| The Ocular Following response (OFR) is a pre-attentive, short latency, visual field holding reflex elicited when there is sudden shift of visual scene in the fronto-parallel plane. Prior studies have concerned the horizontal component of the responses in a small group of healthy human subjects. However, most natural gaze-shifts combine a change in the vertical direction and distance of the fixation point. Moreover, walking forward through the environment induces a strong, vertical optic flow. Therefore, I studied the vertical OFR in 12 healthy human subjects and found a consistent inverse relationship between the OFR amplitude and spatial frequency of the visual stimuli, despite inter-subject variability. Next, I studied the vertical OFR when it was induced during vergence eye movements; and found significant enhancement, typically 40%. I postulate that the increase of OFR during vergence movements reflects enhancement of early cortical motion processing, which serves to stabilize the visual field as the eyes approach their new fixation point. Recent studies have found that the neural pathways subserving OFR are shared with smooth pursuit pathway and so I compared OFR in 6 subjects with Progressive supranuclear palsy (PSP), a parkinsonian disorder with known vertical smooth pursuit deficits, with responses of healthy control subjects. I found that OFR is well-preserved in PSP patients, even thought there is substantial defect in the smooth tracking response. For neither normal subjects nor PSP patients was there any correlation between the OFR and smooth tracking, I propose that OFR is spared in PSP because it is generated by low-level motion processing that is dependent on posterior cerebral cortex, which is less affected in PSP. These findings suggest that further electrophysiological studies are called for to explore the dynamic role of the pontine nuclei, which are involved in PSP, on smooth tracking mechanisms. I also had the opportunity to study an individual with a long-standing defect of motion vision due to a focal lesion affecting posterior cerebral cortex (homologue of middle temporal visual areas, MT, in macaque) and documented, for the first time, that OFR was also impaired. Thus, these studies provide the first reports of the vertical OFR in normal subjects as well as in patients with defects of OFR. Taken together, these results are used to propose and test a hypothetical pathway subserving the human OFR. |
