Saturday, September 22, 2012

Switching channels: Attention in the brain!

Selective visual attention in the brain is connected through the pulvinar area in the thalamus.

For most of us, a primary experience of consciousness is vision, and more specifically selective control of our visual experience. Our eyes take in far more than we can "process" at any time, so it is essential to limit this stream, at the same time that it empowers our sense of sovereignty, even if our attention is pulled by external events and easily manipulated by conjurers, film directors, and advertisers.

Visual pathways, early stages. Signals coming from the retina are channeled through the LGN to the visual cortex at the back of the brain. They percolate up through the V1 to V8 areas and branch into upper (dorsal) and lower (ventral) streams to other areas of the brain.
Vision flows in complicated pathways in the brain, from the eyes though a central waystation (geniculate nuclei in the thalamus) to the visual cortex at the very back of the brain for processing into ever more abstract and meaningful representations, during or after which it forks into two streams. The upper (dorsal stream) coming into the parietal cortex forms most of our unconscious visual awareness of motion and location, directing motor control, while the lower (ventral stream) heads back to the thalamus and into the temporal cortex, and seems to provide identification (what) information, (interconnected with memory), and emotional salience information. It seems to be the more consciously available pathway.

Of course we want to focus on the most emotionally salient information. And we want to notice aspects of both streams at the same time, for unified perception. A recent paper describes pathbreaking techniques in identifying and recording key routes of the visual pathway in macaques, and finds that an area in the thalamus may serve as an orchestrator of visual attention.

What is attention? In neuroscience, it is increasingly recognized as synchronized neural firing among distributed brain regions, binding together various processed aspects of a particular scene. While consciousness per se remains unresolved, attention has been the focus of great, er, attention. Most perceptual pathways appear to have feedback pathways going back from higher abstract thinking levels into their primary processing systems which can either shut off or call forth activity that is synchronized with the calling regions. In the visual system, there is the added problem of synchronization between the two streams.

Location of the pulvinar relative to the rest of the visual pathways.  This image depicts some of the visual pathway, but not the key connections studied in this paper.

That is where the current work comes in. The pulvinar area of the thalamus is sort of a transgressive part of the brain, sending connections to many higher cortical levels rather than residing within linear tracts of visual or other forms of processing. Indeed, most of the cortex maps to various and overlapping regions of the small and central pulvinar nuclei. And lesions in the pulvinar cause attention deficits. A decade ago, the pulvinar nuclei of the thalamus were proposed to play a role in this synchronization and binding for visual attention specifically:
"The scheme requires that multiple groups of neurons, distributed within and across separate areas, be capable of attaining synchronous firing by means of re-entrant circuitry (Tononi et al. 1992). It is by facilitating this process that the pulvinar could play a coordinating role in cortico-cortical communication." 
"Ultimately, the synchronized neural assembly is proposed to mediate the perceptual binding of different object features (von der Malsburg & Schneider 1986; Tononi et al. 1992; Eckhorn 1994; Singer & Gray 1995). If the pulvinar is a key element of the assembly, damage to the pulvinar should have a noticeable effect on feature binding. There is already some preliminary evidence in favour of this prediction, documenting one patient’s report of illusory conjunctions of colour and letter form (i.e. ‘misbinding’) in the visual hemifield contralateral to a pulvinar lesion (Ward et al. 2002)." -From Shipp, 2003

So the current researchers carefully mapped the connections they were interested in, between the ventral stream of the visual pathway (occipital V4 to the temporal-occipital border area, called TEO) and the pulvinar area of the thalamus. Using amazing MRI/DTI imaging, they could do this individually for each monkey they experimented on, finding precisely where the nerve projections from the two cortical areas overlap in the pulvinar (a method called tractography). That is where they stuck their electrodes, taking electrophysiology to a new level of long-range circuit specificity. They had electrodes both in the overlap area in the pulvinar and in the originating locations in the V4 and TEO cortical  areas, tracking precisely connected signals over long distances in a living (more or less!) brain.
Mapped connections between relevant areas, in schematic terms. Red arrows are forward processing (feed forward, FF), going with the flow of visual information, while green arrows are feedback signals (FB). The numbers refer to the cortical layer being targeted, and the greek symbols refer to the frequency band of the nerve firing. TEO refers to the temporal/occipital boundary region which is part of late-stage visual processing. The experiments were oriented to detecting the alpha-band signals going from the pulvinar back into V4 and the TEO, and telling whether they have coordinating effects.

The point was then to test whether the pulvinar leads the synchronization of visual signals going between the cortical areas, as though it generates attention based either on rapid perceptions in lower areas of the visual pathway (say, suddenly seeing a snake/stick), or on signals from executive areas that exert voluntary attention control.

The task the macaques were set was to follow a spot on a video screen and rapidly report whether the subsequent shape at that spot was one of two possibilities, for a juice reward. In the experimental cases, the spot was engineered to happen in what the researchers knew was the receptive field (RF) for the neurons they had previously mapped and stuck with electrodes. So the question was whether the monkey's rapt attention (verified with infrared gaze tracking) raised the level of synchrony among the areas being recorded, and whether the pulvinar played a leading role in setting that synchrony.

The authors present data showing that these visually connected pulvinar neurons fire more when the monkey is paying attention to their receptive fields. They also find that the frequency of this firing is maximal in the alpha band around 10 Hz. Firing during attention was also closely correlated between the points they connected anatomically- the pulvinar and the V4 and temporal-occipital cortices. And finally, they use a relative timing method (Granger causality) to argue that, when the monkey was attending to the receptive field of the recorded neurons, and was in the attending period between presentation of the cue-spot and presentation of the puzzle shapes, neuron firing in the pulvinar significantly led the correlated firing in each cortical area it was tied to, not the reverse.

Inferred causality (i.e. prior in time and closely correlated in activity) of neural firing, between the tested areas of the visual system in macaque. Significant signals and causality in the alpha frequency band (10 Hz) are only seen going from the pulvinar (PUL) to the cortical visual processing areas V4 and TEO which are sequential in the usual sequence of visual feature processing. This supports the theory that the pulvinar drives attention-related synchronicity across the the visual processing system.

So one can ask where the regulatory decision of the pulvinar arises from. One hypothesis is that we have an immaterial soul that directs these things. Just kidding! As mentioned above, our attention may be involuntarily drawn by features of the scene (some visual pathways connect to the amygdala), or by a higher voluntary decision. In either case, the model would be that once a receptive field was decided on, the pulvinar helps pull the perceptual scene together by coordinating the firing of much or all of the visual pathway for that receptive field, and perhaps coordinating it as well with higher levels that receive that information.

Model (from Shipp, 2004) for attention in the visual system. The pulvinar (VP) receives signals and returns feedback from all areas of the linear/parallel visual feature processing pathway, governing which features or receptive fields are synchronized and thus attended to. The pulvinar receives input to drive its selection of what to attend to from both high level (prefrontal and parietal and frontal eye fields; PEF, FEF) and from low-level areas (the superior colliculus, SC and amygdala, not shown.

The alpha frequency band seems central to vision: "Evidence suggests that the rhythmic excitability of alpha oscillations gates visual events, with the phase of the alpha oscillations being critical for the transmission of visual information."

And, since consciousness seems more closely tied with the gamma band of higher frequency oscillations, the current authors add: "Because low-frequency oscillations modulate higher-frequency oscillations, we tested whether attention increased cross-frequency coupling between alpha and gamma oscillations within V4 and TEO. To measure cross-frequency coupling, we calculated the synchronization index between cortical alpha oscillations and the gamma power envelope. Across the population, there was a significantly greater synchronization index for V4 and TEO during the delay period, when attention was directed to the RF location rather than outside the RF (sign tests, P < 0.05; fig. S3, A and B), suggesting that alpha oscillations contributed to the attention effect on gamma frequencies."

So there you have it.. a few steps towards a physical theory to flesh out the "spotlight of attention" theory that has been an important subject of recent cognitive psychology and neurobiology, not to mention armchair philosophy.

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