Fearful expressions enhance recognition memory: electrophysiological evidence.

Facial expressions play a key role in affective and social behavior. However, the temporal dynamics of the brain responses to emotional faces remain still unclear, in particular an open question is at what stage of face processing expressions might influence encoding and recognition memory. To try and answer this question we recorded the event-related potentials (ERPs) elicited in an old/new recognition task. A novel aspect of the present design was that whereas faces were presented during the study phase with either a happy, fearful or neutral expression, they were always neutral during the memory retrieval task. The ERP results showed three main findings: An enhanced early fronto-central positivity for faces encoded as fearful, both during the study and the retrieval phase. During encoding subsequent memory (Dm effect) was influenced by emotion. At retrieval the early components P100 and N170 were modulated by the emotional expression of the face at the encoding phase. Finally, the later ERP components related to recognition memory were modulated by the previously encoded facial expressions. Overall, these results suggest that face recognition is modulated by top-down influences from brain areas associated with emotional memory, enhancing encoding and retrieval in particular for fearful emotional expressions.

The assessment of visual acuity in children with periventricular damage: a comparison of behavioural and electrophysiological techniques.

It has been controversial whether electrophysiology offers better precision than behavioural techniques in measuring visual acuity in children with brain damage. We investigated the concordance between sweep VEPs and Acuity Cards (AC) in 29 children with periventricular leukomalacia (PVL), the most common type of brain damage in preterm infants. An overall good correlation was shown but with relatively better behavioural acuity values. VEP/AC ratio was significantly correlated to corpus callosum posterior thinning. We propose that this result reflects the efficacy of the compensatory mechanisms following early brain damage which may differentially affect the two methods.

Are there separate first-order and second-order mechanisms for orientation discrimination?

In a series of experiments we compared orientation discrimination performance for Gabor stimuli in which the stimulus profile was either matched to the receptive field profile of single V1 simple cells ('simple'), or in which the carrier and envelope orientations were different ('tigertails'). In the first Experiment, using small, high spatial frequency, peripheral stimuli to minimise the number of detectors involved, we found that simple stimuli were more detectable than tigertails of the same contrast energy, and that orientation discrimination thresholds for simple stimuli were lower than for tigertails of equal detectability. In later experiments with larger stimuli we measured thresholds for detecting tilts of the envelope with the carrier fixed in orientation. Envelope thresholds were similar for different carrier orientations, but carrier orientation had a strong biasing effect upon perceived envelope orientation. When the orientation difference between envelope and carrier was small, the carrier orientation was attracted to that of the envelope; when the difference was large (>10 degrees ) repulsion was found. The biases were reduced by half-wave rectifying the stimuli, putatively making the envelope visible to a first-order filter (Experiment 2). Discrimination thresholds for envelope orientation were higher than those for carrier orientation, and this difference was greater for briefly-presented parafoveal stimuli than for long duration foveal stimuli (Experiments 3 and 4). We conclude from these results that there are separate mechanisms for envelope and carrier orientation discriminations for large stimuli, but that first- and second-order mechanisms are not independent in the discrimination of orientation.

Feature-based integration of orientation signals in visual search.

We have measured orientation discrimination in the presence of a variable number of neutral distracters for two distinct tasks: identification of the orientation of a tilted target and location of its position. Both tasks were performed in the presence of visual noise of variable contrasts. Under a range of conditions, subjects could identify the direction of target tilt at thresholds well below those necessary to locate its position. The location thresholds showed only weak dependency on set-size, consistent with a stimulus uncertainty of parallel search of the output of independent orientation analysers, while the identification thresholds showed a much stronger dependency, varying with the square root of set-size over a wide range noise contrasts. The square root relationship suggests perceptual summation of target and distracters. Manipulating the spread of visual noise suggests that the summation is feature-based, possibly operating on the outputs of first-stage orientation analysers. Pre-cueing the target eliminates the effects of set-size, showing that the summation is under rapid attentional control; the visual system can choose between high performance over a limited area and poorer performance over a much larger area.

How the human visual system encodes the orientation of a texture, and why it makes mistakes.

Human observers are exquisitely sensitive to tilt in the orientation of a line. We can detect rotations away from the vertical of 0.5 degrees. It has been suggested [1,2] that this accuracy is a result of the orientation-selectivity of simple cells in the primary visual cortex (V1), many of which have receptive fields with an elliptical shape [3]. However, it is possible to sense the tilt of many stimuli that are unlikely to have their tilt directly encoded by such cells. For example, a garment such as a tie with diagonal stripes would predominantly stimulate cells in V1 tuned to an orientation of the stripes; yet we could tell whether or not the garment as a whole was tilted from the vertical. The perception of oriented textures is subject to systematic errors, however. A striking example is the Fraser 'twisted cord' illusion (Fig. 1) in which we see the global orientation of the horizontal texture-defined lines as being tilted in the direction of its locally tilted segments. If the component segments are at a larger angle (30 degrees) to the global orientation, on the other hand, the perceived shift is in the opposite direction. We have measured these effects psychophysically, and we propose a model in which second-order orientation units receive excitation from V1 units of similar orientation, but inhibition from V1 units of dissimilar orientation. Our model correctly predicts that making the textures different in average brightness from the background will reduce the illusions.