Local non-linear interactions in the visual cortex may reflect global decorrelation
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The classical receptive field in the primary visual cortex have been successfully explained by sparse activation of relatively independent units, whose tuning properties reflect the statistical dependencies in the natural environment. Robust surround modulation, emerging from stimulation beyond the classical receptive field, has been associated with increase of lifetime sparseness in the V1, but the system-wide modulation of response strength have currently no theoretical explanation. We measured fMRI responses from human visual cortex and quantified the contextual modulation with a decorrelation coefficient (d), derived from a subtractive normalization model. All active cortical areas demonstrated local non-linear summation of responses, which were in line with hypothesis of global decorrelation of voxels responses. In addition, we found sensitivity to surrounding stimulus structure across the ventral stream, and large-scale sensitivity to the number of simultaneous objects. Response sparseness across voxel population increased consistently with larger stimuli. These data suggest that contextual modulation for a stimulus event reflect optimization of the code and perhaps increase in energy efficiency throughout the ventral stream hierarchy. Our model provides a novel prediction that average suppression of response amplitude for simultaneous stimuli across the cortical network is a monotonic function of similarity of response strengths in the network when the stimuli are presented alone.
KeywordsFunctional magnetic resonance imaging Efficient coding Ventral stream Information theory Energy consumption
We thank Jarmo Hurri for help in stimulus preparation, mathematics and comments on the manuscript. Marita Kattelus, Lauri Nurminen and Linda Henriksson helped in the measurements, and Linda Henriksson, Lauri Nurminen and Juha Silvanto gave insightful comments on the manuscript. Interpretation of the results have been discussed with Alessandra Angelucci, Jussi Saarinen and Aapo Hyvärinen. Mika Seppä and Lauri Parkkonen helped with mathematics. This study has been supported by the Academy of Finland grant numbers 210347, 124698, 111817, and national centre of excellence-program 2006–2011.
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