OBJECTIVE: Identify the optimal number of pulses necessary to achieve reliable measures of motor evoked potentials (MEPs) in transcranial magnetic stimulation (TMS) studies. METHODS: Retrospective data was obtained from 54 healthy volunteers (30 men, mean age 61.7+/-13.1years) who as part of prior studies had completed three blocks of 30 consecutive TMS stimuli using neuronavigation. Data from four protocols were assessed: single-pulse TMS for measures of amplitude and latency of MEPs; paired-pulse TMS for short-interval intracortical inhibition (sICI) and intracortical facilitation (ICF); and single-pulse TMS to assess the effects of intermittent theta burst stimulation (iTBS). Two statistical methods were used: an internal consistency analysis and probability of inclusion in the 95% confidence interval (CI) around the mean MEPs amplitude. RESULTS: For single-pulse TMS, the minimum number of pulses needed to achieve reliable amplitude and latency MEPs measures was 21 and 23, respectively. For paired-pulse TMS, the minimum number of pulses needed to achieve reliable sICI and ICF measures was 20 and 25, respectively. Finally, the minimum number of pulses needed to achieve reliable amplitude and latency MEPs measures after iTBS was 22 and 23, respectively. CONCLUSIONS: This study provides guidelines regarding the minimum number of pulses needed to achieve reliable MEPs measurements in various study protocols using neuronavigated TMS. SIGNIFICANCE: Results from this study have the potential to increase the reliability and quality of future neuronavigated TMS studies.

Identification of the second of two targets (T1, T2, inserted in a stream of distractors) is impaired when presented within 500 ms after the first (attentional blink, AB). Barring a T1-T2 task-switch, it is thought that T2 must be backward-masked to obtain an AB (Giesbrecht & Di Lollo, Journal of Experimental Psychology: Human Perception and Performance, 24, 1454-1466, 1998). We tested the hypothesis that Giesbrecht & Di Lollo's findings were vitiated by ceiling constraints arising from either response scale (experiment 1) or data limitations (experiment 2). In experiment 1, digit-distractors were replaced with pseudoletters to increase task difficulty, bringing performance below ceiling. An AB occurred without backward masking of T2. In experiment 2, a ceiling-free procedure estimated the number of noise dots needed for 80% T2 identification. An AB was revealed: fewer noise dots were required during the AB period than outside it. Both outcomes confirm that an AB can be obtained without either masking of T2 or task switching.

Object substitution masking (OSM) occurs when an initial display of a target and mask continues with the mask alone, creating a mismatch between the reentrant hypothesis, triggered by the initial display, and the ongoing low-level activity. We tested the proposition that the critical factor in OSM is not whether the mask remains in view after target offset, but whether the representation of the mask is sufficiently stronger than that of the target when the reentrant signal arrives. In Experiment 1, a variable interstimulus interval (ISI) was inserted between the initial display and the mask alone. The trailing mask was presumed to selectively boost the strength of the mask representation relative to that of the target. As predicted, OSM occurred at intermediate ISIs, at which the mask was presented before the arrival of the reentrant signal, creating a mismatch, but not at long ISIs, at which a comparison between the reentrant signal and the low-level activity had already been made. Experiment 2, conducted in dark-adapted viewing, ruled out the possibility that low-level inhibitory contour interactions (metacontrast masking) had played a significant role in Experiment 1. Metacontrast masking was further ruled out in Experiment 3, in which the masking contours were reduced to four small dots. We concluded that OSM does not depend on extended presentation of the mask alone, but on a mismatch between the reentrant signals and the ongoing activity at the lower level. The present results place constraints on estimates of the timing of reentrant signals involved in OSM.

As Human Genome Project exploration continues, the necessity of having a broader spectrum of genomic DNA material from different nationalities to study various aspects of hereditary disease becomes more obvious. The existence of high genetic polymorphism within and between different communities in the world makes it necessary for the gene hunters to investigate many different populations. Iran, a large country with close to 66 million people, is a land of different nationalities, tribes, and religions that offers a highly heterogeneous gene pool to the genetics researcher. The purity of many different races in this country has been highly conserved by geographical borders and by an ancient culture that has always encouraged intrafamilial marriages. All these have created a population that is remarkably heterogeneous yet high in consanguinity rate. During the last five years of investigation we have established a DNA bank, the Iranian Human Mutation Gene Bank (www.IHMGB.com), which contains all genetic diseases studied in Iran that have the Mendelian mode of inheritance. Some of the samples are assigned to common or novel mutations and others belong to patients with clinical profiles associated with particular genetic diseases but undefined mutation. This bank stores samples of DNA from the patient and his/her first-degree relatives together with a comprehensive pedigree and clinical profile for each sample. To facilitate collaboration with other scientists around the world with the same interests, we decided to present our experimental projects online. This DNA bank provides opportunities for us to collaborate with scientists outside Iran. It offers a sample resource to research scientists around the world, at no charge, for the purpose of investigating the various aspects of genetic disorders from prenatal diagnosis to gene structure and function. It is strongly stressed that no commercial benefit is involved in the establishment of this DNA bank and the DNA samples are free of charge. However, to meet our goals and to respect ethical values, DNA samples can only be used under certain conditions stated in the User Consent Form.

Objective
We used complete-linkage cluster analysis to identify healthy subpopulations with distinct responses to continuous theta-burst stimulation (cTBS).

Methods
21 healthy adults (age±SD, 36.9±15.2 years) underwent cTBS of left motor cortex. Natural log-transformed motor evoked potentials (LnMEPs) at 5–50 minutes post-cTBS (T5–T50) were calculated.

Results
Two clusters were found; Group 1 (n=12) that showed significant MEP facilitation at T15, T20, and T50 (p’s<.006), and Group 2 (n=9) that showed significant suppression at T5–T15 (p’s<.022). LnMEPs at T10 and T40 were best predictors of, and together accounted for, 80% of cluster assignment.

In an exploratory analysis, we examined the roles of brain-derived neurotrophic factor (BDNF) and apolipoprotein E (APOE) polymorphisms in the cTBS response. Val66Met participants showed greater facilitation at T10 than Val66Val participants (p=.025). BDNF and cTBS intensity predicted 59% of interindividual variability in LnMEP at T10. APOE did not significantly affect LnMEPs at any time point (p’s>.32).

Conclusions
Data-driven cluster analysis can identify healthy subpopulations with distinct cTBS responses. T10 and T40 LnMEPs were best predictors of cluster assignment. T10 LnMEP was influenced by BDNF polymorphism and cTBS intensity.

Significance
Healthy adults can be sorted into subpopulations with distinct cTBS responses that are influenced by genetics.

The capacity of visual short-term memory (VSTM) is commonly estimated by K scores obtained with a change-detection task. Contrary to common belief, K may be influenced not only by capacity but also by the rate at which stimuli are encoded into VSTM. Experiment 1 showed that, contrary to earlier conclusions, estimates of VSTM capacity obtained with a change-detection task are constrained by temporal limitations. In Experiment 2, we used change-detection and backward-masking tasks to obtain separate within-subject estimates of K and of rate of encoding, respectively. A median split based on rate of encoding revealed significantly higher K estimates for fast encoders. Moreover, a significant correlation was found between K and the estimated rate of encoding. The present findings raise the prospect that the reported relationships between K and such cognitive concepts as fluid intelligence may be mediated not only by VSTM capacity but also by rate of encoding.

A brief target embedded in—and coterminating with—a noise mask is identified easily when the duration of the mask is long but not when it is short (Di Lollo, 1980; inverse-duration effect). Identification has been said to be mediated by the visible persistence of the target, which outlasted that of the mask. We tested an alternative account based on input filtering triggered by the onset and offset of the target, relative to those of the mask, without recourse to visible persistence. The results of Experiment 1 could not be explained wholly in terms of visible persistence but were entirely consistent with input filtering. Identification suffered in Experiment 2 when transient responses were attenuated by “ramping.” In Experiment 3, accuracy improved gradually as a function of leading-mask duration. All results were consistent with a modified version of von Holst’s (1954) hypothesis that a new stimulus (e.g., the present mask) establishes an input filter within the system. Any sudden onsets or offsets then lead to the perception of a new object only when they do not match the input filter, thus becoming segregated from the temporally leading stimulus.