & Blakemore, 1973) and thus well-suited for developmental studies. The swept-contrast cortical VEP technique will be used to estimate contrast thresholds following adaptation. These developmental data will be supplemented with studies of adults which investigate in greater detail the psychophysical and VEP estimates of orientation selectivity. In order to characterize the 2-dimensional extent of the orientation-selective response, additional experiments will investigate the relationship between orientation and spatial frequency tuning.
