Bilateral Superiority in Detecting Gabor Targets Among Gabor Distracters Nestor Matthews Department of Psychology, Denison University, Granville OH USA It has long been known that for a given point of visual fixation, the left visual field projects to the right cortical hemisphere while the right visual field projects to the left cortical hemisphere. Recent research has shown that performance on a motion tracking task is twice as good when the stimuli are distributed across both visual hemi-fields than when the stimuli are restricted to just one hemi-field, i.e., project to just one cortical hemisphere 1. (It is as if the unilaterally presented stimuli “flood the cortex’s zone defense”.) Even more recently, distracter-induced bilateral superiority effects (BSE’s) have been observed on rudimentary orientation discrimination tasks 2,3. The present study was conducted to determine whether distracter-induced BSE’s occur for the most basic visual task – detection. Each trial began with attentional cues marking the potential positions of Gabor targets (SF=1.3 c.p.d., size=3x3 deg window, eccentricity=14.4 deg to nearest edge, 90% contrast, duration=183 msec), which were present with 50% probability. Participants identified a central letter to ensure fixation before indicating whether a target was present or not. Across trials, bilateral and unilateral cues were randomly interleaved, as were Gabor distracters positioned between the target locations. Stimulus variations were examined across four experiments to determine which distracter-features most reliably generate BSE’s in visual detection. Discussion Introduction Across the four experiments, independent groups showed distracter-induced BSE’s in detection. The effect had modest spatial frequency dependence, but no orientation dependence. Distracter-induced BSE’s in detection may reflect neural events that occur earlier in the visual pathway than do those that generate distracter-induced BSE’s in motion tracking 1 and orientation discrimination 2,3. Although the present method did not include standard ‘litmus tests’ for visual crowding 4,5, the results are more consistent with masking (specifically, surround suppression 5 ) than with crowding because detection was impaired 6, and because the distracter effects were stronger for vertical than for horizontal configurations 7. Lastly, although there is fMRI evidence for two spotlights of attention within and across cortical hemispheres 8, the present data are consistent with separate capacity-limited pools of resources within each cortical hemisphere – even for the fundamental visual task of detection. References 1. Alvarez & Cavanagh (2005). Psychol Sci. PMID: Chakravarthi & Cavanagh (2006). VSS Abstracts. # 1216, p Matthews & Cox (2007). VSS Abstracts. # 689, p Levi (2008). Vision Research. PMID: Petrov, Popple, & McKee (2007). Journal of Vision. PMID: Pelli, Palomares, & Majaj (2004). Journal of Vision. PMID: Feng, Jiang, & He (2007). Journal of Vision. PMID: McMains & Sommers (2004). Neuron. PMID: Poster # Abstract # 773 Experiment 1: Two Targets versus One Target Attentional Cue m StimuliNoise Masks 1. Which Letter? 2. Target Present? Yes (y) Or No (n) Response Prompts m Bilateral: Distracters Absent m Bilateral: Distracters Present m Unilateral: Distracters Absent m Unilateral: Distracters Present General Method Stimulus Sequence On Each TrialLaterality & Distracter Conditions Results Experiment 2: Spatial Frequency Effects Experiment 3: Orientation EffectsExperiment 4: Striped versus Solid Distracters * * * * * * * * ** ** Laterality Effects: Two Targets Distracter Absent F(1,23)=1.29, p=0.268, partial  2 =0.053 Distracter Present F(1,23)=23.26, p<0.001, partial  2 =0.503 Laterality Effects: One Target Distracter Absent F(1,23)=1.73, p<0.201, partial  2 =0.070 Distracter Present F(1,23)=21.80, p<0.001, partial  2 =0.487 Laterality Effects: High SF Target High SF Distracter F(1,19)=8.07, p=0.010, partial  2 =0.298 Low SF Distracter F(1,19)=3.21, p=0.089, partial  2 =0.145 Distracter Absent F(1,19)=1.36, p=0.257, partial  2 =0.067 Laterality Effects: Low SF Target Low SF Distracter F(1,19)=20.04, p<0.001, partial  2 =0.513 High SF Distracter F(1,19)=7.89, p=0.011, partial  2 =0.294 Distracter Absent F(1,19)=4.78, p=0.041, partial  2 =0.201 Laterality Effects: Oblique Target Distracter Absent F(1,14)=1.20, p=0.291, partial  2 =0.079 Random Orientation F(1,14)=8.34, p=0.012, partial  2 =0.373 Target Orientation F(1,14)=5.34, p=0.037, partial  2 =0.276 Laterality Effects: Cardinal Target Distracter Absent F(1,14)=0.82, p=0.381, partial  2 =0.055 Orthogonal Configuration F(1,14)=12.16, p=0.004, partial  2 =0.465 Parallel Configuration F(1,14)=17.41, p=0.001, partial  2 =0.554 Laterality Effects: Oblique Target Distracter Absent F(1,39)=1.00, p=0.323, partial  2 =0.025 Gabor F(1,39)=17.97, p<0.001, partial  2 =0.315 Bulls-eye F(1,39)=18.55, p<0.001, partial  2 =0.322 Laterality Effects: Solid Distracters Mixed Polarity F(1,39)=5.60, p=0.023, partial  2 =0.126 All White F(1,39)=2.50, p=0.126, partial  2 =0.060 Chromatic F(1,39)=1.22, p=0.275, partial  2 =0.030