PS222: Introduction and Basics: Receptive Fields

Perception

interaction between the organism and the physical world, dependent upon sense organs and the brain, dependent on evolution as whatever they have gives organisms a selective advantage

Sense organs

contain specialized receptor cells (ex. photoreceptors in the eye), funnel the physical stimulus to specialized receptor cells

Receptor cells

detect the physical stimulus, transduce (translate) the physical stimulus into a neural signal the brain can understand

Physical stimulus energy

light (electromagnetic radiation), vibration, chemicals, pressure/heat/position → need specialized sense organs that can detect the stimuli

Signal chain: stimulus to neuron

Receptor cells in the sensory surface are stimulated → send signal to neuron → neuron gets excited → neuron sends signal

Receptive field of a neuron

• An area on the sensory surface containing receptor cells, when stimulated change the activity of a neuron (only happens when receptor cells are in the receptive field of the neuron)

• A neuron that receives signals from multiple neurons will have an RF equal to the total RFs of those neurons

• The signal can be conveyed directly from the receptor cells themselves, or through other neurons that are connected to those cells

Receptor cells in RF

Any receptor cell can be a part of one or more neurons’ receptive fields; could consist of one receptor cell, thousands, spread out, or close together

Receptive field size

can vary over sensory surface depending on the brain areas receiving the signals

Smaller receptive field

fewer receptive cells, high resolution (finer details), lower sensitivity (level of detection)

Larger receptive field

more receptor cells, low resolution (less detail), higher sensitivity (level of detection)

Neurons in higher brain areas

can put together detailed information from multiple neurons to identify the object anywhere on the stimulus surface → therefore, RFs of these neurons are larger and can be high resolution as well because they get the details from earlier in processing and put them together

Why are we more sensitive in detecting low intensity stimuli?

Evolutionary purposes, makes it easier to detect predators before they get close, and faint smells or quiet sounds help us find resources like food

Two-point sensitivity test

• used to measure spacial acuity—different areas of the body have vastly different thresholds because of the density of mechanoreceptors (touch receptors) and the size of their receptive fields

• high sensitivity (small threshold): fingertips, tongue → densely packed receptors, small receptive fields, so better discrimination

• low sensitivity (large threshold): thigh, shoulder → less receptors, large receptive fields, so two points feel like one

Reactions to low intensity stimulus

neurons with larger receptive fields will have the biggest change from baseline because more of the receptor cells are being stimulated—those with smaller receptive fields have high spatial detail but have lower sensitivity to weak or broad inputs

Reactions to high intensity stimulus

neurons with smaller receptive fields get good information and detailed information about the surface, but it is hard to discriminate from baseline firing—those with larger receptive fields have a harder time pinpointing the exact location of a touch or signal, leading to lower spatial precision and poorer two-point discrimination

Receptive field size can vary as you move up in processing

Neurons in higher brain areas can put together detailed information from multiple neurons to identify the object anywhere on the stimulus surface—therefore, the RFs of these neurons can be high resolution as well because they get the details from earlier in processing and put them together