Letters Response Attentional pointers: response to Melcher Patrick Cavanagh 1, 2 , Amelia R. Hunt 3 , Arash Afraz 4 and Martin Rolfs 5 1 Laboratoire Psychologie de la Perception, Universite ´ Paris Descartes, 45 rue des Saints Pe ` res, 75006 Paris, France 2 Vision Sciences Laboratory, Harvard University, 33 Kirkland Street, Cambridge, MA 02138, USA 3 School of Psychology, William Guild Building, University of Aberdeen, Aberdeen, AB24 2UB, UK 4 McGovern Institute for Brain Research, Massachusetts Institute of Technology, 43 Vassar Street, Cambridge, MA 02139, USA 5 Department of Psychology, New York University, 6 Washington Place, New York, NY 10003, USA Melcher appears to have misunderstood our opinion piece on attention pointers [1], or perhaps we did not state it clearly enough. We did not claim that when it comes to visual stability, the attention system does the work. The oculomotor system does the work and the attention system comes along for the ride. The performance benefits that comprise the central properties of spatial attention appear to be parasitic on the functions of the eye movement control system [2]. As reviewed by Awh et al. [3], stimulation of cells in the saccade control centers triggers attentional benefits at corresponding retinotopic locations. However, when the eyes move, the activity in saccade centers is shifted to the locations that targets of interest will have following the saccade. Attention does not do this shift work, but the consequence is that attentional benefits will then be at the appropriate locations following the saccade. To summarize, the saccade control centers maintain a set of potential target locations as peaks of activity. If the activity at one location crosses a movement threshold (while subcortical neurons are controlling the fixation pause [4]), a saccade is triggered. At lower levels of activity, these location pointers confer attentional benefits at the corresponding retinotopic locations in early visual cortices [3]. That is why we called these peaks of activity in saccade centers attentional pointers. Shifting these pointers at the time of saccades is a service of the oculomotor system that keeps the attention pointers appropriately aligned with targets of interest. By contrast, we were clear that there is nothing in these pointers that relates to features. They are not, in our proposal, feature-specific in any way, although experi- ments might show them to be so in the future. We did note that there must necessarily be a link between the location information of a target (its attention pointer) and some other set of identity information about the target. The combination of the identity with location would correspond to the putative structure of ‘object files’ [5]. The nature of this link is central to the understanding of visual proces- sing in general, as well as spatiotopy in particular, as pointed out by Melcher [1]. We also suggested that these attention pointers, coupled with a link to their target identity, allow high-level spa- tiotopy: we know where the target is after the eye move- ment so we know what its properties are even before we start to re-encode them from its new location. This can easily lead to spatiotopic priming (e.g. [6]). By contrast, it is not clear that the information stored about an object includes the adaptation state of the cells that are encoding it. It is this information that would have to be transferred to generate the spatiotopic aftereffects (e.g. [7]) that have been difficult to replicate (e.g. [8]). Therefore our distinc- tion between the presence of high-level spatiotopy (for identity, in the form of priming) and the absence of low- level spatiotopy (for aftereffects) is straightforward. The link between location and identity is a very general re- quirement of visual processing and the lack of evidence of where or how it works is a challenge for all visual science and is not a weakness of our proposal in particular. Finally, these attention pointers and their shifts at the time of saccades are sufficient to explain that apparent motion seen between two successive stimuli is based on spatial coordinates and not retinal coordinates when a saccade intervenes between the first and second stimulus [9]. However, Melcher points out that some types of appar- ent motion cannot be explained by mere shifts between the centers of the two stimuli. In particular, if the stimuli have different shapes, a transformation is seen between the two [10] even across a saccade [1]. Clearly, a shift of a pointer is not sufficient to explain this phenomenon. Our interpreta- tion is not that the pointers are linked to features such as shape, but that the activity pattern on the saccadic map is shaped like the target. We have evidence of this in a recent remapping study using fMRI [11]. In line with previous fMRI studies of remapping (e.g. [12]), saccades generated BOLD activity in early retinotopic cortices at an expected post-saccadic retinotopic location of a target even though the target was removed before the saccade landed and so was never present at that location. In our case, the target had a wedge shape that changed orientation on each saccade, and indeed the spatial pattern of the remapped activity was correspondingly wedge shaped, rotating in step with the stimulus. Given that the activity pattern confers attentional ben- efits on the corresponding location in early cortex (which is where we were measuring the BOLD activity), this shaped pattern of activity, if it holds up in future studies, also makes sense of object-based attention [13] where attention to one part of an object spreads throughout the object. Admittedly, calling this target-shaped activity pattern a pointer goes beyond the usual meaning of ‘pointer’. Never- theless, it does ‘point’ in that it indexes all locations within the object. A better label could emerge but ‘target-shaped attention pointers’ is not it, so we still favor ‘attention pointers’. Corresponding author: Cavanagh, P. (patrick.cavanagh@parisdescartes.fr). Update Trends in Cognitive Sciences Vol.14 No.11 474