Ti CN revision - Lecture notes 1-15 PDF

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Wang et al., (2016): FYI: THE SUPERIOR PARETIAL LOBULE is closely linked with the occipital lobe, and is important in visuospatial attention (in terms of perception and manipulation of objects perceived in our environment). Aim & Rationale (Summary of Intro): Our attention is biased; we tend to pay more attention to some objects over others. Contralateral (opposite side) Posterior (Further back) Ipsilateral (Belonging to the same side) Spatial neglect: There is nothing wrong with visual perception, but someone with a lesion in their parietal lobe cannot attend to information on the contralateral side.  May draw only one side of the picture.  Only when told to look at the other side, they realise there is more. Before you do anything, you tend to scan the environment (that is a top-down process) as we have to attend to the whole environment before attending to objects of interest, but you select what you want to pay attention to rather than it being an automatic process: TOP-DOWN.

Fail to attend to one side contralateral to the lesion hemifield. We are still unsure as to how the whole top-down processing method helps in visuospatial attention to objects in our environment. Models of visual processing: 1. Hemispherical Asymmetry (Rivalry) Model:  Hemispherical activity balanced through interaction to contralateral hemifield.  Each hemisphere directs attention to the opposite visual field (left  right, and right  left).  This is balanced through interaction, and this interaction evokes competition between hemispheres.  It is important to note previous literature has found there is limited processing capacity in the visual system which evokes this competition as visual stimuli we perceive in our environment compete to be represented in the attentional network via top-down or bottom-up processes. 2. Hemifacial dominance model: Left directs to both left and right Right is biased to the left visual hemifield.  2 parietal lobes: 1. Only directs to the left hemisphere 2. One directs to both the right and the left hemispheres which results in attentional biases to the contralesional visual field.

Right hemisphere lesions in spatial neglect patients cause persistent deficits in visuospatial attentional processing of left hemisphere attended stimuli. There are connections between the prefrontal cortex and the visual cortex; this is the attentional network. Lesions tend to be in the parietal lobe in neglect patients. The interactions between the parietal lobe and the visual cortex need to be investigated in further detail to understand how top-down visuospatial attention network processing occurs.  There is a lot going on in the process between attending to stimuli to processing within the visual cortex; is it reductionist to simply focus on primarily top-down processing to understand the mechanisms involved in visuospatial attention processing  What is underlying the lateralisation (i.e. the neurological basis of lateralisation in visual perception)? Focuses on the fact that in humans, visual perception processing is subject to lateralisation.  Particularly, focusing on the interaction between the superior parietal lobule and visual cortex. Addresses: 1. Controls; normal spatial bias 2. Root of adaptation; does adaptation influence the visuospatial bias demonstrated by patients with neglect? Methods: TMS: Provokes a virtual lesion in control participants 1. High-frequency repetitive TMS: Constant stimulation to inhibit firing. 2. Low-frequency repetitive TMS: A few times a minute to stimulate neuronal firing in response to visual stimuli presented. Facilitates or inhibits neuronal firing in response to visual objects perceived. Event related time: Stimulation of brain during the event (stimulus presentation). Classic tests for spatial neglect: 1. Line bisection (Neglect patients only see half a line) 2. Block drawing MRI: 1. Detailed structural scan to determine where the superior parietal lobule is; to stimulate this part of the parietal lobe to ensure that this stimulation is replicated using a standardised procedure.  Use of software for spatial mapping (enables researcher to accurately stimulate the right region) 2. Analytical aspect; resting state MRI connectivity analysis. When a network is working in the brain what will happen is that there will be temporal synchronisation so one region of the brain will activate another region at the same time.

 They were attempting to determine using resting state functional connectivity analyses whether there was a correlation between activation of the superior parietal lobule in the parietal lobe and activation in the visual cortex.  These analyses imply that there is structural connectivity between these regions but it may not be the case.  If there is demonstrated there is structural connectivity over several event related time periods, there could be functional connectivity between these regions. BUT they could be connected via another region of the brain (this needs to be considered!).  If there is functional connectivity between the parietal lobe and the visual cortex it implies that every time the parietal lobe is active, the visual cortex is activated. TMS is not very spatially accurate, thus MRI is a better method to use to determine the functional connectivity of the superior parietal lobule & the visual system.

Behavioural Task:    

Screen is divided into 4 segments. Participants asked to find one thing; fast moving red dots. Press in response to seeing or not seeing fast moving red dots in the right or left screens. Measuring response time; are participants faster to react seeing fast moving red dots in the left or right screens.  Right hand for the red target dots  Left hand for the non-target dots  If you’re quicker to react to fast moving dots in the left screen you have right visuospatial attentional bias whereas if you’re faster to react to fast moving dots in the right screen you have left visuospatial attentional bias.

Laterality Effect: Measure of attentional bias. If you have laterality in your behavioural measure, you’re faster to attend one side over the other (left vs right, or right vs left visual field visuospatial attentional bias). 1) The more positive the laterality; the more right asymmetric (meaning you’re more likely to pay attention to things in your left visual field over your right visual field). 2) The more negative the laterality; the more left asymmetric (meaning you’re more likely to pay attention to things in your right visual field over your left visual field)

In summary, what they wanted to see if you have a bigger left attentional bias (so you attend faster to dots shown in the right of the screen in the behavioural measure), is there stronger functional connectivity in the left hemisphere between the parietal lobe & visual cortex in processing of these dots & vice versa? Is there a correlation between functional connectivity strength and attentional bias? Figure 1 simply shows how they wanted to look at functional connectivity between the superior parietal lobule and 4 areas within the visual cortex (V1, V2, V3 & V4). Results:

TMS IN NORMAL PARTICIPANTS: FIGURE 2: There is greater connectivity on the right (i.e. most of us have RIGHT asymmetry so we attend to objects on the left faster than those in the right). For all the controls, this was the most significant finding. SHAM TMS behavioural test results: Wanted to see whether when they induce a ‘virtual lesion’ on the right or the left superior parietal lobule using low-frequency TMS will it influence behavioural response to a stimulus? The use of a sham TMS is an important part of this study as without the use of this, it is difficult to determine whether the lesion on the right or the left had any effect. To simply rely only upon the MRI, YOU could only say there is a difference in how the left and the right fronto-parietal connections to the visual cortex activate in response to visual stimuli presented. However, the use of SHAM TMS to reduce the placebo effect in this study with control participants (i.e. trying to convince them that they’ve been lesioned or not) can be criticised as even if you place the 8 figure coil on the region of interest they can tell they are not being lesioned as it involved low-frequency electrical stimulation to the scalp. Use of Tdcs? Potential question could ask whether use of sham Tdcs WOULD have better controlled for the placebo effect…

 Will this “virtual” lesion influence behaviour; how fast participants can detect a target stimulus in the right or left visual space. Figure 3 explained: Figure 3a)     

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The left superior parietal lobule stimulation was a sham; the reaction time was about 0.6ms (SEE FIGURE 3). Middle bar shows when they lesion the left superior parietal lobule (very like sham response time). Right bar shows when they lesion the right superior parietal lobule, it influenced response time to a stimulus presented. They are attempting to stop the retention of visual stimuli to be processed, and this influences response time to recognise and attend to the stimuli being presented. These findings imply that functional connectivity is important in the left than the right; lesions to the right superior parietal lobule affects visuospatial attentional processing to stimuli presented on the left of the visual field. There is right hemispheric dominance in visuospatial attention (in terms of attentional shifting and control).

Figure 3b)

(X axis explanation): More right asymmetry than left as we are left biased (there is more dots right of zero than left of zero). Anyone on zero neither has left or right attentional bias as their response time is the same regardless if the target is on the left or the right. There are more dots on the right which is what is expected as healthy participants generally will attend to more information on the contralateral side (the left). Only a few are right biased (demonstrated by only a few dots below the line of best fit) suggesting that predominantly in healthy participants there is right asymmetry.

(Y axis explanation): Looking at the relationship between functional connectivity and attentional bias. There is a correlation between functional connectivity strength (i.e. how active is the top-down attentional processing network proposed to exist from the SPL in the parietal lobe to the V1 region in the visual cortex) and attentional bias (how fast or slow participants attend to stimuli presented in their left or right visual fields).  The stronger your connection, the more your bias. Trying to show that: 1. If your attention is biased to the left, are you more likely to have weak functional connectivity? 2. The greater your functional connectivity, the greater your bias (left or right); there is a correlation between the two.  If you are much, much slower to respond to targets on the right than the left, your left hemisphere top-down functional connectivity between left posterior SPL, and V1 region is stronger.  If you have stronger connectivity between the parietal lobe and V1 on your right, you have a greater bias towards the left; shown by a greater response time to attending to visual stimuli on the left.  People that have greater connectivity in the right, have a slower response time to stimuli presented in the left as the top-down attentional network between the parietal lobe and the visual cortex (V1) have to work harder to process this contralateral visual field information. Figure 4 explained: 1. Looking at functional connectivity in neglect patients. 2. It is not expected that lesion patients have huge functional connectivity between the parietal lobe and the visual cortex on the side of their lesion (in the right SPL). 3. Because patients with spatial neglect have a lesion on one side, the functional connectivity on the side of the lesion (right SPL and its connections to V1 and V2 significantly) is much reduced. 4. Functional connectivity on the left is much larger for neglect patients, so it takes longer for them to attend to visual information presented to them in their left visual field.

5. In the left, there is greater connectivity from the posterior SPL to four regions of the visual cortex (V1 TO V4) as there is a right hemisphere SPL lesion (most sig in V1 and V2 regions).  Having a SPL lesion in the right side means that there is weaker connectivity between the SPL on the right side, and areas in the visual cortex (weaker top-down attentional network connectivity).

Discussion: 

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The premise of their study was to establish whether top-down networks affected lateralization of visuospatial attention, focusing on the interaction between the superior parietal lobule and V1. Used resting fMRI analyses paired with TMS based behavioural tests for both healthy and neglect (lesion) patients Their overall conclusion states that the connectivity between SPL and V1 was the most relevant form of top-down to behavioural activity. Therefore, attention could directly modulate the activity of V1 to affect perception or search efficiency.

From the results they concluded that in healthy patients, asymmetry of top-down connectivity with V1 were closely correlated with behavioural lateralization of visuospatial attention. They further stated that in neglect patients, contralesional top down connectivity between the SPL and V1 was significantly stronger than ipsilesional connectivities. This suggests that the unbalanced interaction between the top-down networks of SPL and V1 causes the lateralization of visuospatial attention in healthy subjects and causes the behavioural deficit in neglect patients.

Stated that during attention shifting, stronger activity was observed in right SPL than left Suggested that higher order cortex areas involving SPL were strongly correlated with selection biases which facilitate information processing of stimuli at attended locations. Thus resulting in the bias signal causing lateralization of visuospatial attention. However, as stated by Thiebaut de Schotten et al (2011), humans naturally have larger parietal networks (main body of SPL) in the right hemisphere than in the left, suggesting that lateralization of top-down networks and asymmetry of visuospatial attention are a result of unbalanced speed of visuospatial processing This is supported by the findings in the current study. They found that while modelling the virtual lesion using TMS on the right SPL and ipsilateral visual areas, reaction times were significantly longer when applied to the right SPL than the left, suggesting that visuospatial attention is lateralized to the right hemisphere. The opposite was found for the neglect patients; contralesional top-down connectivity was significantly stronger than ipsilesional, suggesting that lateralization of visuospatial attention is on the left due to a lack of processing on the right hemisphere.

Other regions can serve a role in PFC FMRI can detect other top-down influences as a result of the haemodynamic response which may pick up other influences.

Towler et al., (2016) Aim: 

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To look at the N170 in developmental prosopagnosics. To see if the N170 in face recognition and processing is the same or different from neurotypical individuals as previous research such as Towler et al., (2012) suggests that DP patients have the same levels of N170 face sensitivity in the processing of holistic faces as neurotypical individuals. To see if the N170 component network is capable of being released from neural adaptation in developmental prosopagnosics.

Rationale: In prosopagnosia, there is deficits in face processing, but we are yet to identify what these deficits are. The idea is that in developmental prosopagnosia, there are a range of deficits: o o

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These range of deficits exist as DP is categorised as a syndrome. This classification of the range of deficits that are shown by DP patients are generalised, they do not consider the individual severity of face processing deficits which may cause differences in how much DP affects the ability to recognise and process faces. There may be discreet deficits or abnormalities in face recognition and processing which haven’t been identified yet in DP. All we know about DP is there is developmental abnormalities in face processing which could be one of two things: 1. Could be the inability to process the CONFIGURAL aspects of the face 2. Problems with FACIAL MEMORY (i.e. if you’re unable to identify someone based on their face, it could be you do not have a strong representation of that person’s face OR could process faces but cannot access the memory associated with that person’s face or facial features).

Issue as to why this paper is not well written is that there are a range of deficits in DP, and AP which vary from sufferer to sufferer we can only say there is abnormalities in face processing. People with DP have slightly different deficits we do not know if it is deficits in: 1. Holistic processing (recognition of a face as a whole) 2. Configural processing (recognition of a face by looking at individual face features such as eyes and nose) 3. Memory load (memory of faces; disruption to access of face memories). It is also important to note there is differences in acquired proposognosia and developmental proposognosia: 1. In AP, individuals have lesions in the fusiform face area region which makes it harder for them to recognise their own faces but have no other face processing problems.

2. In DP, individuals do not have a lesion as such. It is developmental meaning it is acquired as individuals develop. There is no specific region deficits or lesion in areas specific to face processing we just know that these individuals have problems processing and remembering their own faces, and the faces of others. o Can usually recognise faces but cannot differentiate between faces to give individuals visual identities. o Might be able to differentiate there is differences in the faces of two people, but fail to remember who is who. 

Memory load: Is there a problem with DP patients being able to memorise or access face memories?  One way to determine this is by increasing your memory load. Increasing your cognitive load leads to being unable to access memories.  Without cognitive memory load, it can lead to problems for patients with DP accessing face memories but research is inconclusive in this respect (no one knows!).

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The inversion effect: Neurotypical individuals (without prosopagnosia) are able to identify faces upside down.  N170: Face specific event related potential normally it is larger in inverted face processing and recognition as the brain is supposedly working harder to identify face specific components.  The reason why the N170 is bigger in recognition of inverted faces is because inverted faces are harder to recognise.  This bigger N170 effect in recognition of inverted faces is not necessarily the case for patients with DP as there is an increase in N170 to inverted faces; suggesting they are not able to process inverted faces in the same way.

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N170: Electrical components specific to facial processing. “FACE SPECIFIC ERP”. The N170 is found in the back of the brain. It is not completely face specific but it is face specialised as it always occurs when you are processing faces. ERP: If you time-lock your stimulus presentation in an EEG, you get this temporal specific waveform. Basically, a voltage measurement of a specific time and depending where you measure it you average it (lock it to the event) and you find that you get this pattern of change in amplitude over time. o This way you can examine positive and negative electrical potential differences in neural activity over time which is related to an event (i.e. showing a participant an inverted face vs showing a participant a neutral face).

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