# Show Reference: "Population coding of saccadic eye movements by neurons in the superior colliculus"

Population coding of saccadic eye movements by neurons in the superior colliculus. Nature, Vol. 332, No. 6162. (24 March 1988), pp. 357-360, doi:10.1038/332357a0 by Choonkil Lee, William H. Rohrer, David L. Sparks
@article{lee-et-al-1988,
abstract = {The deeper layers of the superior colliculus are involved in the initiation and execution of saccadic (high velocity) eye movements. A large population of coarsely tuned collicular neurons is active before each saccade. The mechanisms by which the signals that precisely control the direction and amplitude of a saccade are extracted from the activity of the population are unknown. It has been assumed that the exact trajectory of a saccade is determined by the activity of the entire population and that information is not extracted from only the most active cells in the population at a subsequent stage of neural processing. The trajectory of a saccade could be based on vector summation of the movement tendencies provided by each member of the population of active neurons or be determined by a weighted average of the vector contributions of each neuron in the active population. Here we present the results of experiments in which a small subset of the active population was reversibly deactivated with lidocaine. These results are consistent with the predictions of the latter population-averaging hypothesis and support the general idea that the direction, amplitude and velocity of saccadic eye movements are based on the responses of the entire population of cells active before a saccadic eye movement.},
author = {Lee, Choonkil and Rohrer, William H. and Sparks, David L.},
day = {24},
doi = {10.1038/332357a0},
issn = {0028-0836},
journal = {Nature},
keywords = {biology, eye-movements, eye-saccades, motor, population-coding, sc},
month = mar,
number = {6162},
pages = {357--360},
pmid = {3352733},
posted-at = {2013-02-11 11:03:37},
priority = {2},
publisher = {Nature Publishing Group},
title = {Population coding of saccadic eye movements by neurons in the superior colliculus.},
url = {http://dx.doi.org/10.1038/332357a0},
volume = {332},
year = {1988}
}


The direction of a saccade is population-coded in the SC.

There exist two hypotheses for how saccade trajectory is population-coded in the SC:

• the sum of the contributions of all neurons
• the weighted average of contributions of all neurons

The difference is in whether or not the population response is normalized.

According to Lee et al., the vector summation hypothesis predicts that any deactivation of motor neurons should result in hypometric saccades because their contribution is missing.

According to the weighted average hypothesis, the error depends on where the saccade target is wrt. the preferred direction of the deactivated neurons.

Lee et al. found that de-activation of SC motor neurons did not always lead to hypometric saccades. Instead, saccades where generally too far from the preferred direction of the de-activated neurons. They counted this as supporting the vector averaging hypothesis.