An EEG/ERP study of efficient versus inefficient visual search

An EEG/ERP study of efficient versus inefficient visual search Steven Phillips (steve@ni.aist.go.jp) Neuroscience Research Institute (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568 Japan Yuji Takeda (yuji-takeda@aist.go.jp) Institute for Human Science and Biomedical Engineering (AIST), Tsukuba Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8568 Japan Abstract (inefficient search). The way in which bottom-up (possibly parallel) and top-down (possibly serial) processes interact to determine search efficiency, though, remains unclear despite decades of intensive research (see Wolfe, 2003, for a review). EEG/ERP affords a non-invasive alternative for discern- ing visual search subprocesses (e.g., Luck & Hillyard, 1990; Wolber & Wascher, 2003). Although there are well- developed theories of visual search at the behavioural level, relatively little neuroimaging work has provided the neces- sary bridging laws that afford rigourous testing via corre- spondences to the neurophysiological level (Coltheart, 2006). Hence, our approach is primarily exploratory. That is, to map out and contrast the temporal relationships between pu- tative EEG/ERP markers of cognitive processes under ef- ficient and inefficient search conditions, which motivates a more hypothesis-driven approach for future studies. Visual search involves perception, attention, memory, and re- sponse selection. In this EEG/ERP study, we investigate how these components change with the type of search. Participants identified the location of a target item (coloured, oriented rect- angular bar) in search displays also containing non-target dis- tractors. In the condition yielding efficient search, where re- sponse time was independent of set size (two or four items), distractors had no feature in common with the target. In the condition yielding inefficient search, where response time in- creased with set size, distractors shared one feature (either colour, or orientation) with the target. Stimulus- and response- locked event-related potentials (P1, N1, P2, N2, P3, and rLRP) were analyzed. P3, an indicator of working memory engage- ment, showed no significant difference in onset latency be- tween efficient and inefficient conditions. Although P3 peak amplitude was smaller, activity over time was greater for in- efficient search. The duration of response-locked lateralized readiness potential (rLRP), indicating response selection, was also longer in the inefficient condition. By contrast, indicators of selective attention, P1 peak amplitude and N1 peak latency, were sensitive to number of items; and P2 peak amplitude to number of non-target features. (N2 results were inconclusive.) These results suggest a difference between efficient and ineffi- cient search that lies not in the time required to select items as potential targets of search (selective attention), but in the time required to confirm which of those items is indeed the target (target confirmation). Keywords: Visual search; attention; similarity; EEG; ERP; P3; rLRP; synchrony; phase-locking Methods Participants Eleven Japanese university students (aged 24 ± 3.4 years; male; one left handed) with normal, or corrected-to-normal vision, were paid to participate in the experiment. Apparatus and stimuli A standard desktop computer was used to present stimuli. Stimuli were rectangular bars, either red, green, blue, or yel- low; and at an orientation of 0 ◦ , 45 ◦ , 90 ◦ , or 135 ◦ degrees from horizontal. The background color for all trials was grey. The display was divided into four equal quadrants by invisi- ble horizontal and vertical centerlines. A quadrant contained at most one stimulus item, jittered about its center so that the location of the target was clearly identifiable by the contain- ing quadrant. Electrical potentials were collected using a dig- ital electroencephalograph system (Nihon Kohden Neurofax EEG-1100) with an Ag/AgCl electrode cap. Introduction Visual search is generally conceptualized as involving an in- teraction between bottom-up perceptual information process- ing versus top-down processing of contextual information by working memory (see Corbetta & Shulman, 2002; Desimone & Duncan, 1995, for reviews). Reflecting this distinction, some models of visual search include two types of compu- tation based on (1) the relationships between items in the search display independent of the target item; and (2) the re- lationships between display items and the currently searched for target (see, e.g., Duncan & Humphreys, 1989; Wolfe, Cave, & Franzel, 1989). Such models have accounted for the well-established effects of target-distractor and distractor- distractor similarity on search efficiency. Respectively, they are the increase and decrease in search time per display item (i.e., search slope, or conversely efficiency) with more items sharing the same features (e.g., colour, or orientation). When the target is uniquely identifiable by a single feature, search time is typically independent of the number of display items (efficient search), else it increases linearly with item number Conditions and procedure There were two search display type conditions: efficient, where distractors shared no feature with the target so that response times are independent of search set size; and inef- ficient, where distractors shared one feature (either colour, or orientation) with the target so that response times increase with size. Set size was two or four items. The display type- size conditions are labeled, Eff-2, Eff-4, Ineff-2, and Ineff- 4 for efficient and inefficient, two and four item conditions,