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Robert J Snowden

Publications and source records attributed to Robert J Snowden.

At least 19 recordsLinked to original sources
PubMed2006

A new continuous within-participants latent inhibition task: examining associations with schizotypy dimensions, smoking status and gender.

A within-participants latent inhibition task (LI: the retardation in learning that occurs if a stimulus has first been presented without consequence) was developed that produced robust LI, using both reaction time and number of correct responses as the dependent measures. Furthermore, associations were found between the preexposed stimulus and the schizotypy dimension of unusual experiences and smoking status. There were no significant relationships with the non-preexposed stimulus. This pattern of results was found using both reaction time and number of correct responses as the dependent measures which indicates that individuals who smoke and those high in unusual experiences show reduced LI. These findings indicate that the task may have sufficient sensitivity to be used in patients with schizophrenia.

Adolescent↗
PubMed2006

Alerting and orienting in Alzheimer's disease.

Recently, researchers (E. Festa-Martino, B. R. Ott, & W. C. Heindel, 2004; A. Tales, J. L. Muir, A. Bayer, R. Jones, & R. J. Snowden, 2002; A. Tales, J. L. Muir, A. Bayer, & R. J. Snowden, 2002 have found significantly abnormal spatial orienting together with the abolishment of the alerting effect in Alzheimer's disease (AD). However, these research groups differed in their interpretation of the results. A. Tales, J. L. Muir, A. Bayer, R. Jones, and R. J. Snowden (2002) and A. Tales, J. L. Muir, A. Bayer, and R. J. Snowden (2002) explained their data in terms of two independent processes, whereas E. Festa-Martino et al. (2004) interpreted their findings as indicative of an inverse association, namely that the increased spatial orienting effect in AD was the direct result of the abolition of the phasic alerting effect. In this further study examining exogenous spatial orienting and phasic alerting, the authors present evidence to suggest that the increased spatial orienting effect in AD is not the result of a decreased phasic alerting effect.

Aged↗
PubMed2006

Motion perception in the ageing visual system: minimum motion, motion coherence, and speed discrimination thresholds.

We aimed to address two issues: first, to describe how the perception of motion differs in elderly observers as compared to younger ones; and, second, to see if these changes in motion perception could be accounted for by the known changes in the ability of elderly observers to detect patterns (as indexed via contrast sensitivity). The lower threshold of motion, motion coherence, and speed discrimination were measured, alongside contrast sensitivity, in a group of thirty-two older (mean age 61.5 years) and thirty-two younger (mean age 23.2 years) subjects. The older observers showed losses in their ability to detect slow motions as indexed via the lower threshold of motion for random-dot patterns and for gratings of a range of spatial frequencies. They also were impaired on a test of motion coherence, but only for stimuli of a slow to medium speed, whereas faster speeds showed no decline with age. Finally, at all speeds tested the older observers required greater differences in speed in order to discriminate between patterns moving at different speeds. The pattern of losses on motion perception tasks was not predicted by the deficits of the older groups, such as loss of detection thresholds for high spatial and/or temporal frequencies. It is concluded that these hypotheses do not provide an adequate account of the data, and therefore that the losses occurring with age are complex and probably are a result of the loss of several types of cell.

Adolescent↗
PubMed2005

The relevance of irrelevance to schizophrenia.

Jeffrey Gray's neuropsychological theory of the positive symptoms of schizophrenia has been highly influential by enabling a strong link between animal and human research. Central to the development and testing of this theory has been the phenomenon and paradigm of latent inhibition (LI-the retardation of learning that one stimulus predicts the occurrence of another due to pre-exposure of the first stimulus). We review findings relating to its alteration in patients with schizophrenia (acute and chronic), people high on dimensions of schizotypy and the effects of amphetamine and anti-psychotic medication in humans. We suggest that many human-LI paradigms still suffer from theoretical and practical limitations, but that recent developments are beginning to address these. Finally we explore the idea that the paradigm of Learned Irrelevance (LIRR-the retardation of learning that one stimulus predicts the occurrence of another due to pre-exposure of both stimuli but in an unrelated manner) might be used to complement studies on LI in exploring the cognitive distortions suffered by patients with schizophrenia.

Animals↗
PubMed2005

An implicit test of the associations between children and sex in pedophiles.

Pedophiles are motivated to disguise their thoughts and feelings about their sexual beliefs and attraction toward children. New developments using implicit measures of associations have been successful in accessing socially stigmatic beliefs, even in cases in which the participant is resistant to this disclosure. Using an implicit measure, the authors show that pedophiles have an association between children and sex, whereas nonpedophilic offenders have an association between adults and sex. The task can therefore identify a core cognitive abnormality that may underpin some pedophilic deviant sexual behavior.

Adult↗
PubMed2005

Abnormal visual search in mild cognitive impairment and Alzheimer's disease.

Our aim was to further characterize the clinical concept of mild cognitive impairment (MCI). We examined the status of visual attention-related processing in such patients in relation to healthy older adults and patients with Alzheimer's disease (AD) by measuring performance on a computer-based visual search task. We tested 20 older adult control participants, 13 patients with amnestic mild cognitive impairment and 12 patients with AD. Patients with AD and with MCI exhibited a significant detriment in visual search performance compared to the older adult controls. The deficit in visual search was greater for the patients with AD than the patients with MCI. The pattern of results displayed by the MCI group indicates that patients who appear clinically to suffer only from a deficit in memory also display a deficit in visual attention-related processing, which although not as severe as those with AD, represents a significant detriment in such performance compared to that seen in healthy ageing.

Aged↗
PubMed2005

Abnormal spatial and non-spatial cueing effects in mild cognitive impairment and Alzheimer's disease.

Our aim was to further characterize the clinical concept of mild cognitive impairment (MCI). We examined visual attention-related processing in 12 patients with amnestic MCI, 16 healthy older adults and 16 patients with Alzheimer's disease (AD) by measuring performance on computer-based tests of attentional disengagement, alerting ability, and inhibition of return. Unlike the healthy older controls, the patients with AD and the patients with amnestic MCI exhibited a significant detriment in both the ability to disengage attention from an incorrectly cued location and the ability to use a visual cue to produce an alerting effect. The pattern of results displayed by the MCI group indicates that patients who only appear clinically to suffer from a deficit in memory also display a deficit in specific aspects of visual attention-related processing, which closely resemble the magnitude seen in AD.

Aged↗
PubMed2005

Prepulse inhibition of startle and its moderation by schizotypy and smoking.

Abstract The influences of smoking status and schizotypy on prepulse inhibition (PPI) of the startle eye-blink response were assessed in 71 healthy volunteers, across a wide range of prepulse-to-pulse intervals (50-2020 ms). Multiple regression analyses revealed that nonsmoking participants high in cognitive disorganization showed reduced PPI between 50 and 260 ms, whereas at prepulse intervals of 80 and 140 ms individuals high in introvertive anhedonia had greater PPI compared to their low-scoring counterparts. Moreover, there were positive associations in nonsmokers between introvertive anhedonia and latencies to onset and peak response. In contrast, for those individuals who smoked these associations were attenuated or abolished. The results suggest that PPI is altered differentially in psychosis-prone populations who display different symptom profiles, and that these relationships are moderated by smoking status.

Acoustic Stimulation↗
PubMed2004

The effects of saliency and task difficulty on visual search performance in ageing and Alzheimer's disease.

We asked whether the poor performance on visual search tasks typical of patients with Alzheimer's disease (AD) is the result of a selective deficit in the ability to shift attention from item to item, or the consequence of an inefficient processing of each item within the search set. We attempted to manipulate the ease of attention shifting and item processing in a visual search task by manipulating target salience and task difficulty, respectively. Significant effects of both target saliency and task difficulty for both AD patients and age-matched controls were obtained, with the AD group displaying greater effects of both of these manipulations than the controls. This interaction remained even when the reaction time data were log-transformed to account for the overall slower reaction times of the AD group. We conclude that inefficiency in visual search tasks in AD probably represents the product of both attention shifting and target processing factors.

Aged↗
PubMed2004

Relative efficacy of criminological, clinical, and personality measures of future risk of offending in mentally disordered offenders: a comparative study of HCR-20, PCL:SV, and OGRS.

The authors compared the ability of 3 commonly used measures of risk of future offending in a sample of 315 mentally disordered offenders discharged from a medium-secure unit in the United Kingdom. The authors explored whether the same criminogenic factors that predict recidivism in the general population also predict recidivism in mentally disordered offenders. The actuarial measure, using mainly criminological variables, provided the best prediction of recidivism compared with measures based on personality or clinical information, which provided no incremental validity over the actuarial measure. The authors suggest that for maximum efficacy clinical risk should be rated at a time of active symptoms rather than at discharge when symptoms are minimal.

Adult↗
PubMed2003

The extra-retinal motion aftereffect.

Repetitive eye movements are known to produce motion aftereffect (MAE) when made to track a moving stimulus. Explanations typically centre on the retinal motion created in the peripheral visual field by the eye movement. This retinal motion is thought to induce perceived motion in the central test, either through the interaction between peripheral MAE and central target or by adaptation of mechanisms sensitive to the relative motion created between centre and surround. Less attention has been paid to possible extra-retinal contributions to MAE following eye movement. Prolonged eye movement leads to afternystagmus which must be suppressed in order to fixate the stationary test. Chaudhuri (1991, Vision Research, 131, 1639-1645) proposed that nystagmus-suppression gives rise to an extra-retinal motion signal that is incorrectly interpreted as movement of the target. Chaudhuri's demonstration of extra-retinal MAE depended on repeated pursuit to induce the aftereffect. Here we describe conditions for an extra-retinal MAE that follows more reflexive, nystagmus-like eye movement. The MAE is extra-retinal in origin because it occurs in part of the visual field that received no retinal motion stimulation during adaptation. In an explicit test of the nystagmus-suppression hypothesis, we find extra-retinal MAE fails to store over a 30s delay between adaptation and test. Implications for our understanding of motion aftereffects are discussed.

Eye Movements↗
PubMed2003

A demonstration of within-subjects latent inhibition in the human: limitations and advantages.

The magnitude of latent inhibition (LI) (a retardation of associative learning due to prior exposure to the conditioning stimulus) was measured in healthy volunteers using both a within- and a between-subjects version of the task. Reliable LI was demonstrated for the within-subjects paradigm (using a design that fully counter-balanced stimulus of pre-exposure) but the magnitude of the effect was smaller than for the between-subjects version. Measures of schizotypal personality were found to be associated with reduced LI for the between-subjects task, but not for the within-subjects task. We hypothesised that for the within-subjects task learning about the first stimulus-consequence association (usually that for the not pre-exposed (NPE) stimulus) primes learning about the second stimulus, thus reducing the effect of pre-exposure and restricting the range of LI scores. In turn, this restricted range of LI scores does not allow subtle differences on schizotypal personality dimensions to reveal their effect using this within-subjects paradigm. In conclusion, a within-subjects LI task has been developed which is not open to explanation in terms of differences in stimulus salience. However, the limited range of pre-exposure scores in the current within-subject paradigm may severely limit it is use as an indicator of subtle performance changes.

Adolescent↗
PubMed2003

Optokinetic potential and the perception of head-centred speed.

Extra-retinal information about eye velocity is thought to play an important role in compensating the retinal motion experienced during an eye movement. Evidently this compensation process is prone to error, since stimulus properties such as contrast and spatial frequency have marked effect on perceived motion with respect to the head. Here we investigate the suggestion, that 'optokinetic potential' [Perception 14 (1985) 631] may contribute to an explanation of these errors. First, we measured the optokinetic nystagmus induced by each stimulus so as to determine the optokinetic potential. Second, we determined the speed match between two patches of Gaussian blobs presented sequentially. Observers pursued the first pattern and kept their eyes stationary when viewing the second. For stimuli with identical contrast or spatial frequency, the pursued pattern was perceived to move slower than the non-pursued pattern (the Aubert-Fleischl phenomenon). Lowering the contrast or the spatial frequency of the non-pursued pattern resulted in a systematic decrease of its perceived speed. A further condition in which the contrast or spatial frequency of the pursued pattern was varied, resulted in no change to its perceived speed. Pursuit eye movements were recorded and found to be independent of stimulus properties. The results cast doubt on the idea that changing contrast or spatial frequency affects perceived head-centred speed by altering optokinetic potential.

Eye Movements↗
PubMed2003

Prediction of violence and self-harm in mentally disordered offenders: a prospective study of the efficacy of HCR-20, PCL-R, and psychiatric symptomatology.

The efficacy of the Historical, Clinical, and Risk Management Scales (HCR-20; C. D. Webster, D. Eaves, K. S. Douglas, & A. Wintrup, 1995), Psychopathy Checklist-Revised (PCL-R; R. D. Hare, 1991), Beck Hopelessness Scale (BHS; A. T. Beck, A. Weissman, D. Lester, & L. Trexler, 1974), and Brief Psychiatric Rating Scale (BPRS) to predict violence and self-harm in 34 institutionalized mentally disordered offenders was assessed. Both the HCR-20 and BPRS were strong predictors of violence whereas the PCL-R had moderate predictive ability. BHS was the only variable predictive of self-harm. Although risk assessment measures were successful at predicting in-patient violence, a clinical measure of mental state was at least as effective in these mentally disordered offenders.

Adolescent↗
PubMed2002

Phasic visual alertness in Alzheimer's disease and ageing.

An individual's ability to see and react quickly to a target stimulus is enhanced if they are alerted to the arrival of this target by a stimulus that occurs just prior in time to it. This alerting effect is thought to occur due to a phasic increase in alertness mediated by noradrenergic activity. In Alzheimer's disease (AD) there is a dysfunction in the noradrenergic system resulting in a decrease in central levels of noradrenaline. We therefore predicted that patients with AD would not be able to benefit from the prior stimulus to the same extent as that seen in healthy older adults and thus would have a reduced or abolished alerting-effect. We measured reaction times to respond to a visual target that could be preceded (by 200 ms) by a visual alerting cue, in 17 patients with Alzheimer's disease, 19 age-matched controls and 13 younger controls. We found that the alerting cue significantly decreased the reaction times for both the young and old controls, but that this cue had no effect upon the reaction times for those with AD. This marked inability to increase phasic alertness in AD may contribute to the everyday problems faced by these patients, and may provide a simple tool to aid diagnosis and disease progression.

Aged↗
PubMed2002

The partial reinforcement extinction effect in humans: effects of schizophrenia, schizotypy and low doses of amphetamine.

The partial reinforcement extinction effect (PREE) was studied in human subjects. It has been suggested that the PREE depends on neural mechanisms critical to the cognitive dysfunction which underlines acute schizophrenia. We therefore predicted that the PREE should be reduced, through decreased resistance to extinction in the partial reinforcement (PR) condition, in various types of individual: (a) healthy volunteers given low doses of oral amphetamine; (b) those in the acute (but not chronic) phase of a schizophrenic illness and; (c) healthy volunteers with high scores on personality measures of schizotypy. Despite obtaining robust demonstrations of PREE in all experiments, none of these predictions were confirmed. A single, low dose, of amphetamine had no effect on either continuous reinforcement (CR) or partial reinforcement (PR). Acute and chronic schizophrenic patients showed a reduced PREE compared to controls. However this was due to increased resistance to extinction in the CR groups. Finally, high schizotypy scores were associated with greater PREE, attributable to both decreased extinction in the CR condition and increased extinction in the PR condition. The results of these experiments on human PREE provide no support that PREE is a valid paradigm with which to explore the cognitive dysfunction underlying schizophrenia.

Acute Disease↗