The role of the superior colliculus in saccade initiation: a study of express saccades and the gap effect.

Neural mechanisms underlying the initiation of saccadic eye movements were studied by recording the activity of neurons in the superior colliculus of rhesus monkeys that had extensive experience on the gap task using targets restricted to one visual field. The superposition of visual activation upon the increased excitability occurring on gap trials facilitates the occurrence of a motor burst with extremely short latency; the motor burst is tightly coupled to saccade onset for the full range of saccadic reactions times, both regular and express. We found no evidence that express saccades are a special class of saccades triggered directly by visual responses. The low frequency activity, necessary for the occurrence of express saccades, neither initiates express saccades nor serves as an accurate predictor of the direction or latency of saccades. Based upon these findings, the hypothesis that the motor burst of collicular neurons serves as a signal for triggering saccade onset can now be extended to express saccades.

Express saccades: the effects of spatial and temporal uncertainty.

Once trained to make express saccades, monkeys continue to make express saccades when there is a high level of uncertainty about both the location and the time of onset of a peripheral target. Providing additional information about the location and the time of onset of a peripheral target does not increase the frequency of occurrence of short latency saccades. Furthermore, the metrics of saccades with express latencies are typical of visually-guided saccades, rather than anticipatory movements or saccades to the remembered location of a target. Finally, practice on the gap paradigm restricted to one target location always viewed from the same initial eye position does not increase the probability of producing express saccades to the same target from other initial eye positions. Instead, the practice effect is specific to targets that have the same relative location, regardless of initial eye position.

Organization of the projection from the superficial to the deep layers of the hamster's superior colliculus as demonstrated by the anterograde transport of Phaseolus vulgaris leucoagglutinin.

Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) was employed to describe the projection from the superficial to the deep layers of the hamster's superior colliculus (SC). Deposits of PHA-L in the stratum griseum superficiale (SGS) resulted in labelled terminal swellings in the stratum opticum and all of the deep laminae (the stratum griseum intermediate [SGI], stratum albumin intermedium [SAI], stratum griseum profundum [SGP], and stratum albumin profundum [SAP]). Labelled terminals were also visible in the periaqueductal gray (PAG). Reconstructions of individual axons showed that many collateral in the deep laminae arose from axons that projected to targets outside the colliculus. The projection from the superficial to the deep laminae had a loose topographic organization, and the trajectories of interlaminar axons were generally deflected laterally from "projection" lines that were orthogonal to the SC surface. Physiological recording and receptive field mapping were used to determine actual projection lines, which connect neurons in the superficial and deep layers that have receptive fields with the same elevation. These projection lines closely matched the trajectory of the pathway from the superficial to the deep laminae.

The orientation of horizontal cell dendrites in the superior colliculus of the hamster: an analysis based on three-dimensional reconstruction of intracellularly injected neurons.

We have employed intracellular injection of horseradish peroxidase (HRP) and 3-dimensional, computer-assisted reconstruction to delineate the organization of the dendrites of horizontal cells in the superficial laminae (the stratum griseum superficiale-SGS, and stratum opticum-SO) of the hamster's superior colliculus. Fifteen well-filled cells were analyzed. The dendrites of these cells were generally parallel to the frontal plane. An average of 74.8 +/- 13.0% of the total dendritic arbor of the recovered horizontal cells was located within 30 degrees of this plane. The long axis of horizontal cell dendritic trees deviated an average (mean +/- s.d.) of 21.7 +/- 13.2 degrees from the frontal plane and the average extent of the dendritic tree in this plane was 637 +/- 216 microns. This differed significantly from the average dendritic extent in the rostrocaudal axis (358 +/- 146 microns, p less than 0.001). In some cases, portions of the dendritic arbors of horizontal cells appeared to be oriented along lines of isoelevation or isoazimuth of the visual field representation in the superficial laminae. For other cells, there was no clear relationship between dendritic orientation and the visual field map.

Population coding of saccadic eye movements by neurons in the superior colliculus.

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.

Neuromuscular electrophysiology of the filarial helminth Dipetalonema viteae.

1. A body wall preparation is described which permits intracellular recording from the somatic muscle cells of the small filarial nematode, Dipetalonema viteae. Using this preparation, resting membrane potentials were measured and spontaneous muscle depolarizations described. 2. Stimulatory effects noted upon the addition of acetylcholine, or the cholinergic agonists suggest the hypothesis that acetylcholine is the excitatory neurotransmitter. However, in contrast with vertebrate tissues, the cholinergic antagonists, d-tubocurarine, hexamethonium and pentolinium do not inhibit somatic muscle activity of the worm. 3. GABA inhibited somatic muscle depolarizations, suggesting the possibility that it may serve as an inhibitory neurotransmitter. 4. The anthelmintic drug, levamisole, produced a depolarizing block. Effects of other pharmacological agents are described, discussed and compared with effects on vertebrate muscles.

Persistence of La Crosse virus (California encephalitis serogroup) in north-central Illinois.

La Crosse (LAC) virus was first isolated in Illinois from a pool of 50 female Aedes triseriatus mosquitoes collected in July 1976, in Peoria Heights. From 1978 through 1981, 27 strains (11 from males and 16 from females) of LAC virus were recovered from 888 pools containing 22,021 adult A. triseriatus mosquitoes from the same study area. These mosquitoes had developed from larvae and pupae collected from 50 individually identified treeholes. Of the 14 trees that yielded LAC virus-positive mosquitoes, one was positive in 3 of 4 years and another was positive in all 4 years. The latter tree had minimum mosquito field infection rates (MFIR) ranging from 3.4 to 12.7/1,000. Eight (57%) of the trees with positive mosquitoes were red oak (Quercus rubra) while 10 (71%) were in the oak genus (Quercus). The four most productive treeholes accounted for 30% of mosquitoes tested and 52% of the LAC isolations. In 1979, 6,729 A. triseriatus mosquitoes were collected in man-baits and tested for virus. From 1,282 tested in 259 pools (mean = 5), 13 LAC isolates were made, resulting in a field infection rate (FIR) of 11.4/1,000. The remaining 5,447 were tested in 218 pools (mean = 25) and 48 strains of LAC were isolated for a FIR of 9.9/1,000. The relationship of these findings to the occurrence of human LAC encephalitis cases in Peoria County, Illinois is discussed. Repeated recovery of virus from this study area reflects a stable, yet dynamic, focus of LAC virus transmission.