Biomedical subjects

Edward L Keller

Publications and source records attributed to Edward L Keller.

12 recordsLinked to original sources

PubMed2006

Symbolic cue-driven activity in superior colliculus neurons in a peripheral visual choice task.

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.

Animals↗

PubMed2005

Readout of higher-level processing in the discharge of superior colliculus neurons.

The discharge of neurons in the deeper layers of the superior colliculus (SC) was studied while monkeys performed two visual discrimination tasks that required different amounts of cognitive processing. In a search paradigm the animal's task was to saccade to the location of an odd-colored stimulus located in an array of distractors of uniform color (pop-out visual search). The visual stimuli remained on the screen as the discrimination process distinguished target from distractors. In a choice response task the color of a central cue signaled which stimulus from a previously presented array of colored stimuli was to be the target of a saccade. The stimulus array was turned off well before the central cue was presented. Most neurons showed activity aligned on both the visual input and the motor response in single-target tasks. Many of these same neurons showed additional discharge that was correlated with the required higher-level decision processes in both of these more natural visual tasks. In the case of pop-out search the SC has been shown to be functionally involved in the decision processes. The cue-aligned activity in SC in the choice response task is surprising because no transient visual stimulus appeared in the response field of the neuron.

Animals↗

PubMed2004

Properties of saccades generated as a choice response.

Since Hick's original description, many subsequent studies have confirmed the logarithmic relationship that exists between response time and the number of alternatives (NA) for a choice response. In the present study a novel paradigm was used to quantify saccade response time as a function of NA. Normal subjects were required to make a saccade to the remembered location of a visual target whose color was specified by a centrally located cue. The paradigm thus required a stimulus-response transformation similar to that used by Hick. The results show that, when such a transformation was required, a logarithmic relationship was found for saccadic response time. The use of a color-to-location paradigm to study saccade choice response time produced an unexpected additional result that may provide insight into the neural organization of the saccadic system. When the number of alternative choice responses was large (4 or 8), subjects frequently made a two-saccade response instead of a single saccade to the correct location. The first movement in such a sequence was in the correct direction, but was hypometric. A second movement then followed which moved the eyes onto the correct location. These results suggest dynamic dissociations in the mechanisms underlying the triggering of saccades and the specification of their metrics.

Adult↗

PubMed2004

A model of the saccade-generating system that accounts for trajectory variations produced by competing visual stimuli.

Variable saccade trajectories are produced in visual search paradigms in which multiple potential target stimuli are present. These variable trajectories provide a rich source of information that may lead to a deeper understanding of the basic control mechanisms of the saccadic system. We have used published behavioral observations and neural recordings in the superior colliculus (SC), gathered in monkeys performing visual search paradigms, to guide the construction of a new distributed model of the saccadic system. The new model can account for many of the variations in saccade trajectory produced by the appearance of multiple visual stimuli in a search paradigm. The model uses distributed feedback about current eye motion from the brainstem to the SC to reduce activity there at physiologically realistic rates during saccades. The long-range lateral inhibitory connections between SC cells used in previous models have been eliminated to match recent physiological evidence. The model features interactions between visually activated multiple populations of cells in the SC and distributed and topologically organized inhibitory input to the SC from the SNr to produce some of the types of variable saccadic trajectories, including slightly curved and averaging saccades, observed in visual search tasks. The distributed perisaccadic disinhibition of SC from the substantia nigra (SNr) is assumed to have broad spatial tuning. In order to produce the strongly curved saccades occasionally recorded in visual search, the existence of a parallel input to the saccadic burst generators in addition to that provided by the distributed input from the SC is required. The spatiotemporal form of this additional parallel input is computed based on the assumption that the input from the model SC is realistic. In accordance with other recent models, it is assumed that the parallel input comes from the cerebellum, but our model predicts that the parallel input is delayed during highly curved saccadic trajectories.

Animals↗

PubMed2004

Deficits in saccade target selection after inactivation of superior colliculus.

Saccades are rapid eye movements that orient gaze toward areas of interest in the visual scene. Neural activity correlated with saccade target selection has been identified in several brain regions, including the superior colliculus (SC), but it is not known whether the SC is directly involved in target selection, or whether the SC merely receives selection-related signals from cortex in preparation for the execution of eye movements. In monkeys, we used focal reversible inactivation to test the functional contributions of the SC to target selection during visual search, and found that inactivation resulted in clear deficits. When a target appeared in the inactivated field, saccades were often misdirected to distractor stimuli. Control tasks showed that this deficit was not caused by low-level visual or motor impairments. Our results indicate that, in addition to its well-established involvement in movement execution, the SC has an important functional role in target selection.

Action Potentials↗

PubMed2003

Properties of saccadic responses in monkey when multiple competing visual stimuli are present.

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.

Animals↗

PubMed2003

Competition between saccade goals in the superior colliculus produces saccade curvature.

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.

Animals↗

PubMed2003

Shared brainstem pathways for saccades and smooth-pursuit eye movements.

A long-standing belief holds that the saccadic and smooth-pursuit eye movement systems are composed of largely separate premotor circuits, at least in the brainstem. One crucial prediction predicated on this belief is that the tonic discharge of omnipause neurons (OPNs), which are thought to be part of only the saccadic system, should not be modulated during pursuit eye movements. This report shows that the discharge of OPNs, in contradiction, is modulated downward during pursuit movements. In contrast to their behavior during saccades, where they pause completely for the duration of the movement, the downward modulation during pursuit did not totally silence OPNs. The depth of the downward modulation was correlated with the speed of the ongoing pursuit movement. Another type of cell, which we have named saccade/pursuit neurons, was recorded in the paramedian pontine reticular formation near the location of OPNs. This subpopulation of burst cells discharged a cascade of spikes for saccades in a preferred direction. They also displayed a lower-frequency sustained discharge of spikes for the duration of pursuit in the same preferred direction. These data suggest a new type of combined model for the organization of the brainstem saccade/pursuit system. In this new combined model, the OPNs form a common inhibitory mechanism for both types of movements, and the saccade/pursuit neurons participate in the eye-velocity modulation of OPN discharge or membrane polarization during either type of movement.

Animals↗

PubMed2002

Decoding of a motor command vector from distributed activity in superior colliculus.

Several alternative methods for decoding the desired motor command vector from neural networks containing distributed, place-coded information have been suggested. The two most widely discussed candidate mechanisms are vector summation (VS) and a center-of-mass (CM) computation. The latter mechanism has also been called vector averaging. The present paper compares the operation of these two methods in a model of an experimentally well-studied neural structure, the superior colliculus (SC). The SC is one structure that has been shown to be responsible for generating saccadic command vectors in the form of distributed neural activity that is topologically arranged across its surface. It has been suggested that the pattern of eye-movement errors obtained following the placement of a collicular lesion can distinguish between these two mechanisms. As a result of this suggestion, the pattern of saccadic errors produced by lesions in the SC have been widely cited to support the CM hypothesis. In the present paper the placement of a discrete lesion is simulated in a recurrent (dynamic) neural network model of the SC. These dynamic connections in the model SC network produce a systematic shift of the locus of distributed activity away from the site of the lesion. The spatiotemporal shift in the location of SC activity then produces a pattern of saccadic errors that appear to support the CM hypothesis, even though ensemble activity in our model colliculus is decoded by VS. This result demonstrates that, when ensemble activity on the SC motor map is dynamically modulated over space and time by intrinsic collicular circuitry, an explicit CM computation is not needed to reproduce the pattern of physiological results that follow focal SC lesions.

Animals↗

PubMed2002

Neural discharge in the superior colliculus during target search paradigms.

Neural studies of oculomotor function in the past have been conducted with the use of very simple visual stimuli. More recently there has been a new emphasis on using more natural stimuli to extend our knowledge of oculomotor organization. Visual search paradigms are an example of the use of these more natural visual surrounds. In search a subject must locate and saccade to a target that appears simultaneously with an array of distractors. When monkeys are used in this paradigm, it is possible to record from neurons located in various central structures in the brain while the initial visual response, subsequent discrimination processes and final saccadic movement unfold. In the present study we used an array of four visual stimuli, and the target was distinguished by its odd color from three distractors of uniform color. Location of the target within the array and its color were randomly selected on each trial. Neurons located in the deeper layers of the superior colliculus (SC) were recorded by standard methods in blocks of search trials. We found several new features in the discharge of SC neurons using this search paradigm that have not previously been reported in studies using single-target visual displays. (1) In contrast to the "winner take all" behavior previously reported for the SC, we found evidence of concurrent processing of alternative movement vectors. When incorrect movements were made to distractor locations, this concurrent activity was associated with significantly shorter intersaccadic intervals. (2) The discharge profile of the visual response in many units was modified by the appearance of a second prominent peak which followed the initial phasic visual response, but which was clearly differentiated from a third burst in activity associated with a saccade into the cell's response field. In some neurons, the activity in this second peak was discriminatory for the impending saccade vector. That is, it was larger when the target appeared in the response field of the cell than when it contained a distractor. This target selection signal was thus distinct from the burst normally associated with saccades into the movement fields of SC neurons. (3) Some saccades in search had a curved trajectory bowing toward the location of a distractor. These saccades were accompanied by an elevated discharge of neurons coding that distractor.

Animals↗

PubMed2002

Superior colliculus activity related to concurrent processing of saccade goals in a visual search task.

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.

Animals↗

PubMed2002

Saccade target selection in the superior colliculus during a visual search task.

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.

Action Potentials↗