André Achim

Methods for separating temporally overlapping sources of neuroelectric data

SummaryThe localization of intracranial sources of EEG or MEG signals can be misled by the combined effect of several sources, as illustrated by simulated MEG data in which two of the three dipolar sources have slightly out of phase activity and partly complementary scalp topographies. These data were analysed by three different source localization methods. Fitting a single source to each sequential topography worked perfectly when only one source was active; this could also account for as much as 95% of the spatial variance of topographies resulting from two overlapping sources, although the solution was then far from any source. A principal component analysis approach followed by an oblique rotation (fitting one source to the spatial aspect of each component) correctly localized two of the sources but severely mislocated the source that was never active alone. Spatiotemporal source modeling (simultaneously fitting a set of sources to all consecutive topographies) correctly localized all three sources, provided that the parameter optimization method could escape sub-optimal local minima of the error function. Temporally overlapping sources can thus be separated and correctly identified if the mathematical model is adequate and the optimization procedure is well adapted.

Separate memory-related processing for auditory frequency and patterns

Detecting deviant, and potentially meaningful, auditory events depends on transient representations of preceding stimuli. Here, we examined whether the neural circuitry underlying deviance detection system varied as a function of deviance type. In different blocks of trials, participants were presented with a sequence that included standard and deviant tones differing in frequency or a sequence of tones that alternated regularly in frequency with occasional deviant repetitions. Both frequency- and pattern-deviant stimuli elicited a mismatch negativity (MMN) peaking between 120 and 175 ms poststimulus. The MMN amplitude distribution was more frontal for frequency-deviant than for pattern-deviant stimuli. There are two possible explanations for these results. Both frequency- and pattern-deviation MMNs might arise in the same set of generators whose relative strength of activation varies. Alternatively, frequency- and pattern-deviation MMNs could originate in different generators. These alternatives were investigated using principal component analysis and signal identification methods. These methods revealed that no common signal space could account for both of the MMNs, indicating different generator sources for the analysis of frequency and pattern deviance. The results suggest separate memory-related processing for auditory frequency and patterns and indicate that the neural circuit of deviance detection varies as a function of the perceptual context.

Human intracerebral potentials associated with target, novel, and omitted auditory stimuli

SummaryWe recorded late auditory potentials from lateral and medial regions in the frontal, temporal and parietal lobes of patients with temporal lobe epilepsy implanted with horizontal depth electrodes. Tone sequences were presented in three tasks: 1) auditory target detection in a tone sequence, 2) target detection with interspersed novel stimuli, and 3) detection of stimulus omissions. At frontal sites, potentials to targets showed a triphasic response with peak latencies around 200, 270 and 350 ms. At temporal sites, potentials consisted of a generally positive 285 ms peak which was sometimes accompanied by a negative peak at 200 ms or at 400 ms. At parietal sites, potentials were generally triphasic with latencies of about 230, 300, and 370 ms. At most sites, potentials evoked by novel stimuli had shorter latencies than those evoked by targets. The frontal and parietal potentials were either absent or strongly attenuated during stimulus omissions. The results lend further support to the multiple generator hypothesis of late potentials and suggest that some of the cerebral sources of the late potentials are stimulus dependent while others are not.

Intracerebral amplitude distributions of the auditory evoked potential

Intracerebral auditory evoked potentials were recorded from multi-contact electrodes chronically implanted in frontal, temporal, and parietal regions of the brains of 10 epileptic patients. Binaurally presented tones produced a large widely distributed biphasic potential with a close latency correspondence to the scalp vertex response. In each hemisphere, this intracerebral response inverted in polarity from posterior temporal to frontal sites. This response also showed large amplitude changes in the inferior parietal lobe. The distribution of this response is compatible with bilateral posterior temporal sources with a dorso-frontal orientation.

Signal detection in averaged evoked potentials: Monte Carlo comparison of the sensitivity of different methods

Many clinical and research applications rely on detecting evoked potential (EP) signal or EP differences between conditions. Statistical methods for objective signal detection should be sensitive to the presence of signal, but must provide the user strict control on tolerated false alarm rate. The respective sensitivities of 6 signal detection methods were compared through several Monte Carlo simulations involving 2 autocorrelation structures, 5% and 1% significance levels, 8, 10 or 12 replications per study, and increasing signal to noise ratio. The signal detection methods compared were: (1) the Record Orthogonality Test by Permutations (ROT-p), a variant of the Residual Orthogonality Test (Achim et al., 1988), that provides an unbiased estimate of the energy of the signal present in the averaged data, (2) the Tsum2 permutation test of Karniski et al. (1994), (3) a Principal Component Analysis method (PC1) consisting of a t test on the weights of the first principal component, (4) multiple t tests on amplitudes with empirical adjustment for global false alarm rate, and (5-6) the test of Guthrie and Buchwald (1991) on length of consecutive t tests significant at P < 0.05 or 0.01 per-test. The first 3 methods did not exceed their nominal false alarm rate and clearly outperformed the last 3, with the ROT-p method being significantly more sensitive than all others under almost all conditions.

A Study of Theory of Mind in Paranoid Schizophrenia: A Theory or Many Theories?

Social cognitive psychologists (Frith, 1992; Hardy-Baylé et al., 2003) sought to explain the social problems and clarify the clinical picture of schizophrenia by proposing a model that relates many of the symptoms to a problem of metarepresentation, i.e., theory of mind (ToM). Given the differences in clinical samples and results between studies, and considering the wide range of what is considered to constitute ToM, one must ask if there a core function, or is ToM multifaceted with dissociable facets? If, there are dissociable dimensions or facets, which are affected in patients with paranoid schizophrenia? To answer these questions, a group of 21 individuals diagnosed with paranoid schizophrenia and 29 non-clinical control subjects, were tested on a battery of five different measures of ToM. The results confirmed that there was little difference in specificity of three of the tests in distinguishing between the clinical and non-clinical group, but there were important differences in the shared variance between the tests. Further analyses hint at two dimensions although a single factor with the same variance and the same contributing weights in both groups could explain the results. The deficits related to the attribution of cognitive and affective states to others inferred from available verbal and non-verbal information. Further analyses revealed that incorrect attributions of mental states including the attribution of threatening intentions to others, non-interpretative responses and incomplete answers, depending on the test of ToM.

Experimental disentangling of spatial-compatibility and interhemispheric-relay effects in simple reaction time (Poffenberger paradigm)

Spatial-compatibility effects can be obtained in simple reaction time (SRT) provided that spatially distinct responses are frequently required. Since this effect is limited to trials with relatively long reaction times (RTs), Hommel (1996b) proposed that if the response does not occur shortly after stimulus detection, then the spatial code of the stimulus can interfere with that of the response. A series of experiments is reported showing that (a) spatial compatibility in SRT to lateralized stimuli is not an alternative, but rather a complementary, explanation to interhemispheric transfer time (contrary to what Hommel surmised), and (b) the spatial compatibility component is essentially limited to the first trial after shifting response preparation from one-half of the visual fields to the other, suggesting a mechanism akin to an orienting response.

Adjustment of speaker’s referential expressions to an addressee’s likely knowledge and link with theory of mind abilities

To communicate cooperatively, speakers must determine what constitutes the common ground with their addressee and adapt their referential choices accordingly. Assessing another person’s knowledge requires a social cognition ability termed theory of mind (ToM). This study relies on a novel referential communication task requiring probabilistic inferences of the knowledge already held by an addressee prior to the study. Forty participants were asked to present 10 movie characters and the addressee, who had the same characters in a random order, was asked to place them in order. ToM and other aspects of social cognition were also assessed. Participants used more information when presenting likely unknown than likely known movie characters. They particularly increased their use of physical descriptors, which most often accompanied movie-related information. Interestingly, a significant relationship emerged between our ToM test and the increased amount of information given for the likely unknown characters. These results suggest that speakers use ToM to infer their addressee’s likely knowledge and accordingly adapt their referential expressions.

A choice reaction time index of callosal anatomical homotopy.

Tachistoscopically presented bilateral stimulus pairs not parallel to the meridian produced significantly longer RTs on a task requiring discrimination of shapes (Go/no-Go) than pairs emplaced symmetrically on each side of the meridian in Desjardins and Braun [Desjardins, S., & Braun, C. M. J. (2006). Homotopy and heterotopy and the bilateral field advantage in the Dimond paradigm. Acta Psychologica, 121, 125-136]. This was explained by the fact that there are more homotopic than heterotopic fibers in the corpus callosum. However: (1) different parts of the visual field were not equiprobably stimulated, possibly causing subtle biases, (2) the predicted cost of vertical asymmetry was tested only with bilateral stimuli, and (3) interstimulus distance was at the outer limit of callosal midline fusion (10.6 degrees ). Here, a tachistoscopic experiment with 24 normal participants replicated the between-field vertical symmetry advantage [Desjardins, S., & Braun, C. M. J. (2006). Homotopy and heterotopy and the bilateral field advantage in the Dimond paradigm. Acta Psychologica, 121,125-136.], but without irrelevant stimulation conditions and with more proximal stimuli. In addition, a significant specific cost of vertical asymmetry of 7ms was found for between-field integration over within-field integration. As far as we know, this is the first demonstration of an effect of callosal anatomical homotopy with reaction time.

EEG amplitude spectra before near threshold visual presentations differentially predict detection/omission and short–long reaction time outcomes

Performance in simple stimulus detection manifests as both probability of detection and speed of signaling detected stimuli. These two dimensions of performance across trials were examined with respect to brain states just prior to stimulus delivery, using near threshold stimuli targeting the magnocellular or the parvocellular visual streams in an attempt to isolate differential perceptual preparation. The EEG amplitude of 12 university students was analyzed in spectral bands from 2 to 50 Hz at 9 bilateral channel pairs in a window covering -450 ms to +50 ms relative to stimulus onset. A hierarchical statistical procedure was applied to control false positive results. EEG power in the 2, 4, 8 and 10 Hz bands was found significantly lower at the F7-F8 channel pair both before detected compared to omitted stimuli and before the fastest compared to slowest reaction time quartiles, with no stimulus type effect. In addition, the 22 and 24 Hz band activity was lower prior to better performance frontally (F3-F4, F7-F8) in reaction time but not in detection, while it was larger centro-parietally (CP1-CP2, P3-P4) in detection but not in reaction times. Spectral analysis thus shows stimulus detection and response speed to depend partly on common and partly on distinct pre-stimulus brain states.