Dynamics of S-cone contributions to the initiation of saccadic and smooth pursuit eye movements

We investigated the interplay between luminance and heterochromatic brightness in guiding oculomotor behavior, particularly in saccades and smooth pursuit eye movements. We were particularly interested in testing whether mechanisms for eye target selection incorporate contributions from S-cones. Luminance, typically measured using the CIE's luminous efficiency function V(λ), has limitations in representing the perceived brightness of heterochromatic stimuli, especially with bluish and yellowish lights. S-cones do not contribute significantly to luminance but do influence brightness perception. To examine the S-cone contributions to oculomotor behavior, we measured the target choices of saccades and smooth pursuit between equi-luminant bluish and yellowish stimuli, with paradigms producing a wide range of latencies. Our results show that at shorter latencies, luminance primarily drives both eye movements, with equi-luminant bluish and yellowish stimuli being chosen equally often. However, as latency increases, participants tend to choose bluish stimuli more frequently, suggesting that heterochromatic brightness plays a major role in longer-latency eye movements. This indicates that S-cone input may influence target selection as latency increases, highlighting a dynamic interaction between luminance and brightness in oculomotor decisions. We were particularly interested in investigating whether the mechanism responsible for eye movement target selection incorporates S-cone activity.