Sensory Processes Notes

Sensory Processes: Introduction

  • Even in calm circumstances, the world provides vast information.
  • The need to assimilate and interpret information raises two questions:
    • Which environmental aspects register with our senses?
    • How do sense organs efficiently acquire acquirable information?
  • Senses are input systems for acquiring data about the world, determining the environment's character.
  • A distinction is made between sensation (raw experiences) and perception (meaningful interpretation).
    • Sensation: Sense organs and neural pathways for acquiring stimulus information.
    • Perception: Higher levels of the cortex related to meaning.
  • The brain constantly sends messages back to early stages of sensory processing, modifying input processing.
  • The chapter is organized around different senses: vision, hearing, smell, taste, and touch.

Characteristics of Sensory Modalities

  • Any sensory system acquires information from the environment and transduces it into a neural representation.
  • Understanding a sensory system involves understanding the relevant dimensions of environmental information and how they are translated into neural representation.
  • Dimensions include intensity and everything else.

Threshold Sensitivity

  • Intensity is common to all forms of information.
    • Light intensity: Incoming photons per second.
    • Sound intensity: Amplitude of sound pressure waves.
  • More intense stimuli affect sense organs more strongly; sensory psychologists quantify the relation between physical stimulus intensity and sensation magnitude.

Absolute Thresholds: Detecting Minimum Intensities

  • Absolute threshold: Minimum stimulus magnitude reliably discriminated from no stimulus.
  • Sensory modalities are extremely sensitive.
  • Psychophysical procedures measure the relation between physical stimulus magnitude and psychological response.
  • Procedure: Present stimuli varying around the threshold, and record 'yes'/'no' responses.
  • Absolute threshold is defined as the stimulus value detected 50% of the time (e.g., 28 units in Figure 4.1).
  • Establishing a threshold is a first step in an experiment.
  • Hecht, Shlaer, and Pirenne (1942) experiment:
    • Determined the absolute threshold for vision.
    • Showed that human vision is virtually as sensitive as physically possible with only 7 photons needed to contact the critical molecules in the eye.

Difference Thresholds: Detecting Changes in Intensity

  • Change detection measures how much stimulus intensity must be raised from a standard level to be distinguishable.
  • Difference threshold/just noticeable difference (jnd): Minimum difference in stimulus magnitude to tell two stimuli apart.
  • Experiment: Observers respond 'more' or 'less' to comparison stimulus relative to a standard.
  • jnd is estimated as half the distance between the 75% and 25% points on the 'percent brighter' axis.
  • Weber and Fechner found that the larger the standard stimulus, the less sensitive the sensory system is to changes in intensity.
  • Weber-Fechner law: The intensity by which the standard must be increased to be noticed is proportional to the intensity of the standard.
  • Weber fraction: Constant of proportionality (e.g., 8% for light intensity).
  • Different sensory qualities have different jnd's expressed in terms of the Weber fraction (Table 4.2).
  • Psychophysical procedures have real-world applications: Sodium stearol lactylate, polysorbate 60, and calcium sulphate can be added as preservatives without fear of degrading the taste.

Suprathreshold Sensation

  • Scientists investigate the relation between suprathreshold stimulus intensities and corresponding sensory magnitudes.
  • Experiment: Assign a number to reflect how intense a light spot appears.
  • S. S. Stevens investigated suprathreshold sensation and derived Stevens' Law:
    • Perceived psychological magnitude (Y) is a power function of physical magnitude ().
    • Y = k\Phi^r where r is an exponent unique to each sensory modality.
  • If r < 1.0, increasing levels of physical intensity lead to progressively smaller increases in sensation.
  • If r > 1.0$$, increasing levels of physical intensity lead to progressively greater increases in sensation.
  • Relatively benign sensory modalities have less-than-1 exponents, while relatively harmful sensory modalities have greater-than-1 exponents.

Signal Detection Theory

  • A sensory system's job is to register the presence of something important via sensory information.
  • Information consists of signal and noise.
  • The task of the detector is to separate the signal from the noise.
  • Medical malpractice lawsuit example illustrates the distinction between sensation and bias.
  • Task of the radiologist is to detect a signal embedded in noise (Figure 4.5).

Hits and False Alarms

  • It is not possible to carry out the task of detecting a signal perfectly.
  • False alarm: Saying 'yes' to noise only.
  • Hit: Saying 'yes' to noise-plus-signal.
  • False-alarm rate: Proportion of noise-only trials that result in an incorrect 'yes' response.
  • Hit rate: Proportion of noise-plus-signal trials that result in a correct 'yes' response.
  • Observer detects a signal only under those conditions that the hit rate exceeds the false-alarm rate.

Sensitivity and Bias

  • An observer can choose their false-alarm rate.
  • Charlotte (conservative observer): Requires a lot of evidence. Low false-alarm rate, low hit rate.
  • Linda (liberal observer): Claims 'signal' given the slightest shred of evidence. High false-alarm rate, high hit rate.
    *A signal-detection allows separation of bias (β) and sensitivity (d').

Sensory Coding

*Each sensory system translates incoming physical information to an initial neural representation and encodes various features of the information to a corresponding neural representation.
*Receptors are specialized cells in the sense organs that translate incoming physical information into a neural signal.
*A receptor is a specialized kind of nerve cell or neuron, the signal is passed to connecting neurons, and the signal travels until it reaches its receiving area in the cortex.
*Electrical stimulation of the primary somatic sensory cortex can evoke the sensation of touch.

Coding of Intensity and Quality

*Our sensory systems evolved to pick up information about objects and events in the world, including intensity, quality, duration, location, and time of onset.
*The usual means is to record the electrical activity of a single cell while a subject is presented with various inputs or stimuli (Figure 4.6).
*Primary means for coding the intensity of a stimulus is via the number of neural impulses in each unit of time; that is, the rate of neural impulses.
*Stimulus intensity can also be coded by the temporal pattern of electrical impulses or by the number of neurons activated.
*Brain can distinguish between information from different sensory modalities because they involve different sensory nerves.
*Within a sense, coding is likely based on the specific neurons involved and/or the pattern of neural firing.

Interim Summary

  • The senses include seeing, hearing, smell, and taste, plus pressure and temperature, and pain, plus the body senses.
  • For each sense, two kinds of threshold sensitivity can be defined: absolute threshold and different threshold.
  • Sensation is often viewed as the process of detecting a signal that is embedded in noise.
  • Every sensory modality must recode or transduce physical energy into neural impulses.

Vision

  • Each sense responds to a particular form of physical energy, and for vision the physical stimulus is light.

Light and Vision

Electromagnetic energy travels in waves, with wavelengths varying from cosmic rays to radio waves.

  • Visible electromagnetic energy (light) makes up only a very small part of electromagnetic energy (400-700 nanometers).

The Visual System

Parts of the visual system

  • The visual system consists of the eyes, several parts of the brain, and the pathways connecting them.
  • The right half of the visual world is initially processed by the left side of the brain and vice-versa.
  • The eye contains two systems: one for forming the image and the other for transducing the image into electrical impulses.

The Image-Forming system

*Components of the image forming system: cornea, the pupil and the lens
*Analogy is often made between an eye and a camera- misleading
*function- to focus light reflected from an object to form an image of the object on the retina.
*Function is to focus the light rays onto the retina
*The image forming system consists of the cornea, the pupil, and the lens.
*Cornea- the transparent front surface of the eye.
Light enters at the cornea and rays are bent inward.
Completes the process of focusing the light of the retinas.

Neural Receptors in the retina:

*Two types- Rods and Cones
*Rods are for seeing at night
*Cones are for seeing during the day and they respond to high resolution sensations that include color.
*Fovea-the highest resolution region of the eye because of the densily packed cones.

Seeing Light

Sensitivity

  • Determined by the rods and cones.
  • Rods and Cones are activated under the different levels of light.
  • Cones and Rods are specialized for different tasks.

The connection of the gangleon helps ensure detailed form perception under well conditionts
The convergence of many RODS- HELP ENSURE SENSITIVITY TO LIGHT UNDER LOW lighting conditions

  • Dark adaptation improves our ability to see in the dark, with the eye's pupil changing size and photochemical changes in the receptors that increase the receptors' sensitivity to light

Seeing patterns

  • Visual acuity: the eye's ability to resolve details.
    Most common is the found in optometrist offices-Snellen chart
  • method is not good for young children or other people
  • method is designed to test acuity only for objects seen at a distance
  • method does not distinguish between spatial Acuity.

Seeing color

  • Short Wavelength appears blue
  • Medium is Green
  • Long Wave lengths appear RED
  • seeing color
    Color consist of the brain based on the Analysis of wavelenghts of light
  • color experiences dimensions: HUE, BRIGHT, SATURATION
    These Are summerized in a concept review table
    Color Mixture
    Disciminate can be generated by mixing 3 basic colors
    This was demonstrated many years ago Color matching experiment
    We project different colored lights to the same region of the retine
    Addictive color Mixture dot so close their Images on your retina over lap!!
    Color deficiences
    Match a variety Color by using color of only 2 primaries such ppl R dichromatic Vision
    Can still see color!!
    If we cannot tell or discrimenate between wavelength then that means are monochromatics!!( TRULY COLOR BLIND)
    Color blindness occurs more frequent in men than woman B.C genes are recessive locted on X chromosome!!

Theories of Color vision

Trichromatic Theory

R,G,B Three types of receptors , each sensitive to wide range
Joint action of these 3 receptors determins the sensation of color

  • holds quality of color is coded by the pattern of activity of three receptors and Not BY specific receptors for each of a multitude of colors!
    Opopent Color theory
    The visual system contains 2 type of color sensitive units, one to to RED or Green the to blue and yellow!
    Each unit respond to opposite was to its own opponent colors.!