RIGHT PARIETAL CORTEX PLAYS A CRITICAL ROLE IN CHANGE BLINDNESS by Naser Aljundi

INTRODUCTION  Ventral stream activation is a necessary condition for visual awareness  Occipitotemporal cotex, 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”

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 transcranial magnetic 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.

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 ms on 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.

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.

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.

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’sattention 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.

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.

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.

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