Biomedical subjects

Mark Shelhamer

Publications and source records attributed to Mark Shelhamer.

17 recordsLinked to original sources

PubMed2007

Sensory versus motor information in the control of predictive saccade timing.

Humans readily make predictive saccades to periodic alternating targets. This predictive behavior depends on internal monitoring of timing error of past saccades in order to determine the time of initiation of future saccades; our earlier studies have confirmed this by finding correlations between latencies of consecutive predictive saccades. It is natural to consider that timing error is determined by visual detection of the difference between the time the target appears and the time the eyes arrive at the target; this in turn implies that saccades must actually be produced in order for their timing errors to be determined and predictive saccade timing to be established. We tested this hypothesis by having subjects view alternating visual targets while fixating a central target in order to eliminate saccade production. After six alternating target presentations, subjects began tracking the alternating targets. Tracking performance was assessed with an error measure that compared saccade latency and inter-saccade interval with desired values (zero and inter-stimulus interval, respectively). Errors in this Prior Viewing paradigm were compared to those from a conventional De Novo paradigm in which saccades began as soon as the alternating targets were presented. Saccades under Prior Viewing reached a low-error steady predictive state more rapidly than under De Novo tracking. The initial saccade under Prior Viewing had a higher latency than the others, suggesting that this saccade was reactive even though the paradigm is predictable; other reasons for this higher latency include time to disengage from the fixation target and time required to pre-program the initial set of saccades. The results show that visual detection of timing error from an actual motor act (saccades) is not necessary to establish predictive saccadic pacing: sensory-only information from viewing the moving targets can help to establish this predictive state.

Brain↗

PubMed2006

An internal clock generates repetitive predictive saccades.

Previously we demonstrated the presence of a behavioral phase transition between reactive and predictive eye tracking of alternating targets. Prior studies of repetitive movements have proposed that an "internal clock" is the neural mechanism by which interval timing is achieved. In the present report we tested whether predictive oculomotor (saccade) tracking is based on an internal time reference (clock) by examining the effect of transient perturbations to the periodic pacing stimulus. These perturbations consisted of altering the timing of the stimulus (abruptly increasing or decreasing the inter-stimulus interval) or extinguishing the targets altogether. Although reactive tracking (at low pacing rates) was greatly affected by these timing perturbations, once predictive tracking was established subjects continued to time their eye movement responses at the pre-existing rate despite the perturbation. As expected from certain clock models, inter-stimulus intervals for predictive tracking followed Weber's law and the scalar property (timing variability increases in proportion to interval duration), but this was not true for reactive tracking. In addition, the perturbation results show that subjects can establish an internal representation of target pacing (the internal clock) in as little as two eye-movement intervals, which suggests that this mechanism is relevant for real-world situations. These findings are consistent with the presence of an internal clock for the generation of these predictive movements, and demonstrate that the neural mechanism responsible for this behavior is temporally accurate and flexible.

Humans↗

PubMed2006

Responses to noisy periodic stimuli reveal properties of a neural predictor.

In programming motor acts, the brain must consider both internal and external noise sources: inherent variation in sensory estimates and changes within the environment. An interesting question in motor control is how reliable responses can be programmed in the face of noise and how these two noise sources interact. We study this by investigating the generation of sequences of predictive saccades to visual targets. First, eight normal subjects tracked targets that alternated at a pacing frequency (0.9 Hz) that promoted predictive behavior, for 300 trials. When tracking this perfectly periodic stimulus, there was variability in the timing of the saccades (intersaccade intervals) that was distributed around the interval of the stimulus (556 ms). We used this inherent variability to set the timing of subsequent stimuli; subjects completed three additional sessions in which the variance of the stimulus timing (the interstimulus intervals) had the same (1.0 SD), less (0.5 SD), or more (2.0 SD) variability than the subject displayed when tracking the perfectly periodic stimulus. Despite changes in stimulus timing variability, variance of the response timing (intersaccade intervals) was equal to the variance of the stimulus plus "inherent variance" (response variance when tracking a perfectly periodic stimulus). Examining the correlations between saccade latency and interstimulus interval, this relationship is interpreted as a tradeoff between reliance on previous saccade performance (intertrial correlations) and reliance on the current stimulus.

Algorithms↗

PubMed2006

Pursuit and saccadic tracking exhibit a similar dependence on movement preparation time.

Data from previous human and primate studies on saccadic and smooth pursuit eye movements suggest that there are shared internal inputs (for example, perception, attention, expectation, and memory) for the initiation of the two types of movements. Additional reports examining the effect of preparation time on movement responses have shown that when ample time is allowed subjects usually generate long-latency "reactive" responses. When the time allowed to prepare a movement is short, however, subjects respond with reduced latency and often anticipate the stimulus ("predictive" response). Based on these findings, we believe that the shared internal inputs at early stages of movement preparation may result in saccade and pursuit eye movements demonstrating the same dependence on preparation time despite acting through different neural pathways further downstream. Previously we demonstrated a behavioral "phase transition" when normal subjects tracked alternating targets with saccades. When preparation time was long (low-frequency pacing) subjects made reactive saccades (latency approximately 180 ms). As preparation time monotonically decreased (pacing frequency increased), there was an abrupt transition to a predictive response (latency <100 ms). In the present study we show that a similar transition exists in smooth pursuit tracking and that the point of transition between the two behaviors is the same for both systems. In other words, the same behavior (reactive versus predictive) is selected when pursuit and saccade tracking are tested under the same time constraints. This provides further evidence that the two types of movements are different motor outcomes of a common decision process.

Eye Movements↗

PubMed2005

Sequences of predictive eye movements form a fractional Brownian series--implications for self-organized criticality in the oculomotor system.

When human subjects are presented with a pair of visual targets that alternate periodically, they track the targets with rapid eye movements known as saccades. In previous work we demonstrated that at low pacing rates (<0.5 Hz), saccades have a latency of about 180 ms, and the latencies are uncorrelated from trial to trial. At high pacing rates (>0.6 Hz), latencies are much shorter: subjects make predictive saccades that anticipate target motion. The predictive latencies are correlated and appear to form a fractional Brownian motion. Here we confirm this finding by examining the rate of decay of nonlinear forecasting of predictive latencies. We further characterize the nature of predictive saccade latencies through the use of detrended fluctuation analysis and surrogate data. These results lead us to conclude that predictive saccades may exhibit a form of self-organized criticality, which enables rapid response to changes in stimulus timing. We provide an experimental demonstration of this.

Humans↗

PubMed2005

Context-specific adaptation of saccade gain is enhanced with rest intervals between changes in context state.

Dual-state adaptation of motor responses has been known for some time. A more recent development is a form of dual-state adaptation known as "context-specific adaptation," which was explored through the use of saccade gain adaptation. In this model, two different adapted saccade gains are associated with two different states of a context cue, and the gain switches between the two adapted states when the context cue changes state. Such adaptation is imposed by alternating context/adaptation states over the course of an adaptation session. Here, vertical eye position as a context cue for adaptation of horizontal saccade gain is used: gain increase is induced with the eyes up 10 degrees, and gain decrease with the eyes down 10 degrees. This context cue is not very effective: there is interference between context/adaptation conditions such that gain-decrease adaptation with eyes down transfers to the eyes-up (gain-increase) context. It was hypothesized that the juxtaposition in time of the alternating adaptation states exacerbated this interference. In order to test this, one-minute rest breaks were inserted between each change in context/adaptation state. The resulting context-specific adaptation improved dramatically: gain-increase and gain-decrease adaptations were more rapid and more complete. This resembles consolidation of motor learning, which, however, occurs over much longer time spans (hours rather than minutes). Thus, the results may reflect the operation of a novel "short-term" motor consolidation process.

Acclimatization↗

PubMed2005

Automatic detection of camera translation in eye video recordings using multiple methods.

A concern with video eye movement tracking is that movement of the camera headset relative to the head creates an artifact of eye movement in pupil-detection software. We describe the development of, and compare the results of, three automatic image processing algorithms to measure camera movement. The best of the algorithms has an average accuracy of 1.3 pixels, equivalent to 0.49 deg with our eye tracking system.

Automation↗

PubMed2005

Cerebellar influence in oculomotor phase-transition behavior.

The cerebellum plays an important role in predicting and producing regularly timed motor responses. In the present report, we examined the performance of SCA6 cerebellar patients and normal controls in a simple saccade tracking task. Our results imply that the cerebellum plays a role in maintaining predictive motor behavior once it has been established.

Cerebellum↗

PubMed2004

Sequences of predictive saccades are correlated over a span of approximately 2 s and produce a fractal time series.

We previously demonstrated that there is an abrupt (rather than smooth) transition between reactive and predictive modes of eye-movement tracking of target lights (a phase transition). We also found evidence that the sequence of eye movements in the reactive mode was independent, whereas those in the predictive mode were correlated and possibly formed a random fractal sequence. Here we confirm the finding of fractal structure by quantifying the rate of decay of nonlinear forecasting when applied to these data. We also estimate the window over which consecutive trials are correlated and show that the duration of this window is fixed in time rather than number of trials. These results have implications for the neural mechanisms that drive predictive movements.

Fractals↗

PubMed2004

Acquisition of context-specific adaptation is enhanced with rest intervals between changes in context state, suggesting a new form of motor consolidation.

We previously showed that the saccadic system could be adapted in a context-specific manner: two different adapted gains could be associated with two different context cues, with the gain state switched when the context state was switched. This was accomplished by alternating context/adaptation states several times over the course of an adaptation session, and assessing saccade gain in each context state before and after adaptation. One context cue we studied was vertical eye position; an adaptive gain increase was induced with the eyes up 10 degrees, and an adaptive gain decrease with the eyes down 10 degrees. This context cue was only partially effective: there was considerable undesired transfer of adaptation from the eyes-down condition (gain-decrease) to the eyes-up condition (gain-increase), with the result that there was little or no gain-increase adaptation. One explanation for this is that the two context/adaptation states, presented one after the other, interfered with each other. In the present study, we tested this hypothesis by interposing one-minute rest intervals between each alternation in context/adaptation state. The resulting context-specific adaptation is greatly improved (relative to the case when there are no rest intervals): both gain-increase and gain-decrease adaptations are stronger and occur more rapidly. This effect resembles that found in studies on the consolidation of motor learning, although such consolidation is believed to occur over much longer time spans (hours rather than minutes).

Acclimatization↗

PubMed2003

Saccades exhibit abrupt transition between reactive and predictive; predictive saccade sequences have long-term correlations.

To compensate for neural delays, organisms require predictive motor control. We investigated the transition between reaction and prediction in saccades (rapid eye movements) to periodically paced targets. Tracking at low frequencies (0.2-0.3 Hz) is reactive (eyes lag target) and at high frequencies (0.9-1.0 Hz) is predictive (eyes anticipate target); there is an abrupt rather than smooth transition between the two modes (a "phase transition," as found in bistable physical systems). These behaviors represent stable modes of the oculomotor control system, with attendant rapid switching between the neural pathways underlying the different modes. Furthermore, predictive saccades exhibit long-term correlations (slow decay of the autocorrelation function, manifest as a 1/f alpha spectrum). This indicates that predictive trials are not independent. The findings have implications for the understanding of predictive motor control: predictive performance during a given trial is influenced by a feedback process that takes into account the latency of previous trials.

Humans↗

PubMed2003

Short-term adaptation of the VOR: non-retinal-slip error signals and saccade substitution.

We studied short-term (30 min) adaptation of the vestibulo-ocular reflex (VOR) in five normal humans using a "position error" stimulus without retinal image motion. Both before and after adaptation a velocity gain (peak slow-phase eye velocity/peak head velocity) and a position gain (total eye movement during chair rotation/amplitude of chair motion) were measured in darkness using search coils. The vestibular stimulus was a brief ( approximately 700 ms), 15 degrees chair rotation in darkness (peak velocity 43 degrees /s). To elicit adaptation, a straight-ahead fixation target disappeared during chair movement and when the chair stopped the target reappeared at a new location in front of the subject for gain-decrease (x0) adaptation, or 10 degrees opposite to chair motion for gain-increase (x1.67) adaptation. This position-error stimulus was effective at inducing VOR adaptation, though for gain-increase adaptation the primary strategy was to substitute augmenting saccades during rotation while for gain-decrease adaptation both corrective saccades and a decrease in slow-phase velocity occurred. Finally, the presence of the position-error signal alone, at the end of head rotation, without any attempt to fix upon it, was not sufficient to induce adaptation. Adaptation did occur, however, if the subject did make a saccade to the target after head rotation, or even if the subject paid attention to the new location of the target without actually looking at it.

Adaptation, Physiological↗

PubMed2003

Context-specific adaptation and its significance for neurovestibular problems of space flight.

It has been demonstrated that various vestibular and oculomotor responses can be adapted in a context-specific manner: different adapted states are associated with different states of a prevailing context cue, and a change in the context state triggers a change in the response such that each adapted state is associated with a given context state. We review selected literature on context-specific adaptation, including our own recent results on adaptation of saccades, pursuit, and the linear and angular vestibulo-ocular reflexes (LVOR and AVOR), and suggest some ways in which context-specific adaptation might be useful as a countermeasure to the adverse neurovestibular effects of space flight.

Adaptation, Physiological↗

PubMed2002

Sensory, motor, and combined contexts for context-specific adaptation of saccade gain in humans.

Saccadic eye movements can be adapted in a context-specific manner such that their gain can be made to depend on the state of a prevailing context cue. We asked whether context cues are more effective if their nature is primarily sensory, motor, or a combination of sensory and motor. Subjects underwent context-specific adaptation using one of three different context cues: a pure sensory context (head roll-tilt right or left); a pure motor context (changes in saccade direction); or a combined sensory-motor context (head roll-tilt and changes in saccade direction). We observed context-specific adaptation in each condition; the greatest degree of context-specificity occurred in paradigms that used the motor cue, alone or in conjunction with the sensory cue.

Adaptation, Physiological↗

PubMed2002

Context-specific adaptation of saccade gain.

Previous studies established that vestibular reflexes can have two adapted states (e.g., gain) simultaneously, and that a context cue (e.g., vertical eye position) can switch between the two states. The present study examined this phenomenon of context-specific adaptationfor horizontal saccades, using a variety of contexts. Our overall goal was to assess the efficacy of different context cues in switching between adapted states. A standard double-step paradigm was used to adapt saccade gain. In each experiment, we asked for a simultaneous gain decrease in one context and gain increase in another context, and then determined if a change in the context would invoke switching between the adapted states. Horizontal eye position worked well as a context cue: saccades with the eyes deviated to the right could be made to have higher gains while saccades with the eyes deviated to the left could be made to have lower gains. Vertical eye position was less effective. This suggests that the more closely related a context cue is to the response being adapted, the more effective it is. Roll tilt of the head, and upright versus supine orientations, were somewhat effective in context switching; these paradigms contain orientation of gravity with respect to the head as part of the context.

Adaptation, Psychological↗

PubMed2002

Context-specific adaptation of the gain of the oculomotor response to lateral translation using roll and pitch head tilts as contexts.

Previous studies established that vestibular and oculomotor behaviors can have two adapted states (e.g., gain) simultaneously, and that a context cue (e.g., vertical eye position) can switch between the two states. The present study examined this phenomenon of context-specific adaptation for the oculomotor response to interaural translation (which we term "linear vestibulo-ocular reflex" or LVOR even though it may have extravestibular components). Subjects sat upright on a linear sled and were translated at 0.7 Hz and 0.3 gpeak acceleration while a visual-vestibular mismatch paradigm was used to adaptively increase (x2) or decrease (x0) the gain of the LVOR. In each experimental session, gain increase was asked for in one context, and gain decrease in another context. Testing in darkness with steps and sines before and after adaptation, in each context, assessed the extent to which the context itself could recall the gain state that was imposed in that context during adaptation. Two different contexts were used: head pitch (26 degrees forward and backward) and head roll (26 degrees or 45 degrees, right and left). Head roll tilt worked well as a context cue: with the head rolled to the right the LVOR could be made to have a higher gain than with the head rolled to the left. Head pitch tilt was less effective as a context cue. This suggests that the more closely related a context cue is to the response being adapted, the more effective it is.

Adaptation, Psychological↗

PubMedDate not supplied

Context-specific adaptation of saccade gain in parabolic flight.

Previous studies established that vestibular reflexes can have two adapted states (e.g., gains) simultaneously, and that a context cue (e.g., vertical eye position) can switch between the two states. Our earlier work demonstrated this phenomenon of context-specific adaptation for saccadic eye movements: we asked for gain decrease in one context state and gain increase in another context state, and then determined if a change in the context state would invoke switching between the adapted states. Horizontal and vertical eye position and head orientation could serve, to varying degrees, as cues for switching between two different saccade gains. In the present study, we asked whether gravity magnitude could serve as a context cue: saccade adaptation was performed during parabolic flight, which provides alternating levels of gravitoinertial force (0 g and 1.8 g). Results were less robust than those from ground experiments, but established that different saccade magnitudes could be associated with different gravity levels.

Adaptation, Physiological↗