Robert M. Chapman

EP Component Identification and Measurement by Principal Components-Analysis

Between the acquisition of Evoked Potential (EP) data and their interpretation lies a major problem: What to measure? An approach to this kind of problem is outlined here in terms of Principal Components Analysis (PCA). An important second theme is that experimental manipulation is important to functional interpretation. It would be desirable to have a system of EP measurement with the following characteristics: (1) represent the data in a concise, parsimonous way; (2) determine EP components from the data without assuming in advance any particular waveforms for the components; (3) extract components which are independent of each other; (4) measure the amounts (contributions) of various components in observed EPs; (5) use measures that have greater reliability than measures at any single time point or peak; and (6) identify and measure components that overlap in time. PCA has these desirable characteristics. Simulations are illustrated. PCAs beauty also has some warts that are discussed. In addition to discussing the usual two-mode model of PCA, an extension of PCA to a three-mode model is described that provides separate parameters for (1) waveforms over time, (2) coefficients for spatial distribution, and (3) scores telling the amount of each component in each EP. PCA is compared with more traditional approaches. Some biophysical considerations are briefly discussed. Choices to be made in applying PCA are considered. Other issues include misallocation of variance, overlapping components, validation, and latency changes.

Evoked potentials and the study of sentence comprehension

Evoked brain potentials were used to monitor moment-by-moment decisions during language comprehension. Subjects read sentences containing temporary syntactic ambiguities for which one of the possible interpretations was semantically implausible. The N400 component of the evoked potential, which is sensitive to implausibility, was used to discover when during a sentence subjects made a decision about the ambiguity. The results demonstrate that readers try to interpret a syntactic ambiguity early in a sentence rather than waiting for disambiguating information. This introduces a new way to use brain activity to study sentence comprehension processes.

Behavioral and neural analyses of connotative meaning: Word classes and rating scales☆

Abstract Behavioral (semantic differential) and neural (Evoked Potentials, EPs) responses were related to connotative meaning. The approach was based on Osgoods semantic analyses and dimensions of Evaluation (E), Potency (P), and Activity (A). The experimental variables were (1) the semantic class of the stimulus word (E+, E-, P+, P-, A+, A-) and (2) the dimension of the semantic scale (E, P, A) which the subject used to rate the stimulus words. These variables were experimentally combined such that on each trial the subject used a designated semantic scale to judge a specified stimulus word while brain activity was recorded. Using multivariate analyses, the effects on the EPs of stimulus word class, scale dimension, and their interaction were analyzed. The EP effects of stimulus word class were similar whether the subjects were merely saying the words or rating the words on a variety of semantic scales. Different EPs were found for six word classes, three semantic scale dimensions, and the 18 groups formed by their combination. The success rates in EP identification of (1) word class and (2) scale dimension did not depend on whether these two kinds of semantic variables involved the same or different semantic dimensions. The two kinds of semantic effects in EPs were largely independent. The behavioral data supported Osgoods results and showed that our subjects were appropriately processing the semantic information. The common analyses of data from all subjects suggest the universality of the connotative EP effects across individuals. This parallels, at the neural level, the universality of the connotative dimensions found at the behavioral level by semantic differential ratings. The EP effects imply that the neural representation of meaning is similar in different individuals.

Predicting conversion from mild cognitive impairment to Alzheimer's disease using neuropsychological tests and multivariate methods.

Behavioral markers measured through neuropsychological testing in mild cognitive impairment (MCI) were analyzed and combined in multivariate ways to predict conversion to Alzheimers disease (AD) in a longitudinal study of 43 MCI patients. The test measures taken at a baseline evaluation were first reduced to underlying components (principal component analysis, PCA), and then the component scores were used in discriminant analysis to classify MCI individuals as likely to convert or not. When empirically weighted and combined, episodic memory, speeded executive functioning, recognition memory (false and true positives), visuospatial memory processing speed, and visuospatial episodic memory were together strong predictors of conversion to AD. These multivariate combinations of the test measures achieved through the PCA were good, statistically significant predictors of MCI conversion to AD (84% accuracy, 86% sensitivity, and 83% specificity). Importantly, the posterior probabilities of group membership that accompanied the binary prediction for each participant indicated the confidence of the prediction. Most of the participants (81%) were in the highly confident probability bins (.70–1.00), where the obtained prediction accuracy was more than 90%. The strength and reliability of this multivariate prediction method were tested by cross-validation and randomized resampling.

Brain responses related to semantic meaning

Evoked Potentials from electroencephalogram (EEG) recording were averaged to many visually presented word stimuli whose semantic meanings were specified along Osgoods semantic dimensions of Evaluation, Potency, and Activity [Miron & Osgood, 1966, in R. B. Cattell (Ed.), Handbook of multivariate experimental psychology, Chicago: Rand-McNally; Osgood, 1971, Journal of Social Issues, 27, 5–63; Osgood, May, & Miron, 1975, Cross-cultural universals of affective meaning, Urbana, Ill.: University of Illinois Press]. Multivariate analyses classified the Evoked Potentials to six semantic classes with success rates more than twice chance expectation. The pattern of brain activity related to the six semantic classes was similar for (i) two sets of words, (ii) 10 subjects used to develop the analyses, and (iii) an added, new subject.

Spectral mechanisms of the compound eye in the fireflyPhotinus pyralis (Coleoptera: Lampyridae)

SummaryElectroretinograms (ERG) were recorded from dark- and chromatic-adapted compound eyes in the dusk-active firefly,Photinus pyralis ♂, at different wavelengths ranging from 320 to 700 run and over 4.5 log units change in stimulus intensity. ERG waveforms differed in the short (near-UV and violet) and long (yellow) wavelengths (Fig. 1). Waveform differences were quantitated by analysis of rise and fall times as a function of the amplitude of the response. Rise times were found to be relatively constant for all stimulus wavelengths. However, variations in the fall times were detected and followed characteristically different functions for short and long wavelengths (Fig. 2).No significant differences in the slopes of the Vlog-I curves at different stimulus wavelengths were observed (Fig. 3).Spectral sensitivity curves obtained from the ventral sector in dark- and chromatic-adapted conditions revealed peaks in the short (λ max 400 nm: Fig. 4;λ max 430 nm: Fig. 5 A; andλ max 380 nm; Fig. 5B) and long (λ max 570 nm: Figs. 4, 5) wavelengths, suggesting the presence of two spectral mechanisms. The long wavelength (yellow) mechanism was in close tune with the species bioluminescence emission spectrum (Fig. 4B).

Women have Farther to Fall: Gender Differences Between Normal Elderly and Alzheimer's Disease in Verbal Memory Engender Better Detection of Alzheimer's Disease in Women

We analyzed verbal episodic memory learning and recall using the Logical Memory (LM) subtest of the Wechsler Memory Scale-III to determine how gender differences in AD compare to those seen in normal elderly and whether or not these differences impact assessment of AD. We administered the LM to both an AD and a Control group, each comprised of 21 men and 21 women, and found a large drop in performance from normal elders to AD. Of interest was a gender interaction whereby the womens scores dropped 1.6 times more than the mens did. Control women on average outperformed Control men on every aspect of the test, including immediate recall, delayed recall, and learning. Conversely, AD women tended to perform worse than AD men. Additionally, the LM achieved perfect diagnostic accuracy in discriminant analysis of AD versus Control women, a statistically significantly higher result than for men. The results indicate the LM is a more powerful and reliable tool in detecting AD in women than in men.

Familial and sporadic schizophrenia: visual evoked potential differences.

Introduction The hete~genei~ of ~~zop~nia has been iong recognized (Tsuang 1975), and various methods of subclassification have been developed, including combinations of clinical and biological measures (Jeste 1982; Buchsbaum et al. 1983). Elec~ophysiological literature relevant to subtyping in schizophrenia includes: findings of varying electrodermal responses of patients depending on the severity of emotional withdrawal and disorganization (Straube 1979; Bernstein et al. 1981); evoked potential differences between chronic and acute patients, patients with differing clinical symptoms (Landau et al. 1975; Shagass 1980), and familial versus nonfamilial patients (Romani et al. 1986); lateral asymmetries of electroencephalogram (EEG) variables in patients with specific clinicai symptoms (Serafetinides et al. 198 1); and different patterns of evoked potentials and EEG variables in neuroleptic-responsive and unresponsive patients (Saletu 1977; hi1 et al. 1981). The utility of the familial/sporadic distinction in psychosis has recently been reviewed and may be a useful strategy for determining more homogeneous subgroups (Lewis et al. 1987). In the current study, we tested the hypothesis that visual evoked potential (VEP) latencies are prolonged in psychotic patients with a positive family history of psychosis (FH PGS) when compared to equaliy ill patients without a family history of psychosis (FH NEG).

EEG alpha activity influenced by visual input and not by eye position

Abstract Recent investigations have suggested that elevation of the eyes is associated with a marked increase in EEG alpha activity. Our experiments showed that vertical eye elevation had no direct influence on alpha activity. In Preliminary and Main Groups of thirteen and twenty-two subjects, an EEG electronic scorer was used to measure the amount of time that alpha activity was present from the left and right hemispheres in the following comparisons: (a) eye positions ahead vs. up in the light, (b) eyes ahead vs. up in the dark and (c) eyes open vs. closed in the light. In the Main Group, fixation targets for the eye-ahead and eye-up positions and electro-oculogram records of eye position were added. In the dark, where differential visual input was eliminated, the alpha index did not increase when the eyes were elevated. Differences in alpha activity related to eye position in the light condition were decreased when differential visual input was decreased by the use of fixation targets. The effects of variables confounded with eye position, e.g. patterned visual input to the retina, accomodation, fixation, and effort required to maintain a specified eye position, are discussed. In these experiments, the main variable that determined i increase in alpha activity was reduction in visual input, either by closing the eyes or extinguishing t the lights.

Connotative Meaning and Averaged Evoked Potentials

The effects of two kinds of experimental manipulation of semantic meaning were studied in Evoked Potentials (EPs), brain responses recorded from scalp monitors. Both kinds of semantic manipulation were based on Osgood’s rating analyses which described three primary dimensions of connotative meaning: Evaluation, Potency, and Activity (E, P, and A). One kind of experimental variable was the semantic class of the stimulus word (E+, E-, P+, P-, A+, A-). The other kind of experimental variable was the semantic dimension of the rating scale (E, P, A) which the subject used to make semantic judgments about the stimulus words. These variables were experimentally combined in that for each trial the subject used a designated semantic scale to judge a specified stimulus word while brain activity was recorded. Using multivariate procedures, both stimulus word class and scale dimension effects on the EPs were found. Individual subject analyses demonstrated the generality of the results by showing successful discrimination of word classes and scale dimensions for each of the ten subjects analyzed separately.