Abstract
Purpose: :
To study phosphene-inducing electric parameters of different type microelectrodes implanted into striate cortex of felines in chronic experiment.
Methods: :
Nine cats were used in the study. Prior to implantation, all animals were trained behavior food reflex - cat had to raise paw in response to light stimulation to get food. Stimulus represented 1-degree light source, simulating phosphene area. Five cats (group I) were implanted with 7-8 aureate surface electrodes, 1-mm in diameter, into left primary visual cortex. Layers 3-4 and 5-6 of their right visual cortex were implanted with 7-8 nichrome intracortical electrodes, 0.2 - 0.3-mm in diameter. Four cats (group II) were implanted with intracortical 16-channel (4x4 platinum electrodes, 50 µm in diameter) array into left striate cortex. Microelectrodes were placed 700-800 µm apart with penetrating depth of 1-3 mm. Connector was placed at frontal area above orbits. For post-operative electric stimulation both eyes were occluded with patch. In response to stimulation, cats were expected to raise paw as if their phosphene area was stimulated by light.
Results: :
Most cats raised paw in response to electric stimulus as when they were stimulated by light. Stimulus amplitude ranged within 1-100 µA for penetrating electrodes and 0.1-5.0 mA for surface electrodes. Impulse duration of both electrode types ranged within 0.1-4.0 ms. Impulse frequency ranged from 5 to 200 Hz. Stimulus duration was 1-3 seconds. Trained cats responded to electrical stimulus in a wide range of strength-duration parameters. Correct response was achieved in 70-100%. Most reliable response was observed with penetrating electrodes, at 20-100 µA stimulus amplitude and 0.5-1.0 ms impulse duration. Surface electrodes generated reflex behavior at much higher amplitudes: 1.0-4.0 mA.
Conclusions: :
Felines are highly organized mammals with similar to human structure-functional visual system and they are able to create reflex behavior in response to specific stimulus. Thus, feline model represents high value in evaluation of microelectrode array function in visual prosthetic device. We believe that reflex behavior developed by cats in response to electric stimulus delivered from microelectrode array is a response to phosphenes, similar to light stimulus response.
Keywords: clinical laboratory testing • vision and action • neuro-ophthalmology: cortical function/rehabilitation