The Perceptual Process

Sensory Processing

Bottom-Up (Data-Based):Analysis that begins with the sense receptors and works up to the brain’s integration of sensory information.

Top-Down (Knowledge-Based):

Information processing guided by higher-level mental processes, as when we construct perceptions drawing on our experiences and expectations.

Sensation:

The process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.

Perception:

The process of organizing, and interpreting sensory information, enabling us to recognize meaningful objects and events.

Sensation and Perception

Three Steps in the Sensation and Perception of a Stimulus

All Sensory SystemsFollow the Same Plan

•Reception

•Transduction

•Coding

Sensory Reception

A Sensory System should…

• have different receptors for different forms of energy.

• discriminate among different intensities of stimulation.

• respond reliably.

• respond rapidly.

• suppress extraneous information.

A Variety of Sensory Receptors

Some Animals are Able to Sense Stimuli that Humans Cannot

A Sensory Transducer

Sensory Transduction

1. Stimulation causes deformation of the Paccinian corpuscle

2. Deformation opens pours in membrane allowing Na+ in

3. Na+ depolarizes the cell in the form of a generator potential

Generator Potential

•Similar to an EPSP.

•Amplitude is directly proportional to the stimulus strength.

Sensory Adaptation

Adaptation:

A reduction in response caused by prior or continuing stimulation

CodingA set of rules for translating information from one source to another.

Sensory information:

is coded into action potentials via stimulus…

•Intensity•Type•Location•Identity (via learning)

undergoes sensory adaptation and suppression

is summed across cognitive levels

Stimulus Intensity Coding

Nerve cells can fire at different thresholds for stimulus intensity

Each alone can only fire150 action potentials persecond

Combined they can fire asmany as 450 action potentialsper second

Stimulus Type Coding

Labeled Lines Each receptor has a distinct pathway to the brain

Different types of stimuli can be organized in specific areas of the brain

StimulusLocation Coding

SomatosensoryCortex

Homunculus

Sensory Stimuli Summation

Across Levels

Early Philosophy of Perception

Plato’s

“The Allegory of the Cave” (380 BCE)

Early Philosophy of Perception

• Heraclitus (540–480 BCE): “You can never step into the same river twice.”

–One of the first notions of perception

–Everything is always changing

–Idea that perceiver cannot perceive the same event in exactly the same manner each time

–Perception depends also on the qualities of the observer

Early Philosophy of Perception

• Democritus (460–370 BCE):

– The world is made up of atoms that collide with one another, and the sensations caused by these make contact with our sense organs

– Perception is the result of the physical interaction between the world and our bodies – so it should be trusted

– Idea of primary qualities and secondary qualities

Nativism and Empiricism

Nativism: The idea that the mind produces ideas that are not derived from external sources – certain mental abilities must be innate.

Empiricism: The idea that experience from the senses is the only source of knowledge – senses drive human nature.

Nativism and Empiricism

• Descartes’ (1596–1650):dualist view of the world: both mind and body exist

• Mind-Body Dualism:Originated by Descartes, the idea positing the existence of two distinct principles of being in the universe: spirit/soul and matter/body

• Monism:The idea that the mind and matter are formed from, or reducible to, a single ultimate substance or principle of being.

Nativism and Empiricism• Hobbes (1588–1678):

• believed that everything that could ever be known or even imagined had to be learned through the senses

• Materialism: The idea that physical matter is the only reality, and everything including the mind can be explained in terms of matter and physical phenomena

Nativism and Empiricism

• Locke (1632–1704):

• sought to explain how all thoughts, even complex ones, could be constructed from experience with a collection of sensations

• “tabula rasa”

Nativism and Empiricism

• Berkeley (1685–1753):

• studied ways in which perception is limited by the information available to us through our eyes.

• Concluded that all of our knowledge about the world must come from experience, no matter how limited perception may be.

The Dawn of Psychophysics• Weber (1795–1878):

• discovered that the smallest change in a stimulus, such as the weight of an object, that can be detected (Weber’s fraction) is a constant proportion of the stimulus level—“Weber’s Law”.

• Two-Point Threshold: The minimum distance at which two stimuli can be distinguished

• JND (Just Noticeable Difference/Difference Threshold: The smallest detectable difference between two stimuli, or the minimum change in a stimulus that can be correctly judged as different from a reference stimulus. Also known as difference threshold.

The Dawn of Psychophysics

• Fechner (1801–1887):

• invented Psychophysics: the science of defining quantitative relationships between physical and psychological (subjective) events.

• Pioneering work relating changes in the physical world to changes in our psychological experiences.

• Fechner’s Law: suggests that your psychological experience of an energy increases less quickly than the actual physical stimulus.