Orientation anisotropies early in human visual cortex depend on contrast
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1
The University of Sydney, School of Psychology & ARC Centre of Excellence in Vision Science, Australia
Background: The mechanisms of orientation processing in mammalian visual cortex appear matched to the environment, such that larger populations of cells are tuned to the cardinal orientations (horizontal and vertical) than oblique orientations. Perceptually, this property appears to be manifested in poorer sensitivity to the oblique compared to the cardinal orientations in a wide variety of tasks – the so-called oblique effect. Surprisingly, some recent functional magnetic resonance imaging (fMRI) studies have revealed an opposite pattern of anisotropy – namely, an increased response to the oblique orientations over the cardinals: the inverse oblique effect. It has been proposed that this might reflect efficient coding strategies optimised to the particular diet of orientations encountered during natural viewing. As such, it might be expected that the form of anisotropy would change as the quality or strength of the oriented stimulation changes.
Methods: In two experiments, the fMRI BOLD response at 3T was measured in retinotopically-defined regions of the visual cortex of human subjects (n=5) as a function of the orientation of a sinusoidal grating, across different stimulus contrasts (10, 30 & 100% in Experiment 1; 3 & 100% in Experiment 2).
Results: The results revealed a shift from the previously observed inverse oblique effect at high contrast to a pattern of anisotropy resembling an oblique effect at low contrast. In Experiment 1, a significant orientation by contrast interaction was evident only in primary visual cortex. Moving to 3% contrast in Experiment 2 revealed a similar pattern of results extending to subsequent retinotopic visual areas.
Discussion: The qualitative change in the form of orientation anisotropy as a function of stimulus contrast is consistent with the idea that early visual cortex adaptively changes its coding strategy as a function of signal-to-noise ratio.
Acknowledgements
This work was supported by a National Health and Medical Research Council Project Grant (C.W.G.C.; grant number APP1027258), an Australian Research Council (ARC) Future Fellowship (C.W.G.C.; grant number FT110100150) and the ARC Centre of Excellence in Vision Science.
Keywords:
Vision,
fMRI,
Sensory coding,
Visual Cortex,
Spatial Vision
Conference:
ACNS-2013 Australasian Cognitive Neuroscience Society Conference, Clayton, Melbourne, Australia, 28 Nov - 1 Dec, 2013.
Presentation Type:
Oral
Topic:
Sensation and Perception
Citation:
Clifford
CW and
Maloney
RT
(2013). Orientation anisotropies early in human visual cortex depend on contrast.
Conference Abstract:
ACNS-2013 Australasian Cognitive Neuroscience Society Conference.
doi: 10.3389/conf.fnhum.2013.212.00170
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Received:
15 Oct 2013;
Published Online:
25 Nov 2013.
*
Correspondence:
Prof. Colin W Clifford, The University of Sydney, School of Psychology & ARC Centre of Excellence in Vision Science, Sydney, Australia, colin.clifford@unsw.edu.au