Creating visual percepts with electrical stimulation of human visual cortex: Comparison with receptive fields mapped with local field potentials
Tuesday, Oct 16, 2012, 8:00 AM - 9:00 AM
, P. SUN
, I. M. SCHEPERS
, M. S. BEAUCHAMP
, D. YOSHOR
Dept. of Neurosurg., Baylor Col. of Med., Houston, TX;
Dept. of Neurobio. and Anat., Univ. of Texas Hlth. Sci. Ctr. at Houston, Houston, TX
A cortical visual prosthetic has the potential to provide useful visual perception to blind people by directly activating the visual cortex. Electrical stimulation of a single site in early visual cortex produces perception of a spatially restricted spot of light, known as a phosphene. Previously, we demonstrated that the receptive field (RF) of neurons in human visual cortex can be accurately mapped with local field potentials (LFPs) recorded with electrocorticography (Yoshor et al., Cerebral Cortex, 2007). We hypothesized that the RF location of neurons near an electrode (obtained from recording) should correspond to phosphene location (produced by electrical stimulation).
We studied 3 patients implanted with cortical surface electrodes, 52 of which were over visual cortex. First, we randomly presented 3° checkerboards in different locations in the visual hemifield contralateral to the implanted visual cortex. For each electrode, we used the amplitude of the LFP in a 200 ms time window for each checkerboard location to estimate the spatial RF of that electrode. Second, we stimulated each of the electrodes and found that 23 produced phosphene percepts. The average location and spatial extent of the phosphene was determined by asking the patient to draw the phosphene in each of 3 to 5 trials.
To compare the RF and the phosphene locations, we calculated the distance between the RF center and the phosphene center across electrodes. The mean distance was = 3.0° ± 2.3° SD. To determine if there was a consistent offset between the RF and the phosphene locations, we repeated the calculations using polar co-ordinates and obtained mean r = 0.3° ± 2.3° SD (not significantly different from zero, p = 0.54) and mean theta = -0.04° ± 1.1° SD (not significantly different from zero, p = 0.85).
To test if the RF map could be used to predict perception in a behaviorally useful way, we created a forward model of the predicted phosphene using the RF map. Two images were created, one from the forward model, the other from the forward model rotated by 90 degrees. Then, subjects performed a two-interval match to sample task. The percept to match to was produced by stimulation of the electrode, followed by two samples consisting of two images in random order, which were presented 1.5 seconds apart. The patient selected one of the two images as the best match to the phosphene percept using a mouse button press. The forward model image was selected as the target on 7 out of 8 trials (88%, p = 0.07 different from chance level by binomial distribution).
These results show that it is possible to use the spatial RF of an electrode site to predict the spatial form of a percept produced by electrical stimulation of that site.
VA Merit Award
[Authors]. [Abstract Title]. Program No. XXX.XX. 2012 Neuroscience Meeting Planner. New Orleans, LA: Society for Neuroscience, 2012. Online.
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