Bilateral and Unilateral Orientation Dynamics Nestor Matthews & Kristin M. Reardon Department of Psychology, Denison University, Granville OH USA Previous data (shown below) indicate that orientation sensitivity improves across the tens of milliseconds after masked stimuli are presented foveally 1,2. The purpose of the present study was to extend the previous work on foveal orientation dynamics to the periphery. Earlier studies on peripheral stimulation suggested that more neural processing time may be needed for comparisons across the corpus callosum than for comparisons restricted to either lateral hemi- field 3,4. Here, we probed the role of the corpus callosum by comparing bilateral and unilateral orientation sensitivity. Additionally, we manipulated whether the orientational reference was implicit or explicit. (Activity across the corpus callosum was relevant only in the explicit / bilateral condition.) Previous research has suggested that implicit and explicit references generate comparable discrimination thresholds on other low-level tasks (i.e., judging the separation between lines) under foveal stimulation 5. Discussion Background and Purpose The finding that d’ increased with stimulus duration in the periphery confirms and extends previous work in the fovea 1,2. Similarly the finding that there is no cost associated with inter-hemispheric communication (i.e., there was no unilateral superiority effect) on our orientation task is consistent with previous results from a distal misalignment task 6. It may be that an inter- hemispheric cost arises only on tasks that require low- level grouping 6,7. Distracters in the present study impaired performance only selectively. Specifically, the distracter impairment could be “turned off” by either holding the stimulation constant while changing task from explicit to implicit, or by making the distracters differ from the targets in spatial frequency and color. These findings suggest a difference between the mechanisms underlying implicit and explicit orientation comparisons, and also argue against a number-of-item account of the distracter impairment. Intriguingly, the ‘bilateral superiority effect’ in the present explicit / distracter conditions could reflect independent resources in left and right hemispheres 8. References 1. Strong, Kurosawa & Matthews (2006). JOV. PMID: Matthews, Rojewski & Cox (2005). JOV. PMID: Wilson, Blake & Lee (2001). Nature. PMID: Ringo, Doty, Detemer & Simard (1994). Cereb Cortex. PMID: Morgan, Watamaniuk & McKee (2000). Vision Res. PMID: Pillow & Rubin (2002). Neuron. PMID: Butcher & Cavanagh (2004). VSS Abstracts, D34, p Alvarez & Cavanagh (2005). Psychol Sci. PMID: Poster Session F Board # 7 Abstract # 689 Experiment 1Experiment 2Experiment 3 45-degrees? Yes/No Implicit Same/Different Explicit with Distracters Same/Different Explicit 45-degrees? Yes/No Implicit with distracters 45-degrees? Yes/No Implicit Same/Different Explicit with distracters Same/Different Explicit 45-degrees? Yes/No Implicit with Distracters 45-degrees? Yes/No Implicit Same/Different Explicit with Distracters Same/DifferentExplicit