Summer Sheremata

Visual short-term memory (VSTM) is an essential store that creates continuous representations from disjointed visual input. However, severe capacity limits

Perceptual learning (PL), often characterized by improvements in perceptual performance with training that are specific to the stimulus conditions used during training, exemplifies experience-dependent cortical plasticity. An improved understanding of how neuromodulatory systems shape PL promises to provide new insights into the mechanisms of plasticity, and by extension how PL can be generated and applied most efficiently. Previous studies have reported enhanced PL in human subjects following administration of drugs that increase signaling through acetylcholine (ACh) receptors, and physiological evidence indicates that ACh sharpens neuronal selectivity, suggesting that this neuromodulator supports PL and its stimulus specificity. Here we explored the effects of enhancing endogenous cholinergic signaling during PL of a visual texture discrimination task. We found that training on this task in the lower visual field yielded significant behavioral improvement at the trained location. However, a single dose of the cholinesterase inhibitor donepezil, administered before training, did not significantly impact either the magnitude or the location specificity of texture discrimination learning compared with placebo. We discuss potential explanations for discrepant findings in the literature regarding the role of ACh in visual PL, including possible differences in plasticity mechanisms in the dorsal and ventral cortical processing streams.

While the parietal cortex plays an integral role in visual attention and short-term memory, the mechanisms underlying these cognitive functions are elusive. Retinotopic mapping studies have demonstrated a mapping structure of parietal cortex using attended stimuli that traverse the visual field by moving through continguous spatial locations. While phase-encoded mapping methods depend upon this stimulus pattern, newer methods such as population receptive field mapping (pRF) allow for presentation of stimuli in randomized locations. In contrast to parietal cortex, occipital cortical maps have been found using non-contiguous mapping stimuli have been used to demonstrate maps in occipital cortex questioning whether contiguous stimuli offer attentional cues that facilitate retinotopic mapping in the parietal cortex. In order to test this assumption, contiguous and non-contiguous stimuli were used to assess pRFs in the parietal cortex. Participants maintained fixation while a checkerboard bar traversed the visual field and monitored for a dimming of the mapping stimulus. To determine whether discontiguous stimuli elicit retinotopic structure, the preferred location and visual field extent were compared across the two stimulus conditions. Then each condition was split into two sets of runs and the test-retest reliability of each measure was compared. The results suggest only a modest relationship between contiguous and discontiguous stimulus presentations. Furthermore, test-rest reliability was higher for the contiguous stimulus presentation condition. Together these results suggest that contiguous stimuli are most effective for demonstrating map structure in parietal cortex. However, pRF size was larger in the contiguous stimulus presentation condition, supporting claims that visual field maps are susceptible to stimulus motion direction for continuous stimuli. These results demonstrate a benefit for contiguous stimuli for retinotopic mapping in parietal cortex and suggest that retinotopic mapping can yield insights into the cognitive mechanisms underlying acitivity in parietal cortex.

Visual short-term memory (VSTM) and attention are distinct yet interrelated processes. While both require selection of information across the visual field, memory additionally requires the maintenance of information across time and distraction. VSTM recruits areas within human (male and female) dorsal and ventral parietal cortex that are also implicated in spatial selection; therefore, it is important to determine whether overlapping activation might reflect shared attentional demands. Here, identical stimuli and controlled sustained attention across both tasks were used to ask whether fMRI signal amplitude, functional connectivity, and contralateral visual field bias reflect memory-specific task demands. While attention and VSTM activated similar cortical areas, BOLD amplitude and functional connectivity in parietal cortex differentiated the two tasks. Relative to attention, VSTM increased BOLD amplitude in dorsal parietal cortex and decreased BOLD amplitude in the angular gyrus. Additionally, the tasks differentially modulated parietal functional connectivity. Contrasting VSTM and attention, intraparietal sulcus (IPS) 1–2 were more strongly connected with anterior frontoparietal areas and more weakly connected with posterior regions. This divergence between tasks demonstrates that parietal activation reflects memory-specific functions and consequently modulates functional connectivity across the cortex. In contrast, both tasks demonstrated hemispheric asymmetries for spatial processing, exhibiting a stronger contralateral visual field bias in the left versus the right hemisphere across tasks, suggesting that asymmetries are characteristic of a shared selection process in IPS. These results demonstrate that parietal activity and patterns of functional connectivity distinguish VSTM from more general attention processes, establishing a central role of the parietal cortex in maintaining visual information.SIGNIFICANCE STATEMENTVisual short-term memory (VSTM) and attention are distinct yet interrelated processes. Cognitive mechanisms and neural activity underlying these tasks show a large degree of overlap. To examine whether activity within the posterior parietal cortex (PPC) reflects object maintenance across distraction or sustained attention per se, it is necessary to control for attentional demands inherent in VSTM tasks. We demonstrate that activity in PPC reflects VSTM demands even after controlling for attention; remembering items across distraction modulates relationships between parietal and other areas differently than during periods of sustained attention. Our study fills a gap in the literature by directly comparing and controlling for overlap between visual attention and VSTM tasks.

Successful interaction with the environment requires the ability to flexibly allocate resources to different locations in the visual field. Recent evidence suggests that visual short-term memory (VSTM) resources are distributed asymmetrically across the visual field based upon task demands. Here, we propose that context, rather than the stimulus itself, determines asymmetrical distribution of VSTM resources. To test whether context modulates the reallocation of resources to the right visual field, task set, defined by memory-load, was manipulated to influence visual short-term memory performance. Performance was measured for single-feature objects embedded within predominantly single- or two-feature memory blocks. Therefore, context was varied to determine whether task set directly predicts changes in visual field biases. In accord with the dynamic reallocation of resources hypothesis, task set, rather than aspects of the physical stimulus, drove improvements in performance in the right- visual field. Our results show, for the first time, that preparation for upcoming memory demands directly determines how resources are allocated across the visual field.

Visual short-term memory (VSTM) maintains information across eye movements and distraction. However, VSTM holds a very limited amount of information, with memory capacity reaching a maximum of 3 ± 1 objects. Activity in the parietal cortex mirrors these restrictions, plateauing at a subject's maximum capacity. Parietal cortical activity also shows hemispheric asymmetries in representing memory items across the visual field. Consistent with these asymmetries, memory performance demonstrates a left visual field bias for single-feature objects (Sheremata & Shomstein, 2014). It is, however, an open question which coordinate system, retinotopic or spatiotopic, underlies VSTM biases. Eye position was monitored while subjects performed a color change-detection task for items presented to the left and the right of the monitor while subjects fixated different positions across the visual field. In separate conditions, subjects fixated either the center of the screen (control condition) or locations peripheral to the stimuli. If VSTM is represented in spatiotopic coordinates, then visual field asymmetries should be independent of eye position. If, however, VSTM is retinotopically represented, then visual field performance should be modulated by eye position, reversing when subjects fixate peripheral visual field locations. Results supported retinotopic representations in VSTM, as evidenced by a reversal of visual field asymmetries as compared to the control condition. Further manipulations controlling for spatiotopic location and non-spatial encoding corroborated our findings. These results demonstrate that VSTM, like visuo-spatial processing, is coded in retinotopic coordinates. Meeting presented at VSS 2015

Posterior parietal cortex contains several areas defined by topographically organized maps of the contralateral visual field. However, recent studies suggest that ipsilateral stimuli can elicit larger responses in the right than left hemisphere within these areas, depending on task demands. Here we determined the effects of spatial attention on the set of visual field locations (the population receptive field [pRF]) that evoked a response for each voxel in human topographic parietal cortex. A two-dimensional Gaussian was used to model the pRF in each voxel, and we measured the effects of attention on not only the center (preferred visual field location) but also the size (visual field extent) of the pRF. In both hemispheres, larger pRFs were associated with attending to the mapping stimulus compared with attending to a central fixation point. In the left hemisphere, attending to the stimulus also resulted in more peripheral preferred locations of contralateral representations, compared with attending fixation. These effects of attention on both pRF size and preferred location preserved contralateral representations in the left hemisphere. In contrast, attentional modulation of pRF size but not preferred location significantly increased representation of the ipsilateral (right) visual hemifield in right parietal cortex. Thus, attention effects in topographic parietal cortex exhibit hemispheric asymmetries similar to those seen in hemispatial neglect. Our findings suggest potential mechanisms underlying the behavioral deficits associated with this disorder.