Giulia Prete

Perceptual asymmetries and handedness: a neglected link?

Healthy individuals tend to weigh in more the left than the right side of visual space in a variety of contexts, ranging from pseudoneglect to perceptual asymmetries for faces. Among the common explanations proposed for the attentional and perceptual advantages of the left visual field, a link with the prevalence of right-handedness in humans has never been suggested, although some evidence seems to converge in favor of a bias of spatial attention toward the region most likely coincident with another person’s right hand during a face-to-face interaction. Such a bias might imply an increased efficiency in monitoring both communicative and aggressive acts, the right limb being more used than the left in both types of behavior. Although attentional and perceptual asymmetries could be linked to right-handedness at the level of phylogeny because of the evolutionarily advantage of directing attention toward the region where others’ dominant hand usually operates, it is also legitimate to question whether, at the ontogenetic level, frequent exposure to right-handed individuals may foster leftward biases. These views are discussed in the light of extant literature, and a number of tests are proposed in order to assess our hypotheses.

The cerebral correlates of subliminal emotions: an eleoencephalographic study with emotional hybrid faces.

In a high‐resolution electroencephalographic study, participants evaluated the friendliness level of upright and inverted ‘hybrid faces’, i.e. facial photos containing a subliminal emotional core in the low spatial frequencies (< 6 cycles/image), superimposed on a neutral expression in the rest of the spatial frequencies. Upright happy and angry faces were judged as more friendly or less friendly than neutral faces, respectively. We observed the time course of cerebral correlates of these stimuli with event‐related potentials (ERPs), confirming that hybrid faces elicited the posterior emotion‐related and face‐related components (P1, N170 and P2), previously shown to be engaged by non‐subliminal emotional stimuli. In addition, these components were stronger in the right hemisphere and were both enhanced and delayed by face inversion. A frontal positivity (210–300 ms) was stronger for emotional than for neutral faces, and for upright than for inverted faces. Hence, hybrid faces represent an original approach in the study of subliminal emotions, which appears promising for investigating their electrophysiological correlates.

Lateralized hybrid faces: Evidence of a valence-specific bias in the processing of implicit emotions

It is well known that hemispheric asymmetries exist for both the analyses of low-level visual information (such as spatial frequency) and high-level visual information (such as emotional expressions). In this study, we assessed which of the above factors underlies perceptual laterality effects with “hybrid faces”: a type of stimulus that allows testing for unaware processing of emotional expressions, when the emotion is displayed in the low-frequency information while an image of the same face with a neutral expression is superimposed to it. Despite hybrid faces being perceived as neutral, the emotional information modulates observers’ social judgements. In the present study, participants were asked to assess friendliness of hybrid faces displayed tachistoscopically, either centrally or laterally to fixation. We found a clear influence of the hidden emotions also with lateral presentations. Happy faces were rated as more friendly and angry faces as less friendly with respect to neutral faces. In general, hybrid faces were evaluated as less friendly when they were presented in the left visual field/right hemisphere than in the right visual field/left hemisphere. The results extend the validity of the valence hypothesis in the specific domain of unaware (subcortical) emotion processing.

The processing of chimeric and dichotic emotional stimuli by connected and disconnected cerebral hemispheres

Hemispheric asymmetries have been widely explored in both the visual and the auditory domain, but little is known about hemispheric asymmetries in audio-visual integration. We compared the performance of a partially callosotomized patient, a total split-brain patient and a control group during the evaluation of the emotional valence of chimeric faces and dichotic syllables (an emotional syllable in one ear and white noise in the other ear) presented unimodally (only faces or only syllables) or bimodally (faces and syllables presented simultaneously). Stimuli could convey happy and sad expressions and participants were asked to evaluate the emotional content of each presentation, using a 5-point Likert scale (from very sad to very happy). In unimodal presentations, the partially callosotomized patients judgments depended on the emotional valence of the stimuli processed by the right hemisphere, whereas those of the total split-brain patient showed the opposite lateralization; in these conditions, the control group did not show asymmetries. Moreover, in bimodal presentations, results provided support for the valence hypothesis (i.e., left asymmetry for positive emotions and vice versa) in both the control group and the partially callosotomized patient, whereas the total split-brain patient showed a tendency to evaluate the emotional content of the right hemiface even when asked to focus on the acoustic modality. We conclude that partial and total hemispheric disconnections reveal opposite patterns of hemispheric asymmetry in auditory, visual and audio-visual emotion processing. These results are discussed in the light of the right-hemisphere hypothesis and the valence hypothesis.

Hearing it right: Evidence of hemispheric lateralization in auditory imagery

An advantage of the right ear (REA) in auditory processing (especially for verbal content) has been firmly established in decades of behavioral, electrophysiological and neuroimaging research. The laterality of auditory imagery, however, has received little attention, despite its potential relevance for the understanding of auditory hallucinations and related phenomena. In Experiments 1-4 we find that right-handed participants required to imagine hearing a voice or a sound unilaterally show a strong population bias to localize the self-generated auditory image at their right ear, likely the result of left-hemispheric dominance in auditory processing. In Experiments 5-8 - by means of the same paradigm - it was also ascertained that the right-ear bias for hearing imagined voices depends just on auditory attention mechanisms, as biases due to other factors (i.e., lateralized movements) were controlled. These results, suggesting a central role of the left hemisphere in auditory imagery, demonstrate that brain asymmetries can drive strong lateral biases in mental imagery.

The “consonance effect” and the hemispheres: A study on a split-brain patient

The association between musical consonance and pleasantness, and between musical dissonance and unpleasantness (“consonance effect”) is well established. Furthermore, a number of studies suggest the main involvement of the left hemisphere in the perception of dissonance and that of the right hemisphere in the perception of consonance. In the present study, the consonance effect was studied in a callosotomized patient, D. D. C. and in a control group. In binaural presentations, the patient did not attribute different pleasantness judgements to consonant and dissonant chords, differently from the control group who showed the consonance effect. However, in dichotic presentations (e.g. a chord in one ear and white noise in the other ear), a trend towards the consonance effect was found in D. D. C., but only when chords were presented in his right ear (left hemisphere), whereas the control group confirmed the known hemispheric asymmetry in labelling the pleasantness of consonant and dissonant chords. These results suggest that the right-hemispheric superiority in appreciating consonance might hide the inability of the right hemisphere to classify dissonant chords as unpleasant in the split-brain, whereas the left hemisphere seems capable to differently label the pleasantness of consonant and dissonant chords, even if it is more sensitive to dissonance.

Asymmetric cortical adaptation effects during alternating auditory stimulation.

The present study investigates hemispheric asymmetries in the neural adaptation processes occurring during alternating auditory stimulation. Stimuli were two monaural pure tones having a frequency of 400 or 800 Hz and a duration of 500 ms. Electroencephalogram (EEG) was recorded from 14 volunteers during the presentation of the following stimulus sequences, lasting 12 s each: 1) evoked potentials (EP condition, control), 2) alternation of frequency and ear (FE condition), 3) alternation of frequency (F condition), and 4) alternation of ear (E condition). Main results showed that in the central area of the left hemisphere (around C3 site) the N100 response underwent adaptation in all patterns of alternation, whereas in the same area of the right hemisphere the tones presented at the right ear in the FE produced no adaptation. Moreover, the responses to right-ear stimuli showed a difference between hemispheres in the E condition, which produced less adaptation in the left hemisphere. These effects are discussed in terms of lateral symmetry as a product of hemispheric, pathway and ear asymmetries.

Asymmetry for Symmetry: Right-Hemispheric Superiority in Bi-Dimensional Symmetry Perception

A right-hemispheric superiority has been shown for spatial symmetry perception with mono-dimensional stimuli (e.g., bisected lines). Nevertheless, the cerebral imbalance for bi-dimensional stimuli is still controversial, and the aim of the present study is to investigate this issue. Healthy participants and a split-brain patient (D.D.C.) were tested in a divided visual field paradigm, in which a square shape was presented either in the left or right visual field and they were asked to judge whether a dot was placed exactly in the center of the square or off-center, by using the left/right hand in two separate sessions. The performance of healthy participants was better when the stimuli presented in the left visual field (LVF) were on-center rather than off-center. The performance of D.D.C. was higher than chance only when on-center stimuli were presented in the LVF in the left hand session. Only in this condition did his accuracy not differ with respect to that of the control group, whereas in all of the other conditions, it was lower than the controls’ accuracy. We conclude that the right-hemispheric advantage already shown for mono-dimensional stimuli can be extended also to bi-dimensional configurations, confirming the right-hemispheric superiority for spatial symmetry perception.

Modulating adaptation to emotional faces by spatial frequency filtering

In four experiments, we investigated the presence and strength of perceptual aftereffects to emotional faces, using spatial frequency filtering to manipulate awareness of emotional content. We presented angry and happy faces as adapters and used a control condition without adaptation. Participants were subsequently requested to judge the friendliness level of a neutral target face. We confirmed the well-known aftereffect for unfiltered emotional faces in Experiment 1. In the experiment, friendliness judgments were greater for the angry than the happy or the control condition. In Experiment 2, in which the “hidden” emotional expression contained in the low spatial frequencies (LSF) was superimposed to the neutral expression presented in the rest of SF of the same image (emotional hybrid faces), the difference in friendliness judgments between angry and happy was significant, but none of the two conditions significantly differed from the control. In Experiment 3, faces were presented at LSF, confirmed a difference between the two emotions, but only the judgments of angry faces differed from the control condition. In Experiment 4, we extended the initial finding to stimuli presented at middle and high spatial frequencies (M-HSF). Finally, a comparison among all experiments revealed that the aftereffect was stronger with angry faces filtered at M-HSF than all of the other filtering conditions, whereas there were no differences for happy faces. We conclude that spatial frequency filtering influences aftereffects and that these effects are also related to emotional awareness. The results are discussed with reference to the dual route model of visual perception.

Modulation of Illusory Auditory Perception by Transcranial Electrical Stimulation

The aim of the present study was to test whether transcranial electrical stimulation can modulate illusory perception in the auditory domain. In two separate experiments we applied transcranial Direct Current Stimulation (anodal/cathodal tDCS, 2 mA; N = 60) and high-frequency transcranial Random Noise Stimulation (hf-tRNS, 1.5 mA, offset 0; N = 45) on the temporal cortex during the presentation of the stimuli eliciting the Deutschs illusion. The illusion arises when two sine tones spaced one octave apart (400 and 800 Hz) are presented dichotically in alternation, one in the left and the other in the right ear, so that when the right ear receives the high tone, the left ear receives the low tone, and vice versa. The majority of the population perceives one high-pitched tone in one ear alternating with one low-pitched tone in the other ear. The results revealed that neither anodal nor cathodal tDCS applied over the left/right temporal cortex modulated the perception of the illusion, whereas hf-tRNS applied bilaterally on the temporal cortex reduced the number of times the sequence of sounds is perceived as the Deutschs illusion with respect to the sham control condition. The stimulation time before the beginning of the task (5 or 15 min) did not influence the perceptual outcome. In accordance with previous findings, we conclude that hf-tRNS can modulate auditory perception more efficiently than tDCS.