Which way is down? Positional distortion in the tilt illusion.

Contextual information can have a huge impact on our sensory experience. The tilt illusion is a classic example of contextual influence exerted by an oriented surround on a target's perceived orientation. Traditionally, the tilt illusion has been described as the outcome of inhibition between cortical neurons with adjacent receptive fields and a similar preference for orientation. An alternative explanation is that tilted contexts could produce a re-calibration of the subjective frame of reference. Although the distinction is subtle, only the latter model makes clear predictions for unoriented stimuli. In the present study, we tested one such prediction by asking four naive subjects to estimate three positions (4, 6, and 8 o'clock) on an imaginary clock face within a tilted surround. To indicate their estimates, they used either an unoriented dot or a line segment, with one endpoint at fixation in the middle of the surround. The surround's tilt was randomly chosen from a set of orientations (± 75°, ± 65°, ± 55°, ± 45°, ± 35°, ± 25°, ± 15°, ± 5° with respect to vertical) across trials. Our results showed systematic biases consistent with the tilt illusion in both conditions. Biases were largest when observers attempted to estimate the 4 and 8 o'clock positions, but there was no significant difference between data gathered with the dot and data gathered with the line segment. A control experiment confirmed that biases were better accounted for by a local coordinate shift than to torsional eye movements induced by the tilted context. This finding supports the idea that tilted contexts distort perceived positions as well as perceived orientations and cannot be readily explained by lateral interactions between orientation selective cells in V1.

Inhibitory control contributes to "motor"--but not "cognitive"--impulsivity.

Literature on impulsivity regularly claims inhibitory control deficits underlie impulsive behavior. The current study investigated whether taxing inhibitory control will increase reflection (decision making under conditions of uncertainty), temporal (delay of gratification), and motor impulsivity (behavioral disinhibition). Inhibitory control was challenged, via a random letter generation task presented during responding to three impulsivity measures: the Information Sampling Task (IST), Single Key Impulsivity Paradigm, and the Stop Signal Task (SST). Participants (n = 33) were assigned to the inhibitory control challenging (experimental) condition, or to a control condition in which inhibitory control was not challenged. The SST was affected by the inhibitory control challenge: participants in the experimental condition displayed increased motor impulsivity, evidenced in longer stop signal reaction times (SSRTs) compared to the control group. The manipulation did not affect reflection- or temporal- impulsivity measures. These data support the suggestion that the mechanisms underlying the motor subtype of impulsivity are dissociable from the temporal and reflection subtypes, and that engagement of inhibitory control is not necessary to prevent impulsive decision making.

Acute alcohol effects on subtypes of impulsivity and the role of alcohol-outcome expectancies.

RATIONALE: It is well established that alcohol acutely impairs the ability to inhibit a pre-potent response (motor impulsivity), but its effects on cognitive impulsivity, including temporal (delayed gratification) and reflection (decision making) impulsivity, are not clear. An important factor contributing to the effects of alcohol is cognitive expectancies of alcohol-related outcomes. OBJECTIVES: The current study investigated the effect of alcohol, and alcohol outcome expectancies, on subtypes of impulsivity. METHODS: Impulsivity was tested using the Stop Signal, the Single Key Impulsivity and the Information Sampling Task for motor, temporal and reflection impulsivity, respectively. Participants (n = 48) received placebo, a low (0.4 g/kg) or high dose (0.8 g/kg) of alcohol, before completing the impulsivity measures. RESULTS: Motor impulsivity was affected by alcohol dose; participants receiving a high dose displayed reduced inhibitory control. Reflection impulsivity was affected by cognitive alcohol expectancies, but not by alcohol condition; participants expecting greater cognitive and behavioural impairment by alcohol exhibited low impulsivity. Temporal impulsivity was not affected by either alcohol dose or outcome expectancies. CONCLUSIONS: These data suggest that the effects of alcohol on the subtypes of impulsivity are dissociable. Motor impulsivity is sensitive to the pharmacological effects of alcohol, whereas the reflection subtype is affected by cognitive alcohol expectancies. The findings have implications for the understanding of impulsive behaviour under the influence of alcohol.

The short (S) allele of the serotonin transporter polymorphism and acute tryptophan depletion both increase impulsivity in men.

Reduced serotonergic neurotransmission is implicated in impulsive behavior. We studied the triallelic system of the serotonin transporter gene linked polymorphic region (5-HTTLPR) and acute manipulation of serotonin together to further delineate the mechanisms by which serotonergic neurotransmission affects impulsivity. Fifty-two healthy participants (38 men and 14 women) underwent acute tryptophan depletion (ATD) or placebo in a randomized, double-blind, parallel group experiment. Impulsive response style was measured on two versions of the Continuous Performance Task (CPT), and calculated using signal detection theory. We observed a dose-dependent effect for the short (S') allele of the 5-HTTLPR on impulsive response style. Individuals who had the S'/S' genotype were more impulsive than individuals with the L/S' genotype. Participants with the L/S' genotype were more impulsive than those with the L/L genotype. ATD increased impulsivity in men, and decreased impulsivity in women. These data demonstrate for the first time that reduced serotonergic tone as a result of either 5-HTTLPR genotype, or experimental ATD, are both independently and additively, associated with elevated impulsive response style in Caucasian men.

Elevated impulsivity and impaired decision-making in abstinent Ecstasy (MDMA) users compared to polydrug and drug-naïve controls.

Ecstasy (MDMA; 3,4-methylenedioxymethamphetamine) has a well-recognized neurotoxic effect on central serotonergic (5-HT) systems in animals, and there is some evidence of persistent serotonergic dysregulation in human ecstasy users. Serotonin is believed to mediate impulsive behavior and effective decision-making. Thus, the aim of the present study was to investigate impulsive behavior and decision-making in abstinent regular ecstasy users. Three groups were compared: 'ecstasy users' (recreational ecstasy users who reported modest use of illicit drugs other than cannabis), 'polydrug controls' (ecstasy naïve illicit drug users), and 'drug-naïve controls'. All participants completed personal details and general drug history questionnaires, the National Adult Reading Test, Matching Familiar Figures Test (MFF20), a risky decision-making task (RDMT), and the Card Arranging Reward Responsivity Objective Test (CARROT). The groups did not differ on the CARROT measure of responsiveness to financial incentive; however, the ecstasy group displayed significantly elevated MFF20 impulsivity, and showed reduced discrimination between magnitudes of prospective gains and losses when making risky decisions, compared to the 'polydrug' and 'drug-naïve' control groups. These findings may reflect a vulnerability of 5-HT systems in the orbital prefrontal cortex and interconnected corticolimbic circuitry to the cumulative neurotoxic effects of ecstasy and have clinical significance for regular ecstasy users. The combination of elevated impulsivity and impaired use of reinforcement cues in uncertain decision-making may comprise risk factors for continued drug abuse and everyday functioning.