Investigating shape representation in area V4 with HMAX: Orientation and Grating selectivities

The question of how shape is represented is of central interest to understanding visual processing in cortex. While tuning properties of the cells in early part of the ventral visual stream, thought to be responsible for object recognition in the primate, are comparatively well understood, several different theories have been proposed regarding tuning in higher visual areas, such as V4. We used the model of object recognition in cortex presented by Riesenhuber and Poggio (1999), where more complex shape tuning in higher layers is the result of combining afferent inputs tuned to simpler features, and compared the tuning properties of model units in intermediate layers to those of V4 neurons from the literature. In particular, we investigated the issue of shape representation in visual area V1 and V4 using oriented bars and various types of gratings (polar, hyperbolic, and Cartesian), as used in several physiology experiments. Our computational model was able to reproduce several physiological findings, such as the broadening distribution of the orientation bandwidths and the emergence of a bias toward non-Cartesian stimuli. Interestingly, the simulation results suggest that some V4 neurons receive input from afferents with spatially separated receptive fields, leading to experimentally testable predictions. However, the simulations also show that the stimulus set of Cartesian and non-Cartesian gratings is not sufficiently complex to probe shape tuning in higher areas, necessitating the use of more complex stimulus sets

Fluctuation Effects in High Sheet Resistance Superconducting Films

As the normal state sheet resistance, $R_n$, of a thin film superconductor increases, its superconducting properties degrade. For $R_n\simeq h/4e^2$ superconductivity disappears and a transition to a nonsuperconducting state occurs. We present electron tunneling and transport measurements on ultrathin, homogeneously disordered superconducting films in the vicinity of this transition. The data provide strong evidence that fluctuations in the amplitude of the superconducting order parameter dominate the tunneling density of states and the resistive transitions in this regime. We briefly discuss possible sources of these amplitude fluctuation effects. We also describe how the data suggest a novel picture of the superconductor to nonsuperconductor transition in homogeneous 2D systems.Comment: 11 pages, 5 figure

Toward a more biologically plausible model of object recognition

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 105-113).Rapidly and reliably recognizing an object (is that a cat or a tiger?) is obviously an important skill for survival. However, it is a difficult computational problem, because the same object may appear differently under various conditions, while different objects may share similar features. A robust recognition system must have a capacity to distinguish between similar-looking objects, while being invariant to the appearance-altering transformation of an object. The fundamental challenge for any recognition system lies within this simultaneous requirement for both specificity and invariance. An emerging picture from decades of neuroscience research is that the cortex overcomes this challenge by gradually building up specificity and invariance with a hierarchical architecture. In this thesis, I present a computational model of object recognition with a feedforward and hierarchical architecture. The model quantitatively describes the anatomy, physiology, and the first few hundred milliseconds of visual information processing in the ventral pathway of the primate visual cortex. There are three major contributions. First, the two main operations in the model (Gaussian and maximum) have been cast into a more biologically plausible form, using monotonic nonlinearities and divisive normalization, and a possible canonical neural circuitry has been proposed. Second, shape tuning properties of visual area V4 have been explored using the corresponding layers in the model. It is demonstrated that the observed V4 selectivity for the shapes of intermediate complexity (gratings and contour features) can be explained by the combinations of orientation-selective inputs. Third, shape tuning properties in the higher visual area, inferior temporal (IT) cortex, have also been explored. It is demonstrated that the selectivity and invariance properties of IT neurons can be generated by the feedforward and hierarchical combinations of Gaussian-like and max-like operations, and their responses can support robust object recognition. Furthermore, experimentally-observed clutter effects and trade-off between selectivity and invariance in IT can also be observed and understood in this computational framework.(cont.) These studies show that the model is in good agreements with a number of physiological data and provides insights, at multiple levels, for understanding object recognition process in the cortex.by Minjoon Kouh.Ph.D

The interference effect of concurrent working memory task on visual inhibitory control

We examined the interference between inhibitory control of a saccadic eye movement and a working memory task. This study was motivated by the observation that people are suscep-tible to cognitive errors when they are preoccupied. Subjects were instructed to make an anti-saccade, or to look in the opposite direction of a visual stimulus, thereby exercising inhibito-ry control over the reflexive eye movement towards a salient object. At the same time, the subjects were instructed to memorize a random sequence of digits that were read out to them, thereby engaging their working memory. We measured the success of an eye movement by rapidly switching between images and asking the subjects what they saw. We found that these concurrent cognitive tasks significantly degraded anti-saccade performance.We examined the interference between inhibitory control of a saccadic eye movement and a working memory task. This study was motivated by the observation that people are susceptible to cognitive errors when they are preoccupied. Subjects were instructed to make an anti-saccade, or to look in the opposite direction of a visual stimulus, thereby exercising inhibitory control over the reflexive eye movement towards a salient object. At the same time, the subjects were instructed to memorize a random sequence of digits that were read out to them, thereby engaging their working memory. We measured the success of an eye movement by rapidly switching between images and asking the subjects what they saw. We found that these concurrent cognitive tasks significantly degraded anti-saccade performance

Observation of a subgap density of states in superconductor-normal metal bilayers in the Cooper limit

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, $d_{Pb}$ and $d_{Ag}$, that are much less than the superconducting coherence length. The transition temperature, $T_c$, and energy gap, $\Delta$, in the tunneling Density of States (DOS) decrease exponentially with $d_{Ag}$ at fixed $d_{Pb}$. Simultaneously, a DOS that increases linearly from the Fermi energy grows and fills nearly 40% of the gap as $T_c$ is 1/10 of $T_c$ of bulk Pb. This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as $T_c$ goes to 0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.Comment: 5 pages and 4 figures. This version is just the same as the old version except that we try to cut the unnecessary white space in the figures and make the whole paper look more compac

An evaluation of the environmental internship programme (2005- 2013) and it’s prospects for improved graduate employability: the case of interns in the city of Cape town

Magister Artium - MAThis study evaluates the extent to which the Environmental Internship Programme has impacted upon graduate employability in the City of Cape Town. Graduate unemployment crisis has been categorized as one of the socio-economic issues affecting the nation of South Africa with graduates struggling to find jobs in the areas which they graduated from. On the other hand, organisations are suffering from a lack of skills in different spheres. In order for this gap to be bridged, human resource development strategies have been put in place for graduates such as; internships, training and learnership programmes. In 2005, the Environmental Resource Management Department (ERMD) launched a programme called the Environmental Internship Programme (EIP) to address the needs of graduates in the environmental management field. The programme is a yearlong internship aimed at bridging the gap between academic learning and the first steps of a professional career. The aim of the ERMD is to create a pool of skilled professionals to be absorbed into the city as well as provincial, national government and the private sector by making the interns better employable by the time they leave the EIP. It is against this background that this study evaluates the impact of the programme on graduate employability. Within the investigation, the researcher looked at the appropriate legislations, and policies that govern a programme of this nature. The study utilizes primary and secondary research information based on interviews and open-ended questionnaires

A Heterogeneous Communications Network for Smart Grid by Using the Cost Functions

Smart Grids (SG) is an intelligent power grid in which the different SG node types with different communication requirements communicates different types of information with Control Stations (CS). Radio Access Technologies (RATs) due to its advantages are considered as the main access method to be used in order to have bidirectional data transferring between different node types and CS. Besides, spectrum is a rare source and its demand is increasing significantly. Elaborating a heterogeneous in order to fulfill different SG node types communication requirements effectively, is a challenging issue. To find a method to define desirability value of different RAT to support certain node types based on fitness degree between RAT communication characteristics and node type communication requirements is an appropriate solution. This method is implemented by using a comprehensive Cost Function (CF) including a communication CF (CCF) in combination with Energy CF (ECF). The Key Point Indicators which are used in the CCF are SG node type communication requirements. The existing trade of between Eb/N0 and spectral efficiency is considered as ECF. Based on the achieved CCF and ECF and their tradeoffs, SG node types are assigned to different RATs. The proposed assigning method is sensitive to the SG node types densities. The numerical results are achieved by using MATLAB simulation. The other different outcomes of the research output such as cognitive radio in SG and collectors effect number on data aggregation are discussed as well

Incorporation of prior knowledge and habits while solving anagrams

Games and puzzles provide a valuable context for examining human problem- solving behavior. We recorded and analyzed the sequence of letters viewed by the participants of our study while they were solving anagram puzzles. The goal was to examine and understand how people's linguistic habits and prior knowledge influenced their eye movements. The main findings of this study are: (1) People's stereotypical habit of scanning (e.g., adjacent or top viewing) strongly influences their solution-seeking behavior. (2) People tend to incorpo- rate their prior knowledge of letter statistics in a reasonable way, such as looking less frequently at letter combinations that are uncommon in the English lan- guage. Therefore, it was found that people’s prior