Perceptual reversals of Necker stimuli during intermittent presentation with limited attentional resources

During prolonged viewing of ambiguous stimuli, such as Necker cubes, sudden perceptual reversals occur from one perceptual interpretation to another. The role of attention in such reversals is not clear. We tested whether perceptual reversals depend on attentional resources by manipulating perceptual load and recording event-related potentials (ERPs) during intermittent presentation of Necker stimuli. The results did not reveal any influence for perceptual load on the frequency of reversals. The ERPs showed that perceptual load influenced electrophysiological activity over parieto-central areas in the P1 time window (110-140 ms), but load did not modify the early enhancements of positivity (30-140 ms), which correlated with perceptual reversals at occipito-temporal sites. We conclude that disambiguation of ambiguous figures is based on early mechanisms that can work efficiently with only a minimal amount of attentional resources.

Unconscious and Conscious Processing of Color Rely on Activity in Early Visual Cortex: A TMS Study

Chromatic information is processed by the visual system both at an unconscious level and at a level that results in conscious perception of color. It remains unclear whether both conscious and unconscious processing of chromatic information depend on activity in the early visual cortex or whether unconscious chromatic processing can also rely on other neural mechanisms. In this study, the contribution of early visual cortex activity to conscious and unconscious chromatic processing was studied using single-pulse TMS in three time windows 40–100 msec after stimulus onset in three conditions: conscious color recognition, forced-choice discrimination of consciously invisible color, and unconscious color priming. We found that conscious perception and both measures of unconscious processing of chromatic information depended on activity in early visual cortex 70–100 msec after stimulus presentation. Unconscious forced-choice discrimination was above chance only when participants reported perceiving some stimulus features (but not color).

Is selective primary visual cortex stimulation achievable with TMS?

The primary visual cortex (V1) has been the target of stimulation in a number of transcranial magnetic stimulation (TMS) studies. In this study, we estimated the actual sites of stimulation by modeling the cortical location of the TMS-induced electric field when participants reported visual phosphenes or scotomas. First, individual retinotopic areas were identified by multifocal functional magnetic resonance imaging (mffMRI). Second, during the TMS stimulation, the cortical stimulation sites were derived from electric field modeling. When an external anatomical landmark for V1 was used (2 cm above inion), the cortical stimulation landed in various functional areas in different individuals, the dorsal V2 being the most affected area at the group level. When V1 was specifically targeted based on the individual mffMRI data, V1 could be selectively stimulated in half of the participants. In the rest, the selective stimulation of V1 was obstructed by the intermediate position of the dorsal V2. We conclude that the selective stimulation of V1 is possible only if V1 happens to be favorably located in the individual anatomy. Selective and successful targeting of TMS pulses to V1 requires MRI-navigated stimulation, selection of participants and coil positions based on detailed retinotopic maps of individual functional anatomy, and computational modeling of the TMS-induced electric field distribution in the visual cortex. It remains to be resolved whether even more selective stimulation of V1 could be achieved by adjusting the coil orientation according to sulcal orientation of the target site.

Relationship between visual binding, reentry and awareness

Visual feature binding has been suggested to depend on reentrant processing. We addressed the relationship between binding, reentry, and visual awareness by asking the participants to discriminate the color and orientation of a colored bar (presented either alone or simultaneously with a white distractor bar) and to report their phenomenal awareness of the target features. The success of reentry was manipulated with object substitution masking and backward masking. The results showed that late reentrant processes are necessary for successful binding but not for phenomenal awareness of the bound features. Binding errors were accompanied by phenomenal awareness of the misbound feature conjunctions, demonstrating that they were experienced as real properties of the stimuli (i.e., illusory conjunctions). Our results suggest that early preattentive binding and local recurrent processing enable features to reach phenomenal awareness, while later attention-related reentrant iterations modulate the way in which the features are bound and experienced in awareness.

Reversal negativity and bistable stimuli: Attention, awareness, or something else?

Ambiguous (or bistable) figures are visual stimuli that have two mutually exclusive perceptual interpretations that spontaneously alternate with each other. Perceptual reversals, as compared with non-reversals, typically elicit a negative difference called reversal negativity (RN), peaking around 250 ms from stimulus onset. The cognitive interpretation of RN remains unclear: it may reflect either bottom-up processes, attentional processes that select between the alternative views of the stimulus, or it may reflect the change in the contents of subjective awareness. In the present study, event-related potentials in response to endogenous unilateral and bilateral reversals of two Necker lattices were compared with exogenously induced reversals of unambiguous lattices. The RN neither resembled the attention-related N2pc response, nor did it correlate with the content of subjective visual awareness. Thus, we conclude that RN is a non-attentional ERP correlate of the changes in the perceptual configuration of the presented object.

The role of early visual cortex (V1/V2) in conscious and unconscious visual perception

A "late" period of activity in striate cortex (V1) in response to extrastriate feedback has been proposed to act as a marker of visual awareness. It is not clear, however, whether such recurrent activity is associated exclusively with aware perception or whether it is necessary also for unaware visual processing. We investigated the role of the "late" V1 activity in both aware and unaware visual motion perception. Participants were asked to make a forced-choice direction discrimination judgment on a coherently moving random-dot display and additionally rate their subjective awareness of the stimulus. Transcranial magnetic stimulation (TMS) was applied over the early visual cortex (V1/V2) either 20, 40, 60, 80, or 100 ms after motion offset. Visual awareness was impaired at an "early" (20 ms) and a "late" (60 ms) stimulation time window. Participants' forced-choice direction discrimination performance on "unaware" trials was above chance in No TMS baseline condition. Importantly, this performance was impaired by TMS over V1/V2 at the "late" time window. In a second experiment we show that the critical time window of V5/MT falls between the "early" and "late" time windows of V1/V2 activity. The results indicate that recurrent extrastriate-V1 activity is necessary for both aware and unaware perception.

The effect of age on attentional modulation in Dichotic listening

The right-ear advantage (REA) in Dichotic listening (DL) reflects stimulus-driven bottom-up asymmetry in speech processing. The REA can be modified by top-down attentional control. We investigated attentional control in DL task as a function of age. A total of 186 participants between the ages of 5 and 79 years were tested. The youngest children demonstrated a REA that was not modified by attention, suggesting that bottom-up functional asymmetry was present. The 10-11-year-olds began to show ability to voluntarily modify DL, but only young adults were fully capable of doing so. In 59-79-year-olds, this top-down attentional control was lost again.

Mobile phone effects on children's event-related oscillatory EEG during an auditory memory task

PURPOSE

To assess the effects of electromagnetic fields (EMF) emitted by mobile phones (MP) on the 1 - 20 Hz event-related brain oscillatory EEG (electroencephalogram) responses in children performing an auditory memory task (encoding and recognition).

MATERIALS AND METHODS

EEG data were gathered while 15 subjects (age 10 - 14 years) performed an auditory memory task both with and without exposure to a digital 902 MHz MP in counterbalanced order.

RESULTS

During memory encoding, the active MP modulated the event-related desynchronization/synchronization (ERD/ERS) responses in the approximately 4 - 8 Hz EEG frequencies. During recognition, the active MP transformed these brain oscillatory responses in the approximately 4 - 8 Hz and approximately 15 Hz frequencies.

CONCLUSIONS

The current findings suggest that EMF emitted by mobile phones has effects on brain oscillatory responses during cognitive processing in children.

Perception of the electromagnetic field emitted by a mobile phone

Electromagnetic sensibility refers to the ability to perceive the electromagnetic field (EMF) without necessarily developing health symptoms attributed to EMF exposure. A large sample of young healthy adults (n = 84) performed two forced-choice tasks on the perception of the GSM mobile phone EMF (902 MHz pulsed at 217 Hz), "Was the field on?" and "Did the field change?" (3 conditions x 100 trials for each task, n = 600 trials in total). A monetary prize was announced for good performance (correct response rate > or =75%, n = 600 trials). The performance was no better than expected by chance, and thus none of the participants won the prize. Two participants showed extraordinary performance in one of the task conditions ("Was the field on?", n = 100 trials), with correct response rates of 97% (P = 1.28 x 10(-25)) and 94% (P = 9.40 x 10(-22)), but they failed to replicate the result in the retest of six blocks of the same condition 1 month later. Six participants had reported being able to perceive the mobile phone EMF in the preliminary inquiry, but they performed no better than the others. This study provides empirical evidence against the existence of electromagnetic sensibility to the mobile phone EMF, demonstrating the necessity for replication in EMF studies.

The earliest electrophysiological correlate of visual awareness?

To examine the neural correlates and timing of human visual awareness, we recorded event-related potentials (ERPs) in two experiments while the observers were detecting a grey dot that was presented near subjective threshold. ERPs were averaged for conscious detections of the stimulus (hits) and nondetections (misses) separately. Our results revealed that hits, as compared to misses, showed a negativity around 180-350 ms at occipital and posterior temporal sites. It was followed by a positive wave after 400-500 ms, peaking at parietal sites. These correlates were not affected by a manipulation of attention. The early negativity, called 'visual awareness negativity' (VAN), may be a general, primary electrophysiological correlate of visual awareness. The present data show that it can be observed in response to appearance of a stimulus in visual awareness and that it generalizes across different manipulations of stimulus visibility.

How meaning shapes seeing

Inattentional blindness refers to the failure to see an unexpected object that one may be looking at directly when one's attention is elsewhere. We studied whether a stimulus whose meaning is relevant to the attentional goals of the observer will capture attention and escape inattentional blindness. The results showed that an unexpected stimulus belonging to the attended semantic category but not sharing physical features with the attended stimuli was detected more often than a semantically unrelated stimulus. This effect was larger when the unexpected stimuli were words than when they were pictures. The results imply that the semantic relation between the observer's attentional set and the unexpected stimulus plays a crucial role in inattentional blindness: An unexpected stimulus semantically related to the observer's current interests is likely to be seen, whereas unrelated unexpected stimuli are unseen. Attentional selection may thus be driven by purely semantic features: Meaning may determine whether or not one sees a stimulus.

Comparison of event-related potentials in attentional blink and repetition blindness

Attending to the first target in rapid serial visual presentation (RSVP) interferes with processing of the second target so that the participants fail to recognize the second target if the targets are separated by a stimulus onset asynchrony of 200-500 ms. This phenomenon is attentional blink (AB). Repetition blindness (RB) is an additional difficulty to recognize the second occurrence of the same stimulus in RSVP. A controversial issue in studies of both deficits is the processing level at which they occur. To compare the timing and mechanisms of AB and RB directly during the same RSVP stream, we recorded event-related potentials (ERPs) in response to repeated and unrepeated targets. Comparable to earlier ERP studies on visual awareness, the results showed for both types of targets a negative amplitude difference between ERPs to consciously recognized and unrecognized targets during 250-350 ms from stimulus onset, suggesting that both AB and RB are associated with deficits of conscious perception, occurring at earlier stages than access to working memory. However, the perceptual deficit in RB is more severe, which may be related to higher overall negativity in response to repeated targets observed 150-300 ms after stimulus onset, suggesting stronger cortical baseline activation and higher perceptual threshold for repeated targets as compared with unrepeated ones.