title: A normalized contrast-encoding model exhibits bright/dark asymmetries similar to early visual neurons.
abstract: Biological sensory systems share a number of organizing principles. One such principle is the formation of parallel streams. In the visual system, information about bright and dark features is largely conveyed via two separate streams: theONandOFFpathways. While brightness and darkness can be considered symmetric and opposite forms of visual contrast, the response properties of cells in theONandOFFpathways are decidedly asymmetric. Here, we ask whether a simple contrast-encoding model predicts asymmetries for brights and darks that are similar to the asymmetries found in theONandOFFpathways. Importantly, this model does not include any explicit differences in how the visual system represents brights and darks, but it does include a common normalization mechanism. The phenomena captured by the model include (1) nonlinear contrast response functions, (2) greater nonlinearities in the responses to darks, and (3) larger responses to dark contrasts. We report a direct, quantitative comparison between these model predictions and previously published electrophysiological measurements from the retina and thalamus (guinea pig and cat, respectively). This work suggests that the simple computation of visual contrast may account for a range of early visual processing nonlinearities. Assessing explicit models of sensory representations is essential for understanding which features of neuronal activity these models can and cannot predict, and for investigating how early computations may reverberate through the sensory pathways.
authors.affiliation: Department of Psychological & Brain Sciences, Dartmouth College, Hanover, New Hampshire emilycooper@dartmouth.edu.
authors.forename: Emily A
authors.initials: EA
authors.lastname: Cooper
mesh_terms: D000200:Action Potentials; D000818:Animals; D002415:Cats; D003198:Computer Simulation; D015350:Contrast Sensitivity; D003624:Darkness; D005829:Geniculate Bodies / Q000166:cytology / Q000502:physiology*; D006168:Guinea Pigs; D008027:Light; D016014:Linear Models; D008959:Models, Neurological; D017711:Nonlinear Dynamics; D010775:Photic Stimulation; D012165:Retinal Ganglion Cells / Q000502:physiology*; D014795:Visual Pathways / Q000166:cytology / Q000502:physiology*
keywords: Lateral geniculate nucleus; ON/OFF pathways; retinal ganglion cells; visual contrast