Attention-Dependent Hemifield Asymmetries When Judging Numerosity Nestor Matthews & Sarah Theobald Department of Psychology, Denison University, Granville OH USA This study was designed to explore how our ability to enumerate stimuli in cluttered displays depends on the attended hemifield and on attentional grouping criteria. A recent study indicated that enumeration is more accurate when items are bilaterally distributed (across the left and right hemifields) than when unilaterally distributed (entirely within one lateral hemifield) (Delvenne et al., 2011). This bilateral advantage occurred only when more than four items had to be enumerated –a finding that implicates attention. Bilateral advantages have also been reported for other attentionally demanding tasks, such as attentional tracking (Alvarez & Cavanagh, 2005), and detecting or discriminating the orientation of targets among distractors (Reardon, Kelly, & Matthews, 2009; Chakravarthi & Cavanagh, 2009). On the other hand, a unilateral advantage has been reported when stimuli can be grouped preattentively. Examples include tasks involving illusory contours (Pillow & Rubin, 2002) and the detection of motion paths (Butcher & Cavanagh, 2005). In the present study, we held retinal stimulation constant while we manipulated attention in two ways. First, we manipulated spatial attention by requiring participants to select one hemifield while ignoring the other. Second, we manipulated attentional grouping by requiring participants to select either proximal or similarly colored dots. For any given stimulus, our participants made same/different numerosity judgments across eight attentional conditions: four hemifield conditions (bilateral; top vs. bottom: unilateral; left vs. right) by two grouping conditions (proximity vs. similarity). DiscussionIntroduction References Method Stimuli with Markers Results * * * Proximity While stimuli remained constant across all conditions, visual performance depended on hemifield and the attentional grouping task. In the unilateral case, the change from proximity to similarity grouping created an asymmetry – a LVF advantage. Conversely, in the bilateral case, the change from proximity to similarity grouping removed an asymmetry –the “sky ground” effect. A well known asymmetry in the early visual pathway is that there is “more cortex devoted to lower than to upper fields” (Van Essen, Newsome, & Maunsell, 1984 (p429)). Our data suggests that this asymmetry in the early visual pathway might reflect the limiting neural events on the proximity task, but not the similarity task –even when stimulation is identical. We have recently conducted a follow up study in which we compared the performance of video-gamers and non-gamers. Additionally the white and black dots were replaced by red and blue dots. Although the effect of video-game playing was non- significant, the significant sky-ground effect reported here was replicated and, again, specific to the proximity task. By contrast, the left-hemifield advantage reported here on the similarity task did not replicate. Limitations in the present and follow up studies include differences in task difficulty, and the possible use of density or spatial frequency cues on the numerosity judgments. Participants: 20 Denison University undergraduates Within Subjects design Independent Variables: 4 (Hemifield) x 2 (Grouping) Hemifield: Bilateral (Top vs. Bottom) Unilateral (Left vs. Right) Grouping: Proximity vs. Similarity Dependent Variable: dPrime Hits: “Different” response when number of dots differed False Alarms: “Different” response to same number Series of Questions 1.What letter appeared in the center of the screen? 2. Was the number of dots same or different? Similarity Acknowledgements This project was supported by an Anderson Summer Research Award from Denison University. Similarity Proximity Instructions Bilateral – Proximity (Top vs. Bottom): t(19)=4.325, p<.001, partial η 2 =.496, power=.984 Bilateral – Similarity (Top vs. Bottom): t(19)=0.365, p=0.719, partial η 2 =.007, power=.064 n.s. Unilateral – Proximity (Left vs. Right): t(19)=0.587, p=.564, partial η 2 =.018, power=.086 n.s. Unilateral – Similarity (Left vs. Right): t(19)=3.021, p=.007, partial η 2 =.324, power=.817 Participants responded to 480 trials, blocked by condition. Stimuli: 200ms duration, and identical across conditions. Task: Participants reported (1) the center letter and (2) whether the dots were the same or different in number, given the blocked hemifield and grouping condition. Feedback was provided after each same/different response. Delvenne, J.F., Castronovo, J., Demeyere, N., & Humphreys, G. (2011). Bilateral field advantage in visual enumeration. PLoS one, 6(3)e Alvarez, G. & Cavanagh, P., (2005). Independent resources for attentional tracking in the left and right visual hemifields. Psychological Science, 16(8), Reardon, K. M., Kelly, J. G., & Matthews, N. (2009). Bilateral attentional advantage on elementary visual tasks. Vision Research, 49 (7), Chakravarthi, R. & Cavanagh, P. (2009). Bilateral field advantage in visual crowding. Vision Research, 49(13), Pillow, J., & Rubin, N., (2002). Perceptual Completion across the Vertical Meridian and the Role of Early Visual Cortex. Neuron, 33, Butcher, S. J., & Cavanagh, P. (2005). Within-field advantage for detecting matched motion paths. Vision Sciences Society, 267 (abstract). Van Essen, D.C., Newsome, W.T., & Maunsell, J.H. (1984). The visual field representation in striate cortex of the macaque monkey: asymmetries, anisotropies, and individual variability. Vision Research, 24 (5), vss2011matthewstheobald.html * * Abstract #