Available online at www.sciencedirect.com Saccade control after V1 lesion revisited Tadashi Isa and Masatoshi Yoshida A number of previous articles on blindsight have stressed that even after the lesion of the primary visual cortex (V1), subjects can perform visually guided saccades toward the targets in the blind field and that the extrageniculate visual pathway which bypasses the V1 can control the saccades by itself. However, in monkey model of V1 lesion, about two months of time is needed for recovery, suggesting that the extrageniculate visual pathway cannot immediately take over the function of the geniculo-striatal pathway, and on close look at the dynamics of saccades, saccades became ballistic and online corrections of trajectories and velocities were impaired, and that decision threshold for saccade initiation was lowered. On the basis of these findings, we propose that the visual signal through V1 is necessary for deliberate control of saccades. Address Department of Developmental Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan Corresponding author: Isa, Tadashi (tisa@nips.ac.jp) and Current Opinion in Neurobiology 2009, 19:608–614 This review comes from a themed issue on Motor systems Edited by Abdel el Manira and Krishna Shenoy Available online 14th November 2009 0959-4388/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.conb.2009.10.014 Saccadic control systems are hierarchically organized (Figure 1). Reflexive orienting movements of the eye, head, and body to shift the gaze to visual targets that suddenly appear in the surrounding visual scene are most fundamental behaviors required for animals to survive in the wild environment. The superior colliculus or optic tectum in the midbrain and its downstream brainstem gaze center circuits form robust pattern generator for coordinat- ing movements of various body parts to orient the gaze in all the vertebrate species [1–5]. Thus, the basic control system controlling orienting behavior is supposed to be implemented in the brainstem level and such low-level processing is under the control of higher order structures such as fronto-parietal cortices and basal ganglia. The hierarchy also involves the visual input of the system. Two major visual pathways are known to exist; one directly reaches the superior colliculus (retino-tectal or extrageni- culate visual pathway), which is phylogenetically old, and the other reaches the cerebral cortex via the lateral geniculate nucleus (geniculo-striatal visual pathway), which is phylogenetically new. As shown in Figure 1, the extrageniculate pathway is supposed to have access to the higher order systems such as the fronto-parietal system via the pulvinar, and the visual inputs via this pathway seem to be mixed up at this level with the input via the geniculo-striatal pathway. But how these two visual systems differ in the role of regulating the orienting behaviors is still elusive. Higher vertebrates such as primates acquired foveal vision with high acuity and in these animals, saccadic eye-movement system has developed as a highly accurate motor control system. Such changes might parallel the tremendous development of the cerebral cortex in this species. However, in general, studies on the cortical control of saccades have mainly focused on context-de- pendent modulation such as inhibition of reflexive reac- tions [6,7], decision making [8], attentional modulation [9], update of ongoing strategy [10], and working memory by the fronto-parietal system [11]. In contrast, studies on the role of cortical processing, especially that of the primary visual cortex (V1) in the regulation of dynamic aspects of saccades are sparse [12–14]. This might be partly because many literature reported that visually guided saccades remain intact after lesion of V1 as in the case reports of blindsight patients, where some patients with damage to the V1 were shown to retain their ability to localize visual targets in their contrale- sional hemifield by either saccades or pointing with hand [15–18,19  ], suggesting the contribution of extragenicu- late visual pathway to localization of the target in space. In monkeys, it has been reported that lesion of the V1 alone did not disrupt saccades but additional lesion of the superior colliculus permanently impaired saccades [20]. Thus, it has been a general belief that saccades are generated basically by the brainstem neural circuits after V1 lesion. Moreover, in most of the previous articles on the motor ability of the blindsight patients or monkeys, the motor performance was mainly described as success ratio or endpoint accuracy, but kinematic analysis of the movement trajectories and analysis of reaction time has not been systematically performed. Recently we investigated saccades in macaque monkeys with unilateral lesion to the V1 and found several import- ant changes in the characteristics of saccade control in these animals [21  ]. We raised questions whether the deficits are primarily caused by the insufficient visual inputs or involve later stages of the visual-motor proces- sing. In this review, we discuss the limitation of the signal processing via the extrageniculate pathway alone and the Current Opinion in Neurobiology 2009, 19:608–614 www.sciencedirect.com