Katherine Cornes

Perceptual and decisional factors influencing the discrimination of inversion in the Thatcher illusion

The Thatcher illusion (Thompson, 1980) is considered to be a prototypical illustration of the notion that face perception is dependent on configural processes and representations. We explored this idea by examining the relative contributions of perceptual and decisional processes to the ability of observers to identify the orientation of two classes of forms-faces and churches-and a set of their component features. Observers were presented with upright and inverted images of faces and churches in which the components (eyes, mouth, windows, doors) were presented either upright or inverted. Observers first rated the subjective grotesqueness of all of the images and then performed a complete identification task in which they had to identify the orientation of the overall form and the orientation of each of the interior features. Grotesqueness ratings for both classes of image showed the standard modulation of rated grotesqueness as a function of orientation. The complete identification results revealed violations of both perceptual and decisional separability but failed to reveal any violations of within-stimulus (perceptual) independence. In addition, exploration of a simple bivariate Gaussian signal detection model of the relationship between identification performance and judged grotesqueness suggests that within-stimulus violations of perceptual independence on their own are insufficient for producing the illusion. This lack of evidence for within-stimulus configurality suggests the need for a critical reevaluation of the role of configural processing in the Thatcher illusion. (PsycINFO Database Record (c) 2011 APA, all rights reserved).

Discriminating Grotesque from Typical Faces: Evidence from the Thatcher Illusion

The discrimination of thatcherized faces from typical faces was explored in two simultaneous alternative forced choice tasks. Reaction times (RTs) and errors were measured in a behavioural task. Brain activation was measured in an equivalent fMRI task. In both tasks, participants were tested with upright and inverted faces. Participants were also tested on churches in the behavioural task. The behavioural task confirmed the face specificity of the illusion (by comparing inversion effects for faces against churches) but also demonstrated that the discrimination was primarily, although not exclusively, driven by attending to eyes. The fMRI task showed that, relative to inverted faces, upright grotesque faces are discriminated via activation of a network of emotion/social evaluation processing areas. On the other hand, discrimination of inverted thatcherized faces was associated with increased activation of brain areas that are typically involved in perceptual processing of faces.

The influence of experience upon information-sampling and decision-making behaviour during risk assessment in military personnel

We examined the influence of experience upon information-sampling and decision-making behaviour in a group of military personnel as they conducted risk assessments of scenes photographed from patrol routes during the recent conflict in Afghanistan. Their risk assessment was based on an evaluation of Potential Risk Indicators (PRIs) during examination of each scene. We found that both participant groups were equally likely to fixate PRIs, demonstrating similarity in the selectivity of their information-sampling. However, the inexperienced participants made more revisits to PRIs, had longer response times, and were more likely to decide that the scenes contained a high level of risk. Together, these results suggest that experience primarily modulates decision-making behaviour. We discuss potential routes to train personnel to conduct risk assessments in a more similar manner to experienced participants.

The importance of search strategy for finding targets in open terrain

A number of real-world search tasks (i.e. police search, detection of improvised explosive devices (IEDs)) require searchers to search exhaustively across open ground. In the present study, we simulated this problem by asking individuals (Experiments 1a and 1b) and dyads (Experiment 2) to search for coin targets pseudo-randomly located in a bounded area of open grassland terrain. In Experiment 1a, accuracy, search time, and the route used to search an area were measured. Participants tended to use an ‘S’-shaped pattern with a common width of search lane. Increased accuracy was associated with slower, but also variable, search speed, though only when participants moved along the length (as opposed to across the width) of the search area. Experiment 1b varied the number of targets available within the bounded search area and in doing so varied target prevalence and density. The results confirmed that the route taken in Experiment 1a generalizes across variations in target prevalence/density. In Experiment 2, accuracy, search time, and the search strategy used by dyads was measured. While dyads were more accurate than individuals, dyads that opted to conduct two independent searches were more accurate than those who opted to split the search space. The implications of these results for individuals and dyads when searching for targets in open space are discussed.

Detecting the Thatcher illusion in a case of prosopagnosia

We explored configural face processing in a prosopagnosic patient (PHD, Eimer and McCarthy, 1999) who does not produce an N170 is response for faces. In two sets of studies he was presented with two versions of the Thatcher illusion. In the first set, he was asked to detect Thatcherized from matched typical faces from successive single presentations of faces. He also performed a simultaneous 2 alternative forced choice (2AFC) discrimination task with the same stimulus set to address the question of whether pairs of faces were the same or different. In the second set he was asked to detect Thatcherized from matched typical faces. He also performed in control conditions where orientation decisions were made to isolated eye and mouth features, as well as eye and mouth features presented alone but within face outlines. The results were analyzed using d-prime and C to facilitate cross condition comparisons. The data showed PHD unable to detect Thatcherized from matched typical faces in either study 1 or 2. However, he was as sensitive as controls in the 2AFC discrimination condition of Study 1. In study 2 he showed evidence of moderate sensitivity to the identification of orientation for isolated features; this sensitivity was much enhanced for eyes by face outlines but hindered for mouths. We interpret these findings as showing intact feature processing that should be sufficient to allow the detection of the Thatcher illusion, as well as some evidence of relational processing for eyes but not mouths. However, simultaneous presentation of features and face outline does not allow selective attention to eyes that would enable detection of Thatcherized from matched typical faces. The results suggest one aspect of successful face categorization is to determine face-specific configural routines that allocate attention within faces and that these are missing in PHD.