Modeling Reverse-PHI Motion Selective Neurons In Cortex: Double Synaptic Veto Mechanism
Chunhui Mo, C. Koch

Reverse-phi motion is the illusory reversal of perceived direction of movement when the stimulus contrast is reversed in successive frames. Here we proposed a double synaptic veto mechanism that could account for experimental observed responses to reverse-phi motion in V1 cells. We carried out detailed biophysical simulation in NEURON and verified our results with experimental data. (full report)

Towards an Integrated Model of Saliency-based Attention and Object Recognition
Dirk Walther, Maximilian Riesenhuber, Tomaso Poggio, Laurent Itti, Christof Koch

We are working on an integrated model for the dorsal (where) and the ventral (what) pathway in the primate's visual processing system and the interaction between these two pathways. The model will be applied to visual search tasks for detecting objects in cluttered natural scenes. Components of top-down attention will be integrated into the system to achieve this goal. (full report)

Attention Modulation of Visually Responsive Neurons in the Human Medical Temporal Lobe.
Leila Reddy, Patrick Wilken, Christof Koch

Previous work from our laboratory (Kreiman et al.,2000) has shown that neurons in the medical temporal lobe structures are visually responsive to categories of images. We intend to test whether attention modulates cell firing in these neurons.
(full report)

Part 1/ Rapid Visual Categorization In The Absence of Awareness
Part 2/ Processing Capacity For Natural Scenes and Objects in the Human Visual System
Rufin VanRullen

Humans can categorize natural scenes on the basis of the presence of a target object (i.e. animal) so rapidly (150 ms) that such processing has been proposed to rely on the feed-forward propagation of information collected during the first milliseconds of visual stimulation. According to this view, early motor responses should be mostly unaffected by masking the visual stimulus after a few tens of milliseconds. We asked our subjects to respond to masked (SOA 26.6 ms) and unmasked natural scenes when they contained an animal. (full report)

Structural Description of Basic Objects With Features
Christoph Rasche

We explore the representation of basic-level categories using computer vision methods. The category representation is expressed by lines, arcs and a combination thereof. In a bottom-up process we extract such features, in a top-down process we try to match each category representation against the bottom-up output. (full report)

Mapping Contingency Awareness in Fear Conditioning
C.J. Han

The goal of this project is to employ two types of Pavlovian conditioning: trace and delay, to investigate the awareness of the contingency of the conditioned stimulus (CS) and unconditioned stimulus (US). We have successfully established the behavioral and molecular paradigms over the past year and are currently collecting data of the effects of the anterior cingulate cortex lesion and the immediate early gene c-fos expression patterns in the mouse brain. (full report)

Attention as a Result of Distributed Competition.
Fred H. Hamker

Recordings in V4, IT, MT, MST, PFC and FEF reveal influences of attention on the average rate activity of neurons. However, it is still missing a global picture of the process of attention, i.e. the origin of spatial attention and the interactions between feature-based and spatial attention. We investigate the possibility of a spatial stimulus reentry from the frontal eye field into extrastriate visual areas by means of a quantitative comparison between simulations and experimental data. (full report)

Attentional Modulation of Visual Motion Perception Using Novel Wavelet Stimuli Ð Combination Study of Psychophysics and fMRI Imaging
N. Tsuchiya, G. Rees, J. Braun & C. Koch

We have previously characterized the effects of withdrawing attention on detection and discrimination of static visual stimuli (Lee et al. Nat Neuro 1998). Here we report attentional modulation of motion perception in psychophysics experiment. A novel motion stimulus comprising spatio-temporally contrast-modulated Gabor wavelets was used to distinguish attentional effects on mechanisms sensitive to component motion from those sensitive to pattern motion (Schrater et al Nat Neuro 2000). In the second experiment, we confirmed our component stimulus only activates only early visual cortex by functioal magnetic resonance imaging (fMRI) measurement, supporting our argument in psychophysics and consistent with our previous result (Rees et al. Nat Neuro 2000). (full report)

Models of Visual Object Categorization In Humans
Robert J. Peters, Fabrizio Gabbiani, Christof Koch

Previous studies of exemplar, prototype, and decision-bound models of visual object categorization have not resolved the importance of memory capacity and flexibility of decision surfaces in human categorization behavior. We have compared these previous models with our new roaming exemplar model (RXM), according to their abilities to match human observers' categorizations of various 2-D image contours. Unlike past comparisons among categorization models, we explicitly accounted for memory capacity by penalizing models for their number of free parameters with the Akaike information criterion. This revealed that a successful model of human categorization--such as the RXM--did not require a large memory capacity if the orientation of its decision boundary was unconstrained, suggesting that an efficient computer implementation of object categorization could also rely on limited memory storage. (full report)