In:- The Neuroscience of Attention Ed. George R. Mangun
Oxford University Press (2012)
Summary and review of the above chapter
Thalamocortical interaction is argued here to be central to understanding perception and cognition. The chapter focuses on the visual areas of the thalamus, and the ability of attention to select behaviourally relevant visual information from the environment.
The visual thalamus has three main components, the lateral geniculate nucleus (LGN), the thalamic reticular nucleus (TRN) and the pulvinar. The LGN is involved in retinocortical projection, and is often viewed as the gate to the visual cortex. It receives a variety of inputs from the retina, the primary visual cortex, the TRN and the brain stem The input from the visual cortex suggests the possibility of top-down influences into the visual thalamus. The TRN is involved in the interface between thalamus and cortex. The pulvinar has connections to the primary and secondary visual cortex.
At one point, it was thought that only cortical areas were involved with attention, but this has recently been revised to include the thalamus. Research in recent years has suggested that the thalamus has a role in controlling visual attention, and that selective attention modulates the visual system both in the primary and secondary cortex. The picture now is one of selective attention being modulated from a variety of cortical and subcortical directions.
Selective attention is shown to influence visual processing in three different ways. Neural responses to attended stimuli are more pronounced than those to unattended stimuli. These neural changes extend as far as the LGN. This is taken to suggest that the LGN can be effected by top-down signals from the fronto-parietal areas, without there having to be any prior external stimuli. Responses to unattended stimuli reduce in proportion to the resources concentrated on attended stimuli. Neural activity is also increased simply by directing attention to a particular location, even in the abscence of an incoming stimuli. Activity in the LGN particularly is shown to be modulated by selective attention, so as to enhance activity in attended locations, enhance responses to attended stimuli, and reduce responses to unattended stimuli. This is consistent with the suggestion that attention is based on top-down signals operating by feedback across the cortex. The frontal and parietal areas are thought to modulate attention in the sensory cortex and this would also apply to the LGN.
Evidence suggests that higher areas of the visual cortex such as TE are more closely controlled than the primary visual cortex. However, the attentional modulation is greater in the LGN than the primary cortex, suggesting that its modulation is not just from the cortex, but also from other areas such as the TRN region of the thalamus. Within the LGN, attentional modulation is greater in the two magnocellular layers than in the four parvocellular layers, which might be seen as related to the different functioning of the dorsal and ventral visual streams. Analysis of the LGN reveals regions of attentional enhancement and regions of attentional suppression. The LGN is also suggested to have a role in filtering information. The LGN is certainly shown to be the earliest stage of visual processing to be modulated by selective attention, with the same patterns of enhancement and suppression of attention that had previously been discovered in the cortex.
The findings on the LGN mean that for the purposes of attention it cannot just be regarded as a relay system, but can additionally be seen as part of the process of directing attention. The pulvinar, another section of the visual thalamus is important for directing visuospatial attention, and in particularly filtering out irrelevant information. Feedback to the thalamus is seen as coming from the frontoparietal attention network. This feedback may alter the magnitude or timing of thalamic activity, which can in turn influence cortical activity.