Blindsight in subjects with homonymous visual field defects

Brain damage in the visual system can lead to apparently blind visual areas . However, more elaborate testing indicates that some visual ability may st ill exist for specific stimuli in the otherwise blind regions. This phenome non is called "blindsight" if subjects report no conscious awareness of vis ual stimuli but when forced to guess, nevertheless perform better than chan ce. It has mainly been suggested that secondary visual pathways are respons ible for this phenomenon. However, no published study has clearly shown the neural mechanism responsible for blindsight. Furthermore, experimental art ifacts may have been responsible for the appearance of the phenomenon in so me subjects. In the present study, the visual fields of nine subjects were mapped and residual visual performance was examined in many areas using thr ee different experimental procedures. Artifacts such as stray light or eye movements were well controlled. In addition, confidence ratings were requir ed after each trial in the forced-choice tests. The results show that only one subject with a lesion in the optic radiation had blindsight in two disc rete areas of the affected visual field. Spared optic radiation fibers of t he main (primary) geniculo-striate visual pathway were most likely to accou nt for this finding.