Abstract
The oculomotor system utilizes color extensively for planning saccades. Therefore, we examined how the oculomotor system actually encodes color and several factors that modulate these representations: attention-based surround suppression and inherent biases in selecting and encoding color categories. We measured saccade trajectories while human participants performed a memory-guided saccade task with color targets and distractors and examined whether oculomotor target selection processing was functionally related to the CIE (x,y) color space distances between color stimuli and whether there were hierarchical differences between color categories in the strength and speed of encoding potential saccade goals. We observed that saccade planning was modulated by the CIE (x,y) distances between stimuli thus demonstrating that color is encoded in perceptual color space by the oculomotor system. Furthermore, these representations were modulated by (1) cueing attention to a particular color thereby eliciting surround suppression in oculomotor color space and (2) inherent selection and encoding biases based on color category independent of cueing and perceptual discriminability. Since surround suppression emerges from recurrent feedback attenuation of sensory projections, observing oculomotor surround suppression suggested that oculomotor encoding of behavioral relevance results from integrating sensory and cognitive signals that are pre-attenuated based on task demands and that the oculomotor system therefore does not functionally contribute to this process. Second, although perceptual discriminability did partially account for oculomotor processing differences between color categories, we also observed preferential processing of the red color category across various behavioral metrics. This is consistent with numerous previous studies and could not be simply explained by perceptual discriminability. Since we utilized a memory-guided saccade task, this indicates that the biased processing of the red color category does not rely on sustained sensory input and must therefore involve cortical areas associated with the highest levels of visual processing involved in visual working memory.
Highlights
We examined whether the color representations utilized by the oculomotor system for saccade planning were encoded in perceptual color space by analyzing trials on which a correct saccade was made to the target and an isolated, peripheral distractor was within 90◦ of the target
More plausible is that the proportion of errors and response latency metrics are reflective of the perceptual decision component of the task, which is highly related to the discriminability of the stimuli: a perceptual decision threshold is surpassed faster as the target becomes more perceptually discriminable (Hanes and Schall, 1996; Gold and Shadlen, 2000; Ditterich et al, 2003). We find this explanation more plausible given that subsequent analyses suggested that lower error rates and faster latencies for red and blue saccades might be explained by the perceptual discriminability of these colors in color space, as saccadic reaction time (SRT) showed a marginal linear relationship with the discriminability of the current color categories in color space, and there was an insignificant relationship for error rates, a subjective inspection of these means suggests that errors might vary as a sigmoidal or step-like function of discriminability in color space
Our results demonstrated that there is a functional relationship between saccade curvature elicited by an isolated distractor and the CIE (x,y) color space distance between the target and the distractor suggesting that the oculomotor system encodes color in perceptual color space as with the visual system
Summary
Color processing plays an important role in many oculomotor behaviors like pursuit eye movements (Tchernikov and Fallah, 2010), saccadic eye movements (Itti and Koch, 2000; McPeek and Keller, 2001; Ludwig and Gilchrist, 2003; Mulckhuyse et al, 2009), visual search (Green and Anderson, 1956; Treisman and Gelade, 1980; D’Zmura, 1991; Bauer et al, 1996a,b, 1998; Lindsey et al, 2010), and attentional selection (Folk et al, 1992, 1994; Wolfe and Horowitz, 2004; Pomerleau et al, 2014) Examinations of these behaviors have provided evidence that the color representations that influence oculomotor planning are encoded in multi-dimensional color space similar to how perceived color is encoded. Whether the color representations utilized for saccade planning are encoded in a multi-dimensional color space remains unclear
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