How does the human visual system compare the speeds of spatially separated objects?

We measured psychophysical thresholds for discriminating the speeds of two arrays of moving dots. The arrays could be juxtaposed or could be spatially separated by up to 10 degrees of visual angle, eccentricity being held constant. We found that the precision of the judgments varied little with separation. Moreover, the function relating threshold to separation was similar whether the arrays moved in the same, in opposite or in orthogonal directions. And there was no significant difference in threshold whether the two stimuli were initially presented to the same cerebral hemisphere or to opposite ones. How are human observers able to compare stimuli that fall at well separated positions in the visual field? We consider two classes of explanation: (i) Observers’ judgments might be based directly on the signals of dedicated ‘comparator neurons’, i.e. neurons drawing inputs of opposite sign from local regions of the visual field. (ii) Signals about local features might be transmitted to the site of comparison by a shared ‘cerebral bus’, where the same physical substrate carries different information from moment to moment. The minimal effects of proximity and direction (which might be expected to influence local detectors of relative motion), and the combinatorial explosion in the number of comparator neurons that would be required by (i), lead us to favor models of type (ii)

Colour Relations in Form

The orthodox monadic determination thesis holds that we represent colour relations by virtue of representing colours. Against this orthodoxy, I argue that it is possible to represent colour relations without representing any colours. I present a model of iconic perceptual content that allows for such primitive relational colour representation, and provide four empirical arguments in its support. I close by surveying alternative views of the relationship between monadic and relational colour representation

Blue cone monochromacy: causative mutations and associated phenotypes.

PurposeTo perform a phenotypic assessment of members of three British families with blue cone monochromatism (BCM), and to determine the underlying molecular genetic basis of disease.MethodsAffected members of three British families with BCM were examined clinically and underwent detailed electrophysiological and psychophysical testing. Blood samples were taken for DNA extraction. Molecular analysis involved the amplification of the coding regions of the long (L) and medium (M) wave cone opsin genes and the upstream locus control region (LCR) by polymerase chain reaction (PCR). Gene products were directly sequenced and analyzed.ResultsIn all three families, genetic analysis identified that the underlying cause of BCM involved an unequal crossover within the opsin gene array, with an inactivating mutation. Family 1 had a single 5'-L-M-3' hybrid gene, with an inactivating Cys203Arg (C203R) mutation. Family 3 had an array composed of a C203R inactivated 5'-L-M-3' hybrid gene followed by a second inactive gene. Families 1 and 3 had typical clinical, electrophysiological, and psychophysical findings consistent with stationary BCM. A novel mutation was detected in Family 2 that had a single hybrid gene lacking exon 2. This family presented clinical and psychophysical evidence of a slowly progressive phenotype.ConclusionsTwo of the BCM-causing family genotypes identified in this study comprised different hybrid genes, each of which contained the commonly described C203R inactivating mutation. The genotype in the family with evidence of a slowly progressive phenotype represents a novel BCM mutation. The deleted exon 2 in this family is not predicted to result in a shift in the reading frame, therefore we hypothesize that an abnormal opsin protein product may accumulate and lead to cone cell loss over time. This is the first report of slow progression associated with this class of mutation in the L or M opsin genes in BCM

Retinal Ganglion Cells-Diversity of Cell Types and Clinical Relevance.

Retinal ganglion cells (RGCs) are the bridging neurons that connect the retinal input to the visual processing centres within the central nervous system. There is a remarkable diversity of RGCs and the various subtypes have unique morphological features, distinct functions, and characteristic pathways linking the inner retina to the relevant brain areas. A number of psychophysical and electrophysiological tests have been refined to investigate this large and varied population of RGCs. Technological advances, such as high-resolution optical coherence tomography imaging, have provided additional tools to define the pattern of RGC involvement and the chronological sequence of events in both inherited and acquired optic neuropathies. The mechanistic insights gained from these studies, in particular the selective vulnerability and relative resilience of particular RGC subtypes, are of fundamental importance as they are directly relevant to the development of targeted therapies for these invariably progressive blinding diseases. This review provides a comprehensive description of the various types of RGCs, the developments in proposed methods of classification, and the current gaps in our knowledge of how these RGCs are differentially affected depending on the underlying aetiology. The synthesis of the current body of knowledge on the diversity of RGCs and the pathways that are potentially amenable to therapeutic modulation will hopefully lead to much needed effective treatments for patients with optic neuropathies

Construction of an Instrumentation Kit for Identification and Control of DC Motors

This paper presents the development of an instrumentation kit of voltage and current measurement for identification of the dynamic model and control of direct current (DC) motors. In the methodology for the parameters identification is used the responses of input voltage and current, and angular velocity of the DC motor. The validation of the obtained dynamic model is done through the comparison of the simulated and experimental responses, and the application of a control system based on state feedback and complete eigenstructure assignment (tracking system). The responses are compared through the normalized root-mean-square error criterion

Control of a Modified Ball and Beam System Using Tracking System in Real Time with a DC Motor as an Actuator

This paper presents amodified ball and beam system, with the intention of realizing a test bed, to study new control techniques in real-time.The ball and beam system consists of a ball over a long beam where the control objective is to stabilizethe position of the ball on the beam by changing the angular position of the beam.In this paper, the ball of the conventional system is replaced by a cart with an embedded microcontroller, enabling the use of a linear encoder as position sensor and allowing to transmit the position via RF (Radio Frequency). The mathematical model of the ball and beam is obtained through the equations of Newton-Euler and the equations were linearized. The system is controlled using the hardware-in-the-loop technique with MATLAB/Simulink.It is applied a tracking control system with entire eigenstructure assignment to control the position of the cart. The actuator used is a DC motor, and a PID(proportional, integral and derivative) control is used to perform the angular position control of beam.The simulation results and the experimental results are compared to validate the mathematical model. The results obtained were satisfactory with adequate accuracy

Parameters Identification of a Direct Current Motor Using the Trust Region Algorithm

In this paper, the trust region algorithm was used to identify the parameters of the dynamic model of a permanent magnet direct current (PMDC) motor, using the MATLAB/Simulink Parameter Estimation tool. The objective was to estimate the parameters applying the square wave, pseudo-random binary sequence (PRBS) and random signals in the motor excitation. The obtained models were evaluated in open and closed loop, where a speed control project was applied using the entire eigenstructure assignment. The error between the simulated and real curves of velocity and current were evaluated by means of the normalized root mean squared error (NRMSE)

Quantifying camouflage and conspicuousness using visual salience

1. Being able to quantify the conspicuousness of animal and plant colouration is key to understanding its evolutionary and adaptive significance. Camouflaged animals, for example, are under strong selection pressure to minimise their conspicuousness to potential predators. However, successful camouflage is not an intrinsic characteristic of an animal, but rather an interaction between that animal’s phenotype and the visual environment that it is viewed against. Moreover, the efficacy of any given camouflage strategy is determined not by the signaller’s phenotype per se, but by the perceptual and cognitive capabilities of potential predators. Any attempts to quantify camouflage must therefore take both predator perception and the visual background into account. 2. Here I describe the use of species-relevant saliency maps, which combine the different visual features that contribute to selective attention (in this case the luminance, colour and orientation contrasts of features in the visual environment) into a single holistic measure of target conspicuousness. These can be tuned to the specific perceptual capabilities of the receiver, and used to derive a quantitative measure of target conspicuousness. Furthermore, I provide experimental evidence that these computed measures of conspicuousness significantly predict the performance of both captive and wild birds when searching for camouflaged artificial prey. 3. By allowing the quantification of prey conspicuousness, saliency maps provide a useful tool for understanding the evolution of animal signals. However, this is not limited to inconspicuous visual signals, and the same approach could be readily used for quantifying conspicuous visual signals in a wide variety of contexts, including, for example, signals involved in mate choice and warning colouration