Signal Detection Theory

Definition

Signal Detection Theory (SDT) is a mathematical and conceptual framework for understanding how individuals make decisions under conditions of uncertainty and noise. It helps explain the processes involved in detecting stimuli when there is variability in both the signal and the background noise.

Advantages

• Separates Sensitivity from Criterion: SDT distinguishes between a person's ability to detect a signal (sensitivity) and their decision-making strategy (criterion). This separation helps in analyzing how they respond to stimuli, allowing researchers to determine true perceptual capabilities.

• Sensitivity (d'): Also known as d prime, this metric assesses how effectively an individual can discern signal presence from noise, independent of their response bias.

• Criterion (β): Often denoted as beta, this represents the threshold set by the individual for determining whether a stimulus is present or absent, influencing whether they choose to respond to ambiguous stimuli. Individuals can be either liberal (lower criterion; more likely to report signals) or conservative (higher criterion; less likely to report signals).

Example Problem: Sarah's Sensitivity on Two Days

Sarah conducted psychophysical experiments over two days:

• Day 1:

  • Total Trials: 300, Target Present: 50% (150 Trials)

  • Hits: 105 (correctly reported presence)

  • Correct Rejections: 120 (correctly reported absence)

• Day 2:

  • Total Trials: 420, Target Present: 300, Target Absent: 120

  • Hits: 180 (correctly reported presence)

  • Correct Rejections: 108 (correctly reported absence)

Calculations:

• Day 1:

  • Misses: 150 - 105 = 45

  • False Alarms: 300 - 120 = 30

  • Hit Rate: 105 ÷ 150 = 0.70

  • False Alarm Rate: 30 ÷ 150 = 0.20

• Day 2:

  • Misses: 300 - 180 = 120

  • False Alarms: 120 - 108 = 12

  • Hit Rate: 180 ÷ 300 = 0.60

  • False Alarm Rate: 12 ÷ 120 = 0.1

Sensitivity Calculation:

• Day 1 d': 1.366

• Day 2 d': 1.535

Conclusion

Despite a lower hit rate on day 2 (0.60 vs. 0.70), Sarah's sensitivity improved because her criterion changed, which indicates that her ability to distinguish between signal absence and presence improved over time.

Applications of Signal Detection Theory

SDT is not just applicable to psychophysics but has far-reaching implications in various branches of psychology, including clinical, experimental, and sensory processing studies. It is instrumental in fields like telecommunications where signal detection is critical.

The Visual System

In upcoming lectures, we will explore both biological and psychological processes of visual perception through the lens of signal detection theory.

Evolution of the Eye

History

The evolution of eyes can be traced back over 500 million years to early organisms that developed primitive light-sensitive spots, which eventually led to the development of more complex structures in higher species.

Adaptations

Over time, eyes evolved to perceive light and dark and transformed into intricate organs like the human eye, which allows for detailed vision and depth perception.

Eye Position and Function

• Frontally Placed Eyes: Common in predatory animals. For example, humans have frontally placed eyes that facilitate binocular vision, enabling depth perception. Humans possess a visual field of approximately 190 degrees.

• Side Placed Eyes: Predominant in prey animals to maximize their field of view and enhance predator detection. For instance, deer can see in a visual field up to 310 degrees, although their depth perception is limited.

• Crocodiles: Despite having eyes positioned to the side (like prey animals), crocodiles possess adaptations that support their ambush hunting strategy.

Physiology of the Human Eye

Structure

• Sclera: The tough, white outer layer protecting the eye.

• Cornea: The transparent front part that refracts light entering the eye.

• Iris & Pupil: The iris controls the amount of light reaching the retina by adjusting pupil size, depending on lighting conditions.

• Lens: Changes shape to focus light on the retina, allowing clear vision of both near and distant objects.

Common Vision Conditions

• Myopia (Nearsightedness): Results when the lens cannot flatten sufficiently, or the eyeball is too elongated, making distant objects appear blurry.

• Hyperopia (Farsightedness): Occurs if the lens cannot thicken adequately, or the eyeball is too short; causing close objects to seem blurry.

• Astigmatism: Caused by an irregularly shaped cornea, leading to distorted vision as light does not focus evenly.

The Retina and Photoreceptors

Retina

The retina houses over 100 million photoreceptor cells (rods and cones) responsible for converting light into electrochemical signals for visual processing in the brain.

Types of Photoreceptors

• Rods: Numbering around 100 million, they are more sensitive to light and excel in dim conditions but do not detect color, primarily located outside the fovea.

• Cones: Comprising about 6-8 million, cones are responsible for color vision and detail detection, primarily concentrated in the fovea where no rods are present.

Blind Spot

Definition

The blind spot is an area devoid of photoreceptors, where the optic nerve exits the eye.

Overlay Vision

Our brains compensate for this visual gap through binocular vision and the contextual information from surrounding images, preventing us from being aware of the blind spot in our daily vision.

Retinal Cells and Neural Pathway to the Brain

Photoreceptors

Photoreceptors convert captured light into signals, transmitting these to various retinal cells which relay the information to ganglion cells.

Ganglion Cells

Ganglion cells are vital as they transfer visual information from the retina to the brain via the optic nerve.

Neural Processing in the Brain

Optic Chiasm

This crucial structure is where the optic nerves from both eyes intersect, enabling visual information processing in the opposite hemispheres of the brain.

Visual Relay

Around 90% of optic fibers project to the lateral geniculate nucleus (LGN) in the thalamus, which plays a central role in visual processing, while 10% is directed to the superior colliculus for quick reflexive responses.

Split-Brain Studies

Background

This examination of split-brain patients (due to surgical severing of the corpus callosum for epilepsy treatment) reveals distinctive observations in behavior and cognition.

Case Study

A notable case involved a split-brain patient identifying objects presented to each hemisphere separately, highlighting the specialized functions of the left (language) and right (non-verbal cues) hemispheres.

Language and Behavior

The left hemisphere governs language processing, often leading to inconsistent verbal explanations in these patients, who may respond differently based on visual cues presented to either hemisphere.

Conclusion

Research on split-brain patients provides vital insights into the lateralization of brain functions, significantly enhancing our understanding of how perception and cognition work within the human brain.