Right parietal cortex plays a critical role in change blindness

1. Right parietal cortex plays a critical role in change blindness by Naser Aljundi

2. Introduction • Ventral stream activation is a necessary condition for visual awareness • Occipitotemporalcotex, lateral occipital cortex (LOC). • Previous research saying that the information in dorsal system processed unconsciously • However, when subjects viewed bistable images inducing change in awareness (Necker cube or Rubin’s face/vase), parietal lobe was activated. • Recent study using fMRI revealed enhanced bilateral parietal lobe activity when subjects consciously detected change in 2 successive visual scenes separated by a flicker. • Phenomenon called “Change Blindness”

3. Introduction cont. • Question:Is the parietal cortex activated as a consequence of visual awareness? Or does it play a causal role in visual awareness? • Previous fMRI studies only reveal an association between brain region activity and behaviour. • To assess this, repetitive transcranialmagnetic stimulation (rTMS) was applied to either right or left parietal cortex to disrupt neural activity. • Specifically at the Posterior Parietal Cortex (PPC) – previously associated with conscious detection of visual change. • Visual awareness task completed to see if rTMS in particular region affected participant performance.

4. Procedure • 9 right handed subjects (3 females) participated; normal vision. • 1-shot task. • 500 ms fixation cross. • Two 200 ms display of faces were shown, separated by a 100ms blank interval. • 4 faces were shown in each display chosen from set of 5 faces • 2.8 sec from onset of second display to respond to whether one of the faces was changed or not. Press one of two buttons with right hand. • Responses within 300 mson onset of second display were excluded • All 4 face locations were equally likely to be changed. • On 2/3 of trials one face was changed.

5. Procedure cont. • TMS machine delivered 500ms trains of 10 Hz pulses. • Subjects received right parietal TMS, left Parietal TMS and no TMS. • Block order was randomized • All occurred twice and had to be administered once in first half and once in the second half of experiment. • Mean reaction times (RTs) and changes missed were computed for each of right,left, and no TMS. These were measured for both right and left visual field change.

6. Results • Right parietal TMS produced significantly slower change detection responses when compared to either left parietal TMS or no TMS. • Left parietal TMS had no significant difference in response times compared to no TMS. • Same trend was shown for accuracy of change detection where right parietal TMS shows clear decrease in performance.

7. DISCUSSION • This experiment was able to provide evidence that the right PPC has a key role in conscious visual awareness. • Works by interacting with the occipitotemporal cortex and right dorsolateral PFC in two possible ways: 1. Using top-down processing, parietal cortex helps move participant’s attention around changing image, which reduces change blindness. 2. A form of visual short-term memory (VSTM) may be needed for change detection. • Data suggests that PPC may control what enters VSTM from visual scene.

8. Strengths & limitations • First paper to provide evidence that parietal cortex is not only activated but plays a causal role in detecting change of visual stimuli. • Only female faces were used for the visual stimuli. • Only 9 participants in the study.

9. Future research • Using briefer intervals of TMS at different points in the detection trial to help determine exactly what is being disrupted in right parietal cortex. • Use more participants. • Try experiment with male faces or different shapes/objects for visual stimuli.

10. REFERENCE: Beck, D. M., Muggleton, N., Walsh, V., & Lavie, N. (2006). Right Parietal Cortex Plays a Critical Role in Change Blindness. Cerebral Cortex 16, 712-717. QUESTIONS?