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Richard J Krauzlis

Publications and source records attributed to Richard J Krauzlis.

18 recordsLinked to original sources
PubMed2006

Ongoing eye movements constrain visual perception.

Eye movements markedly change the pattern of retinal stimulation. To maintain stable vision, the brain possesses a variety of mechanisms that compensate for the retinal consequences of eye movements. However, eye movements may also be important for resolving the ambiguities often posed by visual inputs, because motor commands contain additional spatial information that is necessarily absent from retinal signals. To test this possibility, we used a perceptually ambiguous stimulus composed of four line segments, consistent with a shape whose vertices were occluded. In a passive condition, subjects fixated a spot while the shape translated along a certain trajectory. In several active conditions, the spot, occluder and shape translated such that when subjects tracked the spot, they experienced the same retinal stimulus as during fixation. We found that eye movements significantly promoted perceptual coherence compared to fixation. These results indicate that eye movement information constrains the perceptual interpretation of visual inputs.

Algorithms↗
PubMed2005

Shared decision signal explains performance and timing of pursuit and saccadic eye movements.

Each voluntary eye movement provides physical evidence of a visuomotor choice about where and when to look. Primates choose visual targets with two types of voluntary eye movements, pursuit and saccades, although the exact mechanism underlying their coordination remains unknown. Are pursuit and saccades guided by the same decision signal? The present study compares pursuit and saccadic choices using techniques borrowed from psychophysics and models of response time. Human observers performed a luminance discrimination task and indicated their choices with eye movements. Because the stimuli moved horizontally and were offset vertically, subjects' tracking responses consisted of combinations of both pursuit and saccadic eye movements. For each of two signal strengths, we constructed speed-accuracy curves for pursuit and saccades. We found that speed-accuracy curves for pursuit and saccades have the same shape, but are time-shifted with respect to one another. We argue that this pattern occurs because pursuit and saccades share a decision signal, but utilize different response thresholds and are subject to different motor processing delays.

Adult↗
PubMed2005

Spatial deployment of attention influences both saccadic and pursuit tracking.

We examined the effects of changing spatial aspects of attention during oculomotor tracking. Human subjects were instructed to make a discrimination on either the small (0.8 degrees ) central or the large (8 degrees ) peripheral part of a compound stimulus (two counter-rotating concentric rings) while the stimulus either translated across the screen or was stationary. During this period, a transient perturbation with either step or ramp movement profile occurred. For perturbations leading to a change in position larger than the small ring, saccades occurred more frequently and had much shorter latencies (by 135 ms) when attention was directed to the small ring than when attention was directed to the large ring. These latency differences were sufficiently great that from a single saccade one can identify the attentional instruction with 94% accuracy. However, with target steps as small as the small ring, saccade latencies differed less. For pursuit, ramp perturbations caused larger changes in eye velocity with little change in latency when attention was directed to the small ring. Finally, when only the motion of the non-attended ring was perturbed, most subjects showed stronger saccadic responses to perturbations of the small than the large ring, and stronger pursuit responses to perturbations of the large than the small ring. By fitting the saccade latency distributions with the Reddi and Carpenter LATER model, we found that our subjects apparently employed at least two distinct strategies for changing latency when attending large vs. small. We propose that the timing of the saccade decision process depends on both the size of the attended object and the magnitude of the perturbation.

Adult↗
PubMed2005

The control of voluntary eye movements: new perspectives.

Primates use two types of voluntary eye movements to track objects of interest: pursuit and saccades. Traditionally, these two eye movements have been viewed as distinct systems that are driven automatically by low-level visual inputs. However, two sets of findings argue for a new perspective on the control of voluntary eye movements. First, recent experiments have shown that pursuit and saccades are not controlled by entirely different neural pathways but are controlled by similar networks of cortical and subcortical regions and, in some cases, by the same neurons. Second, pursuit and saccades are not automatic responses to retinal inputs but are regulated by a process of target selection that involves a basic form of decision making. The selection process itself is guided by a variety of complex processes, including attention, perception, memory, and expectation. Together, these findings indicate that pursuit and saccades share a similar functional architecture. These points of similarity may hold the key for understanding how neural circuits negotiate the links between the many higher order functions that can influence behavior and the singular and coordinated motor actions that follow.

Animals↗
PubMed2004

Manipulating intent: evidence for a causal role of the superior colliculus in target selection.

The superior colliculus (SC) is well known for its role in the motor control of saccades. Recent work has shown that it also plays a role in the selection of saccades, but a causal role in the process of target selection has not been demonstrated. We applied subthreshold microstimulation to the SC while monkeys performed a task requiring them to select a stimulus as the target for a pursuit or saccade movement. Stimulation increased the proportion of selections toward the stimulus that appeared contralateral to the site of stimulation and also decreased their latencies. For pursuit, this stimulation-induced contralateral response bias was with respect to the initial target location and not the direction of eye movement, demonstrating a causal effect on target choice distinct from any effect on motor preparation. These results show that the SC helps decide the object of the next movement, beyond its traditional responsibility of saccade production.

Animals↗
PubMed2004

Activity of rostral superior colliculus neurons during passive and active viewing of motion.

The superior colliculus (SC) has long been known to be important for the control of saccades, and recent findings indicate that the rostral SC (rSC) plays some role in pursuit as well. The recent finding that the prelude activity of some SC neurons exhibits directional selectivity suggests that the rSC might process visual motion signals relevant for the control of pursuit. We have now tested the activity of buildup neurons in the rSC during the passive viewing of motion stimuli placed within their response field and also during the previewing of visual motion stimuli that were subsequently tracked with pursuit eye movements. We found that rSC buildup neurons typically responded well to motion stimuli, but that they exhibited essentially no selectivity for the direction or speed of visual motion, and that they also responded well to stationary flickering dots. However, during the previewing of visual motion prior to the onset of pursuit, many neurons did exhibit a buildup of activity similar to that exhibited before saccades. These results are inconsistent with the notion that the rSC mediates visual motion signals used to drive pursuit, but instead support the idea that visual motion signals can be used by rSC neurons as part of a mechanism for selecting targets for pursuit and saccades.

Animals↗
PubMed2004

Target selection and the superior colliculus: goals, choices and hypotheses.

The intermediate and deep layers of the superior colliculus (SC) compose a retinotopically organized motor map and are known to be important for the control of saccadic eye movements. However, recent studies have shown that the functions of the SC are not restricted to the motor control of saccades. The pattern of activity observed during multi-saccade movements, combined eye-head movements, and pursuit eye movements argue that activity in the SC indicates the location of the goal, but does not specify the particular movements that will be used to acquire the goal. Prior to the onset of a movement, the SC is involved in representing possible targets, as has been shown by the effects of manipulating target probability and the changes in pre-motor activity found during visual search tasks. A major unresolved issue is how target-related activity in the SC is transformed into the signal that triggers the movement. One possibility is that the levels of activity across the SC motor map correspond to the likelihood of the possible targets, and that the movement decision is based on the neural equivalent of hypothesis testing. In one version of this mechanism, the decision to select a new goal would occur each time the null hypothesis, represented by neurons in the rostral SC, was rejected in favor of an alternative represented elsewhere in the SC.

Attention↗
PubMed2004

Recasting the smooth pursuit eye movement system.

Primates use a combination of smooth pursuit and saccadic eye movements to stabilize the retinal image of selected objects within the high-acuity region near the fovea. Pursuit has traditionally been viewed as a relatively automatic behavior, driven by visual motion signals and mediated by pathways that connect visual areas in the cerebral cortex to motor regions in the cerebellum. However, recent findings indicate that this view needs to be reconsidered. Rather than being controlled primarily by areas in extrastriate cortex specialized for processing visual motion, pursuit involves an extended network of cortical areas, and, of these, the pursuit-related region in the frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation. These recent findings suggest that the pursuit system has a functional architecture very similar to that of the saccadic system. This viewpoint provides a new perspective on the processing steps that occur as descending control signals interact with circuits in the brain stem and cerebellum responsible for gating and executing voluntary eye movements. Although the traditional view describes pursuit and saccades as two distinct neural systems, it may be more accurate to consider the two movements as different outcomes from a shared cascade of sensory-motor functions.

Animals↗
PubMed2003

Shared response preparation for pursuit and saccadic eye movements.

Tracking a single target in the visual world requires coordination between pursuit and saccadic eye movements. The constraints imposed on pursuit and saccade decisions by visual processing and response preparation are difficult to compare because latency differences between the two movements provide different amounts of visual sampling time. The present study compares pursuit and saccade decisions when visual processing was directly manipulated. Human observers were asked to select between two stationary stimuli presented simultaneously at two different locations based on which had the higher contrast. The stimuli were presented for a brief, variable interval and then occluded by masks. Because the masks moved horizontally and were offset vertically, subjects were obliged to make both pursuit and saccadic eye movements to track the mask covering the target stimulus. For each of the exposure durations, we constructed oculometric curves for pursuit and saccades. We found that both systems had similar oculometric thresholds and response biases. The initial pursuit decisions differed from the subsequent saccade decisions on 1-13% of the trials but were the same more often than predicted by independent mechanisms. Moreover, pursuit reversed direction on discordant trials, so that the pursuit decision always matched that of the saccade by the time the saccade was started. These results support the view that, in addition to overlap in early visual areas and the final motor pathways, the pursuit and saccadic systems share processing at the level of response preparation. This shared processing may help ensure the coordination of pursuit and saccadic eye movements in selecting a single target.

Adult↗
PubMed2003

Shared motion signals for human perceptual decisions and oculomotor actions.

A fundamental question in primate neurobiology is to understand to what extent motor behaviors are driven by shared neural signals that also support conscious perception or by independent subconscious neural signals dedicated to motor control. Although it has clearly been established that cortical areas involved in processing visual motion support both perception and smooth pursuit eye movements, it remains unknown whether the same or different sets of neurons within these structures perform these two functions. Examination of the trial-by-trial variation in human perceptual and pursuit responses during a simultaneous psychophysical and oculomotor task reveals that the direction signals for pursuit and perception are not only similar on average but also co-vary on a trial-by-trial basis, even when performance is at or near chance and the decisions are determined largely by neural noise. We conclude that the neural signal encoding the direction of target motion that drives steady-state pursuit and supports concurrent perceptual judgments emanates from a shared ensemble of cortical neurons.

Eye Movements↗
PubMed2003

Pursuit of the ineffable: perceptual and motor reversals during the tracking of apparent motion.

Pursuit can be guided by perceived rather than physical motion, but the temporal relationship between motion perception and pursuit is unknown. We used an apparent motion stimulus consisting of a horizontal row of evenly spaced Kanizsa illusory squares (1.44 deg2): the illusory contours appeared at the midpoints of the illusory squares presented in the previous frame, producing bi-directional apparent motion of the illusory contours (21.5 deg/s) that could be reversed at will. We measured eye movements in five subjects asked to (1) track the motion of the illusory squares, and (2) reverse the perceived direction while continuing to track the squares. We measured the timing of the voluntary perceptual reversals and compared this to the time course of the reversal in tracking direction. We found that subjects could smoothly track the apparent motion of illusory squares and also produce saccade-free reversals in pursuit velocity. The time course of these motor reversals closely followed the measurements of the perceptual reversal and, on average, the perceptual reversals preceded the pursuit reversals by 53 ms, a delay shorter than when the perceptual reversal was visually guided. Smooth pursuit and the perception of motion direction were in temporal register and highly correlated, suggesting that pursuit can provide a real-time readout for the state of motion perception.

Adult↗
PubMed2003

Neuronal activity in the rostral superior colliculus related to the initiation of pursuit and saccadic eye movements.

The extinction of the central fixated stimulus before the appearance of a new target stimulus reduces the latency of saccades and pursuit, a phenomenon known as the "gap effect." The superior colliculus (SC) plays a prominent role in the gap effect for saccades, and recent data indicate that this structure also plays some role in the control of pursuit. We now show that the firing rate of buildup neurons in the rostral SC exhibits a gap effect during the initiation of both pursuit and saccadic eye movements to parafoveal targets. Most neurons exhibited an increase in tonic activity starting approximately 100 msec after the offset of the fixation spot, regardless of whether the target later appeared inside or outside of the response field of the neuron. The subsequent appearance of the target in the response field evoked phasic increases in activity that were approximately twice as large as the effects on tonic activity. For both pursuit and saccades, the levels of tonic and phasic activity were inversely correlated with latency on a trial-by-trial basis. These changes in activity provide a neuronal correlate for the shared effects on latency observed previously with the gap paradigm for pursuit and saccades. Finally, the phasic activity at pursuit onset exhibited a gap effect just like the target-evoked response, whereas the burst activity at saccade onset was fixed in amplitude. These results suggest how SC neurons may coordinate the initiation of pursuit and saccades: buildup activity may gate the initiation of pursuit, whereas it indirectly triggers saccades by recruiting a saccade-related burst.

Animals↗
PubMed2003

Going for the goal.

Explore the source record for details and available documents.

Action Potentials↗
PubMed2003

Cancelling of pursuit and saccadic eye movements in humans and monkeys.

The countermanding paradigm provides a useful tool for examining the mechanisms responsible for cancelling eye movements. The key feature of this paradigm is that, on a minority of trials, a stop signal is introduced some time after the appearance of the target, indicating that the subject should cancel the incipient eye movement. If the delay in giving the stop signal is too long, subjects fail to cancel the eye movement to the target stimulus. By modeling this performance as a race between a go process triggered by the appearance of the target and a stop process triggered by the appearance of the stop signal, it is possible to estimate the processing interval associated with cancelling the movement. We have now used this paradigm to analyze the cancelling of pursuit and saccades. For pursuit, we obtained consistent estimates of the stop process regardless of our technique or assumptions--it took 50-60 ms to cancel pursuit in both humans and monkeys. For saccades, we found different values depending on our assumptions. When we assumed that saccade preparation was under inhibitory control up until movement onset, we found that saccades took longer to cancel (humans: approximately 110, monkeys: approximately 80 ms) than pursuit. However, when we assumed that saccade preparation includes a final "ballistic" interval not under inhibitory control, we found that the same rapid stop process that accounted for our pursuit results could also account for the cancelling of saccades. We favor this second interpretation because cancelling pursuit or saccades amounts to maintaining a state of fixation, and it is more parsimonious to assume that this involves a single inhibitory process associated with the fixation system, rather than two separate inhibitory processes depending on which type of eye movement will not be made. From our behavioral data, we estimate that this ballistic interval has a duration of 9-25 ms in monkeys, consistent with the known physiology of the final motor pathways for saccades, although we obtained longer values in humans (28-60 ms). Finally, we examined the effect of trial sequence during the countermanding task and found that pursuit and saccade latencies tended to be longer if the previous trial contained a stop signal than if it did not; these increases occurred regardless of whether the preceding trial was associated with the same or different type of eye movement. Together, these results suggest that a common inhibitory mechanism regulates the initiation of pursuit and saccades.

Adolescent↗
PubMed2003

Effects of learning on smooth pursuit during transient disappearance of a visual target.

Previous research has demonstrated learning in the pursuit system, but it is unclear whether these effects are the result of changes in visual or motor processing. The ability to maintain smooth pursuit during the transient disappearance of a visual target provides a way to assess pursuit properties in the absence of visual inputs. To study the long-term effects of learning on nonvisual signals for pursuit, we used an operant conditioning procedure. By providing a reinforcing auditory stimulus during periods of accurate tracking, we increased the pursuit velocity gain during target blanking from 0.59 in the baseline session to 0.89 after 8 to 10 daily sessions of training. Learning also reduced the occurrence of saccades. The learned effects generalized to untrained target velocities and persisted in the presence of a textured visual background. In a yoked-control group, the reinforcer was independent of the subjects' responses, and the velocity gain remained unchanged (from 0.6 to 0.63, respectively, before and after training). In a control group that received no reinforcer, gain increased slightly after repetition of the task (from 0.63 to 0.71, respectively, before and after training). Using a model of pursuit, we show that these effects of learning can be simulated by modifying the gain of an extra-retinal signal. Our results demonstrate that learned contingencies can increase eye velocity in the absence of visual signals and support the view that pursuit is regulated by extra-retinal signals that can undergo long-term plasticity.

Acoustic Stimulation↗
PubMed2002

Reaching for answers.

The exact function of motor cortex continues to be an enigma. In this issue of Neuron, present provocative data showing that microstimulation of the precentral cortex evokes complex movements, and conclude that the motor and premotor cortex together may form a single map of complex postures.

Animals↗
PubMed2002

Primacy of spatial information in guiding target selection for pursuit and saccades.

Previous studies have examined the facilitative effects of prior spatial information on target selection for saccadic eye movements. More recently, studies have shown that prior spatial information also influences target selection for smooth pursuit. However, direct comparisons of the effects of prior information on target selection for pursuit and saccades have not been made. To this end, we provided different classes of prior information and measured their effects on target selection for pursuit and saccades. In Experiment 1, we assessed the relative effects of spatial cues (indicating the target stimulus' initial location) and color cues (indicating the target stimulus' color) on eye movement latencies. In Experiment 2, we assessed the effects of motion cues (indicating the target stimulus' direction of motion) in addition to spatial cues. For both pursuit and saccades, we found that spatial cues reduced eye movement latencies more than color cues (Experiment 1). Spatial cues also reduced eye movement latencies more than motion cues (Experiment 2), even for pursuit, despite the fact that stimulus motion is essential for the generation of pursuit eye movements. These results indicate that both pursuit and saccades are affected to a greater degree by spatial information than motion or color information. We suggest that the primacy of spatial information for both pursuit and saccades reflects the importance of spatial attention in selecting the stimulus target for both eye movements.

Adolescent↗