Neural activity predicts individual differences in visual working memory capacity

Abstract

Contrary to our rich phenomenological visual experience, our visual short-term memory system can maintain representations of only three to four objects at any given moment. For over a century, the capacity of visual memory has been shown to vary substantially across individuals, ranging from 1.5 to about 5 objects. Although numerous studies have recently begun to characterize the neural substrates of visual memory processes, a neurophysiological index of storage capacity limitations has not yet been established. Here, we provide electrophysiological evidence for lateralized activity in humans that reflects the encoding and maintenance of items in visual memory. The amplitude of this activity is strongly modulated by the number of objects being held in the memory at the time, but approaches a limit asymptotically for arrays that meet or exceed storage capacity. Indeed, the precise limit is determined by each individual's memory capacity, such that the activity from low-capacity individuals reaches this plateau much sooner than that from high-capacity individuals. Consequently, this measure provides a strong neurophysiological predictor of an individual's capacity, allowing the demonstration of a direct relationship between neural activity and memory capacity.

Does visual working memory represent the predicted locations of future target objects? An event-related brain potential study

“…In the no-shift task, a CDA component was expected to emerge during the delay period at posterior electrodes contralateral to the side of the task-relevant memorized objects, similar to previous findings (e.g., Vogel & Machizawa, 2004). The critical question was whether the size and polarity of this CDA component would be different in the horizontal shift task.…”

“…Evidence for this comes from ERP studies of visual working memory (e.g., Vogel & Machizawa, 2004;McCollough, Machizawa, & Vogel, 2007). These studies employed a change detection paradigm where a bilateral memory display containing coloured objects in the left and right visual hemifield is preceded by a precue that instructs participants which side of this memory display they have to remember.…”

“…The observation that CDA components increased with memory load (i.e., the number of objects that have to be remembered on a trial), and the fact that CDA amplitudes were linked to individual differences in working memory capacity (Vogel & Machizawa, 2004) provide strong evidence that this component is a direct electrophysiological correlate of visual working memory maintenance (see also Katus, Grubert, & Eimer, in press, for an analogous tactile CDA component that is elicited during the maintenance of tactile stimuli in somatosensory working memory). Importantly, the fact that CDA components are elicited contralateral to the side where task-relevant objects appeared in memory displays demonstrates that the spatial layout of to-be-remembered sensory information is retained when this information is stored and maintained in working memory.…”

“…The procedures used in this experiment were similar to previous ERP research that employed the change detection paradigm (e.g., Vogel & Machizawa, 2004). On each trial, bilateral memory arrays that contained a set of three differently coloured objects in the left visual field and another set in the right visual field were followed after a 900 ms delay period by bilateral test arrays (see Figure 1).…”

Prefrontal cortex and sensory cortices during working memory: quantity and quality

“…This limit has been attributed to activity in parietal areas [41][42][43] . However, further studies have shown that high-capacity individuals are more effi cient at fi ltering out irrelevant items, while low-capacity individuals cannot efficiently filter out such distractions [44] .…”