Abstract
Models of visual attention postulate the existence of a saliency map whose function is to guide attention and gaze to the most conspicuous regions in a visual scene. Although cortical representations of saliency have been reported, there is mounting evidence for a subcortical saliency mechanism, which pre-dates the evolution of neocortex. Here, we conduct a strong test of the saliency hypothesis by comparing the output of a well-established computational saliency model with the activation of neurons in the primate superior colliculus (SC), a midbrain structure associated with attention and gaze, while monkeys watched video of natural scenes. We find that the activity of SC superficial visual-layer neurons (SCs), specifically, is well-predicted by the model. This saliency representation is unlikely to be inherited from fronto-parietal cortices, which do not project to SCs, but may be computed in SCs and relayed to other areas via tectothalamic pathways.
Highlights
Models of visual attention postulate the existence of a saliency map whose function is to guide attention and gaze to the most conspicuous regions in a visual scene
We hypothesize that visual saliency is coded in the primate superior colliculus (SC) (Fig. 1b) because: (1) it is heavily interconnected with early visual areas[20]; (2) it encodes stimuli in a featureless manner[21,22,23]; (3) it has a well-defined topography[24]; and (4) it has long-range centre-surround organization well suited for a saliency mechanism[25]
To test the hypothesis that SCs represents a visual saliency map, three rhesus monkeys freely viewed a series of high-definition video clips of natural dynamic scenes while we recorded extracellular activity of single SC neurons (Fig. 2a)
Summary
Models of visual attention postulate the existence of a saliency map whose function is to guide attention and gaze to the most conspicuous regions in a visual scene. Empirical and computational modelling studies have reported evidence of a saliency and/or priority map in various cortical brain areas (for example, V1 (refs 5–7); V4 (refs 8,9); lateral intraparietal area, LIP10–12; frontal eye fields, FEF13; dorsolateral prefrontal cortex[14]) While these studies used intuitive definitions of saliency with simple stimuli under restricted viewing conditions, a strong test of the saliency hypothesis would be to use a well-established computational model to correlate neuronal firing rates with model-predicted saliency during unconstrained viewing of natural dynamic scenes. We hypothesize that visual saliency is poorly represented in the SCi because activation of these neurons is highly dependent upon goal-directed attention and gaze behaviour[26,27,28,29], owing to its dominant inputs from frontal/parietal areas and basal ganglia, and its direct output to the brainstem saccade circuit[30] This has led to the hypothesis that SCi best represents a behavioural priority map[4]
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