The visual system rapidly extracts information about objects from the cluttered natural environment. In 5 experiments, the authors quantified the influence of orientation and semantics on the classification speed of objects in natural scenes, particularly with regard to object-context interactions. Natural scene photographs were presented in an object-discrimination task and pattern masked with various scene-tomask stimulus-onset asynchronies (SOAs). Full psychometric functions and reaction times (RTs) were measured. The authors found that (a) rotating the full scenes increased threshold SOA at intermediate rotation angles but not for inversion; (b) rotating object or context degraded classification performance in a similar manner; (c) semantically congruent contexts had negligible facilitatory effects on object classification compared with meaningless baseline contexts with a matching contrast structure, but incongruent contexts severely degraded performance; (d) any object-context incongruence (orientation or semantic) increased RTs at longer SOAs, indicating dependent processing of object and context; and (e) facilitatory effects of context emerged only when the context shortly preceded the object. The authors conclude that the effects of natural scene context on object classification are primarily inhibitory and discuss possible reasons.

In this study we tested predictions of two important theories of visual coding, contrast energy and sparse coding theory, on the dependence of population activity level and metabolic demands on spatial structure of the visual input. With carefully calibrated displays we find that in humans neither the V1 blood oxygenation level dependent (BOLD) response nor the initial visually evoked fields in magnetoencephalography (MEG) are sensitive to phase perturbations in photographs of natural scenes. As a control, we quantitatively show that the applied phase perturbations decrease sparseness (kurtosis) of our stimuli but preserve their root mean square (RMS) contrast. Importantly, we show that the lack of sensitivity of the V1 population response level to phase perturbations is not due to a lack of sensitivity of our methods because V1 responses were highly sensitive to variations of image RMS contrast. Our results suggest that the transition from a sparse to a distributed neural code in the early visual system induced by reducing image sparseness has negligible consequences for population metabolic cost. This result imposes a novel and important empirical constraint on quantitative models of sparse coding: Population metabolic rate and population activation level is sensitive to second order statistics (RMS contrast) of the input but not to its spatial phase and fourth order statistics (kurtosis).