The contribution of the central retina to the laser speckle visual evoked response.

Laser speckle stimuli were used to examine the contribution of the central retina to the visual evoked response (VER). The foveally fixated test stimuli were either small circular spots or annular targets with dark centers. The speckle elements were shifted seven times per second. Reliable responses could be recorded for even the smallest (1 degree) target size. In addition, it was clearly evident that for spots up to 5 degree in diameter, VER amplitude increased linearly with stimulus area. However, data from annular targets revealed that this result was not strictly dependent upon stimulus area. In many instances, large annular stimuli evoked responses with smaller amplitude than spot stimuli with less area. This result may be due to a center-surround antagonistic interaction. These experiments suggest a potential value for laser speckle VER techniques in the assessment of discrete lesions of the macula.