The Temporal Dynamic Relationship Between Attention and Crowding: Electrophysiological Evidence From an Event-Related Potential Study

Dissociable Effects of Endogenous and Exogenous Attention on Crowding: Evidence from Event-Related Potentials

BACKGROUND/OBJECTIVES

Crowding is a common visual phenomenon that can significantly impair the recognition of objects in peripheral vision. Two recent behavioral studies have revealed that both exogenous and endogenous attention can alleviate crowding, but exogenous attention seems to be more effective.

METHODS

The present study employed the event-related potential (ERP) technique to explore the electrophysiological characteristics of the influence of these two types of attention on crowding. In the experiment, participants were required to judge whether the letter "T" was upright or inverted, which may be preceded by an exogenous cue or an endogenous cue indicating the location of the target letter.

RESULTS

The behavioral results showed that while exogenous cues reduced crowding in all stimulus onset asynchronies (SOAs), endogenous attention took effects only in long SOA. The ERP results indicated that both endogenous and exogenous cues significantly alleviated the inhibition of visual crowding on the N1 component. However, the endogenous cue was effective only under long SOA, while the exogenous cue was effective only under short SOA conditions. In addition, invalid exogenous cues induced a larger P3 wave amplitude than valid ones in the short SOA condition, but endogenous attention did not show such a difference.

CONCLUSIONS

These results indicate that both endogenous and exogenous attention can alleviate the effects of visual crowding, but they differ in effect size and temporal dynamics.

The role of parietal beta-band activity in the resolution of visual crowding

Visual crowding is the difficulty in identifying an object when surrounded by neighbouring flankers, representing a bottleneck for object perception. Crowding arises not only from the activity of visual areas but also from parietal areas and fronto-parietal network activity. Parietal areas would provide the dorsal-to-ventral guidance for object identification and the fronto-parietal network would modulate the attentional resolution. Several studies highlighted the relevance of beta oscillations (15-25 Hz) in these areas for visual crowding and other connatural visual phenomena. In the present study, we investigated the differential contribution of beta oscillations in the parietal cortex and fronto-parietal network in the resolution of visual crowding. During a crowding task with letter stimuli, high-definition transcranial Alternating Current Stimulation (tACS) in the beta band (18 Hz) was delivered bilaterally on parietal sites, on the right fronto-parietal network, and in a sham regime. Resting-state EEG was recorded before and after stimulation to measure tACS-induced aftereffects. The influence of crowding was reduced only when tACS was delivered bilaterally on parietal sites. In this condition, beta power was reduced after the stimulation. Furthermore, the magnitude of tACS-induced aftereffects varied as a function of individual differences in beta oscillations. Results corroborate the link between parietal beta oscillations and visual crowding, providing fundamental insights on brain rhythms underlying the dorsal-to-ventral guidance in visual perception and suggesting that beta tACS can induce plastic changes in these areas. Remarkably, these findings open new possibilities for neuromodulatory interventions for disorders characterised by abnormal crowding, such as dyslexia.

Probing the Bottleneck of Awareness Formed by Foveal Crowding: A Neurophysiological Study

Crowding occurs when an easily identified isolated stimulus is surrounded by stimuli with similar properties, making it very difficult to identify. Crowding is suggested as a mechanism that creates a bottleneck in object recognition and awareness. Recently, we showed that brief presentation times at the fovea resulted in a significant crowding effect on target identification, impaired the target's color awareness, and resulted in a slower reaction time. However, when tagging the target with a red letter, the crowding effect is abolished. Crowding is widely considered a grouping; hence, it is pre-attentive. An event-related potential (ERP) study that investigated the spatial-temporal properties of crowding suggested the involvement of higher-level visual processing. Here, we investigated whether ERP's components may be affected by crowding and tagging, and whether the temporal advantage of ERP can be utilized to gain further information about the crowding mechanism. The participants reported target identification using our standard foveal crowing paradigm. It is assumed that crowding occurs due to a suppressive effect; thus, it can be probed by changes in perceptual (N1, ~160 ms) and attentive (P3 ~300-400 ms) components. We found a suppression effect (less negative ERP magnitude) in N1 under foveal crowding, which was recovered under tagging conditions. ERP's amplitude components (N1 and P3) and the behavioral proportion correct are highly correlated. These findings suggest that crowding is an early grouping mechanism that may be combined with later processing involving the segmentation mechanism.

Differential modulation on neural activity related to flankers during face processing: A visual crowding study

In this visual crowding study, we manipulated the perceivability of a central crowded face (a fearful or a neutral face) by varying the similarity between the central face and the surrounding flanker stimuli. We presented participants with pairs of visual clutters and recorded their electroencephalography during an emotion judgement task. In an upright flanker condition where both the central target face and flanker faces were upright faces, participants were less likely to report seeing the target face, and their P300 was weakened, compared to a scrambled flanker condition where scrambled face images were used as flankers. Additionally, at ∼ 120 ms post-stimulus, a posterior negativity was found for the upright compared to scrambled flanker condition, however only for fearful face targets. We concluded that early neural responses seem to be affected by the perceptual characteristics of both target and flanker stimuli whereas later-stage neural activity is associated with post-perceptual evaluation of the stimuli in this visual crowding paradigm.

Cortical mechanisms of talker normalization in fluent sentences

Adjusting to the vocal characteristics of a new talker is important for speech recognition. Previous research has indicated that adjusting to talker differences is an active cognitive process that depends on attention and working memory (WM). These studies have not examined how talker variability affects perception and neural responses in fluent speech. Here we use source analysis from high-density EEG to show that perceiving fluent speech in which the talker changes recruits early involvement of parietal and temporal cortical areas, suggesting functional involvement of WM and attention in talker normalization. We extend these findings to acoustic source change in general by examining understanding environmental sounds in spoken sentence context. Though there may be differences in cortical recruitment to processing demands for non-speech sounds versus a changing talker, the underlying mechanisms are similar, supporting the view that shared cognitive-general mechanisms assist both talker normalization and speech-to-nonspeech transitions.

Parietal tACS at beta frequency improves vision in a crowding regime

Visual crowding is the inability to discriminate objects when presented with nearby flankers and sets a fundamental limit for conscious perception. Beta oscillations in the parietal cortex were found to be associated to crowding, with higher beta amplitude related to better crowding resilience. An open question is whether beta activity directly and selectively modulates crowding. We employed Transcranial Alternating Current Stimulation (tACS) in the beta band (18-Hz), in the alpha band (10-Hz) or in a sham regime, asking whether 18-Hz tACS would selectively improve the perception of crowded stimuli by increasing parietal beta activity. Resting-state electroencephalography (EEG) was measured before and after stimulation to test the influence of tACS on endogenous oscillations. Consistently with our predictions, we found that 18-Hz tACS, as compared to 10-Hz tACS and sham stimulation, reduced crowding. This improvement was found specifically in the contralateral visual hemifield and was accompanied by an increased amplitude of EEG beta oscillations, confirming an effect on endogenous brain rhythms. These results support a causal relationship between parietal beta oscillations and visual crowding and provide new insights into the precise oscillatory mechanisms involved in human vision.