Abstract
Composition and Topographic Organization of Signals Sent From the
Frontal Eye Field to the Superior Colliculus. J. Neurophysiol. 83:
1979-2001, 2000. The frontal eye field (FEF) and superior colliculus
(SC) contribute to saccadic eye movement generation, and much of
the FEF's oculomotor influence may be mediated through the SC. The
present study examined the composition and topographic organization
of signals flowing from FEF to SC by recording from FEF neurons that
were antidromically activated from rostral or caudal SC. The first
and most general result was that, in a sample of 88 corticotectal
neurons, the types of signals relayed from FEF to SC were highly
diverse, reflecting the general population of signals within FEF
rather than any specific subset of signals. Second, many neurons
projecting from FEF to SC carried signals thought to reflect cognitive
operations, namely tonic discharges during the delay period of a
delayed-saccade task (delay signals), elevated discharges during
the gap period of a gap task (gap increase signals), or both. Third,
FEF neurons discharging during fixation were found to project to
the SC, although they did not project preferentially to rostral SC,
where similar fixation neurons are found. Neurons that did project
preferentially to the rostral SC were those with foveal visual responses
and those pausing during the gap period of the gap task. Many of
the latter neurons also had foveal visual responses, presaccadic
pauses in activity, and postsaccadic increases in activity. These
two types of rostral-projecting neurons therefore may contribute
to the activity of rostral SC fixation neurons. Fourth, conduction
velocity was used as an indicator of cell size to correct for sampling
bias. The outcome of this correction procedure suggested that among
the most prevalent neurons in the FEF corticotectal population are
those carrying putative cognitive-related signals, i.e., delay and
gap increase signals, and among the least prevalent are those carrying
presaccadic burst discharges but lacking peripheral visual responses.
Fifth, corticotectal neurons carrying various signals were biased
topographically across the FEF. Neurons with peripheral visual responses
but lacking presaccadic burst discharges were biased laterally, neurons
with presaccadic burst discharges but lacking peripheral visual responses
were biased medially, and neurons carrying delay or gap increase
signals were biased dorsally. Finally, corticotectal neurons were
distributed within the FEF as a function of their visual or movement
field eccentricity and projected to the SC such that eccentricity
maps in both structures were closely aligned. We conclude that the
FEF most likely influences the activity of SC neurons continuously
from the start of fixation, through visual analysis and cognitive
manipulations, until a saccade is generated and fixation begins anew.
Furthermore, the projection from FEF to SC is highly topographically
organized in terms of function at both its source and its termination.
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