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1、1 Active visual processing Does the brain really have to explain the meaning of inputs? Perception is the process whereby the brain interprets the sensations it receives, giving them order and meaning. Marr, 1982, Vision Three levels at which we need to understand perception: 1 the physiological mec
2、hanisms; -hardware 2 the process of perceiving; -algorithms 3 what do we perceive? -theory of the task to be performed Implications from the results of neuroscience studies Feature registration What is figure-ground segregation? 2 Stimuli and procedure Perceptual grouping perceptual grouping What go
3、es with what? Form perceptual units or objects. The role of grouping in visual perception 3 Perception of circle vs. oval depends on which, trapezia vs. square, it goes with. proximitysimilarity common fate Perceptual organization- contributions of Gestalt psychologists Uniform connectedness Palmer
4、19-25 years of age) in Exp. 1-3. Exp. 2 Nontargets (80%)Targets (20%) Discriminate red vs. green peripheral dots 6 Exp. 3 Nontargets (80%)Targets (20%) Discriminate red vs. green fixation Modeling of the generators of ERP components Boundary element model Electrode positions MRI dataERP data Behavio
5、ral results Exp. 1 Proximityvs.similarity 86.9%vs.79.0%F(1,15)=7.14, p 0.02 640vs.687 msF(1,15)=21.79, p 0.09 87.3% vs. 90.9%F(1,15)=4.74, p 0.044 ERP results of Exp. 1 Calcarine cortex (V1) ERP results of Exp. 1 ERP results of Exp. 2 7 ERP results of Exp. 3 Nontargets (80%) Targets (20%) Discrimina
6、te rows vs. columns of targets Localization of perceptual grouping with fMRI fMRI results Perceptual grouping with texture-defined local elements Task 1: identification of orientations of perceptual groups in target stimuli Task 2: identification of colors of fixation in target stimuli Summary 1.Gro
7、uping by proximity is perceived earlier than grouping by similarity; 2. Proximity grouping occurs as early as 80 ms after stimulus onset in the visual cortex; 3 Early proximity grouping has a neural source in the calcarine cortex (V1). 8 4. Grouping-related neural activities in the calcarine cortex
8、are modulated by task relevance. 5. The grouping process supported by the primary visual cortex is eliminated when stimulus arrays are of low task relevance and fall outside the attended area. Face and object recognition ?Stimuli: familiar and unfamiliar faces houses hands (target) 9 The patients re
9、sults (left temporo-parietal lesion) Conclusion Face perception - intracranial ERP recording Allison et al., 1999, 9, 415-458 Subjects: 98 patients, 10-55 years of age Recording: stainless steel electrodes An experimenter sat at the patients bedside to monitor fixation and alertness. 10 Stimuli: (1)
10、 faces; (2) scrambled faces; (3) front ends of cars; (4) scramble cars; (5) butterflies etc ERPs from striate and peristriate cortex P100 - dorsal occipital sites Recordings from the left striate cortex Left vs. right checkerboards Stimuli of varying luminance Recordings from the left striate cortex
11、 Grating and blank stimuli Gratings of varying size Ventral face-specific N200Decrease of the N200 at non-face-specific sites 11 Lateral face-specific N200 Face-specific P350 Ventral face-specific N700 Object-specific N200 Letter-string-specific N200 Summary 12 Conclusion ERP index of face, object,
12、letter string perception Object recognition Question: where objects are represented in the human brain? Evidence from neuroimaging studies Face perception (Halgren, et al., 1999, Human Brain Mapping) Task:passive viewing A box-car design 13 Ishai, et al., 2000, J. Cog. Neurosci. Task: 1. Passive vie
13、wing 2. A delayed match-to-sample task (house, chair, face) Data analysis: event-related fMRI Same viewing from different angle sample match stimuli Brain areas related to object recognition Neural substrates of perceptual learning in humans Tanaka & Curran, 2001, Psychological Science, 12, 43-47 Bird expert vs. Dog expert Does the increased N170 reflect a general form of expert processing that is not unique to faces? ERP recording: 128-channel Geodesic Sensor NetTM Impedance: 50 k Bandpass filter: 0.1-100 Hz ERP results: 14 Topographic distribution of the N170
