Are somatosensory saccades voluntary or reflexive?

Abstract

The present study examines whether the distinction between voluntary (endogenous) and reflexive (stimulus-elicited) saccades made in the visual modality can be applied to the somatosensory modality. The behavioural characteristics of putative reflexive pro-saccades and voluntary anti-saccades made to visual and somatosensory stimuli were examined. Both visual and somatosensory pro-saccades had much shorter latency than voluntary anti-saccades made in the direction opposite to a peripheral stimulus. Furthermore, erroneous pro-saccades were made towards both visual and somatosensory stimuli on approximately 11-13% of anti-saccade trials. The observed difference in pro- and anti-saccade latency and the presence of pro-saccade errors in the anti-saccade task indicates that a somatosensory stimulus can elicit a form of reflexive saccade comparable to pro-saccades made in the visual modality. It is proposed that a peripheral somatosensory stimulus can elicit a form of reflexive saccade and that somatosensory saccades do not depend exclusively on higher level endogenous control processes for their generation. However, a comparison of the underlying latency distributions and of peak-velocity profiles of saccades made to visual and somatosensory stimuli showed that this distinction may be less clearly defined for the somatosensory modality and that modality-specific differences (such as differences in neural conduction rates) in the underlying oculomotor structures involved in saccade target selection also need to be considered. It is further suggested that a broader conceptualisation of saccades and saccade programming beyond the simple voluntary and reflexive dichotomy, that takes into account the control processes involved in saccade generation for both modalities, may be required.