The Stochastic Nature of Single Neurons
Kamran Diba, Christof Koch

Our labs have been very active in furthering our understanding of the biophysical noise in neocortical pyramidal cells. The Hebrew University group traveled to California in March, and Dr. Kamran Diba traveled twice to Jerusalem in April and August to discuss and advance our collaborative research. Theoretically, we have strengthened our understanding of the role of ion channels and synaptic vesicular release in determining the voltage noise fluctuations. Experimentally, we made more measurements under varied pharmacological conditions. We also developed a method for quantifying instrumental noise, and we began measuring the input impedance of the cell with zap currents. We presented a poster at the Society for Neuroscience meeting in November. We are presently working to understand some of the low-frequency noise features that we recently uncovered. (full report)

Line Source Approximation Predicts Extra-Cellular Voltage for CA1 Neurons Recorded In Vivo
Carl Gold, Christof Koch, Darrell Henze, Gyorgy Buzsaki

Abstract. The Line Source Approximation (LSA) is a mathematical method for calculating the extracellular field from a 3-D distribution of membrane current sources. We investigate the use of the LSA combined with detailed compartmental modeling, including a model of the electrodes used, to predict the extracellular voltage waveform shape and magnitude resulting from the spiking activity of individual neurons. This provides an estimate of the maximal distance at which a neuron could be detected by an extracellular electrode. In order to tune the model we compare simultaneous intracellular and extracellular recordings of CA1 neurons recorded in vivo with model predictions for the same cells reconstructed and simulated. The approximate electrode position is estimated from the histologically determined track. We overcome the uncertainty regarding the values of biophysical parameters, such as the extra-cellular conductivity and the membrane Na+ conductance, by comparing the model and experimental results for numerous samples of the same class of neuron. Based upon comparisons with experimental data, we conclude that the compartmental model can accurately simulate the in vivo intracellular action potential and the LSA model can accurately simulate the extracellular fields of individual spiking neurons.

The Involvement of the Anterior Cingulate Cortex in Novelty
Han C.J., Anderson D.J., & Koch, C.

The activation of the anterior cingulate cortex was previously shown to correlate with novelty detection. However, whether the anterior cingulate cortex is necessary to novelty detection is unclear. We set up a novelty object paradigm in mice. Mice were brought to the testing room in their home cage. A group of mice received a novel object (a corning 15 ml tube), a group received the same procedure including lifting the cage lid but not the object, and a group received nothing. We showed that the novel object readily induces the exploratory behaviors of the mouse directed towards the novel object, and cage lid lifting induces general exploratory behaviors. The sum of time that the group receiving the novel object and the group receiving the lid lifting spend in exploratory behaviors are equal, but the exploratory behaviors in the group that received the novel object are mostly directly to the object. c-fos mRNA was used as a surrogate marker to detect neuronal activation by in situ hybridization on brains from each group. Animals from each of the three groups were sacrificed 30 minutes after the first exposure of the stimulus. We discovered that there are more c-fos positive cells in the anterior cingulate cortex of the brain that received the novel object, compared with the other two groups. To answer the question whether the anterior cingulate cortex is necessary for novelty detection, a group of mice received excitotoxic lesions of the anterior cingulate cortex and another group received sham surgery. Behavioral experiments and analyses are being conducted to determine whether the lesions to the anterior cingulate cortex cause any exploratory behavioral changes directed to the novel object.

Rapid Natural Scene Categorization without Attention
Fei Fei Li, Rufin VanRullen, Christof Koch, Pietro Perona

Abstract. What can we see when we do not pay attention? While attention is not necessary for some detection tasks on simple synthetic stimuli, without it we are “blind” even to major aspects of a natural complex scene. It would thus appear that only visual tasks that have an explanation in the early stages of the visual system may be carried out without attention. We report on a complex visual task that requires no attention. Our subjects can rapidly detect animals in briefly presented natural scenes while simultaneously performing another visual task that demands full attention. By comparison, they are unable to discriminate large ‘T’s from ‘L’s in the same conditions. We conclude that attention may not be necessary for some visual tasks that are associated with ‘high level’ cortical areas. (full report)

Mismatch Reduction in an On-Chip Image Processing Chip
Performing Feature Detection
Ania Mitros, Christof Koch

Feature extraction is a first step for many existing computer vision algorithms. This computation is also often one of the most time- and resource-intensive steps because the same local computation must be performed at each pixel. To head towards a real-time, small-size, energy-efficient implementation, Pesavento implemented the Tomasi- Kanade feature extraction algorithm in silicon. Although each feature detector worked splendidly, transistor mismatch killed the performance of the array. I have been re-implementing the blocks of the feature detector with floating gate transistors within each to permanently program away the mismatch. I have implemented mismatch reduction in the photoreceptor and the multiplier; both are tested and function as desired.

Suppressive Effect of Sustained Low-Contrast Adaptation followed by Transient High-Contrast on Peripheral Target Detection
Farshad Moradi, Shinsuke Shimojo, Christof Koch

Filling-in can be induced by high-contrast edge adaptation, or after prolonged adaptation to a peripheral low-contrast object (Troxler fading). Adaptation to sustained low-contrast vs. adaptation to transient high-contrast suggests synergy between contrast and edge adaptation, but the possible interactions are not well understood. We observed that briefly increasing the contrast of a peripheral low-contrast object after a few seconds of strict fixation elicits disappearance of the object, resulting in perceptual filling-in of the location with the surround (Figure 1a). After a short time usually around one second the object reappears. Hence, following sustained adaptation to a low-contrast target, transient high-contrast stimulation can induce perceptual disappearance. (full report)

Inter-stimulus Distance Effects in Visual Search
Lavanya Reddy, Rufin VanRullen, Christof Koch

Abstract. In a previous study, we showed that the attentional requirements of a task, as revealed by the dual-task paradigm, do not necessarily determine whether visual search will be parallel or serial. For example, natural scene categorization can be performed "preattentively" in a dual-task situation (i.e., a single scene containing animals can be discriminated from non-animal scenes even while attention is occupied elsewhere), and yet visual search for an animal scene among a number of non-animal scenes is a serial process. We interpreted these findings as follows: a task can be performed preattentively if there exist specific neuronal populations selective to the target and distractor categories, independent of the level of processing involved (from V1 to IT); when such selectivities exist, visual search is parallel only if the receptive fields of the relevant neurons do not significantly overlap. When receptive fields are too large, target and distractors compete within the same field and search is serial. It follows that search performance should improve if target and distractors can be separated enough to prevent them from falling into the same receptive field. We tested this prediction and found that for preattentive tasks that usually result in serial visual search (e.g., color-orientation conjunction discrimination, upright vs. inverted face discrimination), search performance improved as inter-stimulus distance was increased. For preattentive parallel tasks (color discrimination, orientation discrimination), the effect of increasing inter-stimulus distance was negligible. These results support the idea that for preattentive tasks, competition within the relevant receptive fields can affect visual search performance.

Attentional Selection for Learning and Recognition of Objects in Cluttered Scenes
Ueli Rutishauser, Dirk Walther, Christof Koch, and Pietro Perona

The problem of serial processing of highly complex visual stimuli containing multiple objects is not only faced by humans and other primates, but also by machine vision systems. Advanced object recognition algorithms are capable of achieving very good recognition performance with objects learned from a single image (one-shot learning). These algorithms perform well as long as they are trained on images in which a major part of the image is occupied by the object to be learned and recognized. As soon as major parts of an image are occupied by clutter it becomes impossible to learn from such images without manual pre-labeling. These approaches are thus not suitable in an unsupervised environment, as they would mainly learn background clutter instead of the actual objects. (full report)