Spatial and temporal visual resolution with retina implants estimated from recordings in striate cortex of cats
Reinhard
Eckhorn¹, T. Schanze¹, M. Wilms¹, M. Eger¹, L. Hesse²,
P. Kroll² (D-Marburg)
¹Group of
NeuroPhysics, Department of Physics, Renthof 7
²Department of Ophthalmology, Robert-Koch-Str. 4
Philipps-University, D-35032 Marburg, Germany
Blind persons with photoreceptor degeneration and intact retinal ganglion
cells can perceive phosphenes elicited by intraocular electrical stimulation.
For ethical reasons we avoid explorative experiments to determine perceptual
resolutions feasible with electrical retina stimulation in blind humans.
Instead we use the anesthetized cat as an animal model. It allows to estimate
the spatial and temporal resolutions achievable with retina implants by
electrical retinal micro-stimulation and simultaneously recording of evoked
cortical activations with arrays of micro-electrodes in visual cortical
area V1 (and V2). Spatial resolution: We determine the cortical activation
spread from multiple-site recordings of responses to a single focal retinal
stimulus. Then, the cortical activity profile is transformed to visual
coordinates according to the local retino-cortical magnification factor.
Alternatively, we use multi-focal retina-stimulation and single-site cortical
recording in order to determine the size of cortical electrical receptive
fields, thereby extending the concept of visual receptive fields. Temporal
resolution is determined from the time course of the cortical responses.
After optimizing these methods we obtained spatial resolutions of about
1° visual angle (minimal 0.66°). This resolution may enable many
practical capabilities in everyday life. The measured temporal resolutions
may allow the perception of 20 to 30 picture frames per second if two-level
contrast is sufficient (during analyses of dynamic scenes). If 2 to 3
frames per second suffice, as during normal fixation sequences, about
10 levels of contrast could be encoded. However, we have to keep in mind
that our resolution estimates were made in anesthetized animals with intact
retinae. Whether patients with degenerated photoreceptors will perceptually
achieve similar values can only be determined by direct tests in direct
dialog with patients.