NEUR2020 Neuroscience for Psychologists

Tutorial 3: Psychophysics and Signal Detection Theory

Worksheet Questions

Q1. Why are sulci functionally advantageous for the cerebral cortex? (2 marks)

• Definition of Sulci: Sulci are grooves located in the brain’s cortex.

• **Functionality of Sulci: **

  • Allow for increased cortical folding, which enhances the brain surface area.

  • More neurons and connections can fit into the skull, enhancing cognitive capacity without increasing the head's size.

  • They also optimize neural wiring by reducing the distance between related brain regions, which increases efficiency.

Q2. What brain regions does the central sulcus separate, and what is its relevance for sensory or motor function? (3 marks)

• Division by Central Sulcus:

  • The central sulcus separates the frontal lobe from the parietal lobe.

• Sensory/Motor Functionality:

  • Divides the primary motor cortex, responsible for voluntary movement (motor output), from the primary somatosensory cortex, which manages touch and body position (sensory input).

  • This separation facilitates coordinated sensorimotor functions.

Common Mistakes in Understanding Sulci and Central Sulcus

• Mapping the Brain: Incorrectly stating that sulci only assist in mapping the brain.

• Surface Area Explanation: Merely describing how sulci increase surface area does not provide insight into why this is vital for neurons.

• Identifying Wrong Sulcus: Errors in identifying the longitudinal or lateral sulcus instead of focusing on the central sulcus.

• Lack of Detail: Insufficient information regarding the sensory/motor functions involved or an overemphasis on executive function attributes.

Psychophysics

Definition

• Psychophysics: The study of the relationship between physical stimuli and the sensations and perceptions they produce.

• Historical Context: Considered one of the earliest forms of experimental psychology.

• Key Illusions that Contributed to Psychophysics:

  • Zöllner Illusion, 1860

  • Helmholtz Illusion, 1867

Fundamental Questions of Psychophysics

1. Stimulus Intensity vs. Perceptual Intensity:

   • Example: If I double the luminance of an object, how much brighter does it appear?

2. Sensitivity of Sensory Systems:

   • Example: What is the weakest signal that can be reliably detected?

3. Quantifying Perceptual Experience:

   • Example: Given that loudness is subjective, how can it be reliably quantified?

4. Perceptual Decision Making:

   • Example: What role does sensation play in shaping perception? Are there additional influencing factors?

Perceptual Decisions

• Commonality Among Decisions:

  • All decisions made under uncertainty.

Signal Detection Theory

Definition

• Signal Detection Theory: A framework for analyzing decision-making processes under conditions of uncertainty, particularly in detecting or discriminating signals from noise.

• Types of Noise:

  • External Noise: External environmental factors, such as background noise impeding communication.

  • Internal Noise: Neural responses that can cloud perception of a signal versus responses driven by decision-making processes.

Tumour Detection Test Overview

• Example Scenario:

  • Two doctors assess 100 chest x-rays, with 50 containing tumours and 50 clear of tumours.

• Analysis of Detections:

  • Determine what constitutes the "signal" (tumour present) and the "noise" (tumour not present).

Performance Metrics

• Detection Rates:

  • Doctor A detects 40 out of 50 tumours.

  • Doctor B detects 45 out of 50 tumours.

• Hit Rate Calculation:

  • Hit Rate for Doctor A = rac{40}{50} = 80 ext{%}

  • Hit Rate for Doctor B = rac{45}{50} = 90 ext{%}

Evaluation of Doctors' Performance

• **Identifying the Better Doctor:

  • Based on detection, which doctor would you choose? What are the implications of Hit Rates?**

• Considering False Positives: Each doctor may have misidentified tumours in clear x-rays.

  • Doctor A calls 5 clear x-rays as tumours (False Alarms).

  • Doctor B calls 20 clear x-rays as tumours (False Alarms).

Overall Decision-Making Analysis

• False Alarm Rate Calculation:

  • False Alarm Rate for Doctor A = rac{5}{50} = 10 ext{%}

  • False Alarm Rate for Doctor B = rac{20}{50} = 40 ext{%}

• Understanding Responses Across Outcomes:

  • Classifications:

  • Hit: Tumour detected when present.

  • Miss: Tumour present but not detected.

  • False Alarm: Tumour reported when absent.

  • Correct Rejection: No tumour reported when absent.

Sensitivity and Criterion

• Sensitivity (d’):

  • Measures the ability to distinguish between signal (tumour present) and noise (tumour absent).

  • Example of Distribution Measurement:

  • d’ = 0.5 represents considerable overlap (less sensitivity).

  • d’ = 4 indicates less overlap (greater sensitivity).

• Criterion (c):

  • A measure for response bias, representing how conservative or liberal a response is.

  • Key factors:

  • A larger c indicates a conservative threshold (more cautious in reporting)

  • A smaller c indicates a liberal threshold (more prone to report potential cases).

Example Application**

• Comparative Analysis of Doctor Performance:

  • Doctor A (d’ = 2.12, c = 0.2) is less likely to miss tumours (higher sensitivity, lower false alarms).

  • Doctor B (d’ = 1.53, c = -0.51) tends to be more liberal (higher false alarms, possible misses).

Real-World Applications of Criteria

• Liberal Criterion Benefits: Situations requiring maximum sensitivity (e.g., cancer screening).

• Conservative Criterion Benefits: Scenarios where avoiding false alarms is crucial (e.g., forensic evidence assessment).

Conclusion

• Final Summary of the Tumour Detection Test:

  • Who is ultimately the better doctor can depend on case specifics. The balance of sensitivity versus criterion in real-world situations influences decisions made in healthcare.

Next Steps

• Preparation for Next Week: Continued exploration of Psychophysics and Signal Detection Theory in Part 2.