Deviance detection in congruent audiovisual speech: evidence for implicit integrated audiovisual memory representations.

Detection of deviant speech syllables embedded in continuous noise was investigated in an oddball paradigm. Behavioral results showed improvement of detecting and identifying the syllables when congruent visual speech accompanied the utterances. A centrally maximal negative ERP difference wave peaking at approximately 290ms post-stimulus was elicited by audiovisual but not by auditory- or visual-only task-irrelevant deviant syllables. Whereas the circumstances of the elicitation of this ERP response are similar to those of the mismatch negativity component (MMN and its visual counterpart, vMMN), its scalp distribution differs from that of both unimodal MMNs. Elicitation of an MMN-like ERP response (termed here as the audiovisual MMN: avMMN) suggests that detection of the audiovisual deviants involved integrated audiovisual memory representations. The pattern of behavioral and ERP results suggest that the formation of such cross-modal memory representation does not require voluntary operations and may even proceed for stimuli outside the focus of attention.

Backward masking and visual mismatch negativity: electrophysiological evidence for memory-based detection of deviant stimuli.

Sequences composed of two different colored checkerboard patterns (standard and deviant) were presented to adults. Each pattern was followed by a mask with stimulus onset asynchronies (SOAs) varying between 14 and 174 ms. ERPs were recorded to the deviant and standard stimuli while the participants detected changes of a cross, which was continuously present at the center of the screen. In further experiments, the participants performed a Go-NoGo task detecting the deviant checkerboards. Deviant stimuli elicited an occipital negative component with 124-132 ms mean latency (the visual mismatch negativity, vMMN) at test (standard or deviant)-to-mask SOAs longer than 27 ms. No vMMN amplitude increase was observed beyond 40 ms test-to-mask intervals, whereas detection of deviant checkerboard patterns improved up to 174-ms SOA. Therefore the processes underlying vMMN elicitation cannot fully explain the overt detection of visual deviance.

Visual temporal window of integration as revealed by the visual mismatch negativity event-related potential to stimulus omissions.

We studied whether, similarly to the auditory modality, short-period temporal integration processes occur in vision. Event-related potentials (ERP) were recorded for occasional stimulus omissions from sequences of patterned visual stimuli. A posterior negative component emerged only when the constant stimulus onset asynchrony (SOA) was shorter than 150 ms. This upper limit is comparable with the duration of the temporal window of integration observed in the auditory modality (including experiments studying the effects of stimulus omissions). Parameters of the posterior negativity were highly similar irrespective of whether the stimuli were task-relevant or not (Experiment 1). Thus, we identified this potential as the visual mismatch negativity (vMMN) component, which reflects task-independent detection of violating regularities of the stimulation. vMMN was followed by an anterior positivity (the P3a), indicating attentional shifts induced by the stimulus omissions. In Experiment 2, a posterior negativity similar to that observed in Experiment 1 emerged after the termination of short trains of stimuli, again only when the SOA was shorter than 150 ms. These results support the notion of a temporal integration window in the visual modality, the duration of which is between 150 and 180 ms.

Age and novelty: event-related brain potentials and autonomic activity.

Our aim was to study age-related differences in the habituation of orienting reaction by using novel visual stimuli. We intended to fill a gap in habituation research by recording both autonomic and ERP components of orienting to visual stimuli in the same sample and in highly related paradigms. We report data showing that in young subjects repetition of visual novels yielded fast habituation of both skin conductance responses and ERP components (P3(novel), N2b) whereas elderly people displayed no sign of habituation. However, cardiac deceleration--thought conventionally to be part of the orienting reaction--did not habituate in either group. Overall, most of our results harmonize with those obtained by using auditory stimuli; therefore we conclude that there is no significant modality specificity in age-related deterioration of habituation processes.

ERPs and deviance detection: visual mismatch negativity to repeated visual stimuli.

Previous studies showed a visual analogue of the mismatch negativity (MMN) event-related potential (ERP), which is elicited by violating some sensory regularity. Separating physical change from violating a regularity, here we show that the visual MMN (vMMN) is elicited by regularity violations that do not involve physical stimulus change. Adult participants were presented with a series of red-black and green-black checkerboard patterns delivered regularly in an RRGGRRGG ... order. Infrequently (p=0.1) this regularity was broken by repeating a stimulus one additional time (e.g. RRGGRRR). ERPs elicited by irregular stimulus repetitions were negatively displaced compared to those elicited both by regular repetition and regular change in two latency ranges: 100-140 ms and 220-260 ms. Whereas the first of these two negative ERP differences appears to be sensitive to stimulus repetition per se, the second difference can be identified as a vMMN response to violating the sequential regularity. Thus, similarly to its auditory counterpart, vMMN reflects deviance-(regularity-violation) rather than change-detection processes.

Effects of sweet and bitter gustatory stimuli in anorexia nervosa on EEG frequency spectra.

The possible differences in processing gustatory stimuli in anorexic patients compared to healthy control subjects was investigated by electrophysiological methods. The electroencephalogram (EEG) was recorded in outpatients treated with anorexia nervosa (AN) and age-matched controls after exposure to sweet (milk chocolate) and bitter (black tea) taste stimuli. Power spectrum analysis was performed on EEG epochs recorded in the above conditions. Compared to controls a significantly higher percent of theta, and lower percent of alpha1 band power was found in anorexic patients, irrespective of the kind of taste effects and hemispheric side. The pattern of activation caused by sweet and bitter stimuli was found to be different in these two groups, possibly indicating altered gustatory processing mechanisms in AN.

Nonlinear and linear EEG complexity changes caused by gustatory stimuli in anorexia nervosa.

The objective of the present study was to investigate the effects of pleasant and unpleasant gustatory stimuli on nonlinear and linear complexity measures of the EEG in healthy controls and in anorexia nervosa (AN) patients. The subjects were exposed to unpleasant (bitter tea) and pleasant (chocolate) gustatory stimuli for 2 min. Multichannel EEG was recorded and the dimensional complexity (point-correlation dimension) and Omega complexity were calculated from the EEG epochs corresponding to the above taste conditions. In AN patients lower-dimensional complexity was observed in the majority of recording sites than that seen in controls, independent of taste conditions. Higher Omega complexity was seen in control subjects in the left side irrespective of taste effects. No such hemispheric difference was observed in AN. The lower-dimensional complexity seen in AN patients may be caused by long-lasting effects of malnutrition. The lack of a significant Omega complexity change in response to exposure of sweet taste in the left side seen in AN patients may correspond to a decreased sensitivity to such stimuli in these subjects.

[Spatial standardization of [15O]-butanol PET images using T1-weighted MRI data]. / [15O]-butanol PET-vizsgálatok térbeli standardizálása szegmentált, T1-súlyozott MRI-felvételek segítségével.

Population averaged brain activation studies require the spatial standardization of the individual perfusion PET images. This procedure is usually supported by T1-weighted MRI images. The authors developed a segmentation technique to improve the automatized transformation of the individual MRI images into Talairach space. It was for the first time in Hungary that population averages of standardized T1-weighted MRI and [15O]-butanol PET images of the brain were created after validation of the procedure using data from 23 healthy volunteers. The newly developed method offers a solution for the automatized processing of primary data from brain activation experiments prior to statistical analysis.

[Right prefrontal cerebral hemispheric activation by symmetrical carotid sinus baroreceptor stimulation]. / Jobb agyféltekei praefrontalis aktiváció szimmetrikus carotis sinus baroreceptor-ingerlés hatására.

This study was performed to test the hypothesis of greater right hemispheric involvement in the processing of signals related to baroreceptor stimuli. Carotis sinus baroreceptors were stimulated by rhythmically decreasing air pressure in a neck chamber, and as a control the thorax was stimulated in a similar manner. Changes in regional cerebral blood flow (rCBF) were measured by positron emission tomography (PET). Baroreceptor stimulation resulted in rCBF increase in the right anterior-inferior prefrontal cortex (Brodmann areas [BA] 10/44/47) and bilaterally in BA 6/8. The authors conclude that, at least in some stages of baroreceptor information processing, the right hemisphere plays a greater role than the left one.