The effects of stimulus movement and attention on peripheral stimulus localization by 8- to 26-week-old infants

This study examined the effect of stimulus movement on localization probability and latency during attention and inattention. Forty infants, 10 each at 8, 14, 20, and 26 weeks of age were presented with a central stimulus. Then, a peripheral stimulus was presented (static or dynamic checkerboard). Stimulus movement did not affect localization probability. Infants localized the dynamic peripheral stimulus more quickly than the static peripheral stimulus when there was no focal stimulus. Focal stimulus attention attenuated this difference in localization latency between static and dynamic stimuli. Signal detection analysis showed that sensitivity to the peripheral stimulus increased over this age range along with a decrease in the bias against responding. The effects of attention were on response bias rather than stimulus sensitivity. These results imply attention affected the localization response to the peripheral stimulus but did not affect the sensitivity of the sensory and perceptual pathways to peripheral stimuli.     

The corridor illusion

Contrary to general belief, the classical corridor illusion is not due solely to the perspective and contextual cues provided by the corridor. Additional factors that are equally important are the inherent spatial anisotropies of the visual system and fixation tendencies.     

On the theoretical error bound for estimating psychometric functions

The theoretical limits to the amount of error, or the Cramer-Rao bounds, were derived for estimating psychometric functions. These theoretical error bounds were compared with the variability of psychometric functions estimated from human as well as computer-simulated observers. For the simulated observers, due to the limited efficiency of the sampling strategies, including the placement of the signals and the distribution of the trials, the variances of the estimated parameters are seven times the theoretical bound for threshold and 22 times that for slope. For the human observers, the variance is 18 times the theoretical bounds for threshold and 80 times that for slope. Therefore, a major portion of the variances (60% for threshold and 73% for slope) for the human observers is associated with factors other than sampling strategies. Further improvement of the accuracy for estimating psychometric functions will depend on not only optimizing the sampling strategy, but also better understanding the various sources of error related to the behavior of human observers.     

Increased brain serotonin turnover in panic disorder patients in the absence of a panic attack: reduction by a selective serotonin reuptake inhibitor

Since the brain neurotransmitter changes characterising panic disorder remain uncertain, we quantified brain noradrenaline and serotonin turnover in patients with panic disorder, in the absence of a panic attack. Thirty-four untreated patients with panic disorder and 24 matched healthy volunteers were studied. A novel method utilising internal jugular venous sampling, with thermodilution measurement of jugular blood flow, was used to directly quantify brain monoamine turnover, by measuring the overflow of noradrenaline and serotonin metabolites from the brain. Radiographic depiction of brain venous sinuses allowed differential venous sampling from cortical and subcortical regions. The relation of brain serotonin turnover to serotonin transporter genotype and panic disorder severity were evaluated, and the influence of an SSRI drug, citalopram, on serotonin turnover investigated. Brain noradrenaline turnover in panic disorder patients was similar to that in healthy subjects. In contrast, brain serotonin turnover, estimated from jugular venous overflow of the metabolite, 5-hydroxyindole acetic acid, was increased approximately 4-fold in subcortical brain regions and in the cerebral cortex (P < 0.01). Serotonin turnover was highest in patients with the most severe disease, was unrelated to serotonin transporter genotype, and was reduced by citalopram (P < 0.01). Normal brain noradrenaline turnover in panic disorder patients argues against primary importance of the locus coeruleus in this condition. The marked increase in serotonin turnover, in the absence of a panic attack, possibly represents an important underlying neurotransmitter substrate for the disorder, although this point remains uncertain. Support for this interpretation comes from the direct relationship which existed between serotonin turnover and illness severity, and the finding that SSRI administration reduced serotonin turnover. Serotonin transporter genotyping suggested that increased whole brain serotonin turnover most likely derived not from impaired serotonin reuptake, but from increased firing in serotonergic midbrain raphe neurons projecting to both subcortical brain regions and the cerebral cortex.