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Haxby J. V., Gobbini , M. I., Furey, M. L., Ishai, A., Schouten, J. L., & Pietrini, P. (2001). Distributed and overlapping representations of faces and objects in ventral temporal cortex. Science , 293, 2425-2430. Chiou Yao Ching ( 邱耀慶） 17 October 2005. Preface.
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Haxby J. V., Gobbini , M. I., Furey, M. L., Ishai, A., Schouten, J. L., & Pietrini, P. (2001). Distributed and overlapping representations of faces and objects in ventral temporal cortex. Science, 293, 2425-2430. Chiou Yao Ching (邱耀慶） 17 October 2005
Preface The authors of this research article prepresent fMRI data indicating that the representations of faces and objects in ventral temporal cortex are widely distributed and overlapping.
Evidences from single-cell recording Cells response best to particular shapes. Cells response best to faces at 0o.
Investigating Form Representations in Ventral Cortex by Brain Imaging • Owing to lack of consistent larger scale neural organization for object representation, instead, researchers use functional brain imaging. • 3 models of object representation within Ventral Cortex have been proposed.
Model 1: Specific Area for Specific Categories of stimuli • Limited number of area represent specific categories of stimuli. • FFA & PPA are most representative.
Model 2: Different Area for Different Types of Perceptual Process • FFA is NOT specialized for faces, but for exemplars of any objects. • FFA also respond to non-face stimuli.
Model 2: Different Area for Different Types of Perceptual Process Before training: No activation in novices’ FFA at all. After training: Experts engaged their FFA in the Greeble recognition.
Model 3: Object Form Topography • Representations of faces and different objects are widely distributed and overlapped. • Large- & small-amplitude responses carry info about object appearance and form a distinct neural-firing pattern representing a particular object.
Model 3: Object Form Topography • Object form topography in ventral temporal cortex produce unique representations for unlimited number of categories. • Haxby & et al predict that Each category elicit a distint response pattern is also evident in cortex that responses maximally to other categories.
Analysis of patterns of neural responses to object categories: A fMRI study • A fMRI study with 6 subjects and 8 kinds of different visual stimuli.
Analysis of patterns of neural responses to object categories: A fMRI study • Subjects viewed pictures of each categories under fMRI scanning, data for each one are split into odd & even runs. • Within-categories correlation: the corr of responses to chairs on odd runs with that on even runs. • Between-categories correlation: the corr of responses to categories A on odd runs with that to categories B on even runs. • What if CORRwithin is larger than CORRbbetween ? A indice of “Correct Identification.”
The Results: Part 1 • The response pattern of whole ventral pathway correctly identified the category being viewed in 96% of pairwise comparisons. ( namely, 96% CORRwithin is larger than CORRbetween.) • Does it really mean that subjects were viewing stimuli with no errors?
The Results: Part 2 • What is the info about category A represented in cortex which respond best to category B, C, D …? • For each comparison, maximal-response area to either half were excluded from calculating the corr. (i.e., CORRwithin derived from A’ compares to CORRbetween derived from A’ & B’) • Correct Identification is up to 94%. Category identification based on patterns of nonmaximal reponses is still high.
The Results: Part 3 • What is the response pattern to other categories within a region that respond best to only one category? • Within any subregion in ventral pathway, correct identification can still be achieved over all categories. (namely, CORRwithinof Ais larger than CORRbetweenof A&B and CORRbetweenof A&C even in the “D area.” )
Does the patterns of nonmaximal responses reveal only low-level features? • A re-anlysis of previous study : 實驗刺激─3種物件(臉、房子&椅子) 兩種形式(照片&線條) • Results: Even within nonmaximal-response regions, correct identification can be accomplished. ( namely, CORR照線of 臉is clearly larger than CORR線臉&屋照 and CORR線臉&椅照 . ) • Nonmaximal responses do NOT represent low-level features but information about specific object category.
Object Form Topography • Spatially functional architecture within subregions which respond maximally to a single object category. • Analogue 1:
Object Form Topography • Analogue 2: 強烈的紅-綠激發 中等的黃-藍激發 橘色
What are the attributes underlie population encoding? • 2-D / 3-D primitives (Biederman, 1987) but how to deal with the flaws of RBC? • Configurations of features covary across faces. small response is as important as large ones for specifying details of a perceived face.
Looking from a Clinical Perspective • Since any single subregion in ventral temporal cortex do conveys info about distinguishing among different categories, this finding answers that why a restricted lesion in FFA can lead to impairments for recognizing other kinds of object categories.
Summary • Object form topography exists in ventral temporal cortex. • Cerebral cortex use distributed coding to represent objects in outside world. • The representations of different object categories are widely overlapping.
