Influence of diazepam and methylphenidate on identification of rapidly presented letter strings: diazepam enhances visual masking

GABAergic Inhibition Gates Perceptual Awareness During Binocular Rivalry

Binocular rivalry is a classic experimental tool to probe the neural machinery of perceptual awareness. During rivalry, perception alternates between the two eyes, and the ebb and flow of perception is modeled to rely on the strength of inhibitory interactions between competitive neuronal populations in visual cortex. As a result, rivalry has been suggested as a noninvasive perceptual marker of inhibitory signaling in visual cortex, and its putative disturbance in psychiatric conditions, including autism. Yet, direct evidence causally implicating inhibitory signaling in the dynamics of binocular rivalry is currently lacking. We previously found that people with higher GABA levels in visual cortex, measured using magnetic resonance spectroscopy, have stronger perceptual suppression during rivalry. Here, we present direct causal tests of the impact of GABAergic inhibition on rivalry dynamics, and the contribution of specific GABA receptors to these dynamics. In a crossover pharmacological design with male and female adult participants, we found that drugs that modulate the two dominant GABA receptor types in the brain, GABA_A (clobazam) and GABA_B (arbaclofen), increase perceptual suppression during rivalry relative to a placebo. Crucially, these results could not be explained by changes in reaction times or response criteria, as determined through rivalry simulation trials, suggesting a direct and specific influence of GABA on perceptual suppression. A full replication study of the GABA_B modulator reinforces these findings. These results provide causal evidence for a link between the strength of inhibition in the brain and perceptual suppression during rivalry and have implications for psychiatric conditions including autism.SIGNIFICANCE STATEMENT How does the brain accomplish perceptual gating? Here we use a direct and causal pharmacological manipulation to present insight into the neural machinery of a classic illusion of perceptual awareness: binocular rivalry. We show that drugs that increase GABAergic inhibition in the brain, clobazam (GABA_A modulator) and arbaclofen (GABA_B modulator), increase perceptual suppression during rivalry relative to a placebo. These results present the first causal link between GABAergic inhibition and binocular rivalry in humans, complementing classic models of binocular rivalry, and have implications for our understanding of psychiatric conditions, such as autism, where binocular rivalry is posited as a behavioral marker of disruptions in inhibitory signaling in the brain.

Lorazepam strongly prolongs visual information processing

Lorazepam is a drug that has been widely used for over 30 years. Whereas its therapeutic and amnestic effects are fairly well known, the visuo-perceptual impairments induced by this drug have been studied to a much lesser degree and only little is known about the influence of lorazepam on the time course of visual information processing. To gain a better insight specifically on these temporal characteristics, we used the recently discovered backward masking technique, 'shine-through', in which a vernier target precedes a grating. We tested subjects, treated with lorazepam, diazepam, or a placebo, with masking gratings of various spatial layouts. Our experiments reveal surprising results. First, for the unmasked vernier target, lorazepam induced a strong deterioration of performance compared to both diazepam and placebo. Performance deteriorated even more significantly if a masking grating was presented following the vernier. We observed that vernier discrimination could be completely abolished even if the grating appeared 400 ms after the vernier presentation. Such long time intervals are beyond usual visual masking effects. When performing the task under placebo, the participants perceived the vernier target and the masking grating as two independent time events rather than as a single event. It appears that lorazepam prolongs dramatically the processing of visual targets. The masking effects revealed here are specific to the type of grating and are much stronger under lorazepam than under diazepam.

Diazepam effects on the performance of healthy subjects are not enhanced by treatment with the antihistamine ebastine

1. We have given 12 healthy subjects the H1-antihistamine ebastine (20 mg) or placebo in a randomized double-blind and crossover study for 1 week each. The subjects were tested for drug effects on day 6 of each period, and for interactions of ebastine with oral 15 mg diazepam (DZ) on day 7. On both days, the testing runs were at baseline and 1.5, 3, 4.5 and 6 h after intake. 2. The performance was evaluated both objectively (digit symbol substitution, flicker fusion, Maddox wing, simulated driving, body balance) and subjectively (visual analogue scales, questionnaires). Venous blood was sampled daily during the maintenance and during each testing round for the assay of plasma carebastine (the active metabolite of ebastine) by high pressure liquid chromatography and plasma diazepam by radioreceptor assay. Three-way ANOVA, paired t-test, Wilcoxon rank sign test and Fisher's fourfold table test were used for data analysis. 3. Plasma carebastine reached steady levels from day 3 onwards. The mean concentrations in the morning were 82 micrograms l-1 on day 6 and 85 micrograms l-1 on day 7. The rise (+ 150%) in plasma carebastine after an extra 20 mg ebastine was not modified by DZ. Ebastine did not affect performance objectively or subjectively, yet borderline drowsiness was recorded during the first 3 h. On day 7, plasma DZ concentrations peaked (mean 480 micrograms l-1) at 1.5 h after the intake. DZ produced impaired performance in various objective tests, and drowsiness, weakness, clumsiness, mental slowness and poor performance were reported on visual analogue scales.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific effects of the benzodiazepine midazolam on visual receptive fields in light and dark adapted human subjects

Psychophysical experiments in humans have revealed similar characteristics of visual receptive fields as were found in cats and monkeys from retinal ganglion cell recordings. In addition, in some retinal ganglion cells of cats the GABA antagonist bicuculline decreases the activity of the inhibitory surround. These findings led to two predicitions: 1) benzodiazepines will selectively increase the inhibitory surround of human visual receptive fields, 2) after dark adaptation, no free GABA will be available in the synapses and benzodiazepines will have no effect on the visual system. Characteristics of human receptive fields were determined by subthreshold summation: the contrast threshold of a vertical line was measured dependent on the distance of two parallel flanking lines whose contrast was below threshold. Both hypotheses were confirmed: the threshold in the inhibitory region of receptive fields was specifically increased in a dose-dependent manner by midazolam PO (7.5 mg: P < 0.05; 15 mg: P < 0.01). In dark-adapted subjects no effect of midazolam was found. Control experiments with atropine (1 mg IV), sulpiride (100 mg IM), and levodopa (100 mg PO) showed no specific effect. The visual system may be a model to bridge the gap between animal and human psychopharmacology.

Preattentive deficit in temporal processing by chronic schizophrenics

Preattentive visual filtering by two types of neural cells (transient and sustained) constitutes the basic neural correlates underlying visual perception. Transient cells respond abruptly to temporal modulation and to the low spatial frequency components of visual stimuli. Their function is localization of objects and global perception (i.e., detection). Sustained cells respond slowly and selectively to high spatial frequencies and are believed to be responsible for the "oblique effect" (i.e., reduced responsiveness to obliquely oriented high spatial frequencies), which likely occurs at the striate cortex. Their function is to discern the details of an object (i.e., identification). The present study evaluated early visual processing in normal controls and schizophrenics by presenting vertical and oblique (45 degrees) spatial frequency gratings of 0.9 and 12 c/deg that were presented for 50, 100, 175, and 250-msec pulses. Threshold, in milliseconds, to detect an interstimulus interval (ISI) between two grating pulses was the dependent measure. Schizophrenics required longer ISIs for each grating duration, except for 250 msec, and longer ISIs for the 15 c/deg oblique grating. The results indicate that schizophrenics have a preattentive processing deficit that is further elaborated at the cortical level.

Mechanism of the excessive sedative response of cirrhotics to benzodiazepines: model experiments with triazolam

Mechanisms responsible for disproportional sedation resulting from triazolam administration to patients with cirrhosis were investigated. Ordinary sedative doses (0.25 mg) were given p.o. to 8 cirrhotics and 18 controls. Plasma concentrations of unbound drug were assessed by capillary gas chromatography and equilibrium dialysis. Median apparent oral clearances of unbound triazolam were 14.8 ml per min per kg in cirrhotics and 23.9 ml per min per kg in controls (p less than 0.01). Clearances were significantly correlated with severity of liver disease as assessed by the aminopyrine breath test (Rs = 0.77, n = 17, p less than 0.001). At a time when plasma concentrations of unbound triazolam were the same in both groups, i.e., 2.25 hr after dosing, flicker sensitivity at 5 Hz which was used as an index of CNS performance was impaired by a factor of 3.2 in cirrhotics and 1.4 in controls (p less than 0.01 for group difference). Performance was also significantly lower in cirrhotics with the digit symbol substitution test (p less than 0.05). It is concluded that, in patients with cirrhosis, disproportional sedation after benzodiazepine administration may be due not only to impaired drug elimination, but also to hypersensitivity of the brain.

Possible behavioral consequences of light-induced changes in melatonin availability

Melatonin is a hormone secreted at night, in the dark, by the human pineal organ. This nocturnal release of melatonin, in humans and other species, is rapidly suppressed by exposure to sufficiently bright light. In humans, the function, if any, of this circadian pattern of melatonin release has not been determined. In fact, no function has been definitively attributed to the hormone melatonin in humans. In one study, conducted in our laboratory, pharmacologic doses of oral melatonin (240 mg over two hours) were administered to volunteers, and various behavioral parameters were assessed. Melatonin had substantial, but brief, sedative-like effects on mood and performance. Thus it appears that a mechanism exists, whereby light, of sufficient intensity to affect melatonin release in humans, can affect behavior. It can be hypothesized that sufficiently bright light, acting by way of the suppression of melatonin release, can acutely increase alertness or act as a zeitgeber (synchronizer of circadian cycles). The light intensity necessary to suppress melatonin secretion in humans is well above typical indoor lighting conditions, but well below normal outdoor daytime levels of illumination. Therefore, the hypothesis that light may affect behavior or circadian patterns of sleep and waking, if found to be true, could have considerable impact on the design of interior lighting.

Effects of melatonin on human mood and performance

The function of melatonin, a hormone secreted by the pineal gland primarily at night, has not been definitively established in humans. To determine if pharmacologic doses of melatonin had any behavioral effects it was administered acutely to 14 healthy men. Their mood, performance, memory and visual sensitivity were assessed. Plasma melatonin concentration was assayed as well. Melatonin significantly decreased self-reported alertness and increased sleepiness as measured by the Profile of Mood States and the Stanford Sleepiness Scale self-report mood questionnaires. The effects were brief. Melatonin also affected performance, slowing choice-reaction time but concurrently decreasing errors of commission. Sustained fine motor performance was not impaired after melatonin administration nor were the tests of memory and visual sensitivity that were administered. It is concluded that melatonin, administered orally in pharmacological quantities, has significant but short acting sedative-like properties.