When attention is directed to a particular location or visual feature in the world, visual perception is enhanced for the attended location or feature. Our research in this area is focused on understanding the neurophysiological and neurochemical bases of this enhancement of perception by attention.
Effects of attention on evoked responses and fluctuations in endogenous activity
We have found that continuously allocating spatial attention to a visual stimulus increases the reliability of the fMRI response evoked by the stimulus in a large number of early visual, ventral occipital, lateral occipital, and posterior parietal cortical areas (Bressler and Silver, 2010). This enhanced response reliability by spatial attention is due two a combination of two factors: 1) enhanced amplitude of responses evoked by the visual stimulus, and 2) a reduction in the amplitude of slow endogenous fluctuations in fMRI signals at frequencies below 0.1 Hz. Surprisingly, we have found that the attentional enhancement of evoked responses is not correlated with performance on a target detection task. Rather, behavioral performance is highly correlated with the amount of suppression of slow endogenous fluctuations (Bressler et al., 2020).
Characterizing and modeling effects of voluntary and involuntary spatial attention and their neurochemical substrates
Voluntary attention refers to allocation of attention to a location that is relevant for current behavioral goals, while involuntary attention is automatically drawn to the appearance of salient stimuli. We have studied the effects of the cholinesterase inhibitor and Alzheimer’s medication donepezil. Donepezil inhibits the enzyme that breaks down acetylcholine in the synapse, thereby prolonging the lifetime of this neurotransmitter. This cholinergic enhancement increases the beneficial effects of voluntary attention but does not affect involuntary attention (Rokem et al., 2010).
In addition to enhancing response reliability and perception of attended stimuli, attention can also change the brain’s representations of visual space to optimize stimulus processing and behavior. We tested the effects of voluntary and involuntary attention on critical spacing in visual crowding. Visual crowding is the reduction in the ability to identify visual targets in the periphery when they are flanked by similar stimuli, and critical spacing is the minimum distance between targets and flankers required to obtain a specified level of target identification. We found that involuntary, but not voluntary visual spatial attention reduced critical spacing in an orientation discrimination task (Bowen et al. 2023).
We also developed a computational model of how changes in representations of visual space by attention could improve performance in visual crowding tasks. Previous physiological and neuroimaging studies have documented that spatial attention shifts the positions of receptive fields towards the attended location and also reduces their size. These changes in receptive field properties serve to improve the fidelity of stimulus representations (smaller receptive fields are less likely to include both target and flanker representations) as well as increase redundancy of target representations (more receptive fields that overlap with the target location). These two changes cannot be separately studied with physiological methods, but a computational model can independently estimate them. We found that the increase in receptive field density was more important for the effects of attention on model performance than reductions in receptive field size. Also, increases in redundancy of stimulus representations had more influence on model performance than increases in the fidelity of these representations (Theiss et al., 2021).
Location-dependence of attentional modulation of responses to visual stimulation
It is well established that directing attention to a visual stimulus enhances the response evoked by that stimulus in many brain areas. Although many aspects of visual processing differ significantly between central and peripheral vision, little is known regarding the neural substrates of the eccentricity-dependence of spatial attention effects. In collaboration with Lynn Robertson’s group, we conducted an fMRI study to measure attentional modulation of stimulus-evoked responses in many topographically-organized cortical areas (Bressler et al., 2013). We found that in early visual, ventral, and lateral occipital cortex, attentional modulation was greater for central compared to peripheral eccentricities, possibly reflecting a role for attention in ventral cortical stream areas in resolving fine detail of an attended object in central vision. The opposite pattern was observed in cortical area IPS0, where attentional modulation of positive responses was greater in the periphery, perhaps reflecting the importance of detecting behaviorally relevant objects in the periphery in dorsal stream areas to facilitate planning of motor responses.
Effects of feature-based attention on neural representations of faces
We have measured fMRI responses to individual faces along a morph continuum and found that individual voxels (volumes of tissue of several cubic millimeters) can exhibit significant selectivity for individual faces (Gratton et al., 2013). Also, directing attention to just one of a pair of superimposed faces selectively enhanced responses to the superimposed face pair in voxels previously defined as preferring the attended face.
Hemispheric asymmetries in attentional shifting of visual field representations in topographic parietal cortex
We employed fMRI and the population receptive field (pRF) method to characterize the effects of spatial attention on visual field representations (Sheremata and Silver, 2015). The pRF represents the portion of the visual field that can evoke a visual response in a given voxel. Attending to a stimulus that traversed the visual field increased pRF size in topographically-organized parietal cortex in both hemispheres. In the left hemisphere, attention also shifted pRFs toward the periphery, thereby maintaining their contralateral representation. In contrast, attention did not shift pRF locations in the right hemisphere, so the increased pRF size resulted in more bilateral representations. These results show that spatial attention can induce bilateral representations in right parietal cortex and offer a new approach for investigating visual attention deficits in hemispatial neglect.
Relationships between spatial attention and episodic memory signals in posterior parietal cortex (PPC)
In collaboration with Anthony Wagner’s laboratory, we employed an episodic retrieval task that identified four distinct regions in PPC that tracked different factors associated with retrieval (Hutchinson et al., 2014). Some of the same subjects participated in a spatial attention ask that allowed identification of topographically-organized areas in PPC that exhibit maps of spatial attention signals (Silver et al., 2005; Silver and Kastner, 2009). We found that activity in some of these spatial attention maps (IPS5, SPL1) is related to episodic retrieval outcomes. These findings help define the functional organization of PPC and clarify the relationships between neural correlates of episodic memory and spatial attention.