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Biomedical subjects

R J Croft

Publications and source records attributed to R J Croft.

At least 19 recordsLinked to original sources
PubMed2006

The sensitivity of human event-related potentials and reaction time to mobile phone emitted electromagnetic fields.

There is some evidence to suggest that exposure to mobile phones (MPs) can affect neural activity, particularly in response to auditory stimuli. The current investigation (n = 120) aimed to test recent findings in this area, namely that N100 amplitude and latency would decrease, and that P300 latency and reaction time (RT) would increase under active relative to sham exposure during an auditory task. Visual measures were also explored. A double blind, counterbalanced, crossover design was employed where subjects attended two sessions 1 week apart. In both sessions participants (1) performed auditory and visual oddball tasks while electroencephalogram (EEG) was recorded with a MP set to sham exposure mounted over the temporal region, and (2) performed the same tasks while the handset was set to active/sham. When active, the MP transmitted for 30 min at 895 MHz (average power 250 mW, pulse modulated at 217 Hz, average SAR 0.11 W/kg). Paired t-tests compared difference scores from the sham/sham session to those from the sham/active condition. The study was designed to detect differences of 1\4 of a standard deviation with a power of 0.80. There was no significant difference between exposure conditions for any auditory or visual event related potential (ERP) component or RT. As previous positive findings were not replicated, it was concluded that there is currently no evidence that acute MP exposure affects these indices of brain activity.

Adolescent↗
PubMed2004

The effects of dexamphetamine on simulated driving performance.

RATIONALE: The number of road fatalities related to the presence of amphetamines in drivers has been relatively constant over the past 10 years. However, there remains uncertainty as to the extent that these drugs induce driving impairment, and whether any such impairments translate to an increase in road fatalities. OBJECTIVES: To examine the acute effects of 0.42 mg/kg dexamphetamine on simulated driving performance, and to establish which, if any, simulated driving abilities become impaired following dexamphetamine administration. METHODS: A repeated-measures, counter-balanced, double-blind, placebo-controlled design was employed. Twenty healthy volunteers completed two treatment conditions-0.42 mg/kg dexamphetamine and placebo. Performance was assessed using a driving simulator task. Blood and saliva samples were obtained prior to the driving tasks and immediately after task completion (120 min and 170 min post-drug administration, respectively). RESULTS: Mean dexamphetamine blood concentrations were 83 ng/ml and 98 ng/ml at 120 min and 170 min, respectively. Results indicated a decrease in overall simulated driving ability following dexamphetamine administration during the day-time but not the night-time scenario tasks. Contributing to this performance reduction, "incorrect signalling", "failing to stop at a red traffic light" and "slow reaction times" were the behaviours most strongly affected by dexamphetamine. CONCLUSIONS: The decrease in simulated driving ability observed during the day-time driving tasks are consistent with the perceptual narrowing or tunnel vision that is associated with dexamphetamine consumption.

Adult↗
PubMed2004

Chronic MDMA (ecstasy) use, cognition and mood.

RATIONALE: It has been suggested that 3,4-methylenedioxymethamphetamine (MDMA or ecstasy) causes damage to the serotonergic system, and that this damage results in cognitive and mood impairments. OBJECTIVES: To examine the effect of chronic MDMA usage on a wide battery of cognitive tests and psychological abilities and processes. METHODS: In the present study, the performance of 17 participants with a history of MDMA use was compared to the performance of 15 control subjects on a battery of neuropsychological tests. This battery included tests for depression, immediate word recall, delayed recall, attention and working memory. RESULTS: Results indicated that the MDMA group had significantly higher scores for depression than the control group, and displayed poorer delayed recall and verbal learning than controls after accounting statistically for the effects of cannabis and depression. CONCLUSIONS: These results suggest that MDMA users exhibit difficulties in coding information into long-term memory, display impaired verbal learning, are more easily distracted, and are less efficient at focusing attention on complex tasks.

Adult↗
PubMed2003

The use of a 'phantom scalp' to assess the possible direct pickup of mobile phone handset emissions by electroencephalogram electrode leads.

A 'Phantom Scalp' was constructed from a conducting foam mat to form a layer under a 62-electrode electroencephalogram cap closely approximating the electrical properties of a human scalp. The mat was placed over a polystyrene manikin head to preserve a correct anatomical arrangement. Electrical signals were recorded with a Global System for Mobile (GSM) communications mobile phone handset in place against the side of the 'head'. Amplitude spectra were compared for three phone conditions: 'off', 'standby' and 'transmit'. At 217 Hz, significant differences were obtained between 'transmit' and the other two conditions, but no significant differences were noted for the physiologically important range 0.5-30 Hz. An anomalous difference was noted for one electrode in the range 30-45 Hz.

Cell Phone↗
PubMed2003

Target-to-target interval versus probability effects on P300 in one- and two-tone tasks.

The P3(00) is an electrophysiological index of neural processing that varies with such stimulus parameters as interstimulus interval (ISI) and target probability, with a common view being that it reflects an endogenous form of memory update. Building on previous research, we argue that relations between P3 amplitude and both ISI and probability may be attributable to the target-to-target interval (TTI). Employing between-subject (Experiment 1; N = 24) and within-subject (Experiment 2; N = 10) designs, the present paper addresses this by testing subjects on a standard two-tone auditory oddball task as well as a one-tone task. In both studies, P3 amplitude increased and latency decreased linearly with TTI, and these relations were relatively unaffected by ISI or probability. This suggests that ISI and probability per se do not independently affect P3 amplitude, and that TTI offers a strong explanation of the reported relations between P3 amplitude and both ISI and probability.

Acoustic Stimulation↗
PubMed2001

Associations of P50 suppression and desensitization with perceptual and cognitive features of "unreality" in schizotypy.

BACKGROUND: P50 suppression is an electrophysiologic index of early sensory gating and has consistently been found deficient in schizophrenic patients. This gating deficit is thought to lead to sensory overload and cognitive fragmentation, and correspondingly many symptoms of the disorder. However, the link between P50 suppression deficits and symptomatology is yet to be established, and so this study was designed to determine whether such a relation is present within a nonclinical population. METHODS: P50 suppression and schizotypy measures were obtained from 36 healthy volunteers, and correlation analyses determined whether measures of schizotypy were related to P50 suppression. RESULTS: Consistent with the view that P50 gating deficits are related to schizophrenic symptoms, subjects with poorer P50 suppression reported more perceptual anomalies and magical ideation--an unreality syndrome--in contrast to other positive symptoms and to withdrawal. This study also found a trend to P50 suppression desensitization, and that whereas subjects low on "unreality" demonstrated desensitization to the second of the paired clicks, subjects high on "unreality" demonstrated sensitization. CONCLUSIONS: It is concluded that early sensory gating deficits, in the form of poor P50 suppression, are related to unreality aspects of schizotypy. This supports the view that poor P50 suppression in schizophrenia is related to symptomatology.

Cognition Disorders↗
PubMed2001

The relative contributions of ecstasy and cannabis to cognitive impairment.

RATIONALE: (+/-)-3,4-methylenedioxymethamphet-amine (MDMA; 'ecstasy'), a commonly used recreational drug, has typically been found to be related to poor cognitive function in humans. However, cannabis consumption may not have been adequately controlled for in these studies. OBJECTIVE: The present study was designed to further elucidate the relation between MDMA and cannabis in cognitive impairment. METHODS: Subjects who had used neither MDMA nor cannabis (controls; n=31), cannabis but not MDMA (cannabis users; n=18) and both MDMA and cannabis (MDMA/cannabis users; n=11) were compared on a battery of neuropsychological tests. RESULTS: The cannabis and MDMA/cannabis groups did not differ on any of the tests, whereas the combined cannabis and MDMA/cannabis groups performed more poorly than controls on tests of memory, learning, word fluency, speed of processing and manual dexterity. Further, apart from speed of processing where higher MDMA consumption predicted slower processing, covariate analysis revealed that the deficits were more closely related to cannabis than MDMA usage. CONCLUSION: The results suggest that cannabis is an important confound in studies of MDMA-related cognitive impairment, and that previously reported cognitive impairment in MDMA users may have been caused by coincident cannabis use.

Adolescent↗
PubMed2001

Electrophysiological evidence of serotonergic impairment in long-term MDMA ("ecstasy") users.

OBJECTIVE: "Ecstasy," or 3,4-methylenedioxymethamphetamine (MDMA), causes long-term impairment to the serotonin (5-HT) system in rats, dogs, and nonhuman primates. 5-HT dysfunction has also been observed in human recreational users of the drug, but whether 5-HT dysfunction in humans is caused by MDMA has not been established, since dysfunction may have preceded MDMA exposure. This ambiguity about causation is particularly important in MDMA research, because 5-HT deficiency is a predictor of risky behavior. METHOD: The 5-HT function of 22 long-term MDMA users was compared to that of 20 drug-naive comparison subjects and 19 cannabis users. 5-HT function was assessed with the intensity dependence paradigm, a tool that measures 5-HT-related attenuation of neural response to auditory stimuli (measured with EEG). RESULTS: Long-term MDMA users exhibited 5-HT dysfunction, relative to both cannabis users and drug-naive comparison subjects. This dysfunction was related to total MDMA consumption (after removing the effect of frequency of use) but not to frequency of use (after removing the effect of total consumption). CONCLUSIONS: These data show that 5-HT dysfunction occurs in MDMA users, is related to users' MDMA consumption, and is independent of cannabis use. The results do not suggest that self-medication explains this relationship, because the deficit was related to total MDMA consumption but not frequency of consumption. The results are thus consistent with the thesis that MDMA consumption causes 5-HT impairment in humans.

Acoustic Stimulation↗
PubMed2001

The CE mark for implantable medical devices.

A vital aspect of device and implant regulation is continual surveillance of performance after approval. Most permanent implants seldom reveal their strengths and weaknesses until they have been in clinical use for a number of years. It is vital that clinicians report problems to the manufacturers and regulatory authorities to enable emerging hazards to be promptly identified.

Equipment Safety↗
PubMed2000

Gamma and beta frequency oscillations in response to novel auditory stimuli: A comparison of human electroencephalogram (EEG) data with in vitro models.

Investigations using hippocampal slices maintained in vitro have demonstrated that bursts of oscillatory field potentials in the gamma frequency range (30-80 Hz) are followed by a slower oscillation in the beta 1 range (12-20 Hz). In this study, we demonstrate that a comparable gamma-to-beta transition is seen in the human electroencephalogram (EEG) in response to novel auditory stimuli. Correlations between gamma and beta 1 activity revealed a high degree of interdependence of synchronized oscillations in these bands in the human EEG. Evoked (stimulus-locked) gamma oscillations preceded beta 1 oscillations in response to novel stimuli, suggesting that this may be analogous to the gamma-to-beta shift observed in vitro. Beta 1 oscillations were the earliest discriminatory responses to show enhancement to novel stimuli, preceding changes in the broad-band event-related potential (mismatch negativity). Later peaks of induced beta activity over the parietal cortex were always accompanied by an underlying gamma frequency oscillation as seen in vitro. A further analogy between in vitro and human recordings was that both gamma and beta oscillations habituated markedly after the initial novel stimulus presentation.

Acoustic Stimulation↗
PubMed2000

Removal of ocular artifact from the EEG: a review.

Eye movements cause changes to the electric fields around the eyes, and consequently over the scalp. As a result, EEG recordings are often significantly distorted, and their interpretation problematic. A number of methods have been proposed to overcome this problem, ranging from the rejection of data corresponding temporally to large eye movements, to the removal of the estimated effect of ocular activity from the EEG (EOG correction). This paper reviews a number of such methods of dealing with ocular artifact in the EEG, focusing on the relative merits of a variety of EOG correction procedures. Issues discussed include the distinction between frequency and time domain approaches, the number of EOG channels required for adequate correction, estimating correction coefficients from raw versus averaged data, differential correction of different types of eye movement, the most suitable statistical procedure for estimating correction coefficients, the use of calibration trials for the estimation of correction coefficients, and the distinction between 'coefficient estimation' and 'correction phase' error. A suggested EOG correction algorithm is also described.

Artifacts↗
PubMed2000

EOG correction: comparing different calibration methods, and determining the number of epochs required in a calibration average.

OBJECTIVE: A form of EOG correction called the 'aligned-artifact average' (AAA) method has been advanced by the authors. In contrast to many previous methods, this used a correction coefficient (B), based on an average of eye movements rather than raw data, to remove eye movement related contamination from the EEG. The first part of this study was aimed at determining whether a variation of this procedure that is more easily implemented would produce a similar correction. The second part was designed to determine the number of epochs needed in an average to correct adequately. METHODS: Subjects performed a series of eye movement tasks whilst EEG and EOG data were recorded. Data were manipulated according to either the AAA or an alternate new ERP method (NERP) and the resultant Bs were compared in part A. In part B, averages were created from varying numbers of epochs, and the resultant r(2)-values were compared. RESULTS: The AAA and NERP methods produced the same Bs, and averages with at least 40 epochs were required for adequate B estimation for both VEOG and HEOG. CONCLUSION: There is no difference between the AAA and NERP methods and thus it is acceptable to use the more easily implemented NERP method for EOG correction. It is recommended that when applying this procedure, at least 40 epochs should be used to make up the averages from which to calculate correction coefficients.

Adolescent↗
PubMed2000

EOG correction of blinks with saccade coefficients: a test and revision of the aligned-artefact average solution.

OBJECTIVE: The 'aligned-artefact average' (AAA) procedure was advanced by the authors as a technique suitable for removing eye movement-related artefacts from the EEG. It was proposed that this method would correct both blink and non-blink artefact from the EEG, using the same set of correction coefficients (Bs). However, recent evidence suggests that this simplification is not always accurate. Thus, we test here a revision of the AAA, including an appropriate allowance for the radial EOG (REOG) component, that does allow the use of the same Bs for the correction of blink and non-blink artefact. METHODS: Blink (and saccade) ERP data from 15 subjects were corrected using the AAA method, with Bs calculated from the same blink (and saccade) data set (referent waveforms), or a different set of blink (and saccade) data, or using the new revised AAA procedure (RAAA). RESULTS: AAA Bs calculated from saccades corrected blinks poorly (and vice versa). However, the RAAA Bs corrected blink ERPs better than blink-derived Bs, and saccade ERPs better than saccade-derived Bs. It was also found that irrespective of correction type, inclusion of REOG improved correction. CONCLUSION: EOG correction is more accurate when the radial channel is included, but inclusion of REOG (and/or HEOG) is not sufficient to resolve the discrepancy between blink and saccade correction. Using the RAAA procedure, both blink and non-blink data can be corrected using the same set of Bs.

Adolescent↗
PubMed2000

EOG correction: which regression should we use?

Electrooculogram (EOG) correction is used to remove eye-movement-related contamination from electroencephalograms (EEG). Correction is reliant on the regression procedure, although when multiple EOG channels are used in the correction, the appropriate type of regression to use is not known. In the present study, we aimed to resolve this matter. Computer simulations were used to compare the simultaneous, multiple-stage, and single-channel regression methods of correction. EOG propagation was modeled on prior findings, under conditions of varying vertical and horizontal EOG (VEOG/HEOG) correlation. The dependent variable was the correlation between the uncontaminated and the corrected EEG. The simultaneous regression procedure gave the best correction, with its advantage increasing as a function of VEOG/HEOG correlation. It is recommended that the simultaneous regression procedure be used for EOG correction of the EEG.

Algorithms↗
PubMed1998

EOG correction: a new perspective.

OBJECTIVE: In the field of EOG correction, discrepancies have been found between the propagation rates for different types and frequencies of eye movement. This study attempted to determine whether these differences could be explained by the affect of EOG magnitude on the correction procedure. METHODS: Experiment 1 utilized simulated data to determine whether the combination of EOG magnitude and other forms of interference distorted the estimation of the propagation coefficients (Bs). Experiment 2 used real data to determine if the patterns obtained in Expt. 1 were apparent in real data. RESULTS: Matched t tests found that simulated low power EOG produced inflated Bs as a function of interference in Expt. 1. Experiment 2 found the same relationship between B and EOG magnitude as with the simulations. CONCLUSIONS: The findings are consistent with the thesis that eye movement related fields propagate similarly for a range of EOG types and frequencies, suggesting that the B differences reported in the literature are artifactual, and indicating the need for a new correction procedure.

Adult↗
PubMed1998

EOG correction: a new aligned-artifact average solution.

OBJECTIVE: In the field of EOG correction, discrepancies have been found between the propagation rates for different types and frequencies of eye movement. However, Croft and Barry demonstrated that these differences can be explained by the affect of EOG magnitude on the correction procedure (Croft, R.J. and Barry, R.J. EOG correction: a new perspective. Electroenceph. clin. Neurophysiol., 1998, 107: 387-394). This study utilized a new 'aligned-artifact average' technique (AAA) to examine whether propagation is constant across eye movement types and frequencies, and tested the AAA as an EOG correction tool. METHODS: Two experiments manipulated interference levels in real data sets to determine if interference affected propagation coefficients (Bs). The third tested real data for the effect of forward propagation of eye movement related neural potentials on Bs, and the fourth utilized computer simulations to assess the effectiveness of the new AAA correction procedure. RESULTS: Interference was found to inflate B at low EOG amplitude, and its removal removed B variation and inflation. The forward propagation of eye movement related neural potentials had very little effect on B. The AAA procedure produced near perfect corrections of the simulated data, superior to a comparison method. CONCLUSIONS: EOG propagation is constant across eye movement types and frequencies, and thus only one correction coefficient should be calculated and applied to data. The AAA method provides a more accurate correction and makes possible, for the first time, the adequate correction of posterior sites.

Artifacts↗