Giovanni Caputo

Attentional selection by distractor suppression.

Selective attention was studied in displays containing singletons popping out for their odd form or color. The target was defined as the form-singleton, the distractor as the color-singleton. The task was to discriminate the length of a longer line inside the target. Target-distractor similarity was controlled using a threshold measurement as dependent variable in experiments in which distractor presence vs absence, bottom-up vs top-down selection (through knowledge of target features), and target-distractor distance were manipulated. The results in the bottom-up condition showed that length threshold was elevated when a distractor was present and that this elevation progressively increased as the number of distractors was increased from one to two. This set-size effect was not accounted by the hypothesis that selective attention intervenes only at the stage of decision before response. Selective attention produced a suppressive surround in which discriminability of neighboring objects was strongly reduced, and a larger surround in which discriminability was reduced by an approximately constant amount. Different results were found in the top-down condition in which target discriminability was unaffected by distractor presence and no effect of target-distractor distance was found. On the other hand, response times in both bottom-up and top-down conditions were slower the shorter the target-distractor distance was. On the basis of the experimental results, selective attention is a parallel process of spatial filtering at an intermediate processing level operating after objects have been segmented. This filtering stage explores high level interactions between objects taking control on combinatorial explosion by operating over only a limited spatial extent: it picks out a selected object and inhibits the neighboring objects; then, non-selected objects are suppressed across the overall image. When no feature-based selection is available in the current behavior, this filtering influences perception in decreasing discriminability of non-selected objects. When feature-based selection is available, spatial interactions are set before stimulus arrival, hence only the unmatching objects have their discriminability diminished.

Texture brightness filling-in

The masking procedure by Paradiso and Nakayama (1991) (Vision Research, 31, 1221-1236) was used to investigate brightness filling-in within textures made of line elements: a texture stimulus was masked by a second stimulus containing a square contour. When a uniform texture was presented, the texture region inside the masking square appeared darkened and a small number of texture elements were perceived with a degenerated shape, appearing as dim dots or shorter line elements; it is as if the line element expanded from a bright point to fill the entire region defined by its contour. If the texture stimulus was a texture patch segregating from the surrounding texture by an orientation gradient and this patch was inside the square mask, darkening was not as strong as in the previous condition, and masked line elements preserved their elongated shape. Brightness spreading was measured in two experiments using dichoptic presentations. Experiment 1 used an adjustment task and showed that the brightness of texture line elements spread from equiluminant borders between segregating textures. Experiment 2 used a matching task and demonstrated that spreading was blocked by segregation borders dependent on the orientation gradient between texture line elements. The selectivity for line orientation began 40-80 msec after texture onset and maximal spreading occurred at approximately 120 msec. These findings may indicate that two processes subserve filling-in within textures: the first spreads isotropically the mean stimulus luminance at an initial processing stage of image analysis; at a later stage, the second spreads a texture flow (both brightness and shape of line elements) directed along the orientation of texture line elements. The texture flow mechanism fills in with a texture surface the region bounded by segregation contours.

The glare effect and the perception of luminosity

The impression of self-luminosity in the glare effect was studied in two experiments. In experiment 1 the target (CS) was set to the highest luminance of the field and subjects were asked to adjust the luminance ramp of the inducers (R) against five backgrounds (B) to the point where they began to see CS as self-luminous. It was found that there is a linear relationship between background and luminance ramp. Another group of subjects carried out the same task in experiment 2, but this time CS and R were linked together so that CS would always have the same luminance as the highest luminance level of R, as adjustments were performed. It was found that: (i) adjustments were always lower than the highest luminance available; (ii) the linear relationship between background and luminance ramp was confirmed; (iii) observers reported a compelling impression of self-luminous grays. Data are discussed in relation to Bonato and Gilchrists model for the perception of luminosity. The authors advance the hypothesis that luminance ramps are used at an early stage of encoding for the perception of luminosity.

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.

Glowing greys and surface-white: the photo-geometric factors of luminosity perception.

The perception of luminosity is thought to depend upon the intensity of the stimulus: a surface begins to appear self-luminous when it emits or reflects a certain amount of light. This is known as the luminosity threshold. It is a common opinion among vision scientists that such a threshold is correlated to the intensity of a perceptually white surface, in the sense that only an area of the visual field with luminance higher than perceived surface-white will appear luminous. Here we show grey colours that appear luminous in virtue of surrounding luminance ramps. These ramps are intended to mimic halos seen around light sources in natural environments. The results of three experiments indicate that the phenomenon is in direct contradiction to the aforementioned assumptions and suggest the existence of separate perceptual pathways for self-luminosity perception and for surface-colour perception.

Strange-face-in-the-mirror illusion.

I describe a visual illusion which occurs when an observer sees his/her image reflected in a mirror in a dimly lit room. This illusion can be easily experienced and replicated as the details of the setting (in particular the room illumination) are not critical. These observations were made in a quiet room dimly lit by a 25 W incandescent light. The lamp was placed on the floor behind the observer so that it was not visible either directly or in the mirror. A relatively large mirror (0.5 mT0.5 m) was placed about 0.4 m in front of the observer. Luminance of the reflected face image within the mirror was about 0.2 cd m(-2) and this level allowed detailed perception of fine face traits but attenuated colour perception. The illusion occurred even at higher levels of illumination of observers face (from 0.2 to 1.6 cd m(-2)). The task of the observer was to gaze at his/her reflected face within the mirror. Usually, after less than a minute, the observer began to perceive the strange-face illusion. Phenomenological descriptions were made by fifty naive individuals (age range 21 ^ 29 years; mean 23 years; SD 2.1 years). At the end of a 10 min session of mirror gazing, the participant was asked to write what he or she saw in the mirror. The descriptions differed greatly across individuals and included: (a) huge deformations of ones own face (reported by 66% of the fifty participants); (b) a parents face with traits changed (18%), of whom 8% were still alive and 10% were deceased; (c) an unknown person (28%); (d) an archetypal face, such as that of an old woman, a child, or a portrait of an ancestor (28%); (e) an animal face such as that of a cat, pig, or lion (18%); (f) fantastical and monstrous beings (48%). Language: en

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.

Strange-Face-in-the-Mirror Illusion and Schizotypy During Adolescence

Patients with schizophrenia can sometimes report strange face illusions when staring at themselves in the mirror; such experiences have been conceptualized as anomalous self-experiences that can be experienced with a varying degree of depersonalization. During adolescence, anomalous self-experiences can also be indicative of increased risk to develop schizophrenia-spectrum disorders. To date however, the Mirror-Gazing test (MGT), an experimentally validated experiment to evaluate the propensity of strange face illusions in nonclinical and clinical adults, has yet to be investigated in an adolescent sample. The first goal of the present study was to examine experimentally induced self-face illusions in a nonclinical sample of adolescents, using the MGT. The second goal was to investigate whether dimensions of adolescent trait schizotypy were differentially related to phenomena arising during the MGT. One hundred and ten community adolescents (59 male) aged from 12 to 19 years (mean age = 16.31, SD age = 1.77) completed the MGT and Schizotypal Personality Questionnaire. The results yielded 4 types of strange face illusions; 2 types of illusions (slight change of light/color [20%] and own face deformation [45.5%]) lacked depersonalization-like phenomena (no identity change), while 2 other types (vision of other identity [27.3%], and vision of non-human identity [7.3%]) contained clear depersonalization-like phenomena. Furthermore, the disorganization dimension of schizotypy associated negatively with time of first illusion (first press), and positively with frequency of illusions during the MGT. Statistically significant differences on positive and disorganized schizotypy were found when comparing groups on the basis of degree of depersonalization-like phenomena (from slight color changes to non-human visions). Similarly to experimentally induced self-face illusions in patients with schizophrenia, such illusions in a group of nonclinical adolescents present significant associations to schizotypy dimensions.

Archetypal-Imaging and Mirror-Gazing

Mirrors have been studied by cognitive psychology in order to understand self-recognition, self-identity, and self-consciousness. Moreover, the relevance of mirrors in spirituality, magic and arts may also suggest that mirrors can be symbols of unconscious contents. Carl G. Jung investigated mirrors in relation to the unconscious, particularly in Psychology and Alchemy. However, the relationship between the conscious behavior in front of a mirror and the unconscious meaning of mirrors has not been clarified. Recently, empirical research found that gazing at one’s own face in the mirror for a few minutes, at a low illumination level, produces the perception of bodily dysmorphic illusions of strange-faces. Healthy observers usually describe huge distortions of their own faces, monstrous beings, prototypical faces, faces of relatives and deceased, and faces of animals. In the psychiatric population, some schizophrenics show a dramatic increase of strange-face illusions. They can also describe the perception of multiple-others that fill the mirror surface surrounding their strange-face. Schizophrenics are usually convinced that strange-face illusions are truly real and identify themselves with strange-face illusions, diversely from healthy individuals who never identify with them. On the contrary, most patients with major depression do not perceive strange-face illusions, or they perceive very faint changes of their immobile faces in the mirror, like death statues. Strange-face illusions may be the psychodynamic projection of the subject’s unconscious archetypal contents into the mirror image. Therefore, strange-face illusions might provide both an ecological setting and an experimental technique for “imaging of the unconscious”. Future researches have been proposed.

Visual Perception during Mirror-Gazing at One's Own Face in Patients with Depression

In normal observers, gazing at ones own face in the mirror for a few minutes, at a low illumination level, produces the apparition of strange faces. Observers see distortions of their own faces, but they often see hallucinations like monsters, archetypical faces, faces of relatives and deceased, and animals. In this research, patients with depression were compared to healthy controls with respect to strange-face apparitions. The experiment was a 7-minute mirror-gazing test (MGT) under low illumination. When the MGT ended, the experimenter assessed patients and controls with a specifically designed questionnaire and interviewed them, asking them to describe strange-face apparitions. Apparitions of strange faces in the mirror were very reduced in depression patients compared to healthy controls. Depression patients compared to healthy controls showed shorter duration of apparitions; minor number of strange faces; lower self-evaluation rating of apparition strength; lower self-evaluation rating of provoked emotion. These decreases in depression may be produced by deficits of facial expression and facial recognition of emotions, which are involved in the relationship between the patient (or the patients ego) and his face image (or the patients bodily self) that is reflected in the mirror.