Abstract
Purpose: We have previously shown that right parietal patients are impaired at performing attentive tasks of apparent motion and phase discrimination in both visual fields, not just the left field where they show other attention-related deficits. Left parietal patients performed normally in these tasks. We hypothesized that the right parietal lobe ought to play a selective role in tasks of visual timing. We studied a patient affected by medication-intractable, parietal lobe epilepsy, testing her before and immediately after her right inferior parietal lobe was removed in order to alleviate her seizure episodes. Method: We devised a battery of seven psychophysical experiments and tested the patient two days before and four days after surgery. We also did a follow-up testing at 9, 68 and 103 days post surgery. We tested her on low-level and high-level motion tasks such us apparent motion, visual tracking and biological motion. Furthermore we tested her on a phase discrimination experiment where six squares (three in each visual field) were flickering at the same temporal frequency and the target to be detected was flickering out of phase. The stimuli were reversed sinusoidally at temporal frequencies varying from 2–9 Hz. We used the method of adjustment and varied the temporal frequency progressively until the subject reported the target correctly. Results: The patient showed a severe loss in both hemifields in the phase discrimination task four days after surgery, while her low-level and high-level motion tasks were all within the normal limits immediately after surgery. She showed a significant improvement in the phase discrimination task 9 days post-surgery and she performed normally at 103 days post-surgery. Biological motion was mildly impaired at 4 days but recovered by 9 days post-surgery. Conclusion: Since the patient detected flicker normally, we conclude that the severe temporary deficit she showed immediately after surgery affects a higher level of processing possibly where attentional mechanisms assign transient onsets and offsets to the appearance and disappearance of objects.