David L. Sparks - US grants

A critical question that must be answered before we understand the control of sensoryguided saccades is that of how the spatial (anatomical) code of saccade direction and amplitude contained in the superior colliculus (SC) is transformed into the temporal signals required by motoneurons for the generation of saccadic movements. The long-range goals of the proposed research are directed toward the solution of this improtant problem. Chronic unit recording, microstimulation , and reversible deactivation methods will be used to study the role of the SC and brainstem recipients of inputs from the SC in the control of saccadic eye movements. More specifically, the goals of the project are to: a) Perform a detailed analysis of the alterations in saccade direction and amplitude occurring during reversible deactivation of small regions of the SC before and after unilateral removal of the frontal eye fields (FEFs). b) Characterize the saccaderelated discharges of neurons in tectorecipient areas by: 1) plotting the movement fields of neurons displaying saccaderelated activity; 2) quantifying the relationship between saccaderelated neural activity and saccadic velocity (vectorial velocity as well as component velocity); 3) determining the effects of varying the initial fixation position upon saccaderelated bursts; and 4) determining the effects of deactiviation of neurons in the SC upon the discharge of neurons with saccaderelated activity. c) Examine the connections between SC and neurons in PPRF, cMRF and NRTP using orthodromic and antidromic stimulation methods. d) Determine whether or not specific functional classes of neurons isolated in the PPRF, cMRF and NRTP receive inputs from specific types of collicular neurons using dual recording methods and cross correlation analysis. e) Study the effects of localized reversible deactiviation of small regions in the PPRF, cMRF and NRTP upon the latency, duration, direction, velocity, and amplitude of saccadic eye movements.

Spatial perception and sensory-guided movements are thought to depend upon several different neural representations of the external environment. Recent studies of the oculomotor system suggest that there are at least three neural representations of a visual target to which a saccade is made: representations in retinal, spatial (head or body) and motor coordinates. The major purpose of the experiments outlined in this proposal is to systematically search for a neural representation of the visual environment that is organized in spatial coordinates. The techniques to be used (chronic unit recording, microstimulation and mapping of metabolic activity) were selected to maximize the likelihood of identifying regions of the CNS that are involved in coding the spatial location of objects in the external environment. Other experiments are proposed that will a) obtain a behavioral estimate of the duration and precision of memory for the spatial location of a visual target; b) study the relationship between neuronal activity in the primate superior colliculus and performance on this spatial memory task; and c) examine the neural mechanisms involved in selecting one of several spatially-localized targets as the goal for a motor response. Findings obtained from these studies should contribute, significantly, to our information about the brain areas involved in neurological disturbances of spatial perception and spatially oriented behavior.

The overall goal of this project is to understand how sensory signals are translated into commands for the control of movements. More specifically, most of the proposed experiments are concerned with the role of the primate superior colliculus, the frontal eye fields, and the pontine reticular formation in the control of orienting movements of the eyes and head. The first set of experiments tests the hypothesis that the superior colliculus and frontal eye fields generate a unitary signal of the desired change in gaze direction rather than separate signals for the control of eye movements or head movements. The effects of reversible activation and inactivation of small regions of the superior colliculus on the accuracy, velocity, and latency of combined eye-head gaze shifts will be examined. Both microstimulation and chronic single unit recording methods will be used to test the hypothesis that cells with movement-related activity through out the rostral-caudal extent of the superior colliculus specifies a change in gaze. the frequency and duration of microstimulation trains will be systematically varied to determine if neurons in the frontal eye fields are also generating commands to move the eyes and head instead of just being involved in saccadic eye movements. The final experiment in this group will study the activity of neurons in the pontine reticular formation during visually guided gaze shifts involving coordinated movements of the eyes and head. This region of the pons receives signals from the superior colliculus and the frontal eye fields and neurons in this area are the source of projections to spinal cord and motoneurons innervating neck muscles as well as to the motoneurons innervating the extraocular muscles. A second set of experiments focuses on the transformation of collicular signals into those required by the motoneuron pools controlling the direction of gaze. The collicular projections to the paramedian pontine reticular formation will be studies. Detailed plots of the movement fields of pontine neurons will be obtained and a potentially powerful new approach will be used to directly examine the influence of collicular inputs upon pontine cells. This involves varying the number and frequency of stimulation pulses applied to the SC while recording the activity of pontine neurons. Other experiments will examine the effects of reversible lesions of the omnipause area and the paramedian pontine reticular formation in behaviorally trained animals to test key assumptions of various models of the saccadic system.

A critical question that must be answered before we understand the control of sensoryguided saccades is that of how the spatial (anatomical) code of saccade direction and amplitude contained in the superior colliculus (SC) is transformed into the temporal signals required by motoneurons for the generation of saccadic movements. The long-range goals of the proposed research are directed toward the solution of this improtant problem. Chronic unit recording, microstimulation , and reversible deactivation methods will be used to study the role of the SC and brainstem recipients of inputs from the SC in the control of saccadic eye movements. More specifically, the goals of the project are to: a) Perform a detailed analysis of the alterations in saccade direction and amplitude occurring during reversible deactivation of small regions of the SC before and after unilateral removal of the frontal eye fields (FEFs). b) Characterize the saccaderelated discharges of neurons in tectorecipient areas by: 1) plotting the movement fields of neurons displaying saccaderelated activity; 2) quantifying the relationship between saccaderelated neural activity and saccadic velocity (vectorial velocity as well as component velocity); 3) determining the effects of varying the initial fixation position upon saccaderelated bursts; and 4) determining the effects of deactiviation of neurons in the SC upon the discharge of neurons with saccaderelated activity. c) Examine the connections between SC and neurons in PPRF, cMRF and NRTP using orthodromic and antidromic stimulation methods. d) Determine whether or not specific functional classes of neurons isolated in the PPRF, cMRF and NRTP receive inputs from specific types of collicular neurons using dual recording methods and cross correlation analysis. e) Study the effects of localized reversible deactiviation of small regions in the PPRF, cMRF and NRTP upon the latency, duration, direction, velocity, and amplitude of saccadic eye movements.

vision; biomedical equipment resource; biomedical facility; biomedical equipment development;

DESCRIPTION (provided by applicant): The overall goal of this project is to understand how sensory signals are translated into commands for the control of movements. More specifically, the proposed experiments study the role of the primate superior colliculus (SC) and the pontomedullary reticular formation in the control of orienting movements of the eyes and head. The first set of experiments has two goals: 1) to describe the functional properties of neurons in the tecto-recipient regions of pontomedullary reticular formation during coordinated eye-head movements and during head movements in the absence of a gaze shift; and 2) to study the effects of microstimulation of these pontomedullary regions upon eye and head movements. These studies target the brainstem areas that receive dense projections from intermediate layers of SC and also have a high density of neurons projecting to regions of the spinal cord innervating the neck muscles. In the second set of experiments neurons in the rostral paramedian pontine reticular formation will be reversibly inactivated to see if the activity of these cells is critical for the eye and head components of coordinated eye-head movements or for head movements made in the absence of a gaze shift. The third set of experiments has three goals: a) to record from neurons in the superior colliculus using the same behavioral tasks that were used to record from cells in pontomedullary regions; b) to use a search strategy that will increase the probability of finding cells in deeper layers of superior colliculus with activity related to head movements; and c) to examine the possible contribution of collicular neurons to the unusual eye velocity profiles observed during large gaze shifts. The final set of experiments uses a novel method of varying the speed and amplitude of eye and head movements during a gaze shift while recording the activity of pontomedullary neurons in an attempt to understand how gaze, eye and head amplitude and velocity is encoded in the activity of pontomedullary neurons.