Gael E. Gordon

Configural masking of faces: Evidence for high-level interactions in face perception

The perception of a stimulus can be impaired when presented in the context of a masking pattern. To determine the timing and the nature of face processing, the effect of various masks on the discriminability of faces was investigated. Results reveal a strong configural effect: the magnitude of masking depends on the similarity between mask and target. Masking is absent for non-face masks (noise, houses), modest for scrambled and inverted faces and strongest for upright faces, even when they differ in size, gender or viewpoint from the targets. This suggests an extra-striate location for the masking (possibly FFA). Reduced but significant masking for isolated face parts (internal features or head shape) is consistent with holistic computations in face perception. The duration over which a face mask can impair face discrimination (130 ms) is markedly longer than previously assumed and is sufficient for iterative and feedback computations to be part of face processing.

Visual Noise Selectively Degrades Vision in Migraine

Purpose. Migraine is a disabling condition with underlying neuronal mechanisms that remain elusive. Migraineurs experience hyperresponsivity to visual stimuli and frequently experience visual disturbances. In the present study, the equivalent input noise approach was used to reveal abnormalities of visual processing and to isolate factors responsible for any such deficits. This approach partitions visual sensitivity into components that represent the efficiency of using the available stimulus information, the background internal noise due to irregular neuronal fluctuations, and the neuronal noise induced by the external stimulation. Methods. Ten migraine with aura, ten migraine without aura, and ten age-matched headache-free subjects participated. Performance in detecting luminance targets embedded in visual noise, resembling grainy photographs, was measured at various noise levels. Results. Contrast thresholds of the three subject groups were similar in the absence of noise, but both migraine groups performed worse in the presence of high noise levels, with performance of migraineurs with aura significantly poorer (P < 0.05) than that of control subjects. Data were fitted with a perceptual template model that showed that the model parameter determining the internal (neuronal) noise triggered by the external (stimulus) noise was significantly higher (P < 0.001) in both migraine groups than in the non-migraineur group. Migraineurs without aura also showed a significant (P < 0.05) though weak reduction of sampling efficiency (0.12 +/- 0.02) compared with control subjects (0.17 +/- 0.02). Conclusions. The results revealed substantial external noise-exclusion deficits in migraine with aura and a minor impairment of noise exclusion in migraine without aura. Migraineurs appeared prone to abnormally high variability of neuronal activity. This result provides a promising explanation of observed visual deficits in migraine.

The Useful Field of View Test, Normative Data in Children of School Age

Purpose. The incidence of early brain injury in children is rising, mainly because of changing reproductive demographics and better perinatal care. Early brain injury may be associated with altered visual perception and visuospatial attention. The useful field of view test (UFOV) has been developed to assess visual attention and processing speed in the elderly. The UFOV may have utility in the evaluation of children with early brain injury. The aim of this study was to collect age-specific normative data for the UFOV test. Methods. Subjects were recruited from visitors at Glasgow Science Centre. One hundred thirty-five healthy children and young adults were tested (range: 5–22 years). After a comprehensive visual screening to establish normal visual function, all three subtests of the UFOV (1: Processing Speed; 2: Divided Attention; 3: Selective Attention) were performed. Results. All the children were able to understand and complete all subtests. UFOV scores improved monotonically with age, for all subtests, throughout the primary school years, with subtest 3 showing the greatest improvement. By age 14, UFOV scores had reached adult levels. Discussion. UFOV performance shows measurable improvement during middle to late childhood, as has been previously found for abilities such as visual acuity. This is a simple test, which was easily understood and performed by all children in the study. Thus, we believe that our results are suggestive of continuing development in visual attention and processing during this period. Conclusions. The UFOV is an objective, standardized computer-based test of visual attention. This study indicates suitability for use with children. It remains to be determined how successful this test proves to be in discriminating between normal children and those with early brain injury.

Detecting shapes in noise: tuning characteristics of global shape mechanisms

The proportion of signal elements embedded in noise needed to detect a signal is a standard tool for investigating motion perception. This paradigm was applied to the shape domain to determine how local information is pooled into a global percept. Stimulus arrays consisted of oriented Gabor elements that sampled the circumference of concentric radial frequency (RF) patterns. Individual Gabors were oriented tangentially to the shape (signal) or randomly (noise). In different conditions, signal elements were located randomly within the entire array or constrained to fall along one of the concentric contours. Coherence thresholds were measured for RF patterns with various frequencies (number of corners) and amplitudes (“sharpness” of corners). Coherence thresholds (about 10% = 15 elements) were lowest for circular shapes. Manipulating shape frequency or amplitude showed a range where thresholds remain unaffected (frequency ≤ RF4; amplitude ≤ 0.05). Increasing either parameter caused thresholds to rise. Compared to circles, thresholds increased by approximately four times for RF13 and five times for amplitudes of 0.3. Confining the signals to individual contours significantly reduced the number of elements needed to reach threshold (between 4 and 6), independent of the total number of elements on the contour or contour shape. Finally, adding external noise to the orientation of the elements had a greater effect on detection thresholds than adding noise to their position. These results provide evidence for a series of highly sensitive, shape-specific analysers which sum information globally but only from within specific annuli. These global mechanisms are tuned to position and orientation of local elements from which they pool information. The overall performance for arrays of elements can be explained by the sensitivity of multiple, independent concentric shape detectors rather than a single detector integrating information widely across space (e.g. Glass pattern detector).

Distinct lower visual field preference for object shape

Humans manipulate objects chiefly within their lower visual field, a consequence of upright posture and the anatomical position of hands and arms.This study tested the hypothesis of enhanced sensitivity to a range of stimuli within the lower visual field. Following current models of hierarchical processing within the ventral steam, discrimination sensitivity was measured for orientation, curvature, shape (radial frequency patterns), and faces at various para-central locations (horizontal, vertical, and main diagonal meridians) and eccentricities (5° and 10°). Peripheral sensitivity was isotropic for orientation and curvature. By contrast, observers were significantly better at discriminating shapes throughout the lower visual field compared to elsewhere. For faces, however, peak sensitivity was found in the left visual field, corresponding to the right hemispheric localization of human face processing. Presenting head outlines without any internal features (e.g., eyes, mouth) recovered the lower visual field advantage found for simple shapes. A lower visual field preference for the shape of an object, which is absent for more localized information (orientation and curvature) but also for more complex objects (faces), is inconsistent with a strictly feed-forward model and poses a challenge for multistage models of object perception. The distinct lower visual field preference for contour shapes is, however, consistent with an asymmetry at intermediate stages of visual processing, which may play a key role in representing object characteristics that are particularly relevant to visually guided actions.

Global Shape Processing Deficits Are Amplified by Temporal Masking in Migraine

PURPOSE Individuals with migraine show subtle defects in a range of visual tasks compared to nonmigraineurs. Increased neuronal noise can account for some of these deficits. To examine the generality of increased noise in migraine, masking effects were compared in migraineurs and headache-free controls using a shape discrimination task, thought to involve processing in extrastriate cortical areas. METHODS Nine migraineurs with aura, nine migraineurs without aura, and nine headache-free controls participated. observers had to detect deviations in circular shapes with or without a larger contour mask. The nonoverlapping mask was presented at five temporal intervals (stimulus onset asynchronies, SOA): 0 (simultaneous), 66, 100, 133, and 250 ms. RESULTS Migraineurs with aura performed worse in all tests than migraineurs without aura and controls. Both migraine groups performed poorer than controls at discriminating shapes without masks. Typical masking functions were obtained from all groups, but they were steeper for migraineurs than controls with thresholds raised most dramatically (2.1 and 4.4 times for migraineurs without and with aura relative to controls, respectively) at SOAs where masks had their most detrimental effect (66-100 ms). Modeling the effect of masking showed that raised internal noise alone is insufficient to explain these deficits. Rather, an abnormal nonlinear transducer function (e.g., as part of gain-control) together with increased multiplicative noise is required to capture the data. CONCLUSIONS The findings are consistent with an extrastriate deficit in migraine that cannot be explained completely by defective inhibition.

Long-range inhibitory mechanisms in the visual system are impaired in migraine sufferers:

Background: After viewing dynamic noise surrounding a homogeneous grey patch (artificial scotoma), observers perceive a prolonged twinkling-noise after-image within the unstimulated area. It has been suggested that noise-stimulated neurons induce a long-range inhibition in neurons within the artificial scotoma, which generates a rebound signal perceived as twinkling noise following noise termination. We used this paradigm to test whether migraineurs have enhanced excitability or weakened inhibition. Methods: Twinkling-noise duration was measured in 13 headache-free volunteers, 13 migraineurs with aura and 13 migraineurs without aura. Results: The durations of the after-image were significantly shorter for both migraine groups compared to controls. Discussion: Enhanced excitation of noise-activated neurons in migraineurs would produce stronger rebound activity and longer after-image durations, while weakened inhibitory mechanisms would diminish the rebound activity and shorten the after-image durations compared to control subjects. The results suggest that cortical inhibitory mechanisms might be impaired in migraineurs with and without aura.

The Pulfrich phenomenon in optic disc drusen.

Optic disc drusen are hyaline bodies in the optic nerve head of unknown aetiology,1 which are present in approximately 2% of the population.2 Reduced visual acuity associated with optic disc drusen is rare, although visual field defects are common in cases of visible drusen.3 4 Although the condition is frequently bilateral, asymmetry is usual.5 We report a case of a patient with disabling visual symptoms associated with optic disc drusen. These were found to result from the Pulfrich phenomenon and were resolved by prescribing an appropriate ophthalmic tint. A 20-year-old woman was seen in our optometry clinic and subsequently examined by a consultant ophthalmologist. She was moderately myopic, with good visual acuities (6/5 OU), no history of ocular disease and showed no symptoms …

Contributions of individual face features to face discrimination

HIGHLIGHTSSensitivity was significantly higher for external, relative to internal, features.Sensitivity was poorest for features affected by facial dynamics (e.g. eyebrows).Isolated features were easier to discriminate than those embedded within a face.Holistic processing impedes extraction of featural information from a face context. ABSTRACT Faces are highly complex stimuli that contain a host of information. Such complexity poses the following questions: (a) do observers exhibit preferences for specific information? (b) how does sensitivity to individual face parts compare? These questions were addressed by quantifying sensitivity to different face features. Discrimination thresholds were determined for synthetic faces under the following conditions: (i) ‘full face’: all face features visible; (ii) ‘isolated feature’: single feature presented in isolation; (iii) ‘embedded feature’: all features visible, but only one feature modified. Mean threshold elevations for isolated features, relative to full‐faces, were 0.84x, 1.08, 2.12, 3.34, 4.07 and 4.47 for head‐shape, hairline, nose, mouth, eyes and eyebrows respectively. Hence, when two full faces can be discriminated at threshold, the difference between the eyes is about four times less than what is required when discriminating between isolated eyes. In all cases, sensitivity was higher when features were presented in isolation than when they were embedded within a face context (threshold elevations of 0.94x, 1.74, 2.67, 2.90, 5.94 and 9.94). This reveals a specific pattern of sensitivity to face information. Observers are between two and four times more sensitive to external than internal features. The pattern for internal features (higher sensitivity for the nose, compared to mouth, eyes and eyebrows) is consistent with lower sensitivity for those parts affected by facial dynamics (e.g. facial expressions). That isolated features are easier to discriminate than embedded features supports a holistic face processing mechanism which impedes extraction of information about individual features from full faces.