Search PubMed⌕ Search

Biomedical subjects

A T Smith

Publications and source records attributed to A T Smith.

At least 109 records · Page 6Linked to original sources
PubMed1987

Velocity perception and discrimination: relation to temporal mechanisms.

Measurements of three aspects of velocity detection are presented: the upper threshold of motion (UTM), perceived velocity and velocity discrimination. UTM (the highest velocity at which a drifting periodic pattern has the appearance of coherent motion) is reduced by adaptation to a similar drifting pattern of low but not medium or high velocity. The range of adaptation velocities over which the perceived velocity of a test pattern is reduced is quite different, the reduction being greatest at medium adaptation velocities. True velocity discrimination for gratings is possible only at low velocities; at higher velocities other cues (such as temporal frequency) are used. These findings are discussed in relation to current models of motion perception.

Adaptation, Ocular↗
PubMed1986

Relative motion sensitivity in cat striate cortex as a function of stimulus direction.

This paper presents the second stage of an investigation into the relative motion sensitivity of complex cells in the striate cortex of lightly anaesthetized adult cats. Relative motion between an oriented bar and a background of random visual texture was generated by moving both stimuli, in-phase or in antiphase, with dissimilar speed. Three configurations were compared: motion of both foreground bar and background texture in preferred directions for either the bar or the background, and also where background motion was other than orthogonal to bar orientation. Providing foreground stimuli elicited substantial responses, sensitivity to relative motion was qualitatively but not quantitatively predictable from discrete responses to foreground or background alone, whatever the angular inclination between their respective directions, suggestive of strongly non-linear interactions between the two. Where the foreground evoked no response, or depressed firing, the pattern of sensitivity to relative motion could not be predicted.

Animals↗
PubMed1986

EEG characteristics of epileptic pattern sensitivity and their relation to the nature of pattern stimulation and the effects of sodium valproate.

Seven patients with pattern-sensitive epilepsy were examined repeatedly over a period of 4-12 months during which the dose of sodium valproate was adjusted. Patterns were presented in series in which the size of successive patterns was progressively increased until paroxysmal activity appeared, or until the largest pattern (radius 24 degrees visual angle) had been presented. As valproate dose increased, paroxysmal activity was less likely to occur. When it did occur it was less likely to include a spike; it had a lower voltage, involved fewer electrodes, and lasted a shorter time. The size of the pattern just sufficient to induce paroxysmal activity showed relatively little change with valproate dose. The degree to which the various electrodes were involved in the discharges showed considerable stability. The paroxysmal response to patterns presented in one visual half-field was almost invariably maximal over the contralateral posterior quadrant, usually at the posterior temporal electrode.

Adolescent↗
PubMed1986

Motion after-effects in cat striate cortex elicited by moving texture.

Responses of striate cortical neurons to randomly-textured test patterns (static visual noise), or to bars of optimal orientation and width, moving back-and-forth with fixed velocity, were recorded in the lightly anaesthetized cat. Effects of prior adaptation with textured patterns drifting continuously in each cell's preferred or null directions, were assessed. Alterations of directional bias and responsiveness to the test stimulus were assessed in relation to the degree and time-course of texture adaptation. The effects of adaptation to moving texture were qualitatively similar to our previously published data on adaptation to drifting gratings in the same or similar cells. Responses to the test stimulus were transiently depressed in the direction of adaptation and enhanced following adaptation in the opposite direction, compared with responses following exposure to stationary texture. However, even in cells that were driven strongly by the adapting texture, i.e. particularly the special complex cells in cortical layers III and V, the after-effects were always weak in strength compared with those elicited by moving gratings. We conclude that, as a group, cortical cells strongly sensitive to texture motion are relatively unsusceptible to adaptation.

Action Potentials↗
PubMed1986

Hemifield differences in perceived velocity.

Measurements of the perceived velocity of a drifting grating were obtained as a function of the position of the grating in the visual field. Identical drifting gratings were presented at the same eccentricity in the left, right, upper, and lower hemifields, and the perceived velocities were compared. A group of ten subjects considered together showed no significant hemifield differences in perceived velocity. However, some individual subjects showed marked and systematic hemifield differences, the directions of which varied among the subjects. There were no hemifield differences in susceptibility to adaptation to moving gratings.

Adaptation, Physiological↗
PubMed1986

Flicker masking and developmental dyslexia.

Recent studies have provided evidence that dyslexic children tend to show longer visual persistence than control children when presented with low-spatial-frequency grating stimuli. The possibility that this phenomenon might reflect an impairment of inhibitory Y-cell activity in the visual system of dyslexics has been investigated. A flicker masking technique was used to mask Y-cell activity selectively in a group of dyslexic boys and a group of age-matched controls. There were no overall differences in reaction times to the offsets of grating patterns of various spatial frequencies between the groups, and no differences between subgroups defined by age, degree of reading impairment, or any other criterion. The results show no evidence of abnormal Y-cell function in developmental dyslexia.

Afterimage↗
PubMed1985

Motion after-effects in cat striate cortex elicited by moving gratings.

Responses of striate cortical neurones to bars of optimal orientation and width, moving with fixed velocity, were recorded in the lightly anaesthetized cat. Effects of periods of pre-adaptation with square-wave gratings of variable spatial frequency and velocity, drifting continuously in each cell's preferred or null directions, were investigated. Variations of cells' directional bias and responsiveness to oriented bars were assessed in relation to the degree and time-course of pre-adaptation to drifting gratings, compared with the preceding level of firing when exposed to uniform backgrounds of the same average luminance. All cells showed some susceptibility to pre-adapting moving gratings: subsequent responses to a bar were initially depressed in the direction of pre-adaptation and, in direction-biased or bidirectional cells, were enhanced in the opposite direction, compared with bar responses following exposure merely to a uniform background. These effects were strongest and most consistent amongst standard complex cells and weakest amongst special complex cells: maximal effects were obtained with adapting gratings of optimal velocity and spatial frequency.

Adaptation, Physiological↗
PubMed1985

The pattern specificity of velocity aftereffects.

Interactions between two different visual patterns, a coarse grating and a fine texture pattern, were investigated in the context of velocity aftereffects in human subjects. The perceived velocity shift, in which the perceived velocity of a moving test pattern is reduced following exposure to a similarly moving adaptation pattern, is apparent when the adaptation and test patterns are of the same or different types. The aftereffect transfers interocularly in both cases. The directional tuning of the aftereffect is broad, and has a different profile for texture adaptation than for bar adaptation. When adaptation is to a composite stimulus comprising independently moving bars and texture, the aftereffect varies according to the nature of the test pattern. The results are discussed with reference to interactions between the responses of neurones in feline striate cortex to the two types of pattern.

Adaptation, Physiological↗
PubMed1985

Cold sensitivity in essential hypertension: the effect of beta- and combined alpha- and beta-blockade.

The presence of cold sensitivity was investigated in three groups of patients; untreated hypertensives and hypertensives treated by a beta-adrenoceptor blocker (propranolol) or by a combined alpha- and beta-adrenoceptor blocker (labetalol) at two ambient temperatures. At a comfortable ambient (24 degrees C) one-third of the untreated and those treated with beta-blockade only showed cold sensitivity as compared with 16% of patients on the combined therapy. Under conditions of mild cold stress (20 degrees C) cold sensitivity increased in frequency in all three groups, more than half of the untreated and beta-blocked patients were affected and greater than one-third of those with alpha- and beta-blockade. These findings indicate that in the general population of hypertensives treatment with beta-adrenoceptor blockade alone may have little effect on the peripheral vasculature and that a useful degree of protection may be provided by therapy which blocks both receptors.

Adrenergic alpha-Antagonists↗
PubMed1985

Velocity coding: evidence from perceived velocity shifts.

Measurements of the perceived velocities of moving patterns were made under a variety of conditions in an attempt to gain information concerning the way in which velocity is encoded in the visual system. Adaptation to a pattern moving in the same or opposite direction reduces the perceived velocity of a moving test pattern, but only if the adaptation pattern moves as fast as or faster than the test pattern. The aftereffect peaks at an adaptation velocity slightly higher than the test velocity and then remains constant at higher velocities. Similar results were obtained for several types of pattern. Perceived velocity reduction also follows adaptation to a flickering homogeneous field. The results can be explained in terms of a theory of velocity coding in which two channels are considered in terms of variations in the velocity sensitivities of the neurones they comprise.

Adaptation, Ocular↗
PubMed1985

The effect of repeated doses of temazepam and nitrazepam on human psychomotor performance.

The effects of six nightly doses of temazepam (20 mg), nitrazepam (10 mg) and placebo on saccadic eye movements, critical flicker fusion, choice reaction time and on subjective feelings were studied in a double blind cross-over study in eight volunteers. Testing was performed on nights 1 and 6 of treatment. Nitrazepam, a benzodiazepine with a long elimination half-life produced a residual performance impairment on night 1 that had disappeared by night 6, despite higher serum concentrations of the drug on the sixth night of treatment. Temazepam, with a shorter elimination half-life produced no significant performance impairment on night 1, but by night 6 a significant impairment of saccadic eye movements (P less than 0.05), was seen 1 h after drug intake. We conclude that tolerance developed to the sedative action of nitrazepam over the 6 night period of the study. No evidence of tolerance was seen with temazepam.

Anti-Anxiety Agents↗
PubMed1985

Photoplethysmography of the distal pulp in the assessment of the vasospastic hand.

A procedure for assessing the peripheral vasculature and a new technique for determining the characteristics of the a.c. (arterial) photoplethysmographic (PPG) waveform are described. Comparing 12 normal female volunteers and 12 female patients with Raynaud's phenomenon, PPG amplitude is as good as the results of a standard thermographic test in distinguishing between the groups. The limb arterial flow measured by strain gauge plethysmography (SPG) and the timing of features in the PPG waveform showed differences between the groups, which did not reach statistical significance. The rise time of the PPG waveform, however, was found to be correlated with age independently of vasospasticity.

Adult↗
PubMed1985

Responses of endurance-trained subjects to caloric deficits induced by diet or exercise.

The purpose of this study was to examine the effect of 7 d of caloric deficit (1,000 kcal X d-1) induced by diet or exercise on weight loss and exercise performance of six endurance-trained males. The diet of each subject was controlled during the weeks preceding the dietary restriction and exercise to normalize nitrogen balance. Weight and blood chemistries were monitored daily. Submaximal and maximal exercise responses were tested at the end of each week. Weight loss during the exercise week was 0.76 kg, significantly less (P less than 0.05) than in the dietary restriction week (-2.16 kg). Seven d of cumulative nitrogen loss was greater (P less than 0.05) during the diet week (-24.5 g) than the exercise week (-11.1 g). Resting hematocrit, hemoglobin, total plasma protein, and albumin were significantly reduced during the exercise week compared to all other weeks. Maximal exercise capacity, measured as VO2 (61 ml X kg-1 X min-1) or duration of exercise, was not affected by either method of caloric deficit, but R values and lactates were lower than controls for both dieting and exercise deficits. The results suggest that endurance-trained individuals lose weight more slowly and conserve more protein when using exercise to induce a caloric deficit compared to dietary restriction. The results also suggest the possibility of the sequestering of blood proteins to supplement muscle protein synthesis during periods of exercise-induced caloric deficits.

Adult↗
PubMed1984

Sensitivity of complex cells in cat striate cortex to relative motion.

Sensitivity of 95 complex cells to relative motion between oriented bars and textured backgrounds was investigated monocularly in the striate cortex of lightly anesthetized, paralyzed cats. Cells were classified conventionally. Those in deep layers were either direction-selective, or strongly preferred one direction of motion, and responded well to background texture motion alone: backgrounds potentiated the response to the bar in the cell's preferred direction when moved in phase, or in the opposite direction when moved in antiphase; other combinations depressed the level of response compared with that for the bar alone. The majority of direction-selective or strongly direction-biased cells in superficial layers behaved similarly. The most interesting superficial-layer cells were bidirectional or weakly direction-biased, and recorded closer to the cortical surface than the direction-selective neurons. A majority showed preference for relative motion, some for antiphase, others for in-phase motion, regardless of the absolute direction of motion across the receptive field, which could not be accounted for on the basis of separate responses to bars and backgrounds alone. Three of the superficial-layer direction-selective cells also showed preference for antiphase relative motion. In a few complex cells from superficial laminae, backgrounds were either without influence on responses to oriented stimuli, or purely suppressive. Visual backgrounds against which objects are perceived are usually neither featureless nor motionless: the results suggest that most complex cells in striate cortex are sensitive to the context in which objects are seen and susceptible to relationships between objects and their backgrounds in relative motion.

Animals↗
PubMed1984

The influence of background motion on the motion aftereffect.

The motion aftereffect caused by adaptation to moving bars is visible in a stationary test pattern consisting of static visual noise (texture). The aftereffect resulting from adaptation to moving bars presented on a background of texture is highly dependent on the direction and velocity of motion of the background during adaptation, and less dependent on the nature of the test pattern. Background motion in the same direction as bar motion during adaptation enhances the aftereffect, whilst a stationary background or background motion in the opposite direction suppresses, and in some cases reverses the direction of, the aftereffect. The influence of background motion is greatest using a textured test pattern, a low adapting texture velocity, and a low grating spatial frequency. The physiological implications of these results are discussed.

Adaptation, Ocular↗
PubMed1984

Hyperadrenalism in childhood and adolescence.

Hyperadrenalism in childhood and adolescence has unique features that influence diagnosis and management. We reviewed our experience with 18 patients, ranging in age from 18 months to 18 years. Nine had bilateral adrenal hyperplasia, eight had adrenal neoplasms, and one had micronodular hyperplasia. Patients with congenital adrenal hyperplasia and hyperaldosteronism were excluded. Six patients with Cushing's disease diagnosed in earlier years were treated by total adrenalectomy and recently two patients underwent transsphenoidal removal of pituitary tumors. Bilateral adrenalectomy was carried out in one patient with micronodular hyperplasia and in a second because of elevated adrenocorticotrophic hormone (ACTH) levels from an undefined source. Eight patients had adrenal neoplasms, including five adenomas and three carcinomas. We found no reliable criteria to differentiate before surgery between adrenal adenomas and adrenal carcinomas. The most recognizable characteristic of malignancy was tumor size, specifically weight greater than 75 gms. Of the three patients with adrenal carcinoma, one expired 20 months after adrenalectomy and 8 months after receiving palliative partial hepatectomy for liver metastasis. Two patients are well with normal growth and development at 11 and 20 years following adrenalectomy. With the exception of one patient who died 6 years after surgery from a glioblastoma multiforme, all patients with adrenal adenomas are well. Eight patients underwent bilateral adrenalectomy for hypercortisolism. Five of the six who have reached their adult stature are significantly stunted. Four of six patients with Cushing's disease, treated by total adrenalectomy, have developed Nelson's syndrome at 2, 6, 10, and 12 years after surgery. Of the two patients undergoing transsphenoidal surgery, one had recurrent disease at 2 years and was treated by pituitary irradiation with recovery. The patients undergoing adrenalectomy for micronodular hyperplasia and ectopic ACTH are well at 2 and 4 years, respectively. Cushing's disease in children and adolescents is best treated by transsphenoidal removal of the pituitary adenoma. Adrenalectomy, once the most accepted approach, plays a secondary role and is indicated primarily in micronodular adrenal hyperplasia, in patients with ectopic ACTH production of an undefined source, and in recurrent Cushing's disease following prior pituitary irradiation. The high incidence of Nelson's syndrome in children treated by adrenalectomy mandates that patients at risk be monitored lifelong for the progression of a pituitary tumor.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenoma↗
PubMed1984

Effect of meptazinol and ethanol on human psychomotor performance and mood ratings.

The effect of meptazinol and/or ethanol on human psychomotor performance and mood ratings has been investigated in 8 healthy volunteers in a double blind randomised study. Meptazinol (200 mg 3 hourly for 4 doses) or placebo were administered orally and ethanol (0.8 g/kg) or placebo were given 30 min after the last tablet. Peak saccadic velocity (PSV), saccade duration at 30 degrees of amplitude (SD), smooth pursuit velocity (SPV), critical flicker fusion threshold (CFF), choice reaction time (CRT) and visual analogue scales were assessed. PSV (p less than 0.01), SD (p less than 0.001) and SPV (p less than 0.01) were significantly impaired after ethanol, while CFF, CFT and visual analogue scales showed no significant effect. None of the tests was affected by the meptazinol treatment alone. No changes were observed in the ethanol-induced impairment of PSV, SD and SPV when meptazinol was given in combination with ethanol.

Adult↗