Abstract
The complexity of sensory stimuli has an important role in perception and cognition. However, its neural representation is not well understood. Here, we characterize the representations of naturalistic visual and auditory stimulus complexity in early and associative visual and auditory cortices. This is realized by means of encoding and decoding analyses of two fMRI datasets in the visual and auditory modalities. Our results implicate most early and some associative sensory areas in representing the complexity of naturalistic sensory stimuli. For example, parahippocampal place area, which was previously shown to represent scene features, is shown to also represent scene complexity. Similarly, posterior regions of superior temporal gyrus and superior temporal sulcus, which were previously shown to represent syntactic (language) complexity, are shown to also represent music (auditory) complexity. Furthermore, our results suggest the existence of gradients in sensitivity to naturalistic sensory stimulus complexity in these areas.
Highlights
Research in perception identified stimulus complexity as one of the most important stimulus properties that affect task performance[1]
Measures related to information content such as Kolmogorov complexity estimated as compressed file size of images[9,19,36,37], self-similarity and fractal dimension[38,39,40,41], and Pyramid of Histograms of Orientation Gradients (PHOG) derived measures[20,42] have been frequently used to automatically obtain an estimate of subjective complexity of images
The current study aims to provide a comprehensive account of representations of naturalistic sensory stimulus complexity in sensory cortices by establishing a direct, predictive relationship between objective stimulus complexity measures and stimulus-evoked brain activations as well as characterizing the properties of this relationship under the framework of neural encoding and decoding in functional magnetic resonance imaging[49]
Summary
Research in perception identified stimulus complexity as one of the most important stimulus properties that affect task performance[1]. We develop several neural encoding and decoding models, which embody specific hypotheses about certain stimulus features modulating stimulus-evoked brain activations, to test alternative hypotheses about what, if any, stimulus complexity measures are represented in different brain regions as well as analyzing these models to characterize the properties of these representations. To this end, we present the results of two different fMRI experiments; one with naturalistic image stimuli to characterize the representations of visual complexity and another with music stimuli to determine the those of auditory/music complexity in the brain. Our results reveal four core findings regarding the processing of stimulus complexity in the sensory cortices: i) Stimulus complexity was shown to modulate visual and auditory cortices. ii) A quantification of the complexity sensitivity of individual regions of interest (ROIs) demonstrated a change of sensitivity (from fine grained to coarser) in a gradient from lower to higher areas. iii) It was shown that parahippocampal place area (PPA) has distributed representations of complexity comparable to or better than the ROIs in the early visual cortex, supporting the notion that global scene properties such as complexity plays an important role in scene processing. iv) It was shown that regions of the auditory cortex, which represent syntactic language complexity such as posterior regions of superior temporal gyrus (STG) and superior temporal sulcus (STS), represent music complexity
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