Adaptation of Gaze to Eccentric Fixation in Homonymous Hemianopia

Zangemeister WH, Dannheim F, Kunze K: In: Adaptive Processes in Visual and Oculomotor Systems, ed. By EL Keller and D Zee. Adv. in Biosci. 1986; 57: 247 252, Elsevier Amsterdam New York

Patients with homonymus hemianopia (HH) display typical disabilities to acquire visual targets in their blind hemifield. In a HEAD FIXED situation they develop characteristic strategies of ocular motor control to overcome the HH defect. In a more natural HEAD FREE situation they show additional difficulties to cope with their HH defect. These include stairstep and one-step overshoot- backdrift saccadic strategies for target acquisition in the blind hemifield (BHF), which is also used in a head fixed condition, these patients demonstrate a significant side asymmetry of compensatory eye movements (CEM); CEM gain i.e. ratio of eye to head velocity is increased for gaze shifts to BHF and decreased for gaze shifts to SHF (seeing hemifield). Often this may cause large ‘maladaptive’ gaze errors. HH patients therefore tend to minimize or avoid head movements. Experimentally normal subjects can be forced to use a similarly deranged behavior of ocular motor control. When normal subjects (Ss) learn to achieve eccentric gaze fixation using secondary visual feedback (2ndVFB), i.e. gaze position is displayed simultaneously with the target, they demonstrate similar strategies to superpose target and 2ndVFB—target. Initially they use stairstep saccades and macrosaccadic square wave oscillations that were earlier described as transient responses to achieve and maintain eccentric fixation. After some training they became adapted to the task and now used slow and fast drifts to achieve eccentric fixation; they use microsaccadic square wave jerks or a nystagmus like pattern of small eccentric saccades with following backdrifts to maintain eccentric fixation. We reasoned that early adaptation to and use of eccentric fixation could help hemianopic patients to cope better with their disability. Also they might be able to make better use of their head movements, which they otherwise avoid . To test this hypothesis we compared gaze responses of three normal subjects and three patients with full dense homonymous hemianopia. We compared first their initial transient responses to the task of eccentric fixation using 2ndVFB. Then, after same training in head fixed and head free situations, we looked for their adaptive responses. The results were compared with earlier findings.