Expertise in detecting and recognizing objects in natural scenes, where targets are camouflaged by their backgrounds, is critical for many of our interactions in complex environments: from identifying predators or prey and recognizing poisonous foods, to diagnosing tumors on medical images and finding familiar faces in the crowd. As with many skills, learning has been shown to be a key facilitator in the detection and recognition of targets in cluttered scenes [1–8
]. Previous neurophysiological [9–15
] and imaging [16–19
] studies on object learning have concentrated on the higher stages of visual (inferior temporal cortex) and cognitive processing (prefrontal cortex), providing evidence that the representations of shape features in these areas are modulated by learning. In contrast, computational approaches have proposed that associations between features that mediate the recognition of familiar objects may occur across different stages of visual analysis, from orientation detectors in the primary visual cortex to occipitotemporal neurons tuned to object parts and views [20–22
]. However, the neural implementation of object learning mechanisms across stages of visual analysis is largely unknown, and the question of how the visual brain learns objects in natural cluttered scenes remains open.
The aim of our study was 2-fold: (1) to investigate the neural plasticity mechanisms that mediate shape learning in cluttered scenes across stages of visual processing in the human visual cortex, and (2) to examine the effect of regularities present in natural scenes (i.e., grouping of similar features) that determine the distinctiveness of targets in noisy backgrounds (i.e., perceptual saliency) on this learning-dependent plasticity. To this end, we used human functional magnetic resonance imaging (fMRI) combined with psychophysics. To gain insight into the neural mechanisms that mediate shape-specific learning, we examined fMRI responses evoked when observers detected shapes that they had learned through training compared with responses evoked when observers detected shapes on which they had not been trained. To investigate the effects of learning in the detection of visual shapes in cluttered scenes, we manipulated the salience of the target shapes by altering their distinctiveness from the background ( ). We compared behavioral performance and fMRI responses for low-salience shapes in noise (Experiment 1) and high-salience pop-out targets (Experiment 2).
Our stimuli consisted of shapes defined by a closed contour of similarly oriented Gabor elements that were embedded in a background of Gabor elements. These stimuli (see ) yield the perception of a global figure in a textured background rather than simple paths (i.e., open contours). These aligned contours have been shown to result from the integration of the similarly oriented elements into global configurations [23–25
]. Previous work has shown that these stimuli involve processing in both early retinotopic and higher occipitotemporal regions [26
]. In Experiment 1, observers were presented with low-salience stimuli in which shapes were embedded in a background of randomly positioned and oriented Gabors. In Experiment 2, high-salience stimuli were used in which shapes were embedded in a background of randomly positioned, but uniformly oriented Gabors. In both experiments, observers were required to decide which of two shapes presented on either side of the central fixation point was symmetrical. Initially, observers performed this task in the scanner with two sets of untrained stimuli. Observers were then trained in the laboratory with feedback on three consecutive days on one set of stimuli, and then tested again in the scanner with the trained set and the originally presented, untrained set of stimuli ( ).
Our findings suggest a link between shape-specific perceptual learning and neural plasticity mechanisms in the human visual cortex. Specifically, for low-salience shapes, improved behavioral performance was coupled with increased fMRI responses for trained shapes in early retinotopic areas (V1, V2, Vp, and V4v) and the lateral occipital complex (LOC), a region in the lateral occipital cortex extending anterior in the temporal cortex ( ) that is thought to be involved in the analysis of object shape [27
] and processes of object recognition [19
]. In contrast to the increased responses for trained low-salience shapes, when observers learned high-salience shapes, lower fMRI responses were evoked in the LOC, and no evidence for plasticity in early retinotopic visual areas was observed. These findings provide novel evidence for distributed neural plasticity mechanisms across stages of visual analysis that are adaptable to image regularities which determine the perceptual saliency of targets in cluttered scenes ( ).