Visual search and spatial deficits

Abstract

Studies of visual search with patients with spatial attentional deficits have shown that the ability to bind basic features properly and thus to search for the conjunction of two spatially contiguous features is compromised. However, the effect of spatial deficits on feature search is more controversial. Here, we explore questions raised by the neuropsychological literature regarding feature processing and demonstrate that features “pop out” in the affected visual field, albeit more slowly. The implications for feature processing and selection as well as the relevance for understanding spatial deficits are discussed.

Notes

¹Unilateral visual neglect as defined clinically does not necessarily mean that a patient never attends to the contralesional side of space. They typically can be cued to attend to this side, and rehabilitation measures often include training patients to cue themselves accordingly.

²Also see Cohen and Rafal (1991) for evidence of illusory conjunctions on the neglected side of space in a patient with unilateral right neglect from left hemisphere damage.

³We will refer to the ipsilesional, non-neglected side as the “spared” side, but this is not quite correct. Search rates were slower on both the ipsi- and contralesional sides than normal, age-matched participants in the data reported by Eglin et al. (1989). Search in the ipsilesional side occurred first with a disproportionate delay to begin search on the contralesional side.

⁴SV showed symptoms of visual neglect since the time of her stroke several years prior to testing in the present experiment. She was again administered a standard battery for neglect shortly before the present experiment was run. She bisected horizontal lines an average of 5 cm to the right of centre, extinguished all the left items in a two-item display and missed three-quarters of the left items when only one item appeared either to the left or right of centre. She also demonstrated visual extinction and neglect on standard bedside confrontation testing. Due to travel limitations, a perimetry test of visual fields was performed on a laptop computer by presenting a 0.3° blue circle briefly on a white background. The circle only appeared after she responded to a central mark that was difficult to detect unless fixated. This procedure assured central eye fixation. SV sat approximately 60 cm from the screen, and was asked to press a key on the mouse as soon as she detected a target. The target could appear in one of a total of 44 locations spanning the monitor screen, 10 times in each location.

⁵Presentation time was adjusted in increments of ΔT = 6−[(r + 1) − mod((r + 1),2)]/2 screen frames, where r = the number of reversals encountered and mod(a,b) is the remainder after division of a by b.

⁶Eight naïve individuals were asked to judge which of the bilateral figures created the better circle or whether both were the same. All eight chose the dense patterns over the sparse patterns without hesitation.