1. Bilateral Attentional Advantage in Gabor Detection Nestor Matthews & Jenna Kelly Department of Psychology, Denison University, Granville OH 43023 USA In principle, visual performance could be uniform within (unilateral) and across (bilateral) the left and right hemifields. However, previous research has revealed bilateral advantages on relatively high level visual tasks, such as letter identification (Awh & Pashler, 2000; Chakravarthi & Cavanagh, 2006), and motion tracking (Alvarez & Cavanaugh, 2005). Recently, this bilateral advantage has been demonstrated even on the elementary task of detecting Gabor targets among Gabor distracters (Reardon, Kelly & Matthews, 2009). In the present study, we investigated the extent to which this bilateral superiority in detecting Gabor targets among Gabor distracters is more appropriately attributed to surround suppression or to attention. These two make different predictions, described in turn. Surround suppression is the phenomenon in which sensitivity to a target's luminance contrast is reduced by a spatially displaced distracter (Petrov, Popple & McKee, 2007). In the present detection study, surround suppression would be evidenced by a distracter-induced decrease in hit rates i.e., a decrease in ‘yes’ responses on target-present trials. Attention is the selection of a sensory event. Attentional selection can fail under conditions of “temporal crowding” -a form of inappropriate target-distracter integration that occurs when stimuli are flashed briefly enough to overload attentional selection (Pelli, Palomares & Majaj, 2004). In the present detection study, a failure of attention would be evidenced by a distracter-induced increase in false alarm rates i.e., an increase in ‘yes’ responses on target-absent trials. We further sought to alter the difficulty of peripheral attentional selection by varying the duration of a foveally presented target letter. Discussion Introduction References Attentional Cue m StimuliNoise Masks 1. Which Letter? 2. Target Present? Yes (y) Or No (n) Response Prompts Method Stimulus Sequence On Each Trial Results * * * * Alvarez & Cavanagh (2005). Independent resources for attentional tracking in the left and right visual hemifields. Psychological Science 16(8), 637-643. Awh & Pashler (2000). Evidence for split attentional foci. Journal of Experimental Psychology: Human Perception and Performance 26(2), 834- 846. Chakravarthi & Cavanagh (2006). Hemifield independence in visual crowding. Vision Sciences Society, 274 (abstract). Petrov, Popple & McKee (2007). Crowding and surround suppression: Not to be confused. Journal of Vision, 7(2), 1-19. Pelli, Palomares & Majaj (2004). Crowding is unlike ordinary masking: distinguishing feature integration from detection. Journal of Vision, 4(12), 1136-1169. Reardon, Kelly & Matthews (2009). Bilateral Attentional Advantage on Elementary Visual Tasks. Vision Research, 49(7), 692-702. Toet & Levi (1992). The two-dimensional shape of spatial interaction zones in the parafovea. Vision Research, 32(7), 1349-1357. Bilateral: Horizontal Unilateral: Vertical Bilateral: Vertical Unilateral: Horizontal Target/ Distracter Configurations Experimental Details Display Details Foveal letter to (peripheral) Gabor target: 14.55 deg (center to center) Gabor target to nearest Gabor distractor: 7.1 deg (center to center) Separation exceeded limit for spatial crowding i.e., 0.1 and 0.5 times the target eccentricity in the tangential and radial directions, respectively (Toet & Levi, 1992). Two different configurations for each laterality condition Target-distracter offset: Vertical or Horizontal Noise mask after each target and distracter: 8 ms (1 screen refresh) Gabor target duration: 183 ms (22 screen refreshes, regardless of letter duration) Our primary finding is that the bilateral superiority in Gabor detection reflects an attentional limit, rather than surround suppression. This was evidenced by the finding that the main effect of laterality was significant and large for false alarms (our proxy for attentional selection), but non-significant and small for hits (our proxy for surround suppression). Indeed, bilateral and unilateral hit rates were statistically indistinguishable from each other across duration-by-distracter pairings. Importantly, peripheral stimulation remained identical across variations in the foveal letter’s duration. Consequently, the improvement with foveal letter duration suggests that a neural resource shared by the fovea and the periphery constrained performance. Relative to the bilateral response, the unilateral response at each letter duration exhibited a failure to exclude distracters –not a failure to detect contrast. The pattern of results implicates bilateral superiority in attention, even on this most elementary visual task. Even when the distracters were absent, false alarms (but not hits) at the briefest letter duration were significantly higher unilaterally than bilaterally. The difference in false alarm rates between unilateral and bilateral conditions was inversely related to foveal letter duration. One possible explanation for this is that with decreasing foveal letter duration, the noise mask was more frequently misconstrued as the target itself –a failure of attentional selection. FactorHits Laterality F(1,12)=0.383, p=0.547, pEta^2=0.031, power=0.088 Distractor F(1,12)=11.222, p=0.006, pEta^2=0.483, power=0.867 Letter Duration F(2,24)=4.181, p=0.028, pEta^2=0.258, power=0.679 Lat x Dist F(1,12)=0.00, p=1.00, pEta^2=0.000, power=0.05 Lat x Letter Dur F(2,24)=0.331, p=0.722, pEta^2=0.027, power=0.097 Dist x Letter Dur F(2,24)=11.814, p<0.001, pEta^2=0.496, power=0.988 Lat x Dist x Letter Dur F(2,24)=0.579, p=0.568, pEta^2=0.046, power=0.135 Participants:13 Denison University undergraduates IVs: 2 (Laterality) x 2 (Distracter) x 3 (Letter Duration) Laterality: Bilateral versus Unilateral Distracter: Absent versus Present Letter Duration: 67, 117, or 167 ms (8, 14, or 20 screen refreshes) DVs: Hits: “Yes” response when Gabor target present False Alarms: “Yes” response when Gabor target absent FactorFalse Alarms Laterality F(1,12)=15.026, p=0.002, pEta^2=0.556, power=0.944 Distractor F(1,12)=14.204, p=0.003, pEta^2=0.542, power=0.932 Letter Duration F(2,24)=3.508, p=0.046, pEta^2=0.226, power=0.598 Lat x Dist F(1,12)=3.833, p=0.074, pEta^2=0.242, power=0.437 Lat x Letter Dur F(2,24)=2.305, p=0.121, pEta^2=0.161, power=0.422 Dist x Letter Dur F(2,24)=1.055, p=0.364, pEta^2=0.081, power=0.213 Lat x Dist x Letter Dur F(2,24)=0.458, p=0.638, pEta^2=0.037, power=0.116 * ** http://www.denison.edu/~matthewsn/bilateralsuperiorityvss2009.html Poster # 63.405 Abstract # 1095