Commentary on the article: ``A model of saccade generation based on parallel processing and competitive inhibition'', by John M. Findlay and Robin Walker. Linking Covert and Overt Attention James J. Clark Centre for Intelligent Machines McGill University Montreal, Quebec, Canada email: clark@cim.mcgill.ca Abstract: The target article questions whether covert attention plays any role in normal visual scanning (overt attention). My commentary on this article suggests that there is indeed a very close link between the processes which govern covert and overt attention. ---------------------------------------------------------------------------- I would like to begin this commentary by saying that the model proposed in the target article is very compelling and in line with my personal philosophy of oculomotor control in humans. I would like to take this opportunity to take issue with one aspect of the target article, however. On page 23 of the target article, the authors question whether covert attention plays any role in normal visual scanning (overt attention). One of the reasons that they give for their reluctance to ascribe any role for covert orienting in the generation of saccades is that covert shifts appear to be at least as slow as overt shifts, so that covert shifts would have no advantage in scanning of scenes. I would argue that the similarity in speed in covert and overt attentional shifts indicates a possible common substrate for overt and covert orienting. There have been a number of studies demonstrating a close, if not exact, connection between neural mechanisms underlying overt and covert attention. For example, Desimone (Desimone et al. 1989) has has found that local deactivation of small zones in the superior colliculus impairs an animal's ability to attend to a target. More recently, Kustov and Robinson (Kustov and Robinson, 1996) have demonstrated the effects of attentional manipulation on the trajectories of saccades evoked by electrical stimulation of the superior colliculus. The target article proposes that saccadic latency is determined mainly by the ``conflict resolution'' competitive push-pull interaction between the fixation centre and the move centre. The idea that when a saccade is triggered its metrics are determined by the point of maximum salience is not a new one. It was also proposed in a recently published article of mine (Clark, 1998), as was the idea that the triggering of a saccade is caused by the switching of a winner-take-all competition. The model proposed in the target article differs from that described in (Clark, 1998) in two important ways, however, these being the role of fixation in triggering of a saccade, and the mechanism that determines the latency of a saccade. In my model the transition of a competitive ``winner-take-all'' interaction between competing spatial locations is what triggers a saccade, as opposed to the transition of the interaction between the fixation and move centres. In the model that I proposed in (Clark, 1998), shifts in (covert) spatial attention are associated with the transitions of the winner-take-all competition between locations in the move centre. Thus, the model explicitly provided a link between covert and overt attention. It was shown in (Clark, 1998) that such a model accounts for the quantitative aspects of a wide variety of oculumotor phenomena, including some of those used to support the model of the target article (gap effect, double step). This was supported by computer simulations of the model, something which is missing in the target article. At the very least, it shows that fixation effects might not be neccessary to explain many features of saccadic latency phenomena, although I believe that fixation has some role to play. My model is clearly a simplistic model, as it is evident that not all covert attention shifts result in eye movements. This shortcoming is easily remedied, however, by adding in a fixational process, as is done in the target article. With this addition, there will be a component of the saccadic latency which depends on the functioning of the fixation process, and this may well account for phenomena such as express saccades, which can not easily be explained with my model. I would argue, however, that the bulk of saccadic latency effects are due to the time required for the winner-take-all competition withing the spatial ``move'' maps, i.e. the time taken for covert attentional shifts. In summary, the view that I wish to put forward is a traditional ``premotor'' view of spatial attention (c.f. Rizzolatti 1983), in which each and every covert attentional shift gives rise to a ``command'' to make a saccade. Whether this command gets expressed as an actual eye movement depends on the state of the fixation system. It is my view that the (very nice) model described in the target article could be easily extended to involve covert attention in this way, and in the process, would gain significant explanatory power. References ------------ Clark, J.J., ``Spatial Attention and Latencies of Saccadic Eye Movements", Vision Research, 39(3):583-600, 1998 Desimone, R., Wessinger, M., Thomas, L. and Schneider, W. (1989), ``Effects of deactivation of lateral pulvinar or superior colliculus on the ability to selectively attend to a visual stimulus'', Society for Neuroscience Abstracts, 15:162. Kertzman, C., and Robinson, D.L. (1988), ``Contributions of the superior colliculus of the monkey to visual spatial attention'', Society for Neuroscience Abstracts, 14:831 Kustov, A.A. and Robinson, D.L. (1996), ``Shared neural control of attentional shifts and eye movements'', Nature, 384:74-77 Rizzolati, G. (1983), ``Mechanisms of selective attention in mammals'', in Advances in Vertebrate Neuroethology, Ewart, J.P., Capranica, R.R., and Ingle, D.J., (eds.), Plenum, New York, pp 261-297