Showing papers by "Chris J. Mitchell published in 2012"

Attentional mechanisms in learned predictiveness

Abstract: Cues that reliably predict an outcome in an initial phase of training (Phase 1) are learned faster in a second phase of training (Phase 2) than cues that were unreliable in Phase 1. This result is observed despite objectively equal relationships between the cues and the outcomes in Phase 2, and consequently constitutes a nonnormative bias in learning. The present experiments sought to confirm that this learned predictiveness effect is the product of attentional processes (Experiment 1), and to test further whether these processes are under voluntary control or are automatic in nature (Experiment 2). In addition to the usual outcome prediction measure, eye-gaze behavior was also monitored. The results indicated an important role for top-down strategic attentional processes in the learned predictiveness task. In contrast, no evidence for an automatic attentional bias was found.

Effects of exposure on discrimination of similar stimuli and on memory for their unique and common features.

Abstract: Human participants received exposure to similar visual stimuli (AW and BW) that shared a common feature (W). Experiment 1 demonstrated that subsequent discrimination between AW and BW was more accurate when the two stimuli were preexposed on an intermixed schedule (AW, BW, AW, BW…) than when they were preexposed on a blocked schedule (AW, AW…BW, BW…): the intermixed–blocked effect. Furthermore, memory for the unique features of the stimuli (A and B) was better when the stimuli were preexposed on an intermixed schedule than when they were preexposed on a blocked schedule. Conversely, memory for the common features of the stimuli (W) was better when the stimuli were preexposed on a blocked schedule than when they were preexposed on an intermixed schedule. Experiment 2 again demonstrated the intermixed–blocked effect, but participants were preexposed to the stimuli in such a way that the temporal spacing between exposures to the unique features was equated between schedules. Memory for the unique and common ...

Both trace and delay conditioned eyeblink responding can be dissociated from outcome expectancy.

Abstract: Squire and colleagues have proposed that trace and delay eyeblink conditioning are fundamentally different kinds of learning: trace conditioning requires acquisition of a conscious declarative memory for the stimulus contingencies whereas delay conditioning does not. Declarative memory in trace conditioning is thought to generate conditioned responding through the activation of a conscious expectancy for when the unconditioned stimulus (US) is going to occur. Perruchet (1985) has previously shown that in a 50% partial reinforcement design it is possible to dissociate single cue delay eyeblink conditioning from conscious expectancy for the US by examining performance over runs of reinforced and nonreinforced trials. Clark, Manns, and Squire (2001) claim that this dissociation does not occur in trace eyeblink conditioning. In the present experiment we examined the Perruchet effect for short, moderate, and long trace intervals (600, 1000, and 1400 ms) and for the equivalent interstimulus intervals (ISIs) in a delay conditioning procedure. We found evidence for a dissociation of eyeblink CRs and US expectancy over runs regardless of whether there was a delay or a trace arrangement of cues. The reasons for the Perruchet effect are still unclear, but the present data suggest that it does not depend on a separate nondeclarative system of the type proposed by Squire and colleagues.

Location and salience of unique features in human perceptual learning.

Abstract: Preexposure to intermixed presentations of a pair of similar stimuli (AX and BX, where A and B represent distinctive features, and X the features the stimuli hold in common) facilitates subsequent discrimination between them. This perceptual learning effect has been interpreted as indicating that the loss of effective salience resulting from repeated presentation of a stimulus is attenuated or reversed by a salience-modulation process that operates on the unique stimulus features A and B during intermixed preexposure. In 3 experiments, we examined discrimination after intermixed preexposure to AX and BX, making comparison with a condition in which novel unique features were added to the preexposed background (CX and DX). In all experiments, we also monitored eye gaze during both preexposure and the test. Experiments 1 and 2 found discrimination of the preexposed stimuli to be superior. This result cannot be explained by salience-modulation theories that suppose that intermixed preexposure merely attenuates loss of salience to the unique features A and B; it suggests, rather, that intermixed preexposure to AX and BX enhances the salience of, or attention paid to, the distinctive features. Experiment 3 demonstrated that exposure increases sensitivity to the spatial location of the features, a conclusion confirmed by analysis of eye gaze.

Generating certification authority authenticated public keys in ad hoc networks

Abstract: In an ad hoc network, nodes may face the need to generate new public keys. To be verifiably authentic, these newly generated public keys need to be certified. However, because of the absence of a permanent communication infrastructure, a certification authority (CA) that can issue certificates may not always be reachable. The downside is that secure communication channels cannot be established. Previously proposed solutions do not guarantee that identities contained in certificates are valid or, when they do, they rely on neighbors to validate user-key bindings. However, there is no guarantee that nodes that are known in advance will always be present in the network. Therefore, neighbors are not always able to verify a node's identity before certificate issuance. In this paper we define a scheme that permits nodes to generate, on-demand, and independently of any third entity, public keys that can be authenticated with the aid of a unique certificate, issued by a CA at initialization. This certificate binds a valid identity to a hash code. We then extend this scheme to a solution permitting certificates to be generated, on-demand, and independently of any third entity, that can be authenticated with a unique signature generated by a CA. Finally we solve the problem of updated revocation information. Copyright © 2010 John Wiley & Sons, Ltd.

Method and apparatus for enabling authorised users to access computer resources

Abstract: An authentication system is disclosed for use in authenticating an entity to a relying party, to enable the entity to access a protected resource provided by the relying party via a web page, comprising an authentication component installable in a web browser used by the entity to access the web page, the authentication component comprising (a) a page scanner component which is operable when the entity accesses the web page to scan the web page (and/or to ask the entity) to identify a plurality of authentication systems supported by the web page; and (b) an activator component which is operable when the entity accesses the web page to install an identity system selector component in the web page which is operable to interact with the entity to enable the entity to select which of the plurality of authentication systems to use.

Building General-Purpose Security Services on EMV Payment Cards

Abstract: The Generic Authentication Architecture (GAA) is a standardised extension to the mobile telephony security infrastructures that supports the provision of security services to network applications. We have proposed a generalised version of GAA which enables almost any pre-existing infrastructure to be used as the basis for the provision of generic security services, and have examined a GAA instantiation supported by Trusted Computing. In this paper we study another instantiation of GAA, this time building on the widely deployed EMV security infrastructure. This enables the existing EMV infrastructure to be used as the basis of a general-purpose authenticated key establishment service in a simple and uniform way, and also provides an opportunity for EMV-aware third parties to provide novel security services. We also discuss possible applications and issues of privacy and trust.