A re-evaluation of the mechanisms underlying simple cell orientation selectivity

Area of Science:

• Neuroscience
• Visual Neuroscience
• Computational Neuroscience

Background:

• Gamma-aminobutyric acid (GABA) is a key inhibitory neurotransmitter in the brain.
• Previous evidence suggests GABA plays a role in the visual cortex's processing of visual information.
• Simple cells in the visual cortex exhibit orientation selectivity, responding best to lines of a specific orientation.

Purpose of the Study:

• To investigate the role of inhibitory inputs in generating orientation selectivity in visual cortex simple cells.
• To determine if excitatory or inhibitory mechanisms are primarily responsible for orientation tuning.

Main Methods:

• Iontophoretic application of N-methyl bicuculline (Nmb), a GABA antagonist, to simple cells in the visual cortex.
• Measuring the response of simple cells to visual stimuli of varying orientations before and after Nmb application.
• Assessing receptive field dimensions by comparing responses to optimal and orthogonal orientations.

Main Results:

• Blocking GABAergic inhibition with Nmb abolished the orientation selectivity of previously sharply-tuned simple cells.
• These cells then responded equally to all orientations.
• Receptive field dimensions, after Nmb application, resembled those expected from a single dLGN cell input.

Conclusions:

• Asymmetries in excitatory inputs are not the primary drivers of simple cell orientation selectivity.
• Intracortical inhibitory mechanisms are crucial for generating orientation selectivity in the visual cortex.
• GABAergic inhibition plays a critical role in shaping the orientation tuning properties of simple cells.