Behavioral/Cognitive Electrical Stimulation of the Fusiform Gyrus

Study Overview

  • Title: Behavioral/Cognitive Electrical Stimulation of the Left and Right Human Fusiform Gyrus Causes Different Effects in Conscious Face Perception
  • Authors: Vinitha Rangarajan et al.
  • Institution Affiliations: Stanford University and Universite Catholique de Louvain.
  • Objective: To explore the causal effects of electrical brain stimulation on face perception by evaluating the fusiform gyrus (FG) in humans.

Background

  • Fusiform Gyrus (FG): Known for its role in face perception.
  • Previous Studies: Both imaging and electrophysiological studies indicate bilateral face-selective responses in FG and ventral temporal cortex (VTC). Prosopagnosia is linked to lesions in this area.
  • Methods Used: Electrocorticography (ECoG) and electrical brain stimulation (EBS) on participants implanted with electrodes.

Methodology

Participants
  • Number: 10 subjects (3 female).
  • Group Division: 5 with right FG stimulation and 5 with left FG stimulation.
  • Handedness: Majority were right-handed, one left-handed with left language dominance.
Equipment and Procedures
  • Electrode Placement: Subdural intracranial electrodes placed over FG. High-resolution anatomical MRIs and post-implant CT were used for localization.
  • Experimental Tasks: Subjects performed tasks involving the presentation of various images (faces, cars, houses) while ECoG data were collected.
  • EBS Protocol: Used to identify functional areas during a clinical mapping procedure, providing information on perceptual effects when different areas were stimulated.

Experimental Tasks

  1. Task 1: Grayscale images of faces and objects displayed for 1000 ms with varied interstimulus intervals (600-1400 ms).
  2. Task 2: Block design with rows of faces, numbers, words, and scrambled symbols, with subjects pressing a button upon color changes.
Data Collection and Analysis
  • ECoG Data: Recorded signals were filtered, normalized, and analyzed to determine frequency power related to face-selectivity (HFB) and event-related potentials (ERPs).
  • Perceptual Reports: Participants recorded any visual distortions experienced during stimulation.

Results

  • Face-selectivity Findings: Bilateral face-selective responses observed; however, right FG stimulation led to perceptual changes (e.g. face distortions) while left FG stimulation did not.
  • EBS Effects:
    • Right FG: Induced clear changes in face perception (e.g., described as metamorphosis, changing features).
    • Left FG: Produced non-face-related visual changes (e.g., phosphenes, twinkling).
  • Participants’ responses varied with specific details on what changes were perceived during EBS.

Key Findings

  • Lateralization: Right FG has a critical role in conscious face perception, eliciting strong perceptual distortions. The left FG is less involved, showing more minor visual changes unrelated to face perception.
  • Causal vs. Correlational: This study differentiates causal effects of EBS from correlational evidence provided by imaging studies; stimulation in the right FG consistently produced perceptual changes whereas left FG did not.
  • Electrodes and Selectivity: HFB measurements were stronger in right FG where perceptual changes occurred. ERP readings did not reliably correlate with perceptual changes across sites.

Conclusions

  • Electrical stimulation demonstrates that the right FG is more essential for conscious face perception compared to the left FG, highlighting the differential roles of each hemisphere in processing faces.
  • Further studies are encouraged to investigate the mechanisms behind these lateralized effects and their implications in understanding face perception and related disorders.