Search

The life history of a parasite describes its partitioning of assimilated resources into growth, reproduction, and transmission effort, and its precise timing of developmental events. The life cycle, in contrast, charts the sequence of morphological stages from feeding to the transmission forms. Phenotypic plasticity in life history traits can reveal how parasites confront variable environments within hosts. Within the protist phylum Apicomplexa major clades include the malaria parasites, coccidians, and most diverse, the gregarines (with likely millions of species). Studies on life history variation of gregarines are rare. Therefore, life history traits were examined for the gregarine Monocystis perplexa in its host, the invasive earthworm Amynthas agrestis at three sites in northern Vermont, United States of America. An important value of this system is the short life-span of the hosts, with only seven months from hatching to mass mortality; we were thus able to examine life history variation during the entire life cycle of both host and parasite. Earthworms were collected ( N  = 968 over 33 sample periods during one host season), then parasites of all life stages were counted, and sexual and transmission stages measured, for each earthworm. All traits varied substantially among individual earthworm hosts and across the sites. Across sites, timing of first appearance of infected earthworms, date when transmission stage (oocysts packed within gametocysts) appeared, date when number of both feeding (trophic) cells and gametocysts were at maximum, and date when 100% of earthworms were infected differed from 2-8 weeks, surprising variation for a short season available for parasite development. The maximal size of mating cells varied among hosts and across sites and this is reflected in the number of oocysts produced by the gametocyst. A negative trade-off was observed for the number of oocysts and their size. Several patterns were striking: (1) Prevalence reached 100% at all sites by mid season, only one to three weeks after parasites first appeared in the earthworms. (2) The number of parasites per host was large, reaching 300 × 10 3 cells in some hosts, and such high numbers were present even when parasites first appeared in the host. (3) At one site, few infected earthworms produced any oocysts. (4) The transmission rate to reach such high density of parasites in hosts needed to be very high for a microbe, from >0.33% to >34.3% across the three sites. Monocystis was one of the first protist parasites to have its life cycle described (early 19 th century), but these results suggest the long-accepted life cycle of Monocystis could be incomplete, such that the parasites may be transmitted vertically (within the earthworm's eggs) as well as horizontally (leading to 100% prevalence) and merogony (asexual replication) could be present, not recognized for Monocystis , leading to high parasitemia even very early in the host's season., Competing Interests: The authors declare there are no competing interests., (©2024 Keller and Schall.)

The frontal eye fields (FEF), originally identified as an oculomotor cortex, have also been implicated in perceptual functions, such as constructing a visual saliency map and shifting visual attention. Further dissecting the area's role in the transformation from visual input to oculomotor command has been difficult because of spatial confounding between stimuli and responses and consequently between intermediate cognitive processes, such as attention shift and saccade preparation. Here we developed two tasks in which the visual stimulus and the saccade response were dissociated in space (the extended memory-guided saccade task), and bottom-up attention shift and saccade target selection were independent (the four-alternative delayed saccade task). Reversible inactivation of the FEF in rhesus monkeys disrupted, as expected, contralateral memory-guided saccades, but visual detection was demonstrated to be intact at the same field. Moreover, saccade behavior was impaired when a bottom-up shift of attention was not a prerequisite for saccade target selection, indicating that the inactivation effect was independent of the previously reported dysfunctions in bottom-up attention control. These findings underscore the motor aspect of the area's functions, especially in situations where saccades are generated by internal cognitive processes, including visual short-term memory and long-term associative memory.

Previous studies using muscimol inactivations in the frontal eye fields (FEFs) have shown that saccades generated by recall from working memory are eliminated by these lesions, whereas visually guided saccades are relatively spared. In these experiments, we made reversible inactivations in FEFs in alert macaque monkeys and examined the effect on saccades in a choice response task. Our task required monkeys to learn arbitrary pairings between colored stimuli and saccade direction. Following inactivations, the percentage of choice errors increased as a function of the number of alternative (NA) pairings. In contrast, the percentage of dysmetric saccades (saccades that landed in the correct quadrant but were inaccurate) did not vary with NA. Saccade latency increased postlesion but did not increase with NA. We also made simultaneous inactivations in both FEFs. The results following bilateral lesions showed approximately twice as many choice errors. We conclude that the FEFs are involved in the generation of saccades in choice response tasks. The dramatic effect of NA on choice errors, but the lack of an effect of NA on motor errors or response latency, suggests that two types of processing are interrupted by FEF lesions. The first involves the formation of a saccadic intention vector from associate memory inputs, and the second, the execution of the saccade from the intention vector. An alternative interpretation of the first result is that a role of the FEFs may be to suppress incorrect responses. The doubling of choice errors following bilateral FEF lesions suggests that the effect of unilateral lesions is not caused by a general inhibition of the lesioned side by the intact side.

Selection of identical responses may not use the same neural mechanisms when the number of alternatives (NA) for the selection changes, as suggested by Hick's law. For elucidating the choice mechanisms, frontal eye field (FEF) neurons were monitored during a color-to-location choice saccade task as the number of potential targets was varied. Visual responses to alternative targets decreased as NA increased, whereas perisaccade activities increased with NA. These modulations of FEF activities seem closely related to the choice process because the activity enhancements coincided with the timing of target selection, and the neural modulation was greater as NA increased, features expected of neural correlates for a choice process from the perspective of Hick's law. Our current observations suggest two novel notions of FEF neuronal behavior that have not been reported previously: (1) cells called "phasic visual" that do not discharge in the perisaccade interval in a delayed-saccade paradigm show such activity in a choice response task at the time of the saccade; and (2) the activity in FEF visuomotor cells display an inverse relationship between perisaccadic activity and the time of saccade triggering with higher levels of activity leading to longer saccade reaction times. These findings support the area's involvement in sensory-motor translation for target selection through coactivation and competitive interaction of neural populations that code for alternative action sets.

Recent evidence implicates the superior colliculus (SC) in cognitive processes, such as target selection and control of spatial attention, in addition to the execution of saccadic eye movements. We report here the presence of a cognitive response in some cells in the SC in a task that requires the long-term association of spatial location with an arbitrary color. In this study, using a visual choice response task, we demonstrate that visuomotor neurons in the SC were activated by the appearance of a central symbolic cue delivered outside of the visual response fields of the recorded neurons. This procedure ensures that cognitively generated activity in these SC cells is not confounded with modulation of activity from previous visual stimuli that appeared in the response field of the neurons. The experiments suggest that cognitive signals can activate SC cells by themselves instead of only being able to modulate activities already evoked by visual events. Furthermore, a substantial fraction of these cells accurately reflected cue-aligned target selection in advance of saccade initiation. Our results add further support to other studies that have demonstrated that internally generated signals exist in SC cells.

In vitro activities of ABT-492, ciprofloxacin, levofloxacin, trovafloxacin, moxifloxacin, gatifloxacin, and gemifloxacin were compared. ABT-492 was more potent against quinolone-susceptible and -resistant gram-positive organisms, had activity similar to that of ciprofloxacin against certain members of the family Enterobacteriaceae, and had comparable activity against quinolone-susceptible, nonfermentative, gram-negative organisms. Bactericidal activity of ABT-492 was also evaluated.

Important insights into the neural organization of the saccadic system have been gained when the usually stereotyped movement trajectories of saccades have been altered by experimental manipulation. In the present study we produced trajectory variability in monkeys by using a visual search task in which both the location and color of an odd-colored target were changed randomly trial by trial, and the number of distractors was varied on each trial. We wished to determine whether increasing the number of distractors also increased the movement trajectory variation, i.e., the amount of initial directional deviation, endpoint deviation (averaging), and curvature of saccades. Overall, saccade latencies and the proportion of saccades directed to distractors decreased as the number of homogenous distractors increased. We also found that saccades have much more dispersion in their initial direction when distractors are present in comparison to the case when only a single target without distractors appears. However, initial dispersion decreases systematically as the number of distractors increases. The percentage of averaging saccades produced in the search task was not consistently dependent on the number of distractors. A significant fraction of averaging saccades still occurred for much wider spatial separations of stimuli than in previous studies using two visual stimuli with no specified target. The curvature of saccade trajectories increased dramatically when distractors were present, but the amount of curvature was not systematically affected by the number of distractors. Errors present in saccade trajectory in the search task were only poorly compensated. We conclude that these variable saccade trajectories result from incomplete or inaccurate specification of the target when competing stimuli are present and that a smaller number of more widely spread distractors facilitate saccade variability, perhaps due to the greater difficulty of target selection.

When saccadic eye movements are made in a search task that requires selecting a target from distractors, the movements show greater curvature in their trajectories than similar saccades made to single stimuli. To test the hypothesis that this increase in curvature arises from competitive interactions between saccade goals occurring near the time of movement onset, we performed single-unit recording and microstimulation experiments in the superior colliculus (SC). We found that saccades that ended near the target but curved toward a distractor were accompanied by increased presaccadic activity of SC neurons coding the distractor site. This increased activity occurred approximately 30 ms before saccade onset and was abruptly quenched on saccade initiation. The magnitude of increased activity at the distractor site was correlated with the amount of curvature toward the distractor. In contrast, neurons coding the target location did not show any significant difference in discharge for curved versus straight saccades. To determine whether this pattern of SC discharge is causally related to saccade curvature, we performed a second series of experiments using electrical microstimulation. Monkeys made saccades to single visual stimuli presented without distractors, and we stimulated sites in the SC that would have corresponded to distractor sites in the search task. The stimulation was subthreshold for evoking saccades, but when its temporal structure mimicked the activity recorded for curved saccades in search, the subsequent saccades to the visual target showed curvature toward the location coded by the stimulation site. The effect was larger for higher stimulation frequencies and when the stimulation site was in the same colliculus as the representation of the visual target. These results support the hypothesis that the increased saccade curvature observed in search arises from rivalry between target and distractor goals and are consistent with the idea that the SC is involved in the competitive neural interactions underlying saccade target selection.

Because real-world scenes typically contain many different potential objects of interest, selecting one goal from many is clearly a fundamental problem faced by the saccadic system. We recorded from visual, movement, and visuo-movement (VM) neurons in the superior colliculus (SC) of monkeys performing a reaction-time visual-search task requiring them to make saccades to an odd-colored target presented with distractors. First, we compared the responses of SC neurons in search with their responses when a single target was presented without distractors (single-stimulus task). Consistent with earlier reports, initial visual activity was smaller in search than in the single-stimulus task, while movement-related activity in the two tasks was comparable. Further experiments showed that much of the reduction in the initial visual response during search was due to lateral inhibition, although a top-down task-related component was also evident. Although the initial visual activity did not discriminate the target from the distractors, some neurons showed a biphasic pattern of visual activity. In VM burst neurons, the second phase of this activity was significantly larger when the target, rather than a distractor, was in the response field. We traced the time course of target/distractor discrimination using receiver operating characteristic (ROC) analysis and found that VM burst neurons, VM prelude neurons, and pure movement neurons discriminated the target from distractors before saccade onset but that phasic and tonic pure visual neurons did not. We also examined the relationship between target/distractor discrimination time and saccade latency. Discrimination in VM burst neurons having a biphasic pattern of visual activity and in many VM prelude neurons occurred after a consistent delay that did not depend on saccade latency, suggesting that these neurons are involved in target selection as well as movement initiation. In contrast, VM burst neurons lacking a biphasic pattern of visual activity, pure movement neurons, and a subset of VM prelude neurons discriminated the target at a time that was well correlated with saccade latency, suggesting that this latter group of neurons is involved in triggering movement execution but not in target selection. Thus a mix of signals likely related to target selection and movement initiation co-exists in different groups of SC neurons. This suggests that certain types of SC neurons participate in the target selection process and that the SC as a whole represents a gateway for target selection signals to be converted into a saccadic command.

The premotor pathways subserving saccades and smooth-pursuit eye movements are usually thought to be different. Indeed, saccade and smooth-pursuit eye movements have different dynamics and functions. In particular, a group of midline cells in the pons called omnipause neurons (OPNs) are considered to be part of the saccadic system only. It has been established that OPNs keep premotor neurons for saccades under constant inhibition during fixation periods. Saccades occur only when the activity of OPNs has completely stopped or paused. Accordingly, electrical stimulation in the region of OPNs inhibits premotor neurons and interrupts saccades. The premotor relay for smooth pursuit is thought to be organized differently and omnipause neurons are not supposed to be involved in smooth-pursuit eye movements. To investigate this supposition, OPNs were recorded during saccades and during smooth pursuit in the monkey (Macaca mulatta). Unexpectedly, we found that neuronal activity of OPNs decreased during smooth pursuit. The resulting activity reduction reached statistical significance in approximately 50% of OPNs recorded during pursuit of a target moving at 40 degrees /s. On average, activity was reduced by 34% but never completely stopped or paused. The onset of activity reduction coincided with the onset of smooth pursuit. The duration of activity reduction was correlated with pursuit duration and its intensity was correlated with eye velocity. Activity reduction was observed even in the absence of catch-up saccades that frequently occur during pursuit. Electrical microstimulation in the OPNs' area induced a strong deceleration of the eye during smooth pursuit. These results suggest that OPNs form an inhibitory mechanism that could control the time course of smooth pursuit. This inhibitory mechanism is part of the fixation system and is probably needed to avoid reflexive eye movements toward targets that are not purposefully selected. This study shows that saccades and smooth pursuit, although they are different kinds of eye movements, are controlled by the same inhibitory system.

Saccades are typically separated by inter-saccadic fixation intervals (ISFIs) of > or =125 ms. During this time, the saccadic system selects a goal and completes the preparatory processes required prior to executing the subsequent movement. However, in tasks in which competing stimuli are presented, two sequentially executed movements to different goals can be separated by much shorter ISFIs. This suggests that the saccadic system is capable of completing many of the preparatory requirements for a second saccade concurrently with the execution of an initial movement. We recorded single neurons in the superior colliculus (SC) during rapid saccade sequences made by rhesus monkeys performing a search task. We found that during the execution of an initial saccade, activity related to the goal of a quickly following second saccade can be simultaneously maintained in the SC motor map. This activity appears to signal the selection or increased salience of the second saccade goal even before the initial saccade has ended. For movements separated by normal ISFIs (> or =125 ms), we did not observe activity related to concurrent processing, presumably because for these longer ISFI responses, the goal of the second saccade is not selected until after the end of the first saccade. These results indicate that, at the time of an initial saccade, the SC does not necessarily act as a strict winner-take-all network. Rather it appears that the salience of a second visual goal can be simultaneously maintained in the SC. This provides evidence that selection or preparatory activity related to the goal of a second saccade can overlap temporally with activity related to an initial saccade and indicates that such concurrent processing is present even in a structure which is fairly close to the motor output.

In human subjects, two mechanisms for improving the efficiency of saccades in visual search have recently been described: color priming and concurrent processing of two saccades. Since the monkey provides an important model for understanding the neural underpinnings of target selection in visual search, we sought to explore the degree to which the saccadic system of monkeys uses these same mechanisms. Therefore, we recorded the eye movements of rhesus monkeys performing a simple color-oddity pop-out search task, similar to that used previously with human subjects. The monkeys were rewarded for making a saccade to the odd-colored target, which was presented with an array of three distractors. The target and distractors were randomly chosen to be red or green in each trial. Similar to what was previously observed for humans, we found that monkeys show the influence of a cumulative, short-term priming mechanism which facilitates saccades when the color of the search target happens to repeat from trial to trial. Furthermore, we found that like humans, when monkeys make an erroneous initial saccade to a distractor, they are capable of executing a second saccade to the target after a very brief inter-saccadic interval, suggesting that the two saccades have been programmed concurrently (i.e. in parallel). These results demonstrate a close similarity between human and monkey performance. We also made a new observation: we found that when monkeys make such two-saccade responses, the trajectory of the initial saccade tends to curve toward the goal of the subsequent saccade. This provides evidence that the two saccade goals are simultaneously represented on a common motor map, supporting the idea that the movements are processed concurrently. It also indicates that concurrent processing is not limited to brain areas involved in higher-level planning; rather, such parallel programming apparently occurs at a low enough level in the saccadic system that it can affect saccade trajectory.

Direct projections from the superior colliculus (SC) to the paramedian pontine reticular formation (PPRF) have been demonstrated anatomically. The PPRF contains cells called excitatory burst neurons (EBNs) that execute the final premotoneuronal processing for saccadic eye movements, as well as other burst cells called long-lead burst neurons (LLBNs). Previous electrophysiological tests in monkey have failed to find evidence for monosynaptic connections from the SC to EBNs, but have shown that direct projections to LLBNs exist. The validity of these results has been questioned because EBNs are known to be inhibited during periods of fixation by cells called omnipause neurons (OPNs). In later experiments in cat, the stimulus in the SC was triggered during saccades (when OPNs are off) and direct connections to EBNs were found. The present experiments were conducted to determine whether direct connections from the SC to EBNs could be demonstrated in monkey. LLBNs located near EBNs were also recorded. Single-pulse stimuli were delivered at sites in the SC at current levels well above those required to evoke saccades with pulse train stimuli. The stimuli were triggered shortly after the onset of ipsilateral or contralateral saccades and also slightly after the end of saccades. A sample of 21 EBNs was recorded and none were activated by postsaccadic stimulation or during contralateral saccades. The high spontaneous discharge rates of EBNs during ipsilateral saccades made activation of these cells more difficult to detect; however, when the results were quantified by peristimulus time histograms aligned on stimulus onset, only 1/21 EBNs showed evidence of activation in the monosynaptic range of latencies (<1.6 ms), 13 EBNs were activated at di- or polysynaptic latencies, and 7 were not activated. In contrast, 15/21 LLBNs were activated with latencies in the monosynaptic range. Further evidence supporting the absence of direct connections to EBNs was obtained by realigning the peristimulus time histograms for a subset of EBNs with similar firing rates on the time of occurrence of the last spike before stimulus onset. A subset of EBNs was also studied during drowsy ipsilaterally directed eye drifts, during which these cells were firing at low spontaneous rates and OPNs were off. No evidence for direct connections to EBNs was found in this behavioral state. The variance in results obtained for cat and monkey may be due to a species difference that reflects the more complex signal processing required in the monkey's saccadic system.

Over the past decade, considerable research efforts have been focused on the role of the rostral superior colliculus (SC) in control of saccades. The most recent theory separates the deeper intermediate layers of the SC into two functional regions: the rostral pole of these layers constitutes a fixation zone and the caudal region comprises the saccade zone. Sustained activity of fixation neurons in the fixation zone is argued to maintain fixation and help prevent saccade generation by exciting the omnipause neurons (OPNs) in the brain stem. This hypothesis is in contrast to the traditional view that the SC contains a topographic representation of the saccade motor map on which the rostral pole of the SC encodes signals for generating small saccades (<2 degrees ) instead of preventing them. There is therefore an unresolved controversy about the specific role on the most rostral region of the SC, and we reexamined its functional contribution by quantifying and comparing spatial and temporal trajectories of 30 degrees saccades perturbed by electrical stimulation of the rostral pole and more caudal regions in the SC and of the OPN region. If the rostral pole serves to preserve fixation, then saccades perturbed by stimulation should closely resemble interrupted saccades produced by stimulation of the OPN region. If it also contributes to saccade generation, then the disrupted movements would better compare with redirected saccades observed after stimulation of the caudal SC. Our experiments revealed two significant findings: 1) the locus of stimulation was the primary factor determining the perturbation effect. If the directions of the target-directed saccade and stimulation-evoked saccade were aligned and if the stimulation was delivered within approximately the rostral 2 mm (<10 degrees amplitude) of SC, the ongoing saccade stopped in midflight but then resumed after stimulation end to reach the original visually specified goal with close to normal accuracy. When stimulation was applied at more caudal sites, the ongoing saccade directly reached the target location without stopping at an intermediate position. If the directions differed considerably, both initial and resumed components were typically observed for all stimulation sites. 2) A quantitative analysis of the saccades perturbed from the fixation zone showed significant deviations from their control spatial trajectories. Thus they resembled redirected saccades induced by caudal SC stimulation and differed significantly from interrupted saccades produced by OPN stimulation. The amplitude of the initial saccade, latency of perturbation, and spatial redirection were greatest for the most caudal sites and decreased gradually for rostral sites. For stimulation sites within the rostral pole of SC, the measures formed a smooth continuation of the trends observed in the saccade zone. As these results argue for the saccade zone concept, we offer reinterpretations of the data used to support the fixation zone model. However, we also discuss scenarios that do not allow an outright rejection of the fixation zone hypothesis.

Stimulation of the rostral approximately 2 mm of the superior colliculus (SC) during a large, visual target-initiated saccade produces a spatial deviation of the ongoing saccade and then stops it in midflight. After the termination of the stimulation, the saccade resumes and ends near the location of the flashed target. The density of collicular projections to the omnipause neuron (OPN) region is greatest from the rostral SC and decreases gradually for the more caudal regions. It has been hypothesized that the microstimulation excites the OPNs through these direct connections, and the reactivation of OPNs, which are normally silent during saccades, stops the initial component in midflight by gating off the saccadic burst generator. Two predictions emerge from this hypothesis: 1) for microstimulation triggered on the onset of large saccades, the time from stimulation onset to resumption of OPN discharge should decrease as the stimulation site is moved rostral and 2) the lead time from reactivation of OPNs to the end of the initial saccade on stimulation trials should be equal to the lead time of pause end with respect to the end of control saccades. We tested this hypothesis by recording OPN activity during saccades perturbed by stimulation of the rostral approximately 2 mm of the SC. The distance of the stimulation site from the most rostral extent of the SC and the time of reactivation with respect to stimulation onset were not significantly correlated. The mean lead of reactivation of OPNs relative to the end of the initial component of perturbed saccades (6.5 ms) was significantly less than the mean lead with respect to the end of control (9.6 ms) and resumed saccades (10.4 ms). These results do not support the notion that the excitatory input from SC neurons-in particular, the fixation neurons in the rostral SC-provide the major signal to reactivate OPNs and end saccades. An alternative, conceptual model to explain the temporal sequence of events induced by stimulation of the SC during large saccades is presented. Other OPN activity parameters also were measured and compared for control and stimulation conditions. The onset of pause with respect to resumed saccade onset was larger and more variable than the onset of pause with respect to control saccades, whereas pause end with respect to the end of resumed and control saccades was similar. The reactivated discharge of OPNs during the period between the end of the initial and the onset of the resumed saccades was at least as strong as that following control movements. This latter observation is interpreted in terms of the resettable neural integrator hypothesis.

To help understand how complex visual stimuli are processed into short-latency saccade motor programs, the activity of visuomotor neurons in the deeper layers of the superior colliculus was recorded while two monkeys made express saccades to one target and to two targets. It has been shown previously that the visual response and perimotor discharge characteristic of visuomotor neurons temporally coalesce into a single burst of discharge for express saccades. Here we seek to determine whether the distributed visual response to two targets spatially coalesces into a command appropriate for the resulting saccade. Two targets were presented at identical radial eccentricities separated in direction by 45 degrees. A gap paradigm was used to elicit express saccades. Express saccades were more likely to land in between the two targets than were saccades of longer latency. The speeds of express saccades to two targets were similar to those of one target of similar vector, as were the trajectories of saccades to one and two targets. The movement fields for express saccades to two targets were more broad than those for saccades to one target for all neurons studied. For most neurons, the spatial pattern of discharge for saccades to two targets was better explained as a scaled version of the visual response to two spatially separate targets than as a scaled version of the perimotor response accompanying a saccade to a single target. Only the discharge of neurons with large movement fields could be equally well explained as a visual response to two targets or as a perimotor response for a one-target saccade. For most neurons, the spatial properties of discharge depended on the number of targets throughout the entire saccade-related burst. These results suggest that for express saccades to two targets the computation of saccade vector is not complete at the level of the superior colliculus for most neurons and an explicit process of target selection is not necessary at this level for the programming of an express saccade.

The two-dimensional distribution of population activity in the superior colliculus (SC) during saccadic eye movements in the monkey was estimated using radial basis functions. To make these ensemble activity estimates, cells in the deeper layers of the SC were recorded over much of the rostrocaudal (caudal to 3.8 mm from the rostral tip), mediolateral extent of this structure. The dynamic movement field of each cell was determined at 2-ms intervals around the time of saccades for a wide variety of horizontal and oblique movements. Collicular neurons were divided into partially overlapping dorsal and ventral cell layers on the basis of recorded depth in SC. The pattern of presaccadic activity was used as an additional discriminant to sort the cells in the two layers into separate burst (dorsal) and buildup (ventral) cell classes. Rostrocaudal and medioventral cell location on the colliculus was estimated from the optimal target vector for a cell's visual response rather than from the optimal motor vector. The former technique was more reliable for locating some buildup neurons because it produced locations that compared better with the locations suggested by electrical stimulation. From the movement field data and from the estimates of each cell's anatomic location, a similar algorithm was used to compute the two-dimensional population activity in the two layers of the SC during horizontal and oblique saccades. A subset of the sample of neurons, located near the horizontal meridian of the SC, first was used to compute one-dimensional dynamic population activity estimates for horizontal saccades to allow partial comparison to previous studies. Statistical analyses on the one-dimensional data were limited to saccades of

One proposed role of the superior colliculus (SC) in oculomotor control is to suppress or excite the activity of brain stem omnipause neurons (OPNs) to initiate or terminate saccades, respectively. Although connections from the SC to the OPNs have been demonstrated, the spatial distribution and discharge characteristics of the projecting neurons from the SC remain unknown. We mapped the spatial distribution of the deeper-layer neurons of the SC by stimulating the region of the OPNs to identify antidromic projections and found that the density of direct projections from the SC to the OPNs was greatest in the most rostral region and decreased gradually for more caudal sites. On the basis of saccade-related discharge characteristics, the antidromically driven neurons were predominantly fixation and buildup neurons. The spatially distributed SC projections to the OPNs and the discharge characteristics of the SC neurons suggest that the direct projections from SC to OPNs are excitatory. Finally, we propose how excitation and disfacilitation from SC activity can contribute to modulation of OPN response and control saccades.

The vestibulo-ocular reflex (VOR) was measured in congenitally blind, adventitiously blind, and normally sighted adults to determine how it was affected by loss of vision. VOR gain and phase were measured in subjects rotated sinusoidally in total darkness, while concentrating on an imaginary earth-fixed target. Gain was lower in adventitiously blind subjects than in sighted subjects. The gain reduction in blind subjects was accompanied by an increase in the amount of phase lead at low frequencies. The dominant time constant was typically 3 sec for adventitiously blind subjects and 16 sec for sighted subjects. No convincing vestibulo-ocular responses were measured in congenitally blind subjects except at the highest test frequencies. These findings demonstrate that vision is necessary early in life for development of the VOR, and that vision is also necessary throughout life for ongoing maintenance of the VOR.

Visual evoked potentials (VEP's) recorded from the scalp are sensitive to retinal image sharpness and thus to changes in the refractive state of the eye. Initially the responses to checkerboard flash or reversal stimuli were computer-averaged in order to raise the signal above the noise, primarily the electroencephalogram (EEG). Recently analogue Fourier signal analysis has been proposed for using the VEP for rapid clinical refraction. We have confirmed that this method can measure the spherical refractive state to within +/- 0.50D. However, because of large slow-wave fluctuations, measurements of changes within +/- 1.00D are not always clear. Despite its promise the method does not appear to be clinically useful at present.

The central nervous system is a parallel dynamical system which connects sensory input with motor output for the performance of visual tracking. This paper applies elementary control system tools to extend dynamical neural network models to the visual smooth pursuit system. Observed eye position responses to target motions and characteristics of the plant (eye muscles and orbital mechanics) place dynamical constraints on the interposed neural network controller. In the process of constructing a model for the controller, we show two previous pursuit system models, using efference copy and feedforward compensation, are equivalent from an input-output standpoint. We introduce a controller model possessing a potentially highly parallel implementation and offer an example with supporting neural firing rate data. Changes in time delays or other system dynamics are expected to lead to compensatory adaptive changes in the controller. A scheme to noninvasively simulate such changes in system dynamics was developed. Actual physiologic data of adaptive responses to increased time delay is presented as an example of the utility of this parallel controller. Compensatory changes in our parallel controller model are easily predicted. These results suggest a productive interaction between neural network modeling, neurophysiology, and control systems engineering.