NEUROPHYSIOLOGY, BASIC AND CLINICAL NEUROREPORT 0959-4965 & Lippincott Williams & Wilkins Vol 12 No 9 3 July 2001 2041 The thalamus interrupts top-down attentional control for permitting exploratory shiftings to sensory signals George Andrew Michael, CA Muriel Boucart, Jean-Franc Ëois Degreef 1 and Olivier Godefroy 2 Exploration Fonctionnelle de la Vision and 1 Clinique Neurologique, Ho à pital Roger Salengro, Rue Emile Laine, 59037 Lille Cedex; 2 Centre Hospitalier Universitaire d'Amiens, Amiens, France CA Corresponding Author Received 28 March 2001; accepted 14 April 2001 When attention is involuntarily drawn in a direction different to that of the target, slower motor response times are observed (i.e. the meridian effect). Previous data suggested that the thalamus might participate in the generation of visual salience. What may be the role of the thalamus in the capture by luminance transients when attentional control is in action? A single experiment was administrated in a group of ten healthy volunteers as well as in a group of three patients with unilateral thalamic infarcts. Subjects participated in a task where attentional control was interrupted by a distractor. The meridian effect was present only in the performance of the healthy volunteers and when distractors occurred in the ipsilesional (intact) hemi®eld of the thalamic patients. These results suggest that when an important signal appears during attentional focalization, the thalamus interrupts current focali- zation and permits the compilation of an attentional program in the midbrain aiming at generating an orienting response towards the source of this signal. NeuroReport 12:2041±2048 & 2001 Lippincott Williams & Wilkins. Key words: Attentional capture; Attentional control; Human thalamus; Meridian effect INTRODUCTION An important aspect of the human top-down control of attention is the ability to prevent re¯exive shiftings to- wards irrelevant abruptly presented items by attenuating the incoming signals they generate [1,2]. This is supported by studies in healthy volunteers who were able to comple- tely ignore distracting onsets [1,3], as well as by a case of pathological loss of the top-down attentional control fol- lowing a lesion of the premotor cortex where onsets inevitably captured attention [4]. However, several recent studies [5±7] found that task-irrelevant onsets frequently elicited brief saccadic eye movements towards their loca- tion. This is consistent with the hypothesis of shared neural substrates of attention and oculomotion [2,8±10] and also with data suggesting that the top-down control over attentional capture requires a cortical control of oculomotor programs [4,11]. These ®ndings [4±7,12] suggest that during attentional top-down control, some involuntary attentional/ocular micro-shiftings are produced betraying the activity of a latent mechanism who is actively looking out for important changes occurring outside the current attentional focus. If a sudden event occurs, then this mechanism will brie¯y orient attention towards the source of signals in order to explore their nature. Thus, this mechanism will be capable of making judgments on the event's nature, taking quick decisions for permitting attentional movements and inter- rupting the current attentional focalization. Early studies offer an excellent support to this idea. Segundo et al. [13] applied electrical stimulations on the reticular formation of unanesthetized monkeys who were occupied with random activities. They observed that the animals ceased whatever they were doing, raised their head expectantly and ap- peared to strain their senses in an effort to determine the nature of the unknown stimulus which apparently had come into their environment. If nothing of a threatening nature appeared, the animals returned to their activity [14]. Because of its relay position in the reticulo-cortical diffu- sion system [15], the thalamus may be involved in the initiation of these micro-shiftings. According to Robinson and colleagues [16,17], one of the major roles of the thalamus is to generate visual salience and to participate in the earliest decision to orient attention towards the source of environmental changes [18]. Consistent data were pre- sented in patients with thalamic lesions [19]. It was reported that warning sensory signals occurring in the affected visual ®eld did not improve detection of impeding targets. The authors proposed that the patients were not able to engage their attention on the location of the warning signals. Thus, the thalamus may be at least partially responsible for the brief exploratory attentional shifts observed in tasks requiring attentional control [4,5]. What would happen if the onsets used in previous studies [1] were more salient in nature, such as distractors