Nature Reviews Neuroscience - Reviews
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February 2005 Vol 6 No 2 REVIEWS
Nature Reviews Neuroscience 6, 97-107 (2005); doi:10.1038/nrn1603
There is an erratum (March 2005) associated with this document. Please click here to view.
WORKING MEMORY IN PRIMATE SENSORY
SYSTEMS
Tatiana Pasternak & Mark W. Greenlee about the authors
Abstract
Sensory working memory consists of the short-term storage of
sensory stimuli to guide behaviour. There is increasing evidence that
elemental sensory dimensions -- such as object motion in the visual
system or the frequency of a sound in the auditory system -- are
stored by segregated feature-selective systems that include not only
the prefrontal and parietal cortex, but also areas of sensory cortex
that carry out relatively early stages of processing. These circuits
seem to have a dual function: precise sensory encoding and short-
term storage of this information. New results provide insights into
how activity in these circuits represents the remembered sensory
stimuli.
Summary
 Psychophysical, physiological and imaging studies of sensory working
memory support the idea that elemental sensory dimensions are
represented by distinct memory systems that are likely to include the
same cortical areas as those involved in encoding stimulus features.
 In visual psychophysics, this idea is supported by the differences in
the length of time for which specific stimulus features are retained,
and by studies that use a 'memory masking' approach in which an
interaction between an interference stimulus during the memory delay
and the preceding stimulus reveals the nature of the remembered
stimulus. Memory masking is most effective early in the delay and its
effect is specific to the stimulus attributes that are retained. Studies of
memory for motion show that the remembered stimuli are spatially
localized, indicating the involvement of areas with fairly precise
retinotopy, and also show that the spatial scale of the mechanism that
underlies performance matches the receptive field size of cortical area
MT. Other studies also indicate a separate representation in memory
of simple stimulus dimensions such as spatial frequency, orientation
and speed.
 Single-cell recordings in monkeys and imaging studies in humans also
reveal memory-related activity in regions of visual cortex that are
known to process various attributes of visual stimuli (for example,
areas IT, V4 and MT). Lesions of areas MT/MST in monkeys and of
analogous regions in humans also produce memory-related deficits in
motion discrimination. The involvement of visual cortical areas in
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Nature Reviews Neuroscience - Reviews
retention of visual information is also supported by microstimulation
experiments in monkeys and transcranial magnetic stimulation (TMS)
in humans.
 Psychophysical studies of tactile working memory show robust and
selective retention of tactile signals, implicating the involvement of the
regions that process these signals. Physiological recordings reveal
memory-related delay activity at the earliest stages of processing of
tactile information in S1 for texture and in S2 for vibration
discrimination. Imaging studies also show activation of somatosensory
regions in cortex during working memory tasks, and TMS applied to
S1 during the delay in the vibration discrimination task disrupted
performance.
 Psychophysical studies of auditory working memory indicate that
elementary dimensions, such as frequency, intensity and sound
location, are stored by separate modules. Single-cell recordings in
primary auditory cortex during the memory delay show activity that
carries information about the remembered tone frequency. The
involvement of auditory cortex in working memory for sound is also
supported by magnetoencephalography, imaging and lesion studies in
humans.
 This review provides evidence for participation of sensory cortical
areas in the circuitry that temporarily stores sensory information.
However, the precise nature of this participation is still unclear. The
use of quantitative sensory psychophysics in conjunction with
recording and imaging studies will bring us closer to an understanding
of how the brain deals with sensory storage.
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