Delay period microstimulation perturbs memory-guided saccade behavior
Statistically significant deviations from control were observed in 33 of 52
FEF sites, but only 8 of 43 LIP sites (two animals for each area). Suprisingly,
most of these effects were in the direction away from the recorded RF. Effects
were strongest for targets at adjacent directions (+/- 45 deg). In FEF
sites, effects did not depend on whether significant (p<0.05) memory activity
was recorded at the site (n=23). However, in LIP, the population of sites
where significant memory activity was recorded (n=17) showed a significant
deviation in a direction away from the RF that was also strongest for adjacent
target directions. No effects were observed in the population of LIP sites
where only visual responses were recorded.
Analysis of the direction of stimulation effects across all target locations
revealed that the systematic effect of stimulation for the population was better
explained by a vector antiparallel to the preferred direction observed at the
site (or the suprathreshold evoked direction) than a vector antiparallel to the
RF center. The fact that the deviation is not directly repulsive, but instead antiparallel,
suggests that stimulation may counterfeit an efference copy signal.
The circuitry may respond to subthreshold microstimulation as if a saccade
has been executed in the suprathreshold evoked direction. As a result, the
system 'remaps' the location of the target to compensate for the fictive eye
movement. The result is a deviation of the remembered location that is antiparallel
to the evoked vector.
These results demonstrate that delay period microstimulation is a useful
method for probing spatial information processing in the brain.
Robert L. White, III* and Lawrence H. Snyder
Neurons in the lateral intraparietal area (LIP) and the frontal eye field (FEF)
are believed to play a role in the planning and execution of saccades. During
a delayed saccade task, many neurons in these areas exhibit memory activity
(increased firing rates above baseline during the delay period) after a target
has been flashed in their visual response field (RF). To determine the relative
contribution of LIP and FEF in planning saccades, we performed delay period
microstimulation in each area to assess whether saccade behavior could
be altered by specifically perturbing memory-related neural activity during
the delay. Furthermore, we asked what type of effects microstimulation had
on saccade behavior. We hypothesized that microstimulation in either area
would systematically bias memory-guided saccades towards the RF of neuron(s)
recorded at the stimulation site.
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Presented at the 2004 Annual Meeting on Computational & Systems Neuroscience.