Modulation Of Stimulus Processing Research Articles

Attention Sharpens the Distinction between Expected and Unexpected Percepts in the Visual Brain

Attention, the prioritization of goal-relevant stimuli, and expectation, the modulation of stimulus processing by probabilistic context, represent the two main endogenous determinants of visual cognition. Neural selectivity in visual cortex is enhanced for both attended and expected stimuli, but the functional relationship between these mechanisms is poorly understood. Here, we adjudicated between two current hypotheses of how attention relates to predictive processing, namely, that attention either enhances or filters out perceptual prediction errors (PEs), the PE-promotion model versus the PE-suppression model. We acquired fMRI data from category-selective visual regions while human subjects viewed expected and unexpected stimuli that were either attended or unattended. Then, we trained multivariate neural pattern classifiers to discriminate expected from unexpected stimuli, depending on whether these stimuli had been attended or unattended. If attention promotes PEs, then this should increase the disparity of neural patterns associated with expected and unexpected stimuli, thus enhancing the classifier's ability to distinguish between the two. In contrast, if attention suppresses PEs, then this should reduce the disparity between neural signals for expected and unexpected percepts, thus impairing classifier performance. We demonstrate that attention greatly enhances a neural pattern classifier's ability to discriminate between expected and unexpected stimuli in a region- and stimulus category-specific fashion. These findings are incompatible with the PE-suppression model, but they strongly support the PE-promotion model, whereby attention increases the precision of prediction errors. Our results clarify the relationship between attention and expectation, casting attention as a mechanism for accelerating online error correction in predicting task-relevant visual inputs.

Serotonergic innervation of the amygdala: targets, receptors, and implications for stress and anxiety

The amygdala is a core component of neural circuits that mediate processing of emotional, particularly anxiety and fear-related stimuli across species. In addition, the nuclear complex plays a key role in the central nervous system stress response, and alterations in amygdala responsivity are found in neuropsychiatric disorders, especially those precipitated or sustained by stressors. Serotonin has been shown to shape and fine-tune neural plasticity in development and adulthood, thereby allowing for network flexibility and adaptive capacity in response to environmental challenges, and is implicated in the modulation of stimulus processing and stress sensitivity in the amygdala. The fact that altered amygdala activity patterns are observed upon pharmacological manipulations of serotonergic transmission, as well as in carriers of genetic variations in serotonin pathway-associated signaling molecules representing risk factors for neuropsychiatric disorders, underlines the importance of understanding the role and mode of action of serotonergic transmission in the amygdala for human psychopathology. Here, we present a short overview over organizational principles of the amygdala in rodents, non-human primates and humans, and review findings on the origin, morphology, and targets of serotonergic innervation, the distribution patterns and cellular expression of serotonin receptors, and the consequences of stress and pharmacological manipulations of serotonergic transmission in the amygdala, focusing particularly on the extensively studied basolateral complex and central nucleus.

Visual and spatial modulation of tactile extinction: behavioural and electrophysiological evidence

Crossing the hands over the midline reduces left tactile extinction to double simultaneous stimulation in right-brain-damaged patients, suggesting that spatial attentional biases toward the ipsilesional (right) side of space contribute to the patients' contralesional (left) deficit. We investigated (1) whether the position of the left hand, and its vision, affected processing speed of tactile stimuli, and (2) the electrophysiological underpinnings of the effect of hand position. (1) Four right-brain-damaged patients with spatial neglect and contralesional left tactile extinction or somatosensory deficits, and eight neurologically unimpaired participants, performed a speeded detection task on single taps delivered on their left index finger. In patients, placing the left hand in the right (heteronymous) hemi-space resulted in faster reaction times (RTs) to tactile stimuli, compared to placing that hand in the left (homonymous) hemi-space, particularly when the hand was visible. By contrast, in controls placing the left hand in the heteronymous hemi-space increased RTs. (2) Somatosensory event-related potentials (ERPs) were recorded from one patient and two controls in response to the stimulation of the left hand, placed in the two spatial positions. In the patient, the somatosensory P70, N140, and N250 components were enhanced when the left hand was placed in the heteronymous hemi-space, whereas in controls these components were not modulated by hand position. The novel findings are that in patients placing the left hand in the right, ipsilesional hemi-space yields a temporal advantage in processing tactile stimuli, and this effect may rely on a modulation of stimulus processing taking place as early as in the primary somatosensory cortex, as indexed by evoked potentials. Furthermore, vision enhances tactile processing specifically when the left hand is placed in the hemi-space toward which the patients' attentional biases are pathologically directed, namely rightwards.

Neural mechanisms of intermodal attention in human EEG/MEG

Attention has been conceptualized as a filter mechanism that suffices the fundamental capacity limitation of sensory processing by selecting the input of most behavioral relevance. Recent research has demonstrated that information can be selected by attention based on the sensory modality it is presented in. However, it is not yet known whether this ‘intermodal’ selection relies on common attentional resources. Alternatively, each sensory modality might draw on an independent pool of resources. The present work investigates the neural mechanisms of sustained intermodal attention by pitting the notions of common vs. modality-specific attentional resources against each other. To this end, concurrently presented, frequency-tagged auditory and visual stimuli elicited continuous electrophysiological brain responses in respective early sensory cortices. Three experiments probed (1) whether attention to a particular sensory modality results in a modality-specific modulation of processing, (2) whether this modulation of processing is a facilitation of the attended, an inhibition of the unattended sensory modality or a combination of both factors, and (3) whether stimuli of different sensory modalities enter a competition for processing, thus necessitating intermodal attention to rely on common attentional resources. Attentional modulation of stimulus processing was found to be modality-specific. This modulation likely involved two separate mechanisms: a facilitation of stimuli presented to attended sensory modalities that led to an inhibition of stimuli presented to unattended modalities. As a complementary result, stimuli were found to enter a competition for processing within but not between modalities. In conclusion, the present findings provide evidence for early sensory processing to rely on modality-specific rather than common attentional resources.

Preparatory states in crossmodal spatial attention: spatial specificity and possible control mechanisms.

We used event-related functional magnetic resonance imaging to study the neural correlates of endogenous spatial attention for vision and touch. We examined activity associated with attention-directing cues (central auditory pure tones), symbolically instructing subjects to attend to one hemifield or the other prior to upcoming stimuli, for a visual or tactile task. In different sessions, subjects discriminated either visual or tactile stimuli at the covertly attended side, during bilateral visuotactile stimulation. To distinguish cue-related preparatory activity from any modulation of stimulus processing, unpredictably on some trials only the auditory cue was presented. The use of attend-vision and attend-touch blocks revealed whether preparatory attentional effects were modality-specific or multimodal. Unimodal effects of spatial attention were found in somatosensory cortex for attention to touch, and in occipital areas for attention to vision, both contralateral to the attended side. Multimodal spatial effects (i.e. effects of attended side irrespective of task-relevant modality) were detected in contralateral intraparietal sulcus, traditionally considered a multimodal brain region; and also in the middle occipital gyrus, an area traditionally considered purely visual. Critically, all these activations were observed even on cue-only trials, when no visual or tactile stimuli were subsequently presented. Endogenous shifts of spatial attention result in changes of brain activity prior to the presentation of target stimulation (baseline shifts). Here, we show for the first time the separable multimodal and unimodal components of such preparatory activations. Additionally, irrespective of the attended side and modality, attention-directing auditory cues activated a network of superior frontal and parietal association areas that may play a role in voluntary control of spatial attention for both vision and touch.