ERP effects of spatial attention and display search with unilateral and bilateral stimulus displays

Two experiments were performed in which the effects of selective spatial at tention on the ERPs elicited by unilateral and bilateral stimulus arrays we re compared. In Experiment 1, subjects received a series of grating pattern s. In the unilateral condition these gratings were presented one at a time, randomly to the right or left of fixation. In the bilateral condition, gra tings were presented in pairs, one to each side of fixation. In the unilate ral condition standard ERP effects of visual spatial attention were observe d. However, in the bilateral condition we failed to observe an attention re lated posterior contralateral positivity (overlapping the P1 and N1 compone nts, latency interval about 100-250 ms), as reported in several previous st udies. In Experiment 2, we investigated whether attention related ERP later alizations are affected by the task requirement to search among multiple ob jects in the visual field. We employed a task paradigm identical to that us ed by Luck et al. (Luck, S.J., Heinze, H.J., Mangun, G.R., Hillyard, S.A., 1990. Visual event-related potentials index focused attention within bilate ral stimulus arrays. II. Functional dissociation of Fl and NI components. E lectroencephalogr. Clin. Neurophysiol. 75, 528-542). Four letters were pres ented to a visual hemifield, simultaneously to both the attended and unatte nded hemifields in the bilateral conditions, and to one hemifield only in t he unilateral conditions. In a focused attention condition, subjects search ed for a target letter at a fixed position, whereas they searched for the t arget letter among all four letters in the divided attention condition las in the experiment of Luck et al., 1990). In the bilateral focused attention condition, only the contralateral P1 was enhanced. In the bilateral divide d attention condition a prolonged posterior positivity was observed over th e hemisphere contralateral to the attended hemifield, comparable to the res ults of Luck et al. (1990). A comparison of the ERPs elicited in the focuse d and divided attention conditions revealed a prolonged 'search related neg ativity'. We discuss possible interactions between this negativity and atte ntion related lateralizations. The display search negativity consisted of t wo phases, one phase comprised a midline occipital negativity, developing f irst over the ipsilateral scalp, while the second phase involved two symmet rical occipitotemporal negativities, strongly resembling the N1 in their to pography. The display search effect could be modelled with a dipole in a me dial occipital (possibly striate) region and two symmetrical dipoles in occ ipitotemporal brain areas. We hypothesize that this effect reflects a proce ss of rechecking the decaying information of iconic memory in the occipitot emporal object recognition pathway. (C) 1999 Elsevier Science B.V. All righ ts reserved.