Alba Grieco

Attention modulates psychophysical and electrophysiological response to visual texture segmentation in humans

To investigate whether processing underlying texture segmentation is limited when texture is not attended, we measured orientation discrimination accuracy and visual evoked potentials (VEPs) while a texture bar was cyclically alternated with a uniform texture, either attended or not. Orientation discrimination was maximum when the bar was explicitly attended, above threshold when implicitly attended, and fell to just chance when unattended, suggesting that orientation discrimination based on grouping of elements along texture boundary requires explicit attention. We analyzed tsVEPs (variations in VEP amplitude obtained by algebraic subtraction of uniform-texture from segmented-texture VEPs) elicited by the texture boundary orientation discrimination task. When texture was unattended, tsVEPs still reflected local texture segregation. We found larger amplitudes of early tsVEP components (N75, P100, N150, N200) when texture boundary was parallel to texture elements, indicating a saliency effect, perhaps at V1 level. This effect was modulated by attention, disappearing when the texture was not attended, a result indicating that attention facilitates grouping by collinearity in the direction of the texture boundary.

Perceptual learning modulates electrophysiological and psychophysical response to visual texture segmentation in humans.

We investigated the mechanisms that allow, via perceptual learning, selective modulation of a visual line-texture figure saliency in accordance with task relevance. Learning-dependent saliency increase was inferred by increased accuracy in orientation discrimination with task repetition. As a result of learning, accuracy increase was more pronounced when local and global orientation of the texture figure conflicted, and reached ceiling in both conflict and conflict-free conditions. This psychophysical effect was associated with a decrease in amplitude of negative VEP components in the configurations where global and local orientation conflicted, and to a weak increase of VEPs earliest negative component in the conflict-free condition. The VEP result is a direct demonstration that learning, in addition to increasing response of relevant channels, also reduces the weight of channels whose receptive field size and orientation tuning conflict with the task.

Long-term effects of MDMA (Ecstasy) on the human central nervous system revealed by visual evoked potentials

Several studies indicate long‐term cognitive impairment of MDMA (ecstasy) users. In the present study we attempted to establish whether electrophysiological correlates of low‐level cognitive processes present a long‐term alteration, dependent on the level of use of ecstasy. We addressed this issue by investigating amplitude and latency of VEPs related to a very simple discrimination task involving sustained attention (arousal). Eight heavy‐MDMA users, eight moderate‐MDMA users and 18 drug‐free control subjects were asked to discriminate whether the digit at the centre of the screen was 1 or 2. None of the subjects (except one) had used MDMA in the 6 months previous testing. We measured psychophysical performance and EEG, recorded in Oz and Fz during task execution. The heavy‐MDMA users made significantly more errors than the other two groups (p < .05). Moreover, they presented reduced amplitude but not latency of VEPs in both Oz and Fz. The effect in Oz is present in P200 (for heavy users only, p < .05) and in P300 components (for both MDMA groups; heavy users: p < .001, moderate users: p < .0.5). In Fz, the amplitude effect is present in N250 (for heavy users only, p < .05) and in P300 components (for both MDMA groups; heavy users: p < .05, moderate users: p < .05). The three groups do not differ in early components, reflecting low‐level processing. These results provide evidence of long‐term electrophysiological abnormality displayed by ecstasy users and agree with the suggestion that even typical recreational doses of ecstasy are sufficient to cause long‐term altered cortical activity in humans.

Discrimination of an orientation difference in dynamic textures

We investigated whether the response of a motion sensor was related to the specificity of sensory information (orientation and direction of motion) used to compute motion energy. This was done in two ways. First, we assessed whether orientation discrimination of a target line, which segregated by an orientation difference from a textured background, was improved with two-frame apparent motion stimulation (as compared with static presentation). Second, we investigated whether the amount of improvement (in either orientation or direction of motion discrimination) depends on a particular combination of target orientation and direction of motion (either orthogonal or parallel). We found that the percentage of correct responses in the discrimination task (a) was higher for a moving target than for a static one; (b) was higher when the target was oriented more orthogonally to motion direction than background elements; (c) was little affected by background motion and (d) decreased with frame duration in the direction of motion task whereas it was largely unaffected by frame duration in the discrimination of orientation task. These results suggest that discrimination of moving texture boundaries is based on a motion sensor tuned to a particular combination of orientation and direction of motion, which is capable of signalling the orientation of a moving target more accurately than a static sensor.

Hyper-vision in a patient with central and paracentral vision loss reflects cortical reorganization

SM, a 21-year-old female, presents an extensive central scotoma (30 deg) with dense absolute scotoma (visual acuity = 10/100) in the macular area (10 deg) due to Stargardts disease. We provide behavioral evidence of cortical plastic reorganization since the patient could perform several visual tasks with her poor-vision eyes better than controls, although high spatial frequency sensitivity and visual acuity are severely impaired. Between 2.5-deg and 12-deg eccentricities, SM presented (1) normal acuity for crowded letters, provided stimulus size is above acuity thresholds for single letters; (2) a two-fold sensitivity increase (d-prime) with respect to controls in a simple search task; and (3) largely above-threshold performance in a lexical decision task carried out randomly by controls. SMs hyper-vision may reflect a long-term sensory gain specific for unimpaired low spatial-frequency mechanisms, which may result from modifications in response properties due to practice-dependent changes in excitatory/inhibitory intracortical connections.

Texture segregation on the basis of contrast polarity of odd-symmetric filters.

This is the first study to demonstrate the selectivity of learning for contrast polarity. The finding is the main result of an investigation into the existence of central and peripheral vision mechanisms selective for contrast polarity within the texture-segregation process, using the perceptual learning paradigm in a detection task. Energy models (Malik & Perona, 1990) exclude segregation of textures composed of elements of odd-symmetric luminance profile by contrast polarity differences. Here the target was a Gabor patch (0.8 deg) of 1 cyc/deg in sine phase (odd-symmetry) embedded in a background of mirror-image elements. Our results showed that, in fovea, segregation on the basis of contrast polarity was above threshold from the first session. After learning, the target popped-out in both central and peripheral vision for durations over 10 ms. Our major result is that learning is selective for contrast polarity; it is also selective for orientation and position, all characteristics distinctive of early processing. Since the learning effects were obtained with texture composed of odd-symmetric mirror-image elements, they indicate that the output from odd-symmetric filters was not excluded or inhibited in texture segmentation, but instead played an active role. Our data support models of texture segmentation, in which detection of texture gradient is achieved on the basis of early cortical process, before the non-linear transformation of their output.

Visual Search for Single and Combined Features by Children and Adults: Possible Developmental Inferences

Visual search time was measured in four groups of children 4 to 10 years of age and in young adults, as a function of display size for both target-present and target-absent conditions. The slopes of regression lines in a simple search (search for \ within /) were compared with those obtained in within-dimension conjunction target searches, within either homogeneous (search for ⊥ within ⊺s) or heterogeneous distractors (search for ↑ within ⋎s and ⋏s). Analysis indicated that simple search was always preattentive (slopes near 0) after 5 years of age and serial (with positive slopes) before this age. Searching within heterogeneous distractors apparently involved element-by-element scrutiny and focal attention in all groups whereas searching within homogeneous distractors required distributed attention, perhaps based on parallel grouping of distractors in adults, though distractors were focally attended to by children.

Lines That Induce Phenomenal Transparency

Three neighbouring opaque surfaces may appear split into two layers, one transparent and one opaque beneath, if an outline contour is drawn that encompasses two of them. The phenomenon was originally observed by Kanizsa [1955 Rivista di Psicologia 69 3–19; 1979 Organization in Vision: Essays on Gestalt Psychology (New York: Praeger)], for the case where an outline contour is drawn to encompass one of the two parts of a bicoloured figure and a portion of a background of lightest (or darkest) luminance. Preliminary observations revealed that the outline contour yields different effects: in addition to the stratification into layers described by Kanizsa, a second split, opposite in depth order, may occur when the outline contour is close in luminance to one of the three surfaces. An initial experiment was designed to investigate what conditions give rise to the two phenomenal transparencies: this led to the conclusion that an outline contour superimposed on an opaque surface causes this surface to emerge as a transparent layer when the luminances of the contour and the surface differ, in absolute value, by no more than 13.2 cd m−2. We have named this phenomenon ‘transparency of the intercepted surface’, to distinguish it from the phenomenal transparency arising when the contour and surface are very different in luminance. When such a difference exists, the contour acts as a factor of surface definition and grouping: the portion of the homogeneous surface it bounds emerges as a fourth surface and groups with a nearby surface if there is one close in luminance. The transparency phenomena (‘transparency of the contoured surface’) perceived in this context conform to the constraints of Metellis model, as demonstrated by a second experiment, designed to gather ‘opacity’ ratings of stimuli. The observer judgments conformed to the values predicted by Metellis formula for perceived degree of transparency, a. The role of the outline contour in conveying figural and intensity information is discussed.

The effect of preview eccentricity in a lexical decision task

We report lexical decision experiments in which eye movements and lexical decision time were analysed. The results show that nonwords produced more fixation than words, and that the time to make a lexical decision was also greater for nonwords. Further, a preview of the stimulus was presented on some occasions either at fixation or peripherally. When the presentation was peripheral the number of refixations and lexical decision times were reduced. Parafoveal previews of words also reduced word length effects on refixations and lexical decision time. These effects decreased (for nonwords) when the case of the letters was changed in the preview. These results are compatible with the idea that the peripheral preview benefit derives from word visual structure in addition to information about word length, word envelope, and single letter identities.