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Processing of time intervals by the brain is critical for our interaction with surroundings. Conscious processing of time intervals predicts movements in a vehicular traffic or the temporal delay in finger movements while playing a musical instrument. Processing of time interval also occurs at a subconscious level, which helps to determine the speed of individual muscle contractions, coordinating our movements. To understand how the brain processes various sub- and supra-second intervals, I have proposed inbuilt neuronal oscillators within various circuits in the brain, which helps to form neuronal clock mechanisms. Proposed oscillators include pacemaker neurons, tonic inputs and, synchronized excitation and inhibition of inter-connected neurons. Time is represented within proposed neuronal clock mechanism by intervals, called ‘sensory moment,' between two spikes or spike bursts produced by oscillators. Outputs from oscillators are processed to generate changes in states of next level of circuits, represented by rates of changes in frequency and/or frequency of neuronal activity, which encode time intervals. Inbuilt oscillators are proposed to be calibrated by the (a) feedback processes (b) input of time intervals resulting from rhythmic external sensory stimulation and (c) synchronous effects of feedback processes and evoked sensory activity. The proposed mechanism of calibration also imposes fluctuations on the regular activity of oscillators. Differences with the pacemaker/accumulator model are discussed, which proposes a separate oscillator with a stable frequency. Multiple calibration mechanisms account for redundancies in the timing mechanisms. Furthermore, abnormal feedback processes associated with motor symptoms could be responsible for the impairment of temporal processing as seen in schizophrenics.
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Two successive flashes can sometimes be perceived as three, and predominantly when the delay between flashes is ~100 ms [Bowen, 1989]. This “triple-flash” illusion was proposed to result from the hypothetical superposition of two oscillatory impulse response functions (IRF), one for each stimulus flash. When the delay between flashes matches the period of the oscillation, the superposition enhances a later part of the oscillation that is normally damped; when this enhancement crosses perceptual threshold, a third flash is erroneously perceived. However, so far no electrophysiological evidence supports this theoretical account of the illusion. In Experiment 1, we systematically varied the inter-flash interval (stimulus onset asynchrony, SOA) and validated Bowen’s theory: The subject-specific optimal SOA for illusory perception was strongly correlated with the period of that person’s parietal “perceptual echo” – an oscillatory IRF measured in a separate EEG experiment [as described in VanRullen & Macdonald, 2012]. Although this finding lends support to Bowen’s notion that the illusion reflects a superposition of two oscillatory responses, it does not explain trial-to-trial variability: At the subject-specific optimal SOA, the third flash is only perceived on average half of the time (45%). In Experiment 2, by presenting two flashes at a fixed SOA while recording EEG, we contrasted brain activity for physically identical trials on which the third-flash was either reported, or not. The findings revealed that: (1) Across subjects, the probability of third-flash perception was correlated with subject-specific alpha peak frequency at parietal but not occipital sources (measured during the baseline, prior to the first flash); (2) Significantly stronger alpha-band (7-11 Hz) inter-trial phase coherence at parietal sites 210-250 ms after the first flash was related to the perception of the third illusory flash. Overall, oscillatory reverberations in the brain can create something out of nothing – a third flash where there are only two.
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Aim of Investigation: Neuropsychological and clinical research suggests working memory (WM) function is impaired in chronic pain patients. Yet, information on mnestic cortical representation of potentially noxious painful stimuli is currently lacking. Methods: we recorded electroencephalography (EEG) in human volunteers during a modified delayed-match-to-sample task, which involved sustained maintenance of the task-relevant attributes of selective nociceptive laser stimuli (intensity and location) for subsequent cued-discrimination of either stimulus dimension, in two different memory-load conditions (i.e., two stimuli vs. three stimuli). In addition, we measured performance in a series of neuropsychological tests to be correlated with nociceptive WM. Results: As expected, participants performed better in the lower load condition. Furthermore, time-frequency analysis of EEG responses revealed that this different performance was reflected in systematic modulations of oscillatory activity. Moreover, we found a covariation between oscillatory modulations and performance in neuropsychological memory tests, thus suggesting a potential clinical relevance of the electrophysiological responses. Conclusions: In keeping with tactile WM research, our findings highlight the link between event related nociceptive oscillations and “pain engrams”, a result that can pave the way to investigation of memory of potentially noxious stimuli in conditions of clinical pain.
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