It is suggested that in the brain the internal attentional searchlight, proposed by Treisman and others, is controlled by the reticular complex of the thalamus (including the closely related perigeniculate nucleus) and that the expression of the searchlight is the production of rapid bursts of firing in a subset of thalamic neurons. It is also suggested that the conjunctions produced by the attentional searchlight are mediated by rapidly modifiable synapses here called Malsburg synapses-and especially by rapid bursts acting on them. The activation of Malsburg synapses is envisaged as producing transient cell assemblies, including "vertical" ones that temporarily unite neurons at different levels in the neural hierarchy. This paper presents a set of speculative hypotheses concerning the functions of the thalamus and, in particular, the nucleus reticularis of the thalamus and, the related perigeniculate nucleus. For ease of exposition I have drawn my examples mainly from the visual system of primates, but I expect the ideas to apply to all mammals and also to other systems, such as the language system in man. Visual SystemIt is now well established that in the early visual system of primates there are at least 10 distinct visual areas in the neocortex. [For a recent summary, see Van Essen and Maunsell (1).] If we include all areas whose main concern is with vision, there may be perhaps twice that number. To a good approximation, the early visual areas can be arranged in a branching hierarchy. Each of these areas has a crude "map" of (part of) the visual world. The first visual area (area 17, also called the striate cortex) on one side of the head maps one-half of the visual world in rather fine detail. Its cells can respond to relatively simple visual "features," such as orientation, spatial frequency, disparity (between the two eyes), etc. This particular area is a large one so that the connections between different parts of it are relatively local. Each part therefore responds mainly to the properties of a small local part of the visual field (2).As one proceeds to areas higher in the hierarchy, the "mapping" becomes more diffuse. At the same time the neurons appear to respond to more complex features in the visual field. Different cortical areas specialize, to some extent, in different features, one responding mainly to motion, another more to color, etc. In the higher areas a neuron hardly knows where in the visual field the stimulus (such as a face) is arising, while the feature it responds to may be so complex that individual neurons are often difficult to characterize effectively (3,4).Thus, the different areas analyze the visual field in different ways. This is not, however, how we appear to see the world. Our inner visual picture of the external world has a unity. How then does the brain put together all of these different activities to produce a unified picture so that, for example, for any object the right color is associated with the right shape?The SearchlightThe pioneer work of Treisman and ...

Here we summarize our present approach to the problem of consciousness. After an introduction outlining our general strategy, we describe what is meant by the term 'framework' and set it out under ten headings. This framework offers a coherent scheme for explaining the neural correlates of (visual) consciousness in terms of competing cellular assemblies. Most of the ideas we favor have been suggested before, but their combination is original. We also outline some general experimental approaches to the problem and, finally, acknowledge some relevant aspects of the brain that have been left out of the proposed framework.

Visual awareness is a favorable form of consciousness to study neurobiologically. We propose that it takes two forms: a very fast form, linked to iconic memory, that may be difficult to study; and a somewhat slower one involving visual attention and short-term memory. In the slower form an attentional mechanism transiently binds together all those neurons whose activiry relates to the relevant features of a single visual object. We suggest this is done by generating coherent semisyru:hronous oscillations, probably in the 40-70Hz range. These oscillations then activate a transient short-term (working) memory. We outline several lines of experimental work that might advance the understanding of the neural mechanisms involved. The neural basis of very short-term memory espedally needs more experimental study.

The claustrum is a thin, irregular, sheet-like neuronal structure hidden beneath the inner surface of the neocortex in the general region of the insula. Its function is enigmatic. Its anatomy is quite remarkable in that it receives input from almost all regions of cortex and projects back to almost all regions of cortex. We here briefly summarize what is known about the claustrum, speculate on its possible relationship to the processes that give rise to integrated conscious percepts, propose mechanisms that enable information to travel widely within the claustrum and discuss experiments to address these questions.

It is usually assumed that people are visually aware of at least some of the neuronal activity in the primary visual area, V1, of the neocortex. But the neuroanatomy of the macaque monkey suggests that, although primates may be aware of neural activity in other visual cortical areas, they are not directly aware of that in area V1. There is some psychophysical evidence in humans that supports this hypothesis.