Lecture on Visual Cortex and Neuron Orientation Selectivity

Hubel and Wiesel's Experiment

• Conducted primarily with cats.
• Cat placed on a table looking at stimuli, with electrodes in visual cortex.
• Stimuli: flashing lights in different shapes and positions to record neuron activity.

Mapping Receptive Fields

• Process of identifying the specific area in the visual field that a neuron responds to.
• Neurons show orientation selectivity, preferring certain orientations of bars of light.
• Example: A neuron may respond to vertical bars but not horizontal ones.

Understanding Orientation Selectivity

• Neurons fire more for preferred orientations and less for non-preferred ones.
• Importance: Basic building blocks for understanding visual perception and shape recognition.
• Visual System: Transforms light spots to perceptual representations.

Behavioral Experiments

• Tilt Aftereffect: Staring at certain orientations tires out neurons, affecting subsequent perception.
• Demonstrates neurons' orientation selectivity through behavior, not just direct neuron recording.
• Adaptation: Neurons tire out after prolonged stimulus exposure, reducing their firing rate.

Inferring Neuron Function Without Direct Recording

• Psychophysics: Infer how the visual system works by presenting stimuli and measuring responses.
• Hubel and Wiesel proposed models for orientation selectivity based on retinal ganglion cells aligning in space.

Simple Neural Circuits

• Neurons aligned as row in space detect specific bar orientations.
• Simple cells in V1 show orientation selectivity.
• Circuit: On-center, off-surround spot detectors contribute to V1 neuron detection.

Orientation Columns

• Neurons in V1 are spatially organized, forming orientation columns.
• Columns show smooth progression of orientation selectivity across the cortex.

Human Studies

• High-resolution imaging (7 Tesla) can demonstrate orientation columns in humans.
• Requires sub-millimeter resolution to observe.

Key Takeaways

• Neurons in the primary visual cortex are crucial for processing visual information by detecting orientation.
• These properties can be inferred through both direct neuron recording and behavioral experiments.
• Understanding orientation selectivity helps illuminate the early steps of visual perception and object recognition.