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Biomedical subjects

W Byth

Publications and source records attributed to W Byth.

7 recordsLinked to original sources
PubMed2004

In search of an elusive hard threshold: a test of observer's ability to order sub-threshold stimuli.

The contrast transducer function (d' vs. contrast) for sine gratings was claimed to come up from some non-zero contrast value rather than from the origin. This implies that there is a point (a hard threshold) on the grating contrast axis below which observers could not distinguish between presentations containing gratings and those containing a homogeneous field. We studied the ability to order sub-threshold square wave gratings and found, to the contrary, that observers were able to do this no matter how low the contrasts. At the same time, the observers failed to order the sub-threshold gratings when they were of the same contrast. The latter is inconsistent with signal detection theory which predicts that an observer's judgements are based on the same ordered set of sensory states irrespective of whether the stimuli differ or are the same. On the other hand, these data can be reconciled with the notion of a threshold if the latter is thought of as a fuzzy rather than a sharp margin on the contrast axis.

Contrast Sensitivity↗
PubMed2000

Cholinergic agents and the McCollough effect.

The strength of the McCollough effect (ME), a pattern-contingent colour aftereffect, has been shown to be inversely related to acetylcholine, being significantly strengthened by (anticholinergic) scopolamine and weakened by (cholinergic) physostigmine delivered before adapting to the ME stimuli. The purpose of the present study was (i) to establish whether the effect of pre-adaptation scopolamine is linearly dose-dependent and (ii) to investigate the effects of scopolamine and physostigmine delivered between adaptation and testing. In experiment 1, ten healthy male volunteers who received placebo, or 0.6 mg, 1.2 mg, or 1.8 mg scopolamine before adapting to ME stimuli showed a significant linear dose-dependence over tests repeated from 10 to 70 min after adaptation. In experiment 2 twelve male volunteers adapted to ME stimuli and then received placebo, 1.2 mg oral scopolamine, or 0.75 mg subcutaneous physostigmine. On subsequent repeated testing, strength of the ME was increased by scopolamine and decreased by physostigmine relative to placebo. Both experiments were double-blind double-dummy repeated measures. These data support the view that the ME is a product of inhibitory mechanisms in the visual system rather than processes involved in associative learning.

Adult↗
PubMed1998

The McCollough effect across the menstrual cycle.

The McCollough effect (ME) has been shown to be sensitive to cholinergic agents, being strengthened by hyoscine (antagonist) and weakened by physostigmine (agonist), and possibly to more generalized changes in CNS arousal. We therefore expected the ME to be sensitive to hormonal changes during the menstrual cycle, being strongest in the postovulatory phases when arousal is low. In two experiments we found a highly significant effect of menstrual phase for the normally cycling women, but not for oral contraceptive users: ME strength gradually increased across the cycle, reaching a premenstrual peak. These findings may be explained in terms of hormonally mediated changes in arousal across the menstrual cycle.

Adolescent↗
PubMed1997

The effects of haloperidol on visual search, eye movements and psychomotor performance.

The effects of single doses of haloperidol (2, 4 and 6 mg) were compared with lorazepam 2.5 mg and placebo in 15 healthy subjects. Visual search strategy was measured, along with a range of psychomotor and eye movement tests. Patients with Parkinson's disease have been shown to exhibit a shift from parallel to serial processing in visual search, but we demonstrated that this does not occur following administration of either haloperidol or lorazepam. Haloperidol was detected by visual analogue rating scales and peak saccadic velocity, the latter being the more sensitive measure. Haloperidol had no statistically significant effect on smooth pursuit position error, velocity error or saccadic intrusions. Digit symbol substitution performance was clearly diminished by haloperidol, but there was no effect on the continuous attention test. Lorazepam decreased performance in all tests apart from saccadic latency.

Adolescent↗
PubMed1993

Extraversion and the McCollough effect.

Two experiments are reported which show that extraverts experience significantly stronger McCollough Effects than introverts. In both experiments the strength of the McCollough Effect (ME) was measured by the match-interference method devised by Shute (1979). The technique was found to be predictably sensitive to the eye tested and to exposure of the non-adapted eye. In the first experiment monocular ME strength was measured at 12-min intervals over two hours in four conditions and found to conform to a power function. Sixteen extraverts showed significantly (p < .00005) stronger initial ME strength than 21 introverts and in both groups the effect ceased to be apparent within about two hours. Log-log plots of decrement from initial strength indicated no significant extraversion differences in decay rates, with a function gradient of about 1/2. In a second experiment six extraverts and nine introverts from the original group were retested with binocular presentation with a similar outcome. These findings support Shute's hypothesis that introverts would show weaker MEs than introverts but do not support his hypothesis that introverts' MEs would decay more quickly. They offer some indirect support for Shute's proposal that the ME may be an indicator of central cholinergic activity in man.

Adolescent↗
PubMed1992

The McCollough effect as a measure of central cholinergic activity in man.

The McCollough Effect (ME) is an orientation contingent colour after-effect which has been proposed as an indicator of central neurotransmitter activity. Shute (1979) suggested that the ME could reflect a hippocampal "forgetting" mechanism which should be inhibited by GABAergic neurones and stimulated by cholinergic neurones. The purpose of the present study was to demonstrate that the ME is in fact sensitive to cholinergic and anticholinergic drugs and to compare its sensitivity to more conventional tests of psychomotor and cognitive function. Ten healthy subjects received single doses of physostigmine (0.75 mg SC), hyoscine (1.2 mg), temazepam (20 mg), flecainide (200 mg) or placebo in a double-blind double-dummy presentation. Subjects were tested on a battery of psychomotor and cognitive function tests at baseline and 1 h, and adapted to the ME at 1.5 h. Visual analogue rating scales and conventional tests of psychomotor function and saccadic eye movements indicated that both subjective and objective measures of arousal were impaired by temazepam. The subjective, but not the objective, measures of arousal were also impaired by both hyoscine and physostigmine, but not by flecainide. Initial strength and duration of the ME were decreased by physostigmine and increased by hyoscine and temazepam, relative to placebo (P less than 0.01). Thus, the ME is capable of detecting cholinergic, anticholinergic and GABA mimetic drug effects in man, in therapeutic doses.

Adaptation, Ocular↗