Jay Todd

Capacity limit of visual short-term memory in human posterior parietal cortex.

At any instant, our visual system allows us to perceive a rich and detailed visual world. Yet our internal, explicit representation of this visual world is extremely sparse: we can only hold in mind a minute fraction of the visual scene. These mental representations are stored in visual short-term memory (VSTM). Even though VSTM is essential for the execution of a wide array of perceptual and cognitive functions, and is supported by an extensive network of brain regions, its storage capacity is severely limited. With the use of functional magnetic resonance imaging, we show here that this capacity limit is neurally reflected in one node of this network: activity in the posterior parietal cortex is tightly correlated with the limited amount of scene information that can be stored in VSTM. These results suggest that the posterior parietal cortex is a key neural locus of our impoverished mental representation of the visual world.

Posterior parietal cortex activity predicts individual differences in visual short-term memory capacity

Humans show a severe capacity limit in the number of objects they can store in visual short-term memory (VSTM). We recently demonstrated with functional magnetic resonance imaging that VSTM storage capacity estimated in averaged group data correlated strongly with posterior parietal/superior occipital cortex activity (Todd & Marois, 2004). However, individuals varied widely in their VSTM capacity. Here, we examined the neural basis of these individual differences. A voxelwise, individualdifferences analysis revealed a significant correlation between posterior parietal cortex (PPC) activity and individuals’ VSTM storage capacity. In addition, a region-of-interest analysis indicated that other brain regions, particularly visual occipital cortex, may contribute to individual differences in VSTM capacity. Thus, although not ruling out contributions from other brain regions, the individual-differences approach supports a key role for the PPC in VSTM by demonstrating that its activity level predicts individual differences in VSTM storage capacity.

A central role for the lateral prefrontal cortex in goal-directed and stimulus-driven attention

Attention is the process that selects which sensory information is preferentially processed and ultimately reaches our awareness. Attention, however, is not a unitary process; it can be captured by unexpected or salient events (stimulus driven) or it can be deployed under voluntary control (goal directed), and these two forms of attention are implemented by largely distinct ventral and dorsal parieto-frontal networks. For coherent behavior and awareness to emerge, stimulus-driven and goal-directed behavior must ultimately interact. We found that the ventral, but not dorsal, network can account for stimulus-driven attentional limits to conscious perception, and that stimulus-driven and goal-directed attention converge in the lateral prefrontal component of that network. Although these results do not rule out dorsal network involvement in awareness when goal-directed task demands are present, they point to a general role for the lateral prefrontal cortex in the control of attention and awareness.

Distinguishing between lateralized and nonlateralized brain activity associated with visual short-term memory: fMRI, MEG, and EEG evidence from the same observers.

Previous functional neuroimaging studies have shown that maintenance of centrally presented objects in visual short-term memory (VSTM) leads to bilateral increases of BOLD activations in IPS/IOS cortex, while prior electrophysiological work suggests that maintaining stimuli encoded from a single hemifield leads to a sustained posterior contralateral negativity (SPCN) in electrophysiology and magnetoencephalography. These two findings have never been investigated using the same physiological measures. We recorded the BOLD response using fMRI, magnetoencephalography (MEG), and electrophysiology (EEG), while subjects encoded visual stimuli from a single hemifield of a balanced display. The EEG showed an SPCN. However, no SPCN-like activation was observed in the BOLD signals. The BOLD response in parietal cortex remained bilateral, even after unilateral encoding of the stimuli, but MEG showed both bilateral and contralateral activations, each likely reflecting a sub portion of the neuronal populations participating in the maintenance of information in VSTM. Contrary to the assumption that BOLD, EEG, and MEG responses - that were each linked to the maintenance of information in VSTM - are markers of the same neuronal processes, our findings suggest that each technique reveals a somewhat distinct but overlapping neural signature of the mechanisms supporting visual short-term memory.