Browsing by Author "Sams, Mikko, Professor"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Measures and models of top-down influences in the human auditory cortex
    (Aalto University, 2012) Kauramäki, Jaakko; Jääskeläinen, Iiro, Docent; Lääketieteellisen tekniikan ja laskennallisen tieteen laitos; Department of Biomedical Engineering and Computational Science; Perustieteiden korkeakoulu; School of Science; Sams, Mikko, Professor
    Our senses rapidly adapt to the sensory information and task requirements. These short-term, plastic changes especially in the central nervous system are necessary for goal-directed and active behavior. Nevertheless, partly due to methodological limitations, the neural mechanisms underlying improved perception during selective attention are not well understood. For instance, human auditory cortex activity is enhanced while engaging in an auditory task, but whether such enhancement involves a larger-extend or a more specific response from a sharply tuned neuronal population remains vague. In addition to unimodal attention effects, our senses can influence each others' processing. Seeing a person articulate can both enhance and modify the perception of acoustic speech and even induce activation in the auditory areas. The goal of the Thesis was to study the neural basis of these auditory cortex modulations both due to selective attention and cross-modal interactions while lipreading. This Thesis consists of a series of four studies where the brain activity was recorded with electroencephalography (EEG) and its magnetic counterpart, magnetoencephalography (MEG). The results show that 1) evoked responses mainly from the secondary auditory areas were rapidly enhanced by a feature-specific manner during a discrimination task, where bottom-up and top-down processing were modified by stimulus and task design, 2) lipreading task suppressed auditory-cortex responses both generally and specifically in the case when sound was related to the articulatory gestures, and 3) lipreading and covert speech production tasks dynamically suppressed auditory cortex reactivity, possibly via an efference copy signal from the speech-production system. Together, these studies expand the knowledge on how the human auditory cortex function is modulated under top-down influence. The modulations due to selective attention and cross-modal interactions while lipreading are opposite at the level of evoked responses: selective attention enhances and lipreading suppresses the responses, but both effects can be feature-selective. The results show that the auditory system is highly dependent on the current task requirements, showing adaptive and goal-dependent functioning.
  • Thumbnail Image
    Item
    Representation of emotional valence in human brain
    (Aalto University, 2012) Viinikainen, Mikko; Lääketieteellisen tekniikan ja laskennallisen tieteen laitos; Department of Biomedical Engineering and Computational Science; Perustieteiden korkeakoulu; School of Science; Sams, Mikko, Professor
    All emotions can be evaluated with fair accuracy on the basis of their position on unpleasantness-pleasantness, or valence, dimension. Valence has been considered to be a linear continuum both on experiential and neural level, ranging from very unpleasant to very pleasant. However, using such a model it is difficult to explain complex emotional states, in which we can simultaneously experience unpleasantness and pleasantness, like during a rollercoaster ride or a horror film. Also experiments with rats, where experimenters have been able to cause simultaneous approach and withdrawal behavior, suggest that pleasantness and unpleasantness may be encoded by different mechanisms. In this thesis, the neural mechanisms of emotional valence were investigated in four different experiments with three different valence models: a traditional linear model, a quadratic model, and a model, in which negative valence and positive valence are represented as separate components. Whereas no study gave any support for the validity of the linear model, all four experiments provided evidence of quadratic valence dependency, and two of the experiments found additionally brain regions specifically sensitive to pleasantness and/or unpleasantness. In particular, these regions included bilaterally the dorsal and ventral prefrontal cortex and insula. Brain activity was measured with functional magnetic resonance imaging. Non-linear dependency between valence and brain activity was found for emotional pictures and sounds, observation of others' facial expressions as well as subjective emotional experience. The results demonstrate that valence processing does not occur in the brain simply in linear bipolar fashion, but the phenomenon is non-linear and multifaceted.