Different loci suggested to mediate tilt and spiral motion aftereffects.

Interocular transfer of two figural aftereffects was examined in orthotropes and strabismic subjects. Within both groups there were persons with an appreciable degree of stereoacuity and others who had little or none. Experimental evidence from a variety of sources has suggested that stereopsis depends upon binocularity of units in the geniculostriate system. For the tilt aftereffect, interocular transfer correlated with stereoacuity among both orthotropes and strabismics. For the spiral motion aftereffect, interocular transfer did not correlate with stereoacuity; it was present among orthotropes and absent among strabismic individuals. The correlation of stereoacuity with interocular transfer of the tilt aftereffect agrees with previous observations and is consistent with the interpreation that this effect is mediated at a cortical level. The lack of a correlation between stereoacuity and interocular transfer of the spiral motion aftereffect suggests that this effect is mediated by units other than those responsible for stereopsis. Richards and Smith, on the basis of certain phenomenal differences between the spiral motion aftereffect and other figural aftereffects, have suggested that the former is mediated at a midbrain level. If strabismic persons lack binocular units in midbrain, the present results are consistent with their hypothesis.