Hi Serge,

1. What kind of stimulation protocol are you using? it is possible that you are activating a very large neural population and the monkey ""see"" a big phosphene not clearly spatially localized. 

2. Are you sure the electrode is not moving? Are you compering the RF measured at the beginning and at the end of the stimulation?

3. Are you sure the monkey understood the task? I guess a phosphene can be a very strange stimulus for a monkey. It could just try to make a saccade away from it.

good luck

But why would you expect an electrical stimulus to cortex, likely to stimulate all sorts of cells and networks not involved in saccades, to produce an effect like a stimulus to the motor map in the SC? Have you compared experiments stimulating different sites (eg V1 vs LIP vs FEF vs SC)? A visual perception is not the same as a visual target (ie same point as 3 above).

The phosphene would be static in retinal coordinates, i.e. moving with the eye movement? That's different to the controlled visual stimulation in world coordinates (probably = head coordinates).

Thanks everyone for replies!

Giacomo,

1) I'm using 1 mA, 100 Hz, and 10 pulses per train. Stimulation is done through chronically implanted surface electrodes. The typical thresholds for studies in Brindley et. al and Dobelle et al. were 1-3 mA. So my current is rather at the lower border. Also, the electrodes I'm using are around 0.5 mm in diameter. This corresponds to about 0.3 degrees on the V1's surface at the eccentricity the electrode is located at. So there are reasons to believe that the activated population processes rather a small area in the visual field.

But this is actually an interesting idea. I never looked how large, in terms of visual space, was the visual space covered by electrodes in human studies. Knowing this along with the reported size of phosphenes would hint on the spread of current from the electrode. 

2) The electrode's movement should be fairly limited to that caused by pulsation of blood vessels under it. Do you think this can create enough disturbance to create strange effects in response?

3) In the task with visual stimuli I used all sorts of strangely looking blobs that somewhat similar to verbal phosphene descriptions of human studies. The idea behind showing such strange visual stimuli was to prepare him to respond to whatever strange things he sees. The monkey successfully made saccades to all these stimuli. Do you know anything about whether animals do treat phosphenes very differently from real light-caused sensations?

Paul,

You brought up a very non-intuitive point to me. This one is very interesting. If I understood you right, the follow up to this idea would be a suggestion that the monkey didn't care about the visually perceived identity of the visual targets. The responses to it were rather caused by a direct visual-to-motor activation, which can remain inactivated through direct stimulation of V1. Do you think it would be right, as a solution to this, to incorporate a delay into the original task with visual targets. Such that there is a delay between the moment the visual target goes off and the response with a saccade is allowed?

Peter,

That's right. The phosphenes will be moving with his eyes. But in other studies on V1 phosphenes they never reported any response issues related to this. How, do you think it can explain his tendency to respond to other-than-receptive-field locations?

Dear Serge,

The following papers might help you:

Tehovnik et al. (2003) Eur. J. Neurosci. 17, 870-878.

Tehovnik et al. (2003) Eur. J. Neurosci. 18, 969-979.

Tehovnik and Slocum (2004) Neuroscience and Biobehavioral Reviews 28, 13-25.

These papers can be found on: https://www.researchgate.net/profile/Edward_Tehovnik

Hope this help and if you have any further questions you can e-mail me at:

[email protected]

Ed