The impact of Relative Prevalence on dual-target search for threat items from airport X-ray screening

The probability of target presentation in visual search tasks influences target detection performance: this is known as the prevalence effect (Wolfe et al., 2005). Additionally, searching for several targets simultaneously reduces search performance: this is known as the dual-target cost (DTC: Menneer et al., 2007). The interaction between the DTC and prevalence effect was investigated in a single study by presenting one target in dual-target search at a higher level of prevalence than the other target (Target A: 45% Prevalence; Target B: 5% Prevalence). An overall DTC was found for both RTs and response accuracy. Furthermore, there was an effect of target prevalence in dual-target search, suggesting that, when one target is presented at a higher level of prevalence than the other, both the dual-target cost and the prevalence effect contribute to decrements in performance. The implications for airport X-ray screening are discussed.     

Developmental changes during childhood in single-letter acuity and its crowding by surrounding contours

Crowding refers to impaired target recognition caused by surrounding contours. We investigated the development of crowding in central vision by comparing single-letter and crowding thresholds in groups of 5-year-olds, 8-year-olds, 11-year-olds, and adults. The task was to discriminate the orientation of a Sloan letter E. Single-letter thresholds, defined as the stroke width forming the smallest discriminable E, were worse than those of adults (0.83 arcmin) at 5 years of age (1.05 arcmin) but not at older ages (8-year-olds: 0.81 arcmin; 11-year-olds: 0.78 arcmin). The maximum distances over which crowding occurred, as measured in multiples of threshold stroke width, were smaller in adults (2.83) than in the three groups of children, who did not differ from each other (5-year-olds: 7.03; 8-year-olds: 7.84; 11-year-olds: 7.13). Thus, even 11-year-olds are more affected than adults by surrounding contours despite having single-letter acuity that has been mature for several years. The stronger influence of crowding in children may be caused by immaturities in the brain areas beyond the primary visual cortex (V1) where early visual inputs are combined and may contribute to their slower reading speed.     

The roles of private speech and inner speech in planning during middle childhood: Evidence from a dual task paradigm

Children often talk themselves through their activities, producing private speech that is internalized to form inner speech. This study assessed the effect of articulatory suppression (which suppresses private and inner speech) on Tower of London performance in 7- to 10-year-olds, relative to performance in a control condition with a nonverbal secondary task. Experiment 1 showed no effect of articulatory suppression on performance with the standard Tower of London procedure; we interpret this in terms of a lack of planning in our sample. Experiment 2 used a modified procedure in which participants were forced to plan ahead. Performance in the articulatory suppression condition was lower than that in the control condition, consistent with a role for self-directed (private and inner) speech in planning. On problems of intermediate difficulty, participants producing more private speech in the control condition showed greater susceptibility to interference from articulatory suppression than their peers, suggesting that articulatory suppression interfered with performance by blocking self-directed (private and inner) speech.     

Amodal completion in bonobos

We trained two bonobos to discriminate among occluded, complete, and incomplete stimuli. The occluded stimulus comprised a pair of colored shapes, one of which appeared to occlude the other. The complete and incomplete stimuli involved the single shape that appeared to have been partially covered in the occluded stimulus; the complete stimulus showed the full shape, whereas the incomplete stimulus showed the same portion of the shape that was visible in the occluded stimulus. The correct response was to select the two-part occluded stimulus. Consistent with amodal completion, the bonobos committed a higher percentage of errors to the complete stimuli than to the incomplete stimuli. As well, the percentage of errors to the complete stimuli rose after repeated training with several different shapes. Extensive experience with two-dimensional images enhances amodal completion of partially occluded stimuli in bonobos.     

Attentional modulation of visual-evoked potentials by threat: investigating the effect of evolutionary relevance

In dot-probe tasks, threatening cues facilitate attention to targets and enhance the amplitude of the target P1 peak of the visual-evoked potential. While theories have suggested that evolutionarily relevant threats should obtain preferential neural processing, this has not been examined empirically. In this study we examined the effects of evolutionarily relevant (e.g. spiders and snakes) and irrelevant (e.g. knifes and syringes) threatening cues. Non-predictive threatening cues (in left or right visual field) were paired with non-threatening cues and were followed by neutral targets in the same or opposite location. The amplitude of the target P1 was increased in contralateral electrodes when the target followed in the same location as the threatening cues. This effect did not interact with evolutionary relevance. Both evolutionary relevant and evolutionary irrelevant threats led to increased P1 amplitude, although the effect was stronger for modern threats. We conclude that the threat-superiority effect is robust and largely independent of the type of threatening stimulus.     

Anomalous global effects induced by ‘blind’ distractors in visual hemifield defects

Previous research has revealed that a stimulus presented in the blind visual field of participants with visual hemifield defects can evoke oculomotor competition, in the absence of awareness. Here we studied three cases to determine whether a distractor in a blind hemifield would be capable of inducing a global effect, a shift of saccade endpoint when target and distractor are close to each other, in participants with lesions of the optic radiations or striate cortex. We found that blind field distractors significantly shifted saccadic endpoints in two of three participants with lesions of either the striate cortex or distal optic radiations. The direction of the effect was paradoxical, however, in that saccadic endpoints shifted away from blind field distractors, whereas endpoints shifted towards distractors in the visible hemifields, which is the normal global effect. These results provide further evidence that elements presented in the blind visual field can generate modulatory interactions in the oculomotor system, which may differ from interactions in normal vision.     

Refining the visual-cortical hypothesis in category learning

Participants produce steep typicality gradients and large prototype-enhancement effects in dot-distortion category tasks, showing that in these tasks to-be-categorized items are compared to a prototypical representation that is the central tendency of the participant’s exemplar experience. These prototype-abstraction processes have been ascribed to low-level mechanisms in primary visual cortex. Here we asked whether higher-level mechanisms in visual cortex can also sometimes support prototype abstraction. To do so, we compared dot-distortion performance when the stimuli were size constant (allowing some low-level repetition-familiarity to develop for similar shapes) or size variable (defeating repetition-familiarity effects). If prototype formation is only mediated by low-level mechanisms, stimulus-size variability should lessen prototype effects and flatten typicality gradients. Yet prototype effects and typicality gradients were the same under both conditions, whether participants learned the categories explicitly or implicitly and whether they received trial-by-trial reinforcement during transfer tests. These results broaden out the visual-cortical hypothesis because low-level visual areas, featuring retinotopic perceptual representations, would not support robust category learning or prototype-enhancement effects in an environment of pronounced variability in stimulus size. Therefore, higher-level cortical mechanisms evidently can also support prototype formation during categorization.     

Visual field asymmetries in attention vary with self-reported attention deficits

The purpose of this study was to determine whether an index of self-reported attention deficits predicts the pattern of visual field asymmetries observed in behavioral measures of attention. Studies of “normal” subjects do not present a consistent pattern of asymmetry in attention functions, with some studies showing better left visual field (LVF) performance, others showing no asymmetry or even a right visual field (RVF) advantage. Here we found that a participant variable (a measure of self-reported attention problems) may help to explain these inconsistencies. We used Conners’ Adult ADHD Rating Scales to measure self-reported attention problems in a group of 36 normal (non-ADHD) subjects, and we used two behavioral tasks to directly measure their attentional abilities: a lateralized Attention Network Task (ANT) and a visual search task. Comparing subjects with relatively Low versus High scores on the attention-deficit (AD) scale, we found that subjects with High scores were less efficient in orienting attention to the left visual field. When LVF targets were preceded by a valid spatial cue, response times were positively correlated with AD scores, indicating slower covert-attention shifts as attention-deficit scores increased. Right visual field data showed a different pattern of results. Subjects with High AD scores were at least as efficient as those with Low AD scores in orienting attention to the RVF. This study provides evidence that visual field asymmetries in orienting attention vary across individuals, and that level of self-reported attention problems correlates with behavioral deficiencies in orienting attention to the left visual field.     

Response to briscoe (2010)

In an earlier paper my colleagues and I put forth a theory called "perceiving-the-present" that predicts a systematic pattern across a large variety of illusions, and we presented evidence that the systematic pattern exists. Briscoe puts forth arguments against the theory and the existence of the systematic pattern. Here I provide counterarguments to his criticisms of the theory, and I explain why his arguments do not bear on the existence of the systematic pattern.