Lecture 5, Spatial Vision Flashcards
Receptive Fields and Single Cell Recording
A cell’s receptive field is the area on the retina that elicits a change in the firing rate of the cell when stimulated by light.
The effect of the stimulus on the cell can be either excitatory or inhibitory.
Single cell recording involves inserting an electrode into a neuron to measure its electrical activity.
Activity recorded during single cell recording is that of a SINGLE neuron.
It is not considered ethical to use single cell recording to study the human brain.
Complexity of Receptive Fields
Receptive fields increase in complexity.
Elongated receptive fields respond better to bars and edges.
V1 simple cells respond to orientated bars and edges.
Hubel and Wiesel found orientated bar detectors in V1 of cats using single cell recordings.
Tilt After-Effect
Tilt after-effect is a phenomenon where prolonged exposure to a tilted line causes a vertical line to appear tilted.
Tilt after-effect has 3 components: orientation-tuned neurons, distribution of responses across cells, and adaptation.
Orientation-tuned neurons respond best to their preferred orientation but also respond to other similar orientations.
Perceived orientation is determined by the distribution of responses across cells.
Adaptation refers to a decrease in the cell’s response following prolonged activity.
Evidence for Orientation and Size-Tuned Cells in the Human Visual System
Tilt after-effect provides evidence for the existence of orientation-tuned cells in the human visual system.
The size of the tilt after-effect provides evidence for size-tuned cells in the human visual system.
Size and orientation are fundamental features of parts of the visual scene, and the brain has cells tuned to these features.
Size Perception and Adaptation
Before adaptation, the perceived size of an object is veridical.
Adaptation to fatter bars causes a decrease in the cell’s response.
After adaptation, lines appear thinner due to asymmetrical response distribution.
Size and Spatial Frequency
Size is often defined in terms of spatial frequency.
Spatial frequency refers to the number of bars per unit distance (usually cycles per degree).
Fat bars have a low spatial frequency, while thin bars have a high spatial frequency.
Natural images contain information at many spatial frequencies, which can be used to extract fine or coarse details.
Sensitivity to Spatial Frequencies
We have greater sensitivity to intermediate spatial frequencies, which can be perceived at low contrasts.
We have lower sensitivity to high and low spatial frequencies, which require higher contrast to be perceived.
Retinal vs. Real Size
Spatial frequency tells us about the size of an object on the retina, not in the real world.
The projected size of an object on the retina depends on its distance from the observer.
Any line on the retina could have been produced by an infinite number of different lines in the real world.
