The role of configurality in the Thatcher illusion: an ERP study.

The Thatcher illusion (Thompson in Perception, 9, 483-484, 1980) is often explained as resulting from recognising a distortion of configural information when 'Thatcherised' faces are upright but not when inverted. However, recent behavioural studies suggest that there is an absence of perceptual configurality in upright Thatcherised faces (Donnelly et al. in Attention, Perception & Psychophysics, 74, 1475-1487, 2012) and both perceptual and decisional sources of configurality in behavioural tasks with Thatcherised stimuli (Mestry, Menneer et al. in Frontiers in Psychology, 3, 456, 2012). To examine sources linked to the behavioural experience of the illusion, we studied inversion and Thatcherisation of faces (comparing across conditions in which no features, the eyes, the mouth, or both features were Thatcherised) on a set of event-related potential (ERP) components. Effects of inversion were found at the N170, P2 and P3b. Effects of eye condition were restricted to the N170 generated in the right hemisphere. Critically, an interaction of orientation and eye Thatcherisation was found for the P3b amplitude. Results from an individual with acquired prosopagnosia who can discriminate Thatcherised from typical faces but cannot categorise them or perceive the illusion (Mestry, Donnelly et al. in Neuropsychologia, 50, 3410-3418, 2012) only differed from typical participants at the P3b component. Findings suggest the P3b links most directly to the experience of the illusion. Overall, the study showed evidence consistent with both perceptual and decisional sources and the need to consider both in relation to configurality

Visual field differences in sequential letter classification tasks.

A review of the literature on visual field asymmetries indicated that although the constructs of strategy, processing, and attention had been invoked to account for results there was little objective evidence to support these views. The Posner letter classification tasks provide a methodology which enables such constructs to be tested empirically, and accordingly were employed in this research. Sequential double letter classifications were used because they provide a stable visual match advantage (Kroll, 1975), permitting an evaluation of retention interval effects without the complications of code change. Despite such stability on cognitive dependent variables, visual field effects differed between 9 sec (Experiment I) and those of less than one second (Experiment III). A change in coding bias was induced by the use of irregular time structure (Experiments IV, V, VII) although overall visual field differences were comparable to those obtained when coding was stable (Experiments III and VI). Four of the relevant studies indicated a right field advantage for cross-case (name) matches, and non asymmetric identity or visual match judgements. Single letter stimuli showed a left field advantage for identity matches, and a right field effect for cross-case classifications. The overall pattern of results indicated that visual field differences arose from the time of test stimulus presentation onwards. These findings were incompatible with models of visual field differences which have been advanced hitherto. An integration of strategy, attention and processing hypotheses was advanced and suggestions made for further research

Remembering and forgetting of semantic knowledge in amnesia: a sixteen-year follow-up investigation of RFR

We report our long-term follow-up investigations of RFR, a post-encephalitic case of very grave anterograde and retrograde amnesia. We also describe the results of quantitative neuroimaging of his brain injury that showed bilateral and severe reduction in the hippocampal formation and medial temporal structures with sparing of left lateral/posterior and right posterior temporal cortex. We established that RFR had a persistent severe anterograde and retrograde amnesia for personal and public events. His personal semantic knowledge was relatively spared for the retrograde period. There was a modest and global reduction in RFR's vocabulary for words acquired in adulthood before he became amnesic but there was no evidence of any retrograde gradient. His retrograde knowledge of people was also without any gradient. Remarkably, there had been no change in the extent of his semantic knowledge across a prolonged re-test interval indicating that the loss of semantic knowledge was stable and likely to have arisen at the time of his initial lesion. RFR also showed evidence of a limited but significant ability to acquire new word meanings and a more restricted capacity for learning about new celebrities. While he was able to demonstrate face and name familiarity for newly famous people, he was unable to provide much semantic detail. RFR's amnesia can be partially explained by contemporary theories that allow for parallel cortical and hippocampal memory systems but is difficult to reconcile in detail with any extant view

2D but not 3D: pictorial-depth deficits in a case of visual agnosia

Patients with visual agnosia exhibit acquired impairments in visual object recognition, that may or may not involve deficits in low-level perceptual abilities. Here we report a case (patient DM) who after head injury presented with object recognition deficits. He still appears able to extract 2D information from the visual world in a relatively intact manner; but his ability to extract pictorial information about 3D object-structure is greatly compromised. His copying of line drawings is relatively good, and he is accurate and shows apparently normal mental rotation when matching or judging objects tilted in the picture-plane. But he performs poorly on a variety of tasks requiring 3D representations to be derived from 2D stimuli, including: performing mental rotation in depth, rather than in the picture-plane; judging the relative depth of two regions depicted in line-drawings of objects; and deciding whether a line-drawing represents an object that is ‘impossible’ in 3D. Interestingly, DM failed to show several visual illusions experienced by normals (Muller-Lyer and Ponzo), that some authors have attributed to pictorial depth cues. Taken together, these findings indicate a deficit in achieving 3D intepretations of objects from 2D pictorial cues, that may contribute to object-recognition problems in agnosia

Form completion across a hemianopic boundary: behindsight?

Patients with homonymous hemianopia may report the completion of forms that overlap the vertical meridian of their field defects. While previous investigations of “hemianopic completion” have variously attributed to the disorder to inattention, residual vision or unstable fixation, we believe that our investigation has controlled for such potentially confounding factors. We report patient P.O.V. who experienced hemianopic completion in everyday life following a surgical lesion of his left occipital lobe. He showed normal spatial attention and normal spatial orienting: hemianopic completion can therefore occur in the absence of inattention. His completion was retinotopic and affected partial as well as complete forms: his hemianopic completion cannot be attributed to residual visual input or poor fixation. P.O.V.'s completion was also systematically affected by varying stimulus contrast and pattern masking. We argue that while other explanations may be appropriate for different cases, P.O.V.'s hemianopic completion reflects normal “constructive” visual processes and can be attributed to the unconstrained operation of visual routines that are normally involved in the perception of partially occluded forms. As such, this disorder has the potential to shed light on some of the most basic aspects of visual perception

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

Bilingual aphasia due to spontaneous acute subdural haematoma from a ruptured intracranial infectious aneurysm

We report a case of spontaneous subdural haematoma due to ruptured intracranial infectious aneurysm, presenting with bilingual aphasia and illustrating differential language recovery. A 62-year-old right-handed bilingual gentleman, with a diagnosis of infective endocarditis, developed headache and became expressively aphasic in the English language. Three days later he was receptively and expressively aphasic in both English and Arabic. Cranial MRI scans showed a left-sided acute subdural haematoma with mass effect and midline shift. Contrast CT brain scans showed an enhancing speck adjacent to the clot and cerebral angiogram confirmed a distal middle cerebral artery aneurysm. He underwent image-guided craniotomy, evacuation of the subdural haematoma and excision of the aneurysm. Histopathological examination was consistent with an infectious intracranial aneurysm. Postoperatively his aphasia did not improve immediately. He had widened pulse pressure due to severe aortic regurgitation, confirmed on echocardiography. He underwent aortic valve replacement and mitral valve repair, following which his aphasia recovered gradually. Initially the recovery of his language was limited to Arabic. About a week later he recovered his English language as well. At 3-year follow-up he is doing well and has no neurological deficits. His aphasia has recovered completely. The present case is unique because of (a) presence of pure subdural haematoma, and (b) the differential susceptibility and recovery of native (L1) and acquired language (L2) in presence of a common pathology. The neurology of language in a bilingual is analysed and possible mechanisms discussed