Abstract
Predictive coding theory suggests that predictable responses are “explained away” (i.e., reduced) by feedback. Experimental evidence for feedback inhibition, however, is inconsistent: most neuroimaging studies show reduced activity by predictive feedback, while neurophysiology indicates that most inter-areal cortical feedback is excitatory and targets excitatory neurons. In this study, we asked subjects to judge the luminance of two gray disks containing stimulus outlines: one enabling predictive feedback (a 3D-shape) and one impeding it (random-lines). These outlines were comparable to those used in past neuroimaging studies. All 14 subjects consistently perceived the disk with a 3D-shape stimulus brighter; thus, predictive feedback enhanced perceived contrast. Since early visual cortex activity at the population level has been shown to have a monotonic relationship with subjective contrast perception, we speculate that the perceived contrast enhancement could reflect an increase in neuronal activity. In other words, predictive feedback may have had an excitatory influence on neuronal responses. Control experiments ruled out attention bias, local feature differences and response bias as alternate explanations.
Highlights
Predictive coding is a form of efficient sensory coding[1] that relies on the elimination of predictable neuronal responses and thereby the exclusive processing and transmission of unpredicted portions of the sensory input[2,3,4,5]
Since there is experimental evidence suggesting a monotonic relationship between perceived contrast and neuronal activity in early visual areas[23,24], we speculate that, at least at the moment at which subjects made their perceptual decision about local contrast, predictive feedback was excitatory rather than inhibitory
Since anatomical evidence shows that feedback connections are strongly divergent[25], we reasoned that the influence of predictive feedback might be measurable over the entire disk
Summary
Predictive coding is a form of efficient sensory coding[1] that relies on the elimination of predictable neuronal responses and thereby the exclusive processing and transmission of unpredicted portions of the sensory input[2,3,4,5]. Standard neuronal models of predictive coding hold that the different hierarchical levels interact by excitatory feedforward carrying residual activity and inhibitory feedback carrying predictions. We employed similar stimuli as in Murray et al.: 3D-shape outlines and random-lines versions of the same stimuli[13]. Since there is experimental evidence suggesting a monotonic relationship between perceived contrast and neuronal activity in early visual areas[23,24], we speculate that, at least at the moment at which subjects made their perceptual decision about local contrast, predictive feedback was excitatory rather than inhibitory
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