1. General Principles: The senses as physical instruments
Kate Fischl
9/8/17

2. Sensory Systems
Physical, chemical, and biological methods for gathering information
Interoceptors – monitor internal state
Exteroceptors – monitor external surroundings
Can we learn about these systems from examining their anatomical structure?
Monitor through physical and chemical influence
Psychophysics – how people’s impressions of their measured experience differs from person to person when the stimulus is changed
- Examine physical structures, sometimes allows us to learn about how that system processes info, and sometimes doesn’t

3. Basic Peripheral Sensory Pathway
Sensory systems should be:
Selective
Fast
Sensitive
Reliable
Adaptation – when firing frequency declines even though stimulus remains constant
Maintained discharge – a baseline firing rate which results from spontaneous deploarization of the terminals occuring in the absence of a deliberately applied stimulus

4. Selectivity Respond to: Physical location Temperature
Tuning Curves of Auditory Nerve Fibers
Respond to:
Physical location
Temperature
Mechanical deformation
Injury …
Cone Spectral Sensitivity Curves
Receptive field
- Receptive to temp, mechanical deformation, injury

5. Speed Set limits to sensitivity Determines reaction time

6. Sensitivity
Sensitivity  Average number of extra impulses elicited per unit of applied energy
High sensitivity is needed for reliability
High sensitivity not always advantageous
What limits gathering of important information?
- Although have to consider cases where neuron rarely fires, so when it does, it’s not noise but meaningful signal

7. Selectivity and Sensitivity
Which box is a different color?
Which fidget is a different color?
Which circle is darker?
Receptive field
- Receptive to temp, mechanical deformation, injury
From:

8. Reliability and noise
Use the signal/noise ratio to quantify signal noise
Consider variability not average rate
What is the origin of the noise?
False positives, false negatives

9. Transformations of Scale and Intensity
Sensory surfaces mapped to cerebral cortex
Example transformations:
Fundamental frequency to pitch
Logarithmic scales for intensity
Relative vs. absolute values
We don’t really understand the details of how the brain transforms info though

10. Fourier Transform to Convey Sound
Rapidly changing sound pressure can be represented as the superposition of functions (sine waves)
This is an example of a set of basis functions
Frequencies change more slowly than the pressure values, allowing for biological processing

11. Fourier Transforms to Convey Luminance
Represent a photograph as a sum of a harmonic set of sinusoidal spatial waves

12. Modulation Transfer Function (MTF)
MTF shows how sinusoidal gratings are handled by an optical system
MTF enables characterization of camera lenses and screens, in addition to eyes
Limiting performance is set by the diffraction of light at the pupil
Cut-off frequency = lambda/D
- Temporal contrast sensititivy curves
Spectral sensitivity curves
Audiograms
but these sorts of curves are only good for linear systems and then brain is not a linear system
When you extract these features – i.e. turn it into frequency measurements, it’s no longer a *copy* of the sensory signal in the brain

13. Selectivity for Pattern; trigger features
Location
Frequency
Motion
Map the receptive field of a particular neuron
(+) = a light in this location causes an increase in firing rate
(-) = alight in this location causes a decrease in firing rate

14. Invariance of pattern selectivity
The luminance, contrast, and position doesn’t matter
Responds most to rightward movement
- What excites the cell is called the trigger feature

15. Neurons at different levels in the visual pathway
Hubel and Wiesel found properties:
Orientation selectivity
Directional selectivity
Size selectivity
Binocularity
Simple vs. complex
Area 17 – primary visual cortex
Area 18 and 19 – secondary visual areas
This is for a cat, but a monkey is similar
Simple cells mostly “on” or “off” for their receptive fields, complex cells have both “on” and “off” regions in one receptive field
- Originally believed that simple cells fed to complex cells but now they think it’s more complicated

16. Conscious Perception
Input from sensory systems can be processed differently by different people, resulting in different observations
What enters your sensory pathway is different than what is perceived
What goes in via your sensory pathway is different than what you actually perceive