[Abstract Title]. - Society for Neuroscience

ecognition might depend on representations that are tied to limited ranges of retinotopic
positions. Thus, the same object presented in different positions might evoke only partially
overlapping or even completely distinct representations, leading to distinct physiological
responses in anterior object-selective cortex and limited behavioral position invariance. To test
our behavioral hypothesis, participants were asked to indicate whether a briefly presented (66ms)
stimulus was a whole or scrambled line drawing of an object. Many trials later (e.g. 256)
participants saw the same stimulus again in either the same or a different position. The amount of
priming for the repeated stimuli scaled with the distance between the initial and repeated
positions. Priming was also qualitatively reduced when the positions were in different
hemifields. To test our physiological hypothesis, we presented 24 of the objects used in our
behavioral studies in each of the four quadrants (5° from fixation) in an event-related fMRI
paradigm. The objects were divided evenly between six categories (faces, mammals, body parts,
objects, tools, vehicles), allowing us to test for the relative effects of category and position
changes. Participants performed a color-matching task on the stimuli and fixation cross. In a set
of independently defined ventral stream ROIs (LO, PFs, FFA), we then established the similarity
of responses to the stimulus conditions using multivariate pattern analysis. We correlated the
response pattern of each of the 96 stimulus conditions (24 objects * 4 quadrants) with every other
condition within each ROI. We found a very clear effect of hemifield, with far lower correlations
between presentations of objects occurring in different hemifields. Response patterns could also
be grouped within a hemifield into the upper and lower quadrants. In many cases, the effect of
position was stronger than that of category. The results indicate the presence of positiondependent
object representations in anterior regions of the ventral stream, and establish that these
representations have predictable behavioral consequences.
Disclosures: D.J. Kravitz, None; N. Kriegeskorte, None; C.I. Baker, None.
Poster
260. Objects and Faces in Humans II
Time: Sunday, November 16, 2008, 1:00 pm - 5:00 pm
Program#/Poster#: 260.8/BB20
Topic: D.04.j. Processing of objects and faces
Support: NIH-NEI
Title: A semantic encoding model that provides an unbiased description of object category
selectivity in visual cortex
Authors: *R. J. PRENGER 1 , T. NASELARIS 2 , K. KAY 3 , M. D. OLIVER 4 , A. T. VU 5 , J. L.
GALLANT 2 ;