title: The tuning of human visual cortex to variations in the 1/fαamplitude spectra and fractal properties of synthetic noise images.
abstract: Natural scenes share a consistent distribution of energy across spatial frequencies (SF) known as the 1/fαamplitude spectrum (α≈0.8-1.5, mean 1.2). This distribution is scale-invariant, which is a fractal characteristic of natural scenes with statistically similar structure at different spatial scales. While the sensitivity of the visual system to the 1/f properties of natural scenes has been studied extensively using psychophysics, relatively little is known about the tuning of cortical responses to these properties. Here, we use fMRI and retinotopic mapping techniques to measure and analyze BOLD responses in early visual cortex (V1, V2, and V3) to synthetic noise images that vary in their 1/fαamplitude spectra (α=0.25 to 2.25, step size: 0.50) and contrast levels (10% and 30%) (Experiment 1). To compare the dependence of the BOLD response between the photometric (intensity based) and geometric (fractal) properties of our stimuli, in Experiment 2 we compared grayscale noise images to their binary (thresholded) counterparts, which contain only black and white regions. In both experiments, early visual cortex responded maximally to stimuli generated to have an input 1/f slope corresponding to natural 1/fαamplitude spectra, and lower BOLD responses were found for steeper or shallower 1/f slopes (peak modulation: 0.59% for 1.25 vs. 0.31% for 2.25). To control for changing receptive field sizes, responses were also analyzed across multiple eccentricity bands in cortical surface space. For most eccentricity bands, BOLD responses were maximal for natural 1/fαamplitude spectra, but importantly there was no difference in the BOLD response to grayscale stimuli and their corresponding thresholded counterparts. Since the thresholding of an image changes its measured 1/f slope (α) but not its fractal characteristics, this suggests that neuronal responses in early visual cortex are not strictly driven by spectral slope values (photometric properties) but rather their embedded geometric, fractal-like scaling properties.
authors.affiliation: School of Psychology, UNSW Australia, Sydney, NSW 2052, Australia; Neuroscience Research Australia, Randwick, NSW 2031, Australia. Electronic address: z.isherwood@student.unsw.edu.au.
authors.forename: Zoey J
authors.initials: ZJ
authors.lastname: Isherwood
authors.affiliation: Neuroscience Research Australia, Randwick, NSW 2031, Australia; School of Psychology, University of Wollongong, Wollongong, NSW 2522, Australia.
authors.forename: Mark M
authors.initials: MM
authors.lastname: Schira
authors.affiliation: School of Psychology, UNSW Australia, Sydney, NSW 2052, Australia.
authors.forename: Branka
authors.initials: B
authors.lastname: Spehar
mesh_terms: D000328:Adult; D001931:Brain Mapping; D015350:Contrast Sensitivity / Q000502:physiology; D005260:Female; D017709:Fractals; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D010775:Photic Stimulation; D014793:Visual Cortex / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult
keywords: 1/f amplitude spectrum; Fractals; Natural scene statistics; Vision; fMRI