Microstimulation

Results from neurophysiological experiments vary on whether neuronal activity in earlier or later stages of visual cortex correlates more closely with the behavioral choices that animals make in visual tasks. To examine how readily changes in activity in different stages of visual processing can influence perception, we measured behavioral thresholds for detecting electrical microstimulation in multiple visual cortical areas.

Monkeys were trained to do a temporal two alternative forced choice task. During each trial, the animal held its gaze on a white spot centered on an unstructured gray computer display during two 250 ms periods. A stimulus was presented during one of the two periods. At the end of each trial, the animal indicated which period contained the stimulus by making a saccade to one of two response targets. An eccentric, low-contrast visual stimulus was used for training, and was replaced with a train of cortical microstimulation during data collection (constant current 200 µs biphasic pulses at 200 Hz for 250 ms). We randomly varied the current across 6-8 levels between trials. Behavioral threshold was taken as the current corresponding to 82% correct performance on a best-fitting psychometric function.

Over 50 stimulation sites were tested in each of several areas: V1, V2, V3A, MT and anterior inferotemporal cortex (IT). After an initial learning period of a few days for each area, electrical stimulation produced reliable thresholds at every site in each area. Mean thresholds in µA were 5.9 for V1, 7.2 for V2, 8.1 for V3A, 9.2 for MT and 12.2 for IT. Thresholds were similar in both animals

While the mean threshold for detecting microstimulation differs between areas, the change is modest. This suggests that different areas in visual cortex are comparable in their ability to contribute to detectable signals.

In similar experiments we have found that thresholds for detecting microstimulation of sites in the frontal eye fields are comparable to those found in visual cortex. This raises the possibility that thresholds vary little across all of neocortex. We have recent begun experiments in which we are microstimulating human cerebral cortex through intracranial surface electrodes.

2005 Society for Neuroscience Poster on Microstimulation of Visual Cortex

2006 Society for Neuroscience Poster on Microstimulation of the Frontal Eye Fields