Towards the cortical representation of form and motion stimuli generated by a retina implant

A retina implant for restoring basic visual perception in patients who are blind due to photoreceptor loss should not only evoke focal phosphenes at high resolution, but should also generate cortical representations of form and motion. We are currently exploring these potential capabilities in anaesthetised cats.

Fibre electrodes were inserted through a small scleral incision onto the retinal surface for stimulation. For the recording of cortical population activities we placed up to 16 fibre electrodes in areas 17 and/or 18. Retinal and cortical electrodes were adjusted to corresponding sites, i.e., overlapping receptive fields. Electrical stimuli were charge-balanced impulses (200 micros, 10-100 microA). Basic form stimuli were generated by the selective and synchronous activation of some of the seven retinal stimulation electrodes. Movement stimuli were generated by spatio-temporal shifting of form stimuli. From multiple microelectrode recordings we computed stimulus-related spatio-temporal cortical activation profiles. We used these profiles to estimate the relations between stimulation distance and spatial resolution (form) and between stimulus velocity and spatio-temporal resolution (movement). Influences by the retino-cortical pathway were assessed by comparing cortical activations evoked by true form or motion stimuli with synthetic responses composed by superpositioning of responses to appropriate subsets of form and motion stimuli. In addition, we compared cortical responses to form and motion stimuli by a receptive-field-based backprojection of cortical activities.

We confirmed our previous finding that electrical retina stimulation may yield a spatial resolution of 1-5 degrees visual angle and a temporal resolution of about 20 ms. We found that the spatio-temporal cortical activation profiles are commonly related to retinal form and motion stimuli. Cortical activity analyses showed that for two-point form stimuli the neuronal interaction depends on the stimulation electrodes' distance and that local cortical group activities can exhibit some tuning to the directions or the velocities of moving electrical bars'. Projections of cortical activations to visual space were consistent with electrical form and motion stimulation of the retina.

Our data indicate that retinal stimulation with electrical form and motion stimuli can lead to spatio-temporally related cortical activations. However, the selective activation of single cortical neurones with specific visual tuning properties by electrical retina stimulation and the potential adaptation of the visual system to long-term stimulation with retina implants should be addressed in future work.