Neural mechanisms of attention Flashcards
what is bottom-up attention
stimulus driven
what is top-down attention
- These factors are not properties of the stimuli themselves but are defined by the observer’s relationship with the stimulus
what other factors can determine ease of access into consciousness
- Personal relevance
- Emotional significance
- Goal relevance
- Semantic relevance
Individual neurons in monkey temporal cortex show …
‘preferences’ for particular stimuli
what was Chelazzi et al’s 1993 study
Monkeys made eye movements to a target
- Either a neuron’s preferred stimulus or non-preferred stimulus
what did Chelazzi et al find
Response of a single neuron in inferior temporal cortex
in chelazzi’s study, when is firing rate high
if the preferred stimulus is the target, the neuron’s firing remains high. But if the nonpreferred stimulus is the target, the response of the neuron is suppressed.
what did Chelazzis results suggest
The results suggest that the attentional template is formed by modulation of brain regions that process the relevant object – in this case enhanced neuronal firing in shape-selective cortex.
in chelazzis study, what happens when the non-preferred item is the target
if the non-preferred item is the target, neuronal firing drops off.
what does chelazzis study tell us
- Individual neurons show competitive interactions during attentional selection
- Competition occurs not in a separate ‘attentional’ brain region but in the brain regions that process the visual features of relevant (and irrelevant) objects
- The same neurons that process the visual features of an object (its shape, colour, orientation etc.) are co-opted by the attention system to resolve the competition for selection
what was Brefczynski and DeYoe (1999) ‘s study
Neuroimaging (fMRI) studies have shown modulation of a variety of different cortical regions by attention
Attentional effects on early visual processing have also been found with fMRI. In this study, different regions of a circular area were cued and subjects had to make judgments on stimuil that subsequently appeared in that region. The cues were auditory (they learned numbers corresponding to each segment and the cue involved hearing a particular segment number over headphones)
Using a technique called retinotopic mapping, it is possible to map the different regions of primary visual cortex with extremely high precision according to which regions of space they are sensitive to.
what did Brefczynski and DeYoe (1999) find
The authors were able to show that there was an analogue map in V1 corresponding to attentional effects in the circular processing field. In other words, when the left side of the circle was cued, the area of V1 that is sensitive to items appearing in that location showed enhanced activation, and when the opposite side was cued, another location in V1 showed enhanced activation. Importantly, the cues were auditory, so nothing visual actually appeared in those spatial locations that could have caused the V1 effects.
The spatial pattern of activation in visual cortex during attentional cueing (left) closely matched what in Deyoes study
the spatial pattern of activation during simple visual stimulation (right)
who presnted fmri evidence for competitivve interactions in V4
Kastner et al. (1998)
what did kastner et al find
Activation in V4 (colour sensitive visual cortex) was lower when items presented simultaneously than sequentially (bottom up competition)
how did kastner test the baised competition in humans using fmri
First they tested whether stimuli compete for attention – they found that when items are presented sequentially, responses in visual cortex were higher than when they were presented simultaneously.
However, when they instructed subjects to attend to one of the objects, this effect disappeared – the response to the object they were attending to (in the presence of the other objects) was equally high as when it was presented alone.
This again demonstrates that attention modulates competitive interactions at the neuronal level in visual cortex.
These data were interpreted as showing that competitive interactions in visual and temporal cortex play a role in attentional selection.
what did Wojciulik want to study
In this study, Wojciulik wanted to see whether attention influenced brain activation at higher levels of processing, in category specific visual processing regions.
They relied on the well known finding that faces and houses activate separate regions of inferior temporal cortex.
Subjects performed a matching task on the two faces or two houses.
They found that activation in FFA was higher when subjects performed the task on faces than houses, even though the houses were well within their spatial field of vision.
what did Wojciulik show
They found that activation in FFA was higher when subjects performed the task on faces than houses, even though the houses were well within their spatial field of vision.
This shows that attention can operate on higher level visual processing regions by modulating activation to whole objects.
what did O’Craven et al 1999 do
Presented subjects with overlapping faces and houses.
The face or the house moved
Subejcts attended to either the face, the house or the direction of motion.
Here you have two brain regions that ‘prefer’ different categories of objects. The FFA prefers faces (shows higher activation to faces) and the PPA prefers houses.
what did O’Craven et al find
Now, when you ask subjects to attend to the moving object, and the preferred object (the face) moves, activation in the FFA is elevated. But if you ask the subject to attend to the moving object and the non-preferred object (the house) moves, activation in the FFA is suppressed.
what did hopfinger et al do
Hopfinger et al used event related fMRI to look at activation that was time-locked to the cue, not the target
They presented subjects with a directional cue and asked them to make a judgment on checkerboards (are there any grey checks?) but only when they appeared in the cued location.
Now instead of looking at target locked activation they looked at cue-locked activation
what did hopfinger find
They found extensive activation across frontal and parietal cortex time locked to the cue. Thus, these regions were activated it seemed in preparation for the upcoming target stimulus
Could this be the source of the top-down bias signals observed in the work of Duncan and Desimone?
what was taylor et al 2006 study
This study provided evidence for a causal role of prefrontal cortex activation on attentional signals in visual cortex.
The subjects performed a Posner cueing task in which a central cue pointed left or right and a target then appeared on the left or right. The researchers used TMS to stimulate the frontal eye fields between the cue and target
We tested whether the frontal eye field (FEF) is critical in controlling visual processing in posterior visual brain areas during the orienting of spatial attention. Short trains (5 pulses at 10 Hz) of transcranial magnetic stimulation (TMS) were applied to the right FEF during the cueing period of a covert attentional task while event-related potentials (ERPs) were simultaneously recorded from lateral posterior electrodes, where visual components are prominent. FEF TMS significantly affected the neural activity evoked by visual stimuli, as well as the ongoing neural activity recorded during earlier anticipation of the visual stimuli. The effects of FEF TMS started earlier and were greatest for brain activity recorded ipsilaterally to FEF TMS and contralaterally to the visual stimulus. The TMS-induced effect on visual ERPs occurred at the same time as ERPs were shown to be modulated by visual attention. Importantly, no similar effects were observed after TMS of a control site that was physically closer but not anatomically interconnected to the recording sites. The results show that the human FEF has a causal influence over the modulation of visual activity in posterior areas when attention is being allocated.
what was Buschman & Miller’s 2007 study
In this study the researchers got monkeys to perform a visual search task involving either popout (easy) or conjunction search (difficult).
They examined the extent to which regions in the parietal (LIP) and prefrontal cortex oscillated at the same frequency, shown here by the measure of coherence.
They found that coherence was higher in the middle frequency band during conjunction search but coherence was higher in the upper frequency band during popout search.
This provides a mechanistic explanation for the two different types of attentional selection – top down, voluntary attention depends on frequency synchronisation between parietal and prefrontal cortex in the middle frequency band (beta band) whilst bottom-up, reflexive attention depends on frequency synchronisation between these regions in the upper frequency band (gamma band).
This is a network level explanation as it explains how distal, remote regions of the brain can work together – via the process of neuronal synchronisation.
ABSTRACT:
Attention can be focused volitionally by “top-down” signals derived from task demands and automatically by “bottom-up” signals from salient stimuli. The frontal and parietal cortices are involved, but their neural activity has not been directly compared. Therefore, we recorded from them simultaneously in monkeys. Prefrontal neurons reflected the target location first during top-down attention, whereas parietal neurons signaled it earlier during bottom-up attention. Synchrony between frontal and parietal areas was stronger in lower frequencies during top-down attention and in higher frequencies during bottom-up attention. This result indicates that top-down and bottom-up signals arise from the frontal and sensory cortex, respectively, and different modes of attention may emphasize synchrony at different frequencies.
