Center for Mind and Brain, and Departments of Psychology and Neurology, University of California, Davis, USA
A commentary on
Spatial attention can bias search in visual short-term memory
by Anna C. Nobre, Ivan C. Griffin and Anling Rao
In his visionary writings in the 19th century, American psychologist James (1890)
wrote about attention describing it in part as:
“…the taking possession of the mind, in clear and vivid form, one out of what seem several simultaneously possible objects or trains of thought.”
Over the course of the ensuing century, especially in the past 60 years, researchers investigating the mechanisms of attention have identified important behavioral and neural correlates of attention, which include the findings that attention influences the processing of sensory stimuli by improving perception and performance for attended stimuli versus unattended stimuli (e.g., Cherry, 1953
; Posner, 1978
), and that such effects can involve changes in sensory-neural signals early in the sensory hierarchy for auditory (e.g., Hillyard et al., 1973
), visual (e.g., Van Voorhis and Hillyard, 1977
) and somatosensory (e.g., Desmedt and Robertson, 1977
) stimuli.
However, as James’ quote makes it clear, selective attention not only involves selecting between competing external signals, but also acting to select between internal and external signals, and perhaps as well between competing internal signals held in short- or long-term memory stores. I recall vividly that my late father George H. Mangun, a biochemist, could withdraw almost completely from the welter of our living room when my brother, sister and I were engaged in childhood mischief and mayhem. When he was focused on a difficult scientific problem, we had to physically leap on him to capture his attention. I asked him about this once, in amazement of his formidable mental sound-proofing, and he told me that he perfected the skill in college in order to study without being distracted. As a child I was not wholly convinced, but as an attention researcher (and a parent of two young boys myself!), I now understand the powerful nature of the human attention system for modulating sensory processing. Surprisingly, in contrast to work on the effects of attention on sensory inputs, very little work has addressed how attention can be turned inward to select from purely mental representations. There is no doubt that this paucity of research has to do with the simple fact that it is challenging to develop reliable measures of mental representations in the first place, and still harder to measure how such representations may be affected by other cognitive factors such as momentary attention.
Nobre et al. (2008)
, building on their and others’ earlier work, have conducted a very interesting study that does just this; they have investigated, using behavioral and physiological measures, the effects of attention on representations held in one form of mental store – visual short term memory (VSTM). VSTM is a short-term (a few seconds) limited-capacity store of visual information (e.g., Zhang and Luck, 2008
). Nobre et al. (2008)investigated whether the retrieval of information from VSTM could be influenced by focused attention. The authors utilized a paradigm similar to those used in studies of the effects of covert selective attention on vision using predictive precues (e.g., Mangun and Hillyard, 1991
). However, instead of precuing the location to which attention should be directed in future, the authors used retrodictive cues (spatial retro-cues) that indicated the location of a relevant target in an array presented in the past (1–2 s previously) with 100% probability. In comparison to neutral retro-cues that gave no information about the likely location in the array of the relevant item, performance to indicate whether a subsequently presented probe stimulus had in fact been anywhere in the array was improved with the spatial retro-cues. Importantly, the spatial retro-cues were found to increasingly benefit the observers when the number of items in the array increased.
Recordings of event-related potentials (ERPs) also revealed brain potentials related to search in VSTM. Nobre et al. (2008)
found a brain potential and labeled it the N3RS, which showed systematic changes in amplitude and duration that mirrored the behavioral findings during search. Interestingly, the ERP covaried with the degree of search required during the neutral retro-cue trials in which subjects did not benefit from retrodictive spatial cues.
Overall, the findings add support to the idea that voluntary attention can influence internal mental processing, like search through VSTM. Models that argue for immutable storage of VSTM are not compatible with the findings. However, the research suggests that William James’ introspections on attention are as rich as they appear at first reading, and that the focus of the mind’s eye can be turned inward, to affect internal mental processes, as well as outward to modulate the processing of sensory inputs from the world around us.