What is the primary difference between sensory sensitivity and sensory resolution?

Sensitivity is the ability to detect a stimulus at low intensities (the 'threshold'), while resolution is the ability to distinguish between two separate stimuli applied close together in space.

How does the size of a receptive field affect the resolution of touch?

Smaller receptive fields result in higher resolution because the brain can pinpoint the stimulus to a smaller area. Areas like fingertips have many small receptive fields, while the back has fewer, larger ones.

Compare the expected results of a two-point discrimination test on the fingertip versus the forearm.

The fingertip will have a much smaller minimum distance threshold (high resolution) due to high receptor density, whereas the forearm will require a much larger distance to perceive two distinct points.

Why is it a mistake to allow a subject to see the stimulus during a touch receptor investigation?

Visual input provides the brain with information about the stimulus location and timing, which can lead the subject to 'feel' the touch even if the sensory receptors haven't reached their threshold, creating a false result.

What error occurs if the pressure applied during a habituation test is inconsistent?

Inconsistent pressure changes the intensity of the stimulus; if pressure increases, it may re-activate receptors and reset the habituation timer, making it impossible to accurately measure the duration of the response.

Why might a subject report feeling 'warmth' from a thermometer at $37^{\circ}C$ but only 'touch' from one at $33^{\circ}C$?

Temperature receptors detect deviations from body temperature. Since $37^{\circ}C$ is closer to internal body temp but higher than typical skin surface temp, it triggers a response, while $33^{\circ}C$ may be too close to the skin's surface temp to be perceived as a change.

Define 'Sensory Habituation' in the context of touch receptors.

It is the decrease in the frequency of nerve impulses sent to the brain in response to a stimulus that is maintained at a constant intensity over a period of time.

What is a 'Generator Potential'?

It is the initial change in the potential difference across the receptor cell membrane caused by a stimulus. If this potential reaches a certain threshold, it triggers an action potential in the sensory neurone.

What does the 'Two-Point Threshold' measure?

It measures the minimum distance at which two simultaneous points of contact can be reliably distinguished as separate, serving as an indicator of receptor density.

Why is a rest period required between repeated habituation trials?

A rest period allows the receptors and the associated neural pathways to recover from their habituated state, ensuring that the next trial measures the baseline sensitivity rather than a lingering suppressed state.

Investigating Touch and Temperature Receptors | AQA A-Level Biology

1. Definition & Core Concepts

Sensory Receptors: These are specialized cells or nerve endings designed to detect specific environmental changes, such as mechanical pressure or thermal energy. In the skin, different receptor types are dedicated to distinct stimuli, ensuring that the nervous system receives precise information about the external environment.
Receptor Specificity: Each receptor type is tuned to a particular modality; for instance, thermoreceptors respond to temperature changes while mechanoreceptors respond to physical deformation. This specificity allows the brain to distinguish between a hot surface and a sharp edge based on which neural pathways are activated.
Spatial Resolution: This refers to the ability of the sensory system to distinguish between two closely spaced stimuli. It is directly proportional to the density of receptors in a given area of skin, with higher densities allowing for finer discrimination.

2. Underlying Principles of Sensory Detection

Thermal Deviation Principle: Temperature receptors in the skin do not measure absolute temperature but rather detect significant deviations from the body's current surface temperature. When a stimulus is close to skin temperature, the receptors remain relatively inactive, and the brain perceives only the mechanical touch of the object.
Sensory Habituation: This is a physiological process where the neural response to a constant, unchanging stimulus decreases over time. It allows the nervous system to filter out background information, such as the feeling of clothing, to remain sensitive to new or changing stimuli.
Two-Point Discrimination: The ability to perceive two distinct points of contact depends on the size of the receptive fields. If two points fall within the same receptive field of a single neuron, they are perceived as one; if they fall into separate fields, they are perceived as two.

Diagram showing the two-point discrimination test where two points close together are perceived as one, while points further apart are perceived as two distinct contacts.

3. Methods & Techniques

Temperature Sensitivity Testing

Procedure: Use water baths set to specific temperatures (e.g., $45^{\circ}C$ and $5^{\circ}C$ ) to heat or cool a probe. Apply the probe to various skin sites and record whether the subject perceives 'warmth/cold' or merely 'touch'.
Analysis: Compare the threshold of detection across different body parts to map thermoreceptor distribution.

Habituation Measurement

Procedure: Apply a consistent, light mechanical stimulus (like a soft brush) to a fixed spot and use a stopwatch to measure the time until the subject no longer perceives the sensation. Ensure the subject's eyes are covered to prevent visual bias.

Resolution Mapping (Two-Point Threshold)

Procedure: Use a calibrated instrument (like a compass or modified paperclip) to apply one or two points of pressure simultaneously. Gradually reduce the distance between the points until the subject can no longer distinguish them as separate.

4. Key Distinctions

Sensitivity vs. Resolution: Sensitivity refers to the minimum intensity of a stimulus required for detection (e.g., how light a touch can be felt), whereas resolution refers to the spatial precision of that detection (e.g., how close two touches can be).
Habituation vs. Adaptation: While often used interchangeably in general contexts, habituation specifically refers to the psychological or physiological decrease in response to a repeated stimulus, whereas adaptation often refers to the sensory receptor's decreased firing rate.

Feature	High Resolution Areas (e.g., Fingertips)	Low Resolution Areas (e.g., Back)
Receptor Density	Very High	Low
Receptive Field Size	Small	Large
Minimum Gap Detected	$\approx 2-3\text{ mm}$	$\approx 40-50\text{ mm}$

5. Exam Strategy & Tips

Identify Control Variables: In sensory experiments, always ensure the intensity of pressure and the surface area of the stimulus are kept constant to ensure valid comparisons between body regions.
Address Subjectivity: Recognize that these experiments rely on self-reporting. To improve reliability, use a double-blind approach where possible or include 'sham' trials (applying no stimulus) to check for subject bias.
Randomization: When testing two-point discrimination, use a random sequence of one-point and two-point applications (e.g., determined by a die roll) to prevent the subject from predicting the answer and skewing the results.

6. Common Pitfalls & Misconceptions

Confusing Touch and Temperature: Students often forget that a very hot or cold object still activates pressure receptors. The experiment must distinguish between the mechanical sensation of contact and the thermal sensation of temperature.
Assuming Uniformity: A common error is assuming that habituation rates or resolution thresholds are the same for everyone. In reality, there is significant biological variation based on age, skin thickness, and individual neural processing.
Visual Cues: Failing to properly blindfold the subject is a major procedural error. Visual confirmation of a stimulus significantly lowers the perceived threshold and invalidates the data regarding pure tactile or thermal receptors.

Investigating Touch and Temperature Receptors

Summary

1. Definition & Core Concepts

2. Underlying Principles of Sensory Detection

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Investigating Touch and Temperature Receptors

Summary

1. Definition & Core Concepts

2. Underlying Principles of Sensory Detection

3. Methods & Techniques

Temperature Sensitivity Testing

Habituation Measurement

Resolution Mapping (Two-Point Threshold)

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Temperature Sensitivity Testing

Habituation Measurement

Resolution Mapping (Two-Point Threshold)