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Biomedical subjects

P C Knox

Publications and source records attributed to P C Knox.

17 recordsLinked to original sources
PubMed2001

The effect of scleral search coil lens wear on the eye.

BACKGROUND/AIM: Scleral search coils are used to measure eye movements. A recent abstract suggests that the coil can affect the eye by decreasing visual acuity, increasing intraocular pressure, and damaging the corneal and conjunctival surface. Such findings, if repeated in all subjects, would cast doubt on the credibility of the search coil as a reliable investigative technique. The aim of this study was to reassess the effect of the scleral search coil on visual function. METHODS: Six volunteer subjects were selected to undergo coil wear and baseline measurements were taken of logMAR visual acuity, non-contact tonometry, keratometry, and slit lamp examination. Four drops of 0.4% benoxinate hydrochloride were instilled before insertion of the lens by an experienced clinician. The lens then remained on the eye for 30 minutes. Measurements of the four ocular health parameters were repeated after 15 and 30 minutes of lens wear. The lens was then removed and the health of the eye reassessed. RESULTS: No obvious pattern of change was found in logMAR visual acuity, keratometry, or intraocular pressure. The lens did produce changes to the conjunctival and corneal surfaces, but this was not considered clinically significant. CONCLUSION: Search coils do not appear to cause any significant effects on visual function. However, thorough prescreening of subjects and post-wear checks should be carried out on all coil wearers to ensure no adverse effects have been caused.

Adult↗
PubMed2001

Modification of smooth pursuit initiation by a nonvisual, afferent feedback signal.

PURPOSE: To investigate the role of extraocular muscle afferent signals in the initiation and early maintenance of smooth-pursuit eye movements. METHODS: A suction scleral contact lens was used to impede the movements of the right eye while subjects tracked small targets in a step-ramp pursuit paradigm. Movements of the left eye were measured by infrared oculography. Pursuit latency, eye acceleration, and velocity were analyzed trial-by-trial and compared before, while, and after the right eye was impeded. RESULTS: When the right eye was impeded, initial acceleration and eye velocity were reduced. Pursuit latency was unchanged. The velocity effect had a rapid onset and offset; there was no evidence that the effects built up over a number of trials. Detailed analysis suggested that the reduction in velocity occurred approximately 40 msec after pursuit was initiated. CONCLUSIONS: These results are consistent with the hypothesis that extraocular muscle afferent signals provide a feedback signal of the movements of the eyes that may be used to modify the initiation and early maintenance of smooth pursuit on-line. It appears that for pursuit, as with saccades, the priority in these conditions is to maintain conjugacy.

Adult↗
PubMed2000

Afferent signals from the extraocular muscles affect the gain of the horizontal vestibulo-ocular reflex in the alert pigeon.

We have shown previously that the gain of the horizontal vestibulo-ocular reflex (HVOR) is modified by afferent signals from extraocular muscle proprioceptors in the decerebrate pigeon. We have now analysed the variability of the HVOR in intact, alert pigeons and, using the artificial vestibulo-ocular reflex method, have found that in all of the pigeons tested afferent signals from the extraocular muscle proprioceptors modify the gain, but not the phase, of the HVOR. While this effect was seen in a given bird only on some occasions, when present it was consistent in magnitude and direction and closely similar to our previous observations on decerebrate pigeons. These results from alert, intact birds strengthen the evidence that extraocular muscle afferent signals play a part in the control of the vestibulo-ocular reflex.

Animals↗
PubMed2000

Modification of visually guided saccades by a nonvisual afferent feedback signal.

PURPOSE: To investigate the role of extraocular muscle afferent signals in the control of saccadic eye movements. METHODS: A suction scleral contact lens was used to impede the movements of the right eye while subjects executed visually guided saccades to briefly presented targets. Movements of the left eye were measured using infrared oculography. Saccade amplitude, peak velocity, and duration were analyzed trial by trial and compared before, during, and after the right eye was impeded. RESULTS: When the right eye was impeded, the amplitudes of saccades executed by the left eye were reduced. There was no alteration in the main sequence relationships. The amplitude effect had a rapid onset and offset. There was no evidence that the effects built up over a number of trials, nor was there evidence that individual saccades were modified on-line. CONCLUSIONS: These results are consistent with the hypothesis that extraocular muscle afferent signals provide a feedback signal of the movements of the eyes that is used to produce rapid adjustments of oculomotor output when required.

Adult↗
PubMed1999

Smooth pursuit latency in gap and non-gap conditions in schizophrenic subjects.

It has been demonstrated in normal subjects that smooth pursuit latency is reduced in gap pursuit tasks. We have now measured smooth pursuit latency in a group of schizophrenic subjects in both gap and non-gap conditions. In non-gap tasks pursuit latency was longer in the schizophrenic subjects than in controls. While the addition of gaps produced reductions in pursuit latency in the schizophrenic subjects, the effect was more variable than in controls, with a greater asymmetry between rightward and leftward pursuit latencies. Our results are consistent with the hypothesis that pursuit initiation is modified in schizophrenia and that as with the gap effect on saccades, the gap effect on pursuit is also modified.

Adult↗
PubMed1999

Modification of smooth pursuit initiation by target contrast.

It has been demonstrated that in gap pursuit tasks, smooth pursuit latency is reduced. This 'gap effect' is modified by factors such as gap duration and task context. We have now investigated whether it is also modified by an important visual parameter--the contrast of the pursuit and fixation targets. We found that while pursuit target contrast is an important determinant of pursuit latency, fixation target contrast had very little effect on pursuit latency. Neither pursuit nor fixation target contrast altered the gap effect on pursuit latency. Our results suggest that while visual parameters, like contrast, may modify the visual processing underlying pursuit initiation, the processing underlying the gap effect is separate and distinct.

Attention↗
PubMed1998

Stimulus predictability and the gap effect on pre-saccadic smooth pursuit.

There is disagreement in the literature as to whether smooth pursuit latency is reduced when a temporal gap is introduced between the extinction of a central fixation target and the illumination of an eccentric moving target. This study confirms that in human subjects smooth pursuit latency is reduced by gaps and that the magnitude of the reduction is related to the duration of the gap. However, latency is not solely determined either by visual factors or by task parameters such as spatial predictability, but is affected by task context. The results suggest a role for non-visual factors such as attention in the initiation of pursuit.

Adult↗
PubMed1997

The directional effects of passive eye movement on the directional visual responses of single units in the pigeon optic tectum.

We have investigated the visual responses of 184 single units located in the superficial layers of the optic tectum (OT) of the decerebrate, paralysed pigeon. Visual responses were similar to those reported in non-decerebrate preparations; most units responded best to moving visual stimuli, 18% were directionally selective (they had a clear preference for a particular direction of visual stimulus movement), 76% were plane-selective (they responded to movement in either direction in a particular plane). However, we also found that a high proportion of units showed some sensitivity to the orientation of visual stimuli. We examined the effects of extraocular muscle (EOM) afferent signals, induced by passive eye movement (PEM), on the directional visual responses of units. Visual responses were most modified by particular directions of eye movement, although there was no unique relationship between the direction of visual stimulus movement to which an individual unit responded best and the direction of eye movement that caused the greatest modification of that visual response. The results show that EOM afferent signals, carrying information concerning the direction of eye movement, reach the superficial layers of the OT in the pigeon and there modify the visual responses of units in a manner that suggests some role for these signals in the processing of visual information.

Afferent Pathways↗
PubMed1996

The effect of the gap paradigm on the latency of human smooth pursuit of eye movement.

When a temporal gap is introduced between the extinction of a central fixation target and the illumination of an eccentric target (the gap paradigm), normal human subjects initiate saccadic eye movements towards the eccentric target at lower latency than when there is no gap. The aim of this study was to examine the latency of human smooth pursuit eye movements using a modified gap paradigm. Smooth pursuit latency was reduced in gap tasks, and the magnitude of reduction was related to the duration of the gap. The distribution of smooth pursuit latencies was also altered. It thus appears that human smooth pursuit latency is modulated in a similar manner to saccade latency in gap tasks.

Adult↗
PubMed1993

Afferent signals from the extraocular muscles of the pigeon modify the vestibulo-ocular reflex.

Although the extraocular muscles (EOM) contain stretch receptors it is generally thought that the afferent signals which they provide play no role in the control of eye movement. We have previously shown that these afferent signals do modify both the vestibular responses of single units in the oculomotor control system and the electromyographic responses of the EOM during the vestibulo-ocular reflex (VOR). We have now investigated the effect of EOM afferent signals on the VOR itself, by recording the electro-oculogram of one eye while imposing movements on the other eye during the VOR. Moving the eye in a manner which mimics the slow phase of the VOR, we have found that, as the peak velocity of the imposed eye movement increases, the amplitude of eye movement of the other eye decreases. These results confirm that the output of the VOR itself, expressed as movement of the globe, and not merely some of its component parts, is modified by EOM afferent signals.

Afferent Pathways↗
PubMed1993

Evidence for corrective effects of afferent signals from the extraocular muscles on single units in the pigeon vestibulo-oculomotor system.

The role of extraocular muscle (EOM) afferent feedback signals in the control of eye movement is still controversial. We recorded from 106 single units in the vestibular nuclei, oculomotor nuclei and reticular formation of 80 decerebrate, paralysed pigeons. EOM afferents were stimulated by passive eye movement (PEM) during vestibular stimulation by sinusoidal oscillation in the horizontal plane. We found that EOM afferent signals profoundly modified the vestibular responses of 91 (86%) of the single units recorded. As well as using PEM to simulate eye movements similar to saccades, we moved the eye in a manner which mimicked the slow phase of the vestibulo-ocular reflex (artificial VOR, AVOR). We have found evidence that, as well as providing signals closely related to the parameters of eye movement, PEM alters the vestibular responses of cells during AVOR in a manner which suggests that EOM afferent signals may play a corrective role in the moment-to-moment control of eye movement in the vestibulo-ocular reflex.

Animals↗
PubMed1991

Afferent signals from the extraocular muscles of the pigeon modify the electromyogram of these muscles during the vestibulo-ocular reflex.

There is no general agreement on whether afferent signals from the extraocular muscles play any part in oculomotor control. However, we have previously shown that they modify the responses of cells in the oculomotor control system during the vestibulo-ocular reflex (VOR). If, as we suspect, these signals have an important role in the control of the VOR from moment-to-moment, we should be able to demonstrate similar, functionally significant, modifications at the output of the reflex. We have recorded the electromyographic activity of several extraocular muscles of the right eye during the VOR and while imposing movements on the left eye. We describe how the activity of the muscles, reflected in the electromyogram, is modified in specific ways depending on the parameters of the imposed eye movements. The effects of the extraocular afferent signals on the eye-muscle responses to vestibular drive during the slow phase of the VOR appear to be corrective. Thus the present results provide strong evidence that afferent signals from the extraocular muscles are concerned in the control of the reflex from moment-to-moment, and suggest that the wider question of their role in oculomotor control merits further consideration.

Afferent Pathways↗
PubMed1991

Afferent signals from pigeon extraocular muscles modify the vestibular responses of units in the abducens nucleus.

Although the extraocular muscles contain stretch receptors it is generally believed that their afferents exert no influence on the control of eye movement. However, we have shown previously that these afferent signals reach various brainstem centres concerned with eye movement, notably the vestibular nuclei, and that the decerebrate pigeon is a favourable preparation in which to study their effects. If the extraocular muscle afferents do influence oculomotor control from moment-to-moment they should exert a demonstrable effect on the oculomotor nuclei. We now present evidence that extraocular muscle afferent signals do, indeed, alter the responses of units in an oculomotor nucleus (the abducens, VI nerve nucleus, which supplies the lateral rectus muscle) to horizontal, vestibular stimulation induced by sinusoidal oscillation of the bird. Such stimuli evoke a vestibulo-ocular reflex in the intact bird. The extraocular stretch receptors were activated by passive eye movement within the pigeon's saccadic range; such movements modified the vestibular responses of all 19 units studied which were all, histologically, in the abducens nucleus. The magnitude of the effects, purely inhibitory in 15 units, depended both on the amplitude and the velocity of the eye movement and most units showed selectivity for particular combinations of plane (e.g. horizontal versus vertical) and direction (e.g. rostral versus caudal) of eye movement. The results show that an afferent signal from the extraocular muscles influences vestibularly driven activity in the abducens nucleus to which it carries information related to amplitude, velocity, plane and direction of eye movement in the saccadic range. They thus strongly support the view that extraocular afferent signals are involved in the control of eye movement.

Abducens Nerve↗
PubMed1990

Optimization and application of particle beam high-performance liquid chromatography/mass spectrometry to compounds of pharmaceutical interest.

Particle beam high-performance liquid chromatography/mass spectrometry (HPLC/MS) parameters were optimized for the sensitive analysis of several drugs in agricultural products such as milk and tissue. Sensitivity of the particle beam interface was greatest for solvents with low heat capacities (methanol greater than acetonitrile greater than isopropanol greater than water). Furthermore, optimal sensitivity was obtained at low solvent flow rates (about 0.4-0.6 ml min-1). Parameters such as desolvation temperature, helium flow rate to the nebulizer and nebulizer position resulted in minimal change in sensitivity. The source temperature was optimized to obtain suitable vaporization with minimal thermal degradation (200-300 degrees C). The determination of a variety of compounds (including beta-lactams, cephapirin, tetracyclines, methylene blue, furosemide, spectinomycin, cytidine, 2-chloro-4-nitrobenzamide and thiamine) was possible using the particle beam interface. Under full-scan conditions, detection limits were in the 100 ng range for most drugs. With selected ion monitoring, particle beam HPLC/MS was demonstrated for the analysis of p.p.m. levels of these drugs in milk and tissue extracts. Precision of the particle beam analysis was usually better than 15% RSD. For the same compounds, HPLC/MS with a thermospray interface often resulted in less structural information (single ion spectra) then obtained by particle beam with thermospray detection limits varying from 10 ng to 1 microgram.

Animals↗
PubMed1990

Directionally-specific effects of afferent signals from the extraocular muscles upon responses in the pigeon brainstem to horizontal vestibular stimulation.

The responses of single units in the brainstem of the decerebrate, paralysed, pigeon were studied. Natural vestibular stimulation was provided by horizontal, sinusoidal, oscillation of the bird and extraocular muscle afferents of the ipsilateral eye were activated by passive eye-movement. Unit responses to vestibular and/or orbital stimuli were examined in sets of peristimulus time histograms interleaved in time. Of 352 units in the brainstem, in the region of the vestibular nuclei, which were exposed to the effects of both vestibular stimuli and passive eye-movement, 40 (11%) responded only to the latter; the other 312 units (89%) responded to vestibular stimulation at 0.4 Hz (amplitude +/- 8 degrees). Of these 312 units, 129 (41%) were affected only by vestibular stimuli; in the other 183 units (59%) passive eye-movement produced clear modification of the vestibular responses by adding excitation or inhibition, or both. There were phasic modifications in most units; in 77 there were longer-lasting changes in the vestibular responses, often following a phasic response. In 124 units whose responses were subjected to statistical analysis, the vestibular responses of 42 (34%) were modified only by horizontal eye-movement and eight (6%) were affected only by vertical movement. A further 18% showed larger effects from horizontal than from vertical eye-movement; in 2% vertical eye-movement was preferred. Further examination of the specificity of the effects of eye-movement in planes between the vertical and horizontal was possible in 29 units which showed various degrees of "tuning" of the effect. In some units there was additional specificity for eye-movement in (a) particular directions (towards the beak rather than towards the tail, for example); (b) in particular arcs of the orbit (centre-to-temporal rather than nasal-to-centre, for example). Note that all these effects were upon the responses of the units to horizontal vestibular stimulation. Thus, the modifications of the vestibular responses depended upon specific characteristics of the passive eye-movement. The exact recording sites of 29 units were determined histologically; some were in the medial vestibular nucleus but many were in the adjacent reticular formation. The principal interest of the results is that they provide more detailed information than was available previously on the specificity of the effects of afferent signals from the extraocular muscles upon the vestibular responses of units in regions of the brainstem known to be involved in oculomotor control. The decerebrate pigeon proves to be a particularly good preparation in which to study these effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMedDate not supplied

Using naltrexone in inpatient alcoholism treatment.

Naltrexone has been used successfully in outpatient settings as an adjunct to alcoholism treatment. This study examines the efficacy of using naltrexone in an inpatient treatment setting. Sixty-three alcohol-dependent patients who volunteered for a double-blind, placebo-controlled study were followed over the course of their 20 days in treatment and six months follow-up. A comparison group of 59 patients who did not volunteer were also studied over the same period of time. Patients in the study group were randomly assigned to receive naltrexone or placebo. Information was gathered daily on alcohol craving, drug craving and moods on self-reporting forms from the naltrexone and placebo groups, and from the comparison group. Follow-up data was gathered through self-report and through Washington State's TARGET management information system. No significant differences were found in craving scores while in treatment, nor in recidivism after treatment.

Adolescent↗