Impurity-driven cone formation during laser sputtering of graphite.

Sputtering of highly oriented pyrolytic graphite (HOPG) by 248-nanometer laser radiation was studied. Neutral carbon atoms and small clusters were ejected with significantly higher translational energies than were expected from a simple model of thermal vaporization in the absence of a potential barrier. The HOPG also developed a remarkable surface morphology that consists of regular cones and domes. Cone formation appears to be initiated by trace metal impurities that serve as heat shunts in this highly anisotropic material.

Voluntary attention modulates fMRI activity in human MT-MST.

How does voluntary attention to one attribute of a visual stimulus affect the neural processing of that stimulus? We used functional magnetic resonance imaging to examine the attentional modulation of neural activity in the human homolog of the MT-MST complex, which is known to be involved in the processing of visual motion. Using a visual stimulus containing both moving and stationary dots, we found significantly more MT-MST activation when subjects attended to the moving dots than when they attended to the stationary dots, even though the visual stimulus was identical during the two conditions.

Right hemisphere language in a neurologically normal dextral: a fMRI study.

Dextrals with right cerebral hemisphere dominance for language are rare. Eight neurologically intact dextrals underwent BOLD-fMRI while being presented auditory and visual words. Fortuitously, in one subject, right hemisphere activations with visually presented words were seen in the inferior frontal, premotor regions together with predominantly left cerebellar activation. These were a mirror image of activations obtained from the seven other dextrals. Also mirrored was temporal activation from auditory words which extended more posteriorly on the right side than the left. These results showing mirror organization of language were replicated in another scanning session and also by using a second word task. Although rare, mirrored organization of language can occur in normal dextrals without penalizing language function.

A functional magnetic resonance imaging study of auditory vigilance with low and high information processing demands.

This study identified the brain activations associated with auditory vigilance tasks, using functional magnetic resonance imaging. We created auditory continuous performance tests (CPTs) in which a demanding task (working memory task) was made more difficult than a simple vigilance task by increasing working memory and interference filtering demands. Two cohorts of normal male controls performed significantly worse on the working memory CPT than on the vigilance task. Compared to the vigilance task, performance of the working memory task produced significant signal change in lateral and medial prefrontal cortex, precentral cortex, temporal lobe, including insula and hippocampus, parietal-occipital cortex, cingulate, thalamus, and superior colliculus. Performance and degree of activation was associated with an estimate of IQ. Further research should clarify the contributions of working memory and interference filtering to the activated network.

History and future directions of human brain mapping and functional neuroimaging.

It has long been known that there is some degree of localisation of function in the human brain, as indicated by the effects of traumatic head injury. Work in the middle of the 20th century, notably the direct cortical stimulation of patients during neurosurgery, suggested that the degree and specificity of such localisation of function were far greater than had earlier been imagined. One problem with the data based on lesions and direct stimulation was that the work depended on the study of what were, by definition, damaged brains. During the second half of the 20th century, a collection of relatively non-invasive tools for assessing and localising human brain function in healthy volunteers has led to an explosion of research in what is often termed "Brain Mapping". The present article reviews some of the history associated with these tools, but emphasises the current state of development with speculation about the future.

Voluntary switching between identities in dissociative identity disorder: A functional MRI case study.

Patients who suffer from dissociative identity disorder present unique scientific and clinical challenges for psychology and psychiatry. We have been fortunate in working with a patient who-while undergoing functional MRI-can switch rapidly and voluntarily between her main personality (a middle-aged, high-functioning woman) and an alternate personality (a 4-6-year-old girl). A unique task was designed to isolate the processes occurring during the switches between these personalities. Data are from two imaging sessions, conducted months apart, each showing the same activated areas during switches between these personalities. The activated areas include the following: the primary sensory and motor cortex, likely associated with characteristic facial movements made during switching; the nucleus accumbens bilaterally, possibly associated with aspects of reward connected with switching; and prefrontal sites, presumably associated with the executive control involved in the switching of personalities.

Normal McCollough effect in Alzheimer's disease and global amnesia.

The McCollough effect (ME), a long-lasting, pattern-contingent aftereffect in normal human vision, was examined in persons with known deficits in memory. We induced MEs in 11 subjects, 5 patients with various severities of Alzheimer's disease (AD), H.M. (a patient who has global amnesia due to bilateral medial temporal lobectomy and who has been studied for 35 years since his operation), and 5 control subjects. H.M. and the AD patients showed MEs of strength and duration comparable to those of the control subjects. These results demonstrate a dissociation between learning mechanisms that mediate recall and recognition versus mechanisms that mediate the ME. Furthermore, knowledge about the sites of neuropathology in H.M. and in AD are consistent with other sources of evidence implicating early visual areas, especially V1, as a critical locus of the ME.

Visibility of low-spatial-frequency sine-wave targets: Dependence on number of cycles.

The number of cycles in a low-frequency sinusoidal display is a crucial variable in determining the visibility of the display. In particular, the threshold contrast is essentially independent of spatial frequency for these displays. We have extended the above experiments, using more cycles and a variety of targets and observer tasks. The results confirm previous findings; they also show that the type of target or task has little influence. For low-frequency sinusoids that contain up to about 3 cycles, the threshold contrast is determined by the number of cycles. For high-number-of-cycles targets with spatial frequencies above 6-10 cycles per degree, visibility is predominantly dependent on the spatial frequency. The results suggest that the low-frequency decrease in reported MTF's is due to the decrease of the number of cycles used in determining them.

Influence of intraocular scattered light on lightness-scaling experiments.

Following Munsell's bisection procedure [J. Opt. Soc. Am. 23, 394 (1933)], we established a nine-step gray scale in which each step is an equal increment in lightness. We calculated retinal illuminances after intraocular scatter by using the point-spread function of Vos et al. [Vision Res. 16, 215-219 (1976)]. After this correction for intraocular scatter, we find a logarithmic relationship between retinal illuminance and achromatic lightness scales that are determined by the bisection method. Additional bisection experiments with a series of different backgrounds corroborate this result. We find that lightness depends linearly on the logarithm of scatter-corrected retinal illuminance, with different slopes for backgrounds of different lightness. This study also highlights the importance of using scatter-corrected illuminance in any quantitative model of lightness.

Auditory and visual word processing studied with fMRI.

Brain activations associated with semantic processing of visual and auditory words were investigated using functional magnetic resonance imaging (fMRI). For each form of word presentation, subjects performed two tasks: one semantic, and one nonsemantic. The semantic task was identical for both auditory and visual presentation: single words were presented and subjects determined whether the word was concrete or abstract. In the nonsemantic task for auditory words, subjects determined whether the word had one syllable or multiple syllables. In the nonsemantic task for visual words, subjects determined whether the word was presented in lower case or upper case. There was considerable overlap in where auditory and visual word semantic processing occurred. Visual and auditory semantic tasks both activated the left inferior frontal (BA 45), bilateral anterior prefrontal (BA 10, 46), and left premotor regions (BA 6) and anterior SMA (BA 6, 8). Left posterior temporal (middle temporal and fusiform gyrus) and predominantly right-sided cerebellar activations were observed during the auditory semantic task but were not above threshold during visual word presentation. The data, when averaged across subjects, did not show obligatory activation of left inferior frontal and temporal language areas during nonsemantic word tasks. Individual subjects showed differences in the activation of the inferior frontal region while performing the same task, even though they showed similar response latency and accuracy.

Plurality and resemblance in fMRI data analysis.

We apply nine analytic methods employed currently in imaging neuroscience to simulated and actual BOLD fMRI signals and compare their performances under each signal type. Starting with baseline time series generated by a resting subject during a null hypothesis study, we compare method performance with embedded focal activity in these series of three different types whose magnitudes and time courses are simple, convolved with spatially varying hemodynamic responses, and highly spatially interactive. We then apply these same nine methods to BOLD fMRI time series from contralateral primary motor cortex and ipsilateral cerebellum collected during a sequential finger opposition study. Paired comparisons of results across methods include a voxel-specific concordance correlation coefficient for reproducibility and a resemblance measure that accommodates spatial autocorrelation of differences in activity surfaces. Receiver-operating characteristic curves show considerable model differences in ranges less than 10% significance level (false positives) and greater than 80% power (true positives). Concordance and resemblance measures reveal significant differences between activity surfaces in both data sets. These measures can assist researchers by identifying groups of models producing similar and dissimilar results, and thereby help to validate, consolidate, and simplify reports of statistical findings. A pluralistic strategy for fMRI data analysis can uncover invariant and highly interactive relationships between local activity foci and serve as a basis for further discovery of organizational principles of the brain. Results also suggest that a pluralistic empirical strategy coupled formally with substantive prior knowledge can help to uncover new brain-behavior relationships that may remain hidden if only a single method is employed.

Generalizable patterns in neuroimaging: how many principal components?

Generalization can be defined quantitatively and can be used to assess the performance of principal component analysis (PCA). The generalizability of PCA depends on the number of principal components retained in the analysis. We provide analytic and test set estimates of generalization. We show how the generalization error can be used to select the number of principal components in two analyses of functional magnetic resonance imaging activation sets.

Blinded review of retrospectively visible unreported breast cancers: an eye-position analysis.

PURPOSE: To determine whether unreported retrospectively identified cancers on mammograms receive prolonged visual attention and can be reliably detected in a blinded review. MATERIALS AND METHODS: Four experienced mammographers performed a blinded review of a test set of 20 retrospective cases where the cancer was not detected until the next mammographic evaluation, 10 prospective cases where the cancer was initially detected, and 10 cancer-free cases. Two views were digitized and displayed on a workstation. The experiment consisted of an initial impression, during which eye position was monitored, and a final impression, during which viewers zoomed on regions of interest and localized suspicious lesions. Eye-position data were analyzed to determine whether retrospectively visible cancers attracted attention to the same degree as prospectively visible cancers. The initial impression used 1,000 msec as the eye-fixation dwell criterion for detecting a lesion. RESULTS: Initially, 70% of retrospective cancers and 50% of prospective cancers did not attract prolonged visual attention. In prospective cases, detailed examination significantly improved the mean receiver operating characteristic area, from.73 to.88 (P <.01), but in retrospective cases, the mean receiver operating characteristic area barely increased, from.60 to.68, due to a high true-positive-to-false-positive ratio. CONCLUSION: At blinded review, detection of retrospectively visible cancers was significantly inferior to that of prospective cancers. It cannot be assumed that retrospectively identified cancers are intrinsically detectable, because they do not draw prolonged visual attention during visual search for breast cancers.

Detection of cortical activation during averaged single trials of a cognitive task using functional magnetic resonance imaging.

Functional neuroimaging studies in human subjects using positron emission tomography or functional magnetic resonance imaging (fMRI) are typically conducted by collecting data over extended time periods that contain many similar trials of a task. Here methods for acquiring fMRI data from single trials of a cognitive task are reported. In experiment one, whole brain fMRI was used to reliably detect single-trial responses in a prefrontal region within single subjects. In experiment two, higher temporal sampling of a more limited spatial field was used to measure temporal offsets between regions. Activation maps produced solely from the single-trial data were comparable to those produced from blocked runs. These findings suggest that single-trial paradigms will be able to exploit the high temporal resolution of fMRI. Such paradigms will provide experimental flexibility and time-resolved data for individual brain regions on a trial-by-trial basis.