Lecture notes, lecture 7 - Presentation on the inversion effect PDF

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Presentation on the inversion effect...

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The role of lateral occipital face and object areas in the face inversion effect Introduction Object recognition and face perception. Object recognition = the recognising of an object (see video lady – the pink shirt) Face perception = makes us recognise our face THE DEVICES

TMS In short, it consists of placing magnets near the participant’s head and applying a magnetic field targeted at a specific brain region. The current generated by the magnetic field can pass through the skull and disrupts the targeted brain region. The whole thing is non-invasive and not painful.  Transient magnetic field (0.1~1.5 Tesla)  Generates a transient electrical field in underlying brain tissue  Temporarily disrupts normal processing  Two forms: repetitive or single pulses

fMRI

Assumption: increased brain activity is reflected by increased oxygenation Oxyhemoglobin and deoxyhemoglolin differ in their magnetic resonance signal

BOLD = Blood Oxygenation Level Dependent

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Brain uses 20% of total O2 and glucose Assumption in fMRI: Increased brain activity in area X leads to increased oxygenation in area X

fMRI just like PET is the marriage between function and structure, it basically measures the changes in the brain. Or so to say, it measures the changes in the blood/metabolism (measures O2) as we need this to make the brain properly function. Thus it uses the blood flow to measure the local active parts of the brain. The procedure is similar to that of MRI (radiowaves cause hydrogens to oscillate and the instrument measures the local energy fields, which later return to their original position. Nevertheless, the difference between MRI and fMRI is that in fMRI, we use the deoxinated form of hemoglobine and in MRI we use oxidized hemoglobine (which would be paramagnetic). The detectors measure the differences in the oxygenated: deoxygenated ratio. The BOLD effect shows that there will be more oxygenated blood around the active parts thus we can report on the ratio in a certain part of the brain – knowing which one will be active & oxygenated.

THE STUDY This study also looks at the face-selective and object selective cortical areas, using TMS causing neuronal disruption. Other studies As some patients can recognise inverted but not normal faces and others can recognise upright faces, but no normal faces, it has been concluded that.. The two differ from each other and are not the same mechanism. Upright faces are represented by mechanisms specialized for

Participants were asked to perform a match-to-sample discrimination task with upright and inverted faces while TMS was delivered over the functionally localized right occipital face area (rOFA) and right lateral occipital cortex (rLO). Occipital face area is believed to play a role in the early stages of face perception, TMS disturbing this area, creates disruption of the discrimination in upright faces, but not of non-face stimuli which were upright (e.g. houses). The occipital face area shows a similar natural response to upright and inverted faces. Therefore the hypothesis of the article we are discussing is that TMS would disrupt both upright and inverted discrimination.

Methods PARTICIPANTS

“10 right handed participants (half male, half female) with normal vision” MATERIALS

matched face stimuli, which were also used in the researchers former experiment. TWO BY THREE DESIGN & PROCEDURE 

Three blocks (see above) of 40 trails (20 showing the same, 20 showing something different from the first stimuli). The order within one block was varied.

No TMS = baseline The participants were seated and their head was stabilized by a chin rest.

The researchers showed the participants those pictures and the inverted version of those pictures and the goal was for the participants to match the faces. The participants could click/indicate with their finger if the previous stimuli was the same as the old stimuli. The goal was to respond as quickly and accurately as possible. So they had to match the inverted and upright faces with TMS over the right lateral occipital cortex (rLO), TMS over the right occipital face area (rOFA) or no TMS. Maria, will tell you in a bit what the outcomes were.

IMAGING

Every different participant is slightly different when it comes to the rOFA and the rLO. (right occipital face area (rOFA) and right lateral occipital cortex (rLO)). Thus the researchers used a standard fMRI localization task to identify the rOFA and the rLO in the different brains. The functional imaging covered the entire brain. (Slices were aligned with the anterior/posterior commissure.) The imaging of one brain took 4 trails of 234 seconds each. Each functional run contained two sets of five consecutive dynamic stimulus blocks, containing two sets of 5 dynamic stimuli blocks (faces, bodies, scenes, objects and scrambled objects). Thus important to notice here, is that in order to find these places, the participants were also checked on other objects or scenes and not only faces.

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Then they processed this information and made a brain image out of this?

The rOFA was identified using a contrast of dynamic faces greater than dynamic objects and was always located on the lateral surface of the occipital lobe posterior to the face selective rFFA. The rLO was identified using a contrast of dynamic objects greater than scrambled objects and was always located on the lateral surface of the occipital lobe and was superior to the rOFA. The variation of the peak responses that were identified between participants were identified. TMS STIMULATION AND SITE LOCALIZATION The rOFA and rLO were localized by -

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overlaying individual activation maps from the fMRI localizer task for the face and object analysis and identifying the voxel exhibiting the greatest activation in each category-selective region.

The surface coil locations were then marked on each participant’s head. To ensure accurate coil placement during the experiment the position of the coil was tracked and monitored during half of the TMS blocks using the Brainsight system. In TMS blocks, TMS was delivered at a frequency of 10 Hz for 500 ms. The start of the TMS started when the assignment started....