Consequences of covert orienting to non-informative stimuli of different modalities: a unitary mechanism?

Reaction time (RT) to visual targets is lengthened following non-informative cues presented in the same location, or in different locations but in the same hemifield as the targets. RT lengthening is best accounted for by the voluntary suppression of an overt orienting toward the location of the cue: this veto produces an inhibition of the overall motor reactivity towards stimuli presented in the entire hemifield of the cue. This paper shows that ipsilateral inhibition is not unique to the visual system, since the same directional constraints in motor readiness are induced with somatosensory stimulation. RT is slower when a somatic target delivered on a shoulder is preceded by an ipsilateral somatic cue compared to a contralateral one. The neural control of these orienting tendencies may involve the superior colliculus, which contains overlapping maps of the visual, somatosensory and auditory peripheries. This suggestion is reinforced by the presence of cross-modal inhibitory effects in paradigms involving visual cues and somatic targets or somatic cues and visual targets. While the time course of ipsilateral inhibition is similar in the visual and the somatic modalities, cross-modal inhibitory effects are different and somehow complementary when visual cues precede somatic targets (early short-lasting inhibition) or, respectively, somatic cues precede visual targets (late, long-lasting inhibition). An additional finding is that crossed-uncrossed RT differences (CUDs), presumably due to the anatomical relations between stimulus and response, are present in both modalities.