Biomedical subjects

C J S Collins

Publications and source records attributed to C J S Collins.

6 recordsLinked to original sources

PubMed2006

The occluded onset pursuit paradigm: prolonging anticipatory smooth pursuit in the absence of visual feedback.

Humans can produce anticipatory smooth pursuit (ASP) for a few hundred ms prior to the appearance of a moving target. Once visual feedback is available, however, it is difficult to distinguish ASP from the visually-driven response with which it merges. Here we have developed a paradigm that extends the anticipatory period to show unequivocally how ASP can evolve over periods of up to 600 ms before being influenced by visual feedback. ASP was evoked by repeated presentation of constant velocity (ramp) stimuli preceded by auditory cues. The target was occluded during the initial part of the ramp, so that when it became visible it had already moved to an eccentric position. The occlusion period (T occ) varied from 0 to 500 ms in 100 ms increments; for each period ramps were presented in blocks of 8 with velocity held constant at 8, 16, 24 or 32 degrees/s. Eye displacement trajectories showed that subjects attempted to match the unseen target trajectory with a mixture of saccades and smooth pursuit. The smooth component was initiated progressively earlier in relation to target appearance as T occ increased, leading to progressively higher ASP gains by the time the target became visible. This prolongation of ASP throughout the occlusion period reveals the underlying internal drive that produces it, a drive that under normal circumstances quickly becomes masked by visual feedback.

Cues↗

PubMed2005

Attention and selection for predictive smooth pursuit eye movements.

Humans cannot typically produce smooth eye movements in the absence of a moving stimulus. However, they can produce predictive smooth eye movements if they expect a target of a known velocity to reappear. Here, we observed that participants could extract velocity information from two simultaneously presented moving targets in order to produce a subsequent predictive smooth eye movement for one of the two targets. Subjects fixated a stationary cross during the presentation of two targets, moving rightward at different velocities. In the next presentation, a single target was presented, which participants tracked with their eyes. A static cue, presented 700 ms before the moving target, indicated which of the two targets would be presented. Predictive eye movements were of an appropriate velocity, even when participants did not know in advance which of the two targets would subsequently be cued. However, the scaling of predictive eye velocity was marginally less accurate in this divided attention condition than when participants knew the identity of the cued target in advance, or a single target was presented during fixation. In a second experiment, we found that the velocity cued on the previous trial had a greater effect than the uncued velocity on the current trial. The negligible effect of the uncued velocity indicates that participants were extremely effective at selectively reproducing one of two recently viewed velocities. However, other influences, such as past history, also affected predictive smooth eye movements.

Adult↗

PubMed2005

Scaling of smooth anticipatory eye velocity in response to sequences of discrete target movements in humans.

We investigated the ability to generate anticipatory smooth pursuit to sequences of constant velocity (ramp) stimuli of increasing complexity. Previously, it was shown that repeated presentation of sequences composed of four ramps with two speeds in two directions, evoked anticipatory smooth pursuit after only one or two presentations. Here, sequences of four or six ramps, each having a choice of four speeds and either one or two directions (uni- or bi-directional) were examined. The components of each sequence were presented as discrete ramps (duration: 400 ms; randomised velocity: 10-40 degrees/s), each starting from the centre with 1,200 ms periods of central fixation between ramps, allowing anticipatory activity to be segregated from prior eye movement. Auditory warning cues occurred 600 ms prior to each target presentation. Anticipatory smooth eye velocity was assessed by calculating eye velocity 50 ms after target onset (V 50), prior to the availability of visual feedback. Despite being required to re-fixate centre during inter-ramp gaps, subjects could still generate anticipatory smooth pursuit with V 50 comparable to single speed control sequences, but with less accuracy. In the steady state V 50 was appropriately scaled in proportion to upcoming target velocity for each ramp component and thus truly predictive. Only one to two repetitions were required to attain a steady-state for unidirectional sequences (four or six ramps), but three or four repeats were required for bi-directional sequences. Results suggest working memory can be used to acquire multiple levels of velocity information for prediction, but its use in rapid prediction is compromised when direction as well as speed must be retained.

Acoustic Stimulation↗

PubMed2005

Predicting the duration of ocular pursuit in humans.

This study examines the effects of expectation on the timing of ocular pursuit termination. Human subjects pursued repeated, constant velocity (15 or 30 degrees/s) target motion stimuli (ramps), moving left or right. Ramps were of constant duration (RD = 240, 480, 720 or 960 ms), resulting in anticipatory slowing of eye velocity prior to ramp termination and target extinction. At unexpected intervals RD was increased or decreased, but velocity remained constant. When RD increased eye velocity continued to decline, even though the target remained visible and continued to move. It took approximately 180 ms before eye velocity started to recover towards the steady state velocity level for the continued target motion. When RD decreased, eye velocity continued as if for a longer ramp duration, again taking approximately 180 ms before eye velocity started to decrease. These results suggest that timing of the response to the expected ramp duration had been pre-programmed on the basis of prior experience of ramp duration. Moreover, adjustments to timing occurred rapidly, within the second presentation of the new RD. Responses were compared to control conditions with randomised ramp duration. Eye velocity declined later in the controls for RD < or = 720 ms, as expected, but exhibited similar decline in predictable and randomised conditions for RD = 960 ms. Further controls established that eye velocity could only be reliably maintained until the end of the ramp when the target was expected to continue in motion after the end of the ramp. The results suggest that estimates of stimulus duration are made continuously in all conditions, based on expectancy of target termination.

Adult↗

PubMed2004

Target selection for predictive smooth pursuit eye movements.

Previous work has indicated that after exposure to a moving stimulus, people are able to produce predictive smooth eye movements prior to reappearance of the stimulus. Here, we investigated whether subjects are able to extract relevant velocity information from two simultaneously presented targets and use this information to produce a subsequent predictive response. A trial consisted of a series of two or five presentations of moving stimuli, preceded 500 ms earlier by an audio warning cue. In the first one or four presentations, subjects fixated during the presentation of two moving targets and in the final presentation they tracked a single moving target. During fixation, two moving targets were presented concurrently, originating from the fixation point and moving horizontally to the right at differing velocities (10, 20, 30 or 40 degrees /s), with each target being presented at the same velocity throughout a trial. In the tracking presentation, the fixation cross was extinguished and only a single target was presented, which the subjects were required to track with their eyes. To cue which of the two targets would be presented, the appropriate target was presented statically at the same time as the audio warning cue. A significant relationship was found between eye velocity 100 ms after the start of the tracking target (i.e. prior to visual feedback) and the cued target velocity. Thus, subjects were able to make predictive eye movements that were of appropriate velocity for the cued target, despite fixating and being uncertain which target was relevant, during previous exposure.

Adult↗

PubMed2002

Factors affecting the longevity of a short-term velocity store for predictive oculomotor tracking.

Fast (up to 30 degrees /s) anticipatory smooth pursuit eye movements can be built up with repeated transient motion stimuli. It is thought that such stimuli charge a putative internal store of velocity information that can then drive anticipatory movements in the absence of a target. The aim of this study was to investigate the longevity of this store. Previous experiments with single ramp stimuli (Wells and Barnes 1998) suggested that the store lasts for only a few seconds before decaying to a baseline level. In the current study we investigate the possibility that the store was not maximally charged by single stimuli, precipitating its decay. The magnitude of the anticipatory response was indexed by smooth eye velocity 100 ms after target onset ( V(100)). In experiment 1 the build-up of the anticipatory response was examined by presenting sets of stimuli (comprising from one to five ramps) within a tracking phase and leaving a dark period (the 'gap') of 9.6 s between successive tracking phases. Each ramp was preceded by an audio warning cue and was accompanied throughout its 480 ms duration by an audio tone. Audio cues continued during the gap to reinforce timing information. V(100) for the first and last ramps of each set increased as the number of ramps was increased from one to three but reached an asymptotic level thereafter, suggesting that the velocity store is maximally charged after three presentations. In experiment 2 the store was maximally charged by presenting five ramps in each tracking phase and its decay was examined by leaving gaps of either 7.2 s or 14.4 s between successive tracking phases. V(100) was not diminished after either gap interval. In experiment 3 the velocity store was less well consolidated during tracking phases comprising two ramps. V(100) for the first response after the gap was unaffected by the 7.2 s gap interval but was significantly reduced when the gap interval was 14.4 s. The interval between the warning cue and ramp onset strongly influenced the magnitude of the anticipatory response, the optimum level being elicited by a cue time of 600 ms. In conclusion, this study has shown that the internal velocity store can be sustained for periods as long as 14.4 s provided that it is initially charged to a sufficiently high level and that accurate external timing cues are provided. Furthermore, we provide evidence to suggest that this process may be controlled by a two-part sample and hold mechanism.

Acceleration↗