Sensation and Perception: Principles and Vision

Lecture 4: Sensation & Perception I

Outline

• Principles of Sensation & Perception

• Vision

Principles of Sensation & Perception

Introduction

Sensation and perception are foundational topics in psychology, describing how we detect and interpret information from the environment. Sensation refers to the process of receiving physical stimuli, while perception involves organizing and interpreting these sensory inputs to form meaningful experiences.

Sensation vs. Perception

• Sensation: The biological process of detecting physical stimuli from the external world and converting it into neural signals.

• Perception: The cognitive process of organizing and interpreting sensory information to understand our environment and guide behavior.

• Dissociation: Sensation and perception can be dissociated in certain neurological conditions (e.g., prosopagnosia, where face perception is impaired despite intact sensation).

• Neural Pathway: Sensory receptors → thalamus → cortex.

Example: The famous 'dress' illusion demonstrates how perception can differ even when the sensory input is the same, highlighting the ambiguity and interpretive nature of perception.

Psychophysics and Thresholds

Psychophysics is the study of the relationship between physical stimulus properties and psychological experience (perception).

• Absolute Threshold: The minimum intensity of a stimulus required for it to be detected 50% of the time.

• Difference Threshold (Just Noticeable Difference, JND): The smallest difference in stimulus intensity that can be detected 50% of the time.

• Weber's Law: The ability to detect a difference depends on the initial intensity of the stimulus. The law is expressed as:

$\frac{\Delta I}{I} = k$ Where $\Delta I$ is the change in intensity, $I$ is the initial intensity, and $k$ is a constant (Weber fraction).

• Examples of Absolute Thresholds:

Sense	Threshold Example
Vision	A candle flame 30 miles away on a dark, clear night
Hearing	A watch ticking 20 feet away
Smell	A drop of perfume in a six-room house
Taste	A teaspoon of sugar in a gallon of water
Touch	The wing of a fly falling on your cheek from 1 cm

Signal Detection Theory

Signal detection theory explains how decision-making occurs in the presence of uncertainty. It considers both the sensitivity to the stimulus and the decision criteria set by the observer.

• Hit: Correctly detecting a present stimulus

• Miss: Failing to detect a present stimulus

• False Alarm: Incorrectly detecting a stimulus when none is present

• Correct Rejection: Correctly identifying that no stimulus is present

Vision

Introduction

Vision is one of the most studied senses in psychology, involving the detection and interpretation of light stimuli. The process includes several anatomical structures and neural pathways.

Visual Pathway

• Light enters through the cornea and passes through the pupil (regulated by the iris).

• Light is focused onto the retina, which contains photoreceptors (rods and cones).

• Rods: Sensitive to low light, responsible for greyscale and peripheral vision.

• Cones: Sensitive to color and detail, concentrated in the fovea.

• Photoreceptors → bipolar cells → ganglion cells → optic nerve.

• Optic nerve → optic chiasm → lateral geniculate nucleus (LGN) of the thalamus → primary visual cortex (V1, occipital lobe).

Color Vision Theories

• Trichromatic Theory (Young-Helmholtz): Three types of cones sensitive to short (blue), medium (green), and long (red) wavelengths.

• Opponent-Process Theory: Color perception is controlled by opposing systems (red-green, yellow-blue, white-black).

Visual Processing Pathways

• Ventral Pathway ('What'): Occipitotemporal route, responsible for object perception and recognition.

• Dorsal Pathway ('Where'): Occipitoparietal route, responsible for object location and spatial processing.

• Lesion Studies: Damage to occipitotemporal areas impairs object identification (e.g., prosopagnosia), while occipitoparietal lesions affect spatial localization.

Gestalt Principles of Perception

Gestalt psychology emphasizes that the whole is different from the sum of its parts. Key principles include:

• Figure-Ground: Differentiating an object (figure) from its background (ground).

• Proximity: Objects close together are perceived as a group.

• Similarity: Similar objects are grouped together.

• Continuity: Perceiving smooth, continuous patterns rather than discontinuous ones.

• Closure: Filling in gaps to perceive complete objects.

Additional info:

• Examples such as the ambiguous cube and 'the dress' illustrate perceptual ambiguity and individual differences in interpretation.

• Signal detection theory is crucial for understanding how we make decisions under uncertainty, relevant for fields such as clinical psychology and neuroscience.