Biomedical subjects

R Q Quiroga

Publications and source records attributed to R Q Quiroga.

5 recordsLinked to original sources

PubMed2001

Wavelet entropy in event-related potentials: a new method shows ordering of EEG oscillations.

In this work we show the application of a measure of entropy defined from the wavelet transform, namely the wavelet entropy (WS), to the study of event-related potentials (ERPs). WS was computed for ERPs recorded from nine healthy subjects with three different types of stimuli, among them target stimuli in a cognitive task. A significant decrease of entropy was correlated with the responses to target stimuli (P300), thus showing that these responses correspond to a more "ordered" state than the spontaneous EEG. Furthermore, we propose the WS as a quantitative measure for such transitions between EEG ("disordered state") and ERP ("ordered state").

Adult↗

PubMed2001

Bisensory stimulation increases gamma-responses over multiple cortical regions.

In the framework of the discussion about gamma (approx. 40 Hz) oscillations as information carriers in the brain, we investigated the relationship between gamma responses in the EEG and intersensory association. Auditory evoked potentials (AEPs) and visual evoked potentials (VEPs) were compared with bisensory evoked potentials (BEPs; simultaneous auditory and visual stimulation) in 15 subjects. Gamma responses in AEPs, VEPs and BEPs were assessed by means of wavelet decomposition. Overall maximum gamma-components post-stimulus were highest in BEPs (P < 0.01). Bisensory evoked gamma-responses also showed significant central, parietal and occipital amplitude-increases (P < 0.001, P < 0.01, P < 0.05, respectively; prestimulus interval as baseline). These were of greater magnitude when compared with the unisensory responses. As a correlate of the marked gamma responses to bimodal stimulation we suggest a process of 'intersensory association', i.e. one of the steps between sensory transmission and perception. Our data may be interpreted as a further example of function-related gamma responses in the EEG.

Acoustic Stimulation↗

PubMed2000

Learning driver-response relationships from synchronization patterns.

We test recent claims that causal (driver-response) relationships can be deduced from interdependencies between simultaneously measured time series. We apply two recently proposed interdependence measures that should give results similar to cross predictabilities used by previous authors. The systems that we study are asymmetrically coupled simple models (Lorenz, Roessler, and Hénon models), the couplings being such that they lead to generalized synchronization. If the data were perfect (noise-free, infinitely long), we should be able to detect, at least in some cases, which of the coupled systems is the driver and which the response. This might no longer be true if the time series has finite length. Instead, estimated interdependencies depend strongly on which of the systems has a higher effective dimension at the typical neighborhood sizes used to estimate them, and causal relationships are more difficult to detect. We also show that slightly different variants of the interdependence measure can have quite different sensitivities.

Electroencephalography↗

PubMed2000

Kulback-Leibler and renormalized entropies: applications to electroencephalograms of epilepsy patients.

Recently, "renormalized entropy" was proposed as a novel measure of relative entropy [P. Saparin et al., Chaos, Solitons and Fractals 4, 1907 (1994)] and applied to several physiological time sequences, including electroencephalograms (EEGs) of patients with epilepsy. We show here that this measure is just a modified Kullback-Leibler (KL) relative entropy, and it gives similar numerical results to the standard KL entropy. The latter better distinguishes frequency contents of, e.g., seizure and background EEGs than renormalized entropy. We thus propose that renormalized entropy might not be as useful as claimed by its proponents. In passing, we also make some critical remarks about the implementation of these methods.

Biophysical Phenomena↗

PubMed1999

Functions and sources of event-related EEG alpha oscillations studied with the Wavelet Transform.

OBJECTIVES: By using the Wavelet Transform, a time frequency representation with nearly optimal resolution, we studied responses to stimulation in the 'alpha' range (10 Hz). METHODS: Visual evoked responses of 10 healthy subjects were studied with 3 different stimulus types (no-task VEP, non-target and target stimulus). RESULTS: Upon all the stimulus types, event-related responses in the 10 Hz ('alpha') range were distributed in the whole scalp, best defined in the occipital locations, the responses on the anterior electrodes being less pronounced and delayed. In some subjects, these event-related responses were prolonged upon target stimulation in posterior locations. CONCLUSIONS: These results point towards a distributed origin of event-related alpha oscillations with functional relation to sensory processing, and possibly to further processes.

Brain↗