Biomedical subjects

Paul C Knox

Publications and source records attributed to Paul C Knox.

6 recordsLinked to original sources

PubMed2006

The effect of Kanizsa's compression illusion on reflexive saccades.

When a horizontal line is occluded by a surface, subjects misperceive the line as being compressed, that is, they perceive it to be shorter than it really is (Kanizsa's compression illusion). The size of the compression effect usually ranges from approximately 5 to 10%. In this experiment, subjects executed reflexive saccades from one end of a line, presented at fixation, to the other end, with and without an occluding square. Saccade amplitude was reduced in all subjects in the presence of an occluding square; the effect averaged about 5%. These results demonstrate that saccade amplitude is modified by the Kanizsa compression illusion. They provide further evidence that reflexive saccade amplitude can be altered by illusion inducing stimuli, to the same degree as perceptual effects, even in circumstances in which other motor behaviours resist the illusion. They are difficult to reconcile with any strong version of the "two visual systems hypothesis".

Brain↗

PubMed2005

Age-related changes in smooth pursuit initiation.

Quantitative analysis of eye movements is a useful tool for examining the behavioural effects of ageing. Although the effect of ageing on saccadic eye movement has been examined in some detail, the effect of ageing on a second class of eye movement, smooth pursuit (SP), has received less attention. We examined the initiation of SP in a group of fifteen healthy older people (mean age 72 years) and compared their performance with that of ten young controls (mean age 21 years). Although their performance was qualitatively similar, pursuit latency was increased in the older group. Investigation of the gap effect on pursuit revealed that, while the gap effect was present in the older group, it seemed to be directionally asymmetrical. When the longer absolute latencies were taken into account, although the gap effect in the two groups was identical for leftward tasks, for rightward tasks it was reduced in the older group, although this did not reach statistical significance. The difference between the old and young groups was driven by some of the older subjects. At the longest gap duration employed (400 ms), while there was a clear gap effect for leftward tasks in these subjects, there was no reduction in latency, or increases in latency, for rightward tasks. This asymmetry was not related to chronological age within the older group. These results suggest an age-related alteration in SP initiation that is more complex than general slowing of information processing in ageing. They may be indicative of additional ageing effects specific to the oculomotor or closely related systems.

Adult↗

PubMed2005

Detection of the slipped extraocular muscle after strabismus surgery.

PURPOSE: To estimate the frequency of slipped extraocular muscle (SM) in cases of repeat strabismus surgery and to compare the results of a number of putative preoperative tests for the detection of SM, with direct intraoperative inspection and histologic confirmation. DESIGN: Retrospective analysis of audit records followed by prospective interventional case series. PARTICIPANTS: Case records of 715 adults presenting for repeat surgical correction of horizontal strabismus. Five patients suspected of having horizontal SM were consecutively recruited for the prospective case series. METHODS: Slipped extraocular muscle frequency was determined retrospectively from surgical outcome data in 715 adult horizontal strabismus surgeries in 1 center. A separate prospective study examined 5 participants with putative SM in detail. Gaze-dependent clinical tests (squint magnitude, eye movement range, palpebral fissure widening, naked-eye saccadic velocity estimation, intraocular pressure change) and saccade main sequence parameters (infrared eye-tracking) were recorded before and 2 weeks after corrective surgery. Intraoperative diagnosis was made by direct muscle examination and confirmed by histologic examination. Length of slippage and forced duction tests were intraoperative outcome measures. RESULTS: Slipped extraocular muscle frequency was 10.6%. The muscles of 3 participants in the prospective series were slipped. Direct intraoperative inspection accurately identified all SM cases. All other clinical tests produced false-positive and false-negative results, although SM was suggested preoperatively by limited eye movement range. Distorted saccadic velocity profiles were significantly (P<0.0001) associated with prior strabismus surgery. Saccade main sequence parameters were not diagnostic for SM. CONCLUSIONS: Direct intraoperative inspection can accurately diagnose SM. Preoperative diagnosis of long-standing SM was not possible. In contradiction to a previous report, SM was not reliably associated with reduced peak saccadic velocity. The association between distorted saccadic velocity profiles and previous strabismus surgery is, to our knowledge, reported here for the first time.

Adult↗

PubMed2003

Spatial mapping of the remote distractor effect on smooth pursuit initiation.

In order to extract information from the visual world, it is necessary to bring the images of objects of interest to rest on the high acuity part of the retina, the fovea. Primates, including humans, use two types of eye movement, saccades and smooth pursuit, to accomplish this. While classically conceived of as being separate and distinct, various lines of evidence indicate a close linkage between these two eye movement systems. They are often investigated at a behavioural level by presenting subjects with single targets to saccade or to track. We investigated the effect of presenting a single stationary distractor at various positions in the visual field at the same time as a moving target which subjects were instructed to track. We found that while a stationary distractor presented in the contralateral visual field and part of the ipsilateral visual field increased pursuit latency in an eccentricity dependent manner, a distractor presented in the ipsilateral visual field, within 45 degrees of the axis along which the pursuit target moved, had no effect on latency. We found no evidence that within this region distractors modified eye velocity during the early part of the pursuit response. This spatial pattern of the effect of a stationary distractor on pursuit latency is very similar to the effect of distractors on saccade latency. Our results provide behavioural evidence supporting the hypothesis that the processes that determine when an eye movement is made are linked, but that those determining the form of that eye movement are substantially independent.

Analysis of Variance↗

PubMed2002

Non-target influences on the initiation of smooth pursuit.

Smooth pursuit is usually regarded as a relatively stereotyped oculomotor response in which the early part of the response reflects primarily the properties of the visual stimulus and cortical motion processing. We have investigated pursuit initiation in human subjects using the gap paradigm to alter fixation conditions, and single stationary distractors to alter visual context. The results suggest that a number of processes, distinct from motion processing, are involved in pursuit initiation. The processes which are modified by gaps and distractors are closely related and interact with each other. They may be shared with the saccade system.

Adult↗

PubMed2002

Oculomotor control in a group of very low birth weight (VLBW) children.

VLBW infants are at risk of lesions including intraventricular haemorrhage and periventricular leucomalacia. Those with normal IQ still present with reading difficulties. Oculomotor performance was assessed on 14 VLBWs (IQ > 85) and 15 full-term age-matched controls. Anti-saccade errors were significantly higher for the VLBWs (78%) compared to full terms (62%) (P = 0.02). Smooth pursuit latency was longer for the VLBWs compared to the full terms. Greater anti-saccade errors may be indicative of a lesion affecting the frontal cortex or developmental delay. Oculomotor deficits in VLBW children may be associated with the higher incidence of reading difficulties that have been reported.

Child↗